LTC3812-5 synchronous step-down switching regulator controller that di
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LTC3812-5 Current Mode Synchronous Switching Regulator Controller DESCRIPTION
LTC3812-5 synchronous step-down switching regulator controller that directly step down voltages from input, making ideal telecom automotive applications. LTC3812-5 uses constant on-time valley current control architecture deliver very duty cycles with accurate cycle-by-cycle current limit without requiring sense resistor. precise internal reference provides 0.5% accuracy. high bandwidth (25MHz) error amplifier provides very fast line load transient response. Large gate drivers allow LTC3812-5 drive large power MOSFETs higher current applications. operating frequency selected external resistor compensated variations VIN. shutdown allows LTC3812-5 turned reducing supply current <230A. Integrated bias control generates gate drive power from input supply during start-up when output shortcircuit occurs, with addition small external SOT23 MOSFET. When regulation, power derived from output higher efficiency.
Lare registered trademarks Linear Technology Corporation. other trademarks property their respective owners. Protected U.S. Patents, including 5481178, 5847554, 6304066, 6476589, 6580258, 6677210, 6774611.
High Voltage Operation: Large Gate Drivers Current Sense Resistor Required Dual N-Channel MOSFET Synchronous Drive Extremely Fast Transient Response ±0.5% 0.8V Voltage Reference Programmable Soft-Start Generates 5.5V Driver Supply Selectable Pulse Skip Mode Operation Power Good Output Voltage Monitor Adjustable On-Time/Frequency: tON(MIN) 100ns Adjustable Cycle-by-Cycle Current Limit Undervoltage Lockout Driver Supply Output Overvoltage Protection Thermally Enhanced 16-Pin TSSOP Package
APPLICATIONS
Telecom Base Station Power Supplies Networking Equipment, Servers Automotive Industrial Control Systems
TYPICAL APPLICATION
High Efficiency High Voltage Step-Down Converter
110k 100k PGOOD PGOOD VRNG RUN/SS 1000pF 47pF 200k 1.89k SGND PGND BOOST EXTVCC INTVCC Si7850DP MBR1100 Si7850DP 4.7H LTC3812-5 0.1F NDRV 100k ZXMN10A07F
Efficiency Load Current
EFFICIENCY
VOUT
COUT 270F
LOAD CURRENT
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LTC3812-5 ABSOLUTE MAXIMUM RATINGS
(Note
Supply Voltages INTVCC -0.3V (INTVCC PGND), (BOOST -0.3V BOOST (Continuous) -0.3V BOOST (400ms) -0.3V EXTVCC -0.3V (EXTVCC INTVCC). -12V (NDRV INTVCC) Voltage. -0.3V Voltage (Continuous). Voltage (400ms) Voltage (Continuous) -0.3V
Voltage (400ms) -0.3V RUN/SS Voltage -0.3V PGOOD Voltage -0.3V VRNG, Voltages -0.3V Voltage -0.3V 2.7V INTVCC, EXTVCC Currents .50mA Operating Temperature Range (Note LTC3812E-5 -40°C 85°C LTC3812I-5 -40°C 125°C Junction Temperature (Notes 125°C Storage Temperature Range. -65°C 150°C Lead Temperature (Soldering, sec) 300°C
CONFIGURATION
VIEW VRNG PGOOD RUN/SS SGND BOOST PGND INTVCC EXTVCC NDRV
PACKAGE 16-LEAD PLASTIC TSSOP TJMAX 125°C, 38°C/W EXPOSED (PIN GND, MUST SOLDERED
ORDER INFORMATION
LEAD FREE FINISH LTC3812EFE-5#PBF LTC3812IFE-5#PBF LEAD BASED FINISH LTC3812EFE-5 LTC3812IFE-5 TAPE REEL LTC3812EFE-5#TRPBF LTC3812IFE-5#TRPBF TAPE REEL LTC3812EFE-5#TR LTC3812IFE-5#TR PART MARKING* 3812EFE-5 3812IFE-5 PART MARKING* 3812EFE-5 3812IFE-5 PACKAGE DESCRIPTION 16-Lead Plastic TSSOP 16-Lead Plastic TSSOP PACKAGE DESCRIPTION 16-Lead Plastic TSSOP 16-Lead Plastic TSSOP TEMPERATURE RANGE -40°C 85°C -40°C 125°C TEMPERATURE RANGE -40°C 85°C -40°C 125°C
Consult Marketing parts specified with wider operating temperature ranges. *The temperature grade identified label shipping container. more information lead free part marking, http://www.linear.com/leadfree/ more information tape reel specifications,
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LTC3812-5
denotes specifications which apply over full operating temperature range, otherwise specifications 25°C, INTVCC VBOOST VRNG VEXTVCC VNDRV VFCB unless otherwise specified.
SYMBOL Main Control Loop INTVCC IBOOST INTVCC Supply Voltage INTVCC Supply Current INTVCC Shutdown Current BOOST Supply Current Feedback Voltage RUN/SS 1.5V (Notes RUN/SS RUN/SS 1.5V (Note RUN/SS (Note 85°C -40°C 85°C -40°C 125°C (I-grade) INTVCC (Note VRNG 0.76V VRNG 0.76V VRNG INTVCC, 0.76V VRNG 0.84V VRNG 0.84V VRNG INTVCC, 0.84V 0.8V (Note VFCB Rising RUN/SS INTVCC Rising, INDRV 100A INTVCC Rising, NDRV INTVCC EXTVCC INTVCC Rising, NDRV INTVCC, EXTVCC INTVCC Falling 100A 300A 2500A Rising Falling Returning IPGOOD -7.5 12.5 -12.5
ELECTRICAL CHARACTERISTICS
PARAMETER
CONDITIONS
4.35
UNITS
0.796 0.794 0.792 0.792 0.800 0.800 0.800 0.800 0.002 -300 -200 0.75 1.85 4.05 4.05 8.70
0.804 0.806 0.806 0.808 0.02
VFB,LINE VSENSE(MAX)
Feedback Voltage Line Regulation Maximum Current Sense Threshold
VSENSE(MIN)
Minimum Current Sense Threshold
IVFB AVOL(EA) VFCB IFCB VRUN/SS IRUN/SS VVCCUV
Feedback Current Error Amplifier Open-Loop Gain Error Unity Gain Crossover Frequency Threshold Current Shutdown Threshold RUN/SS Source Current INTVCC Undervoltage Lockout Linear Regulator Mode External Supply Mode Trickle-Charge Mode
0.85 4.35 4.35 9.30
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Oscillator tON(MIN) tOFF(MIN) Driver IBG,PEAK RBG,SINK ITG,PEAK RTG,SINK PGOOD Output VFBOV VFB,HYST VPGOOD PGOOD Upper Threshold PGOOD Lower Threshold PGOOD Hysterisis PGOOD Voltage Driver Peak Source Current Driver Pull-Down RDS(ON) Driver Peak Source Current Driver Pull-Down RDS(ON) On-Time Minimum On-Time Minimum Off-Time 1.55 2.15
LTC3812-5
denotes specifications which apply over full operating temperature range, otherwise specifications 25°C, INTVCC VBOOST VRNG VEXTVCC VNDRV VFCB unless otherwise specified.
SYMBOL IPGOOD Delay Regulators VEXTVCC EXTVCC Switchover Voltage EXTVCC Rising EXTVCC Hysterisis INTVCC Voltage from EXTVCC VEXTVCC VINTVCC Dropout INTVCC Load Regulation from EXTVCC INTVCC Voltage from NDRV Regulator INTVCC Load Regulation from NDRV Current into NDRV Linear Regulator Timeout Enable Threshold Maximum Supply Voltage Maximum Current into NDRV/INTVCC Trickle Charger Shunt Regulator Trickle Charger Shunt Regulator, INTVCC 16.7V (Note VEXTVCC 20mA, VEXTVCC 20mA, VEXTVCC Linear Regulator Operation 20mA, VEXTVCC VNDRV VINTVCC
ELECTRICAL CHARACTERISTICS
PARAMETER PGOOD Leakage Current PGOOD Delay
CONDITIONS VPGOOD Falling
UNITS
0.25 0.01 0.01
VINTVCC,1 VEXTVCC,1 VLOADREG,1 VINTVCC,2 VLOADREG,2 INDRV INDRVTO VCCSR ICCSR
Note Stresses beyond those listed under Absolute Maximum Ratings cause permanent damage device. Exposure Absolute Maximum Rating condition extended periods affect device reliability lifetime. Note LTC3812E-5 guaranteed meet performance specifications from 85°C. Specifications over -40°C 85°C operating temperature range assured design, characterization correlation with statistical process controls. LTC3812I-5 guaranteed meet performance specifications over full -40°C 125°C operating temperature range. Note calculated from ambient temperature power dissipation according following formula: LTC3812-5: 38°C/W) PARAMETER Maximum MOSFET Gate Drive INTVCC INTVCC LTC3810 100V 6.35V 6.2V
Note LTC3812-5 tested feedback loop that servos reference voltage with forced voltage between Note dynamic input supply current higher power MOSFET gate charging being delivered switching frequency fOSC). Note Guaranteed design. subject test. Note This includes overtemperature protection that intended protect device during momentary overload conditions. Junction temperature will exceed 125°C when overtemperature protection active. Continuous operation above specified maximum operating junction temperature impair device reliability. Note current into NDRV INTVCC.
LTC3810-5 4.5V 4.2V
LTC3812-5 4.5V 4.2V
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LTC3812-5 TYPICAL PERFORMANCE CHARACTERISTICS
Load Transient Response
20V/DIV VOUT 50mV/DIV IOUT 5A/DIV INTVCC, VOUT 2V/DIV 2A/DIV 10s/DIV FRONT PAGE CIRCUIT LOAD STEP
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Start-Up
VOUT 5V/DIV INTVCC VOUT SS/TRACK 2V/DIV 5A/DIV
Short-Circuit/Fault Timeout Operation
2ms/DIV FRONT PAGE CIRCUIT ILOAD 0.5A
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5ms/DIV FRONT PAGE CIRCUIT RSHORT
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Short-Circuit/Foldback Operation
VOUT 5V/DIV 5A/DIV VOUT 100mV/DIV
Pulse Skip Mode Operation
Efficiency Input Voltage
FRONT PAGE CIRCUIT 250kHz ILOAD FORCED CONTINUOUS ILOAD 0.5A FORCED CONTINUOUS ILOAD 0.5A PULSE SKIP
2A/DIV
EFFICIENCY
0.5A/DIV
200s/DIV FRONT PAGE CIRCUIT
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20s/DIV FRONT PAGE CIRCUIT IOUT 100mA INTVCC
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INPUT VOLTAGE
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Efficiency Load Current
EFFICIENCY FREQUENCY (kHz)
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Frequency Input Voltage
FRONT PAGE CIRCUIT LOAD FREQUENCY (kHz)
Frequency Load Current
FRONT PAGE CIRCUIT FORCED CONTINUOUS
PULSE SKIP
LOAD
VOUT INTVCC 250kHz LOAD CURRENT
INPUT VOLTAGE
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LOAD CURRENT
LT1108 TPC12
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LTC3812-5 TYPICAL PERFORMANCE CHARACTERISTICS
Voltage Load Current
VOLTAGE CURRENT SENSE THRESHOLD (mV) VRNG FRONT PAGE CIRCUIT VRNG -100 -200 -300 -400 VOLTAGE 1000 CURRENT 10000
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Current Sense Threshold Voltage
10000
On-Time Current
INTVCC
1.4V ON-TIME (ns) 0.7V 0.5V 1000
LOAD CURRENT
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On-Time Temperature
MAXIMUM CURRENT SENSE THRESHOLD (mV) 300A
Current Limit Foldback
VRNG INTVCC MAXIMUM CURRENT SENSE THRESHOLD (mV)
Maximum Current Sense Threshold VRNG Voltage
ON-TIME (ns)
TEMPERATURE (°C)
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VRNG VOLTAGE
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Maximum Current Sense Threshold Temperature
MAXIMUM CURRENT SENSE THRESHOLD (mV) VRNG INTVCC 0.803 0.802 REFERENCE VOLTAGE 0.801 0.800 0.799 0.798
Reference Voltage Temperature
Driver Peak Source Current Temperature
VBOOST VINTVCC
PEAK SOURCE CURRENT
TEMPERATURE (°C)
0.797
TEMPERATURE (°C)
TEMPERATURE (°C)
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LTC3812-5 TYPICAL PERFORMANCE CHARACTERISTICS
Driver Pull-Down RDS(ON) Temperature
1.75 1.50 1.25 RDS(ON) 1.00 0.75 0.50 0.25 VBOOST VINTVCC PEAK SOURCE CURRENT RDS(ON)
Driver Peak Source Current Supply Voltage
Driver Pull-Down RDS(ON) Supply Voltage
TEMPERATURE (°C)
DRVCC/BOOST VOLTAGE
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DRVCC/BOOST VOLTAGE
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EXTVCC Switch Resistance Temperature
RESISTANCE INTVCC CURRENT (mA)
INTVCC Current Temperature
INTVCC
INTVCC Shutdown Current Temperature
INTVCC
INTVCC CURRENT
TEMPERATURE (°C)
TEMPERATURE (°C)
TEMPERATURE (°C)
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INTVCC Current INTVCC Voltage
INTVCC CURRENT (mA) INTVCC VOLTAGE INTVCC CURRENT
INTVCC Shutdown Current INTVCC Voltage
INTVCC VOLTAGE
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LTC3812-5 TYPICAL PERFORMANCE CHARACTERISTICS
RUN/SS Pull-Up Current Temperature
RUN/SS SHUTDOWN THRESHOLD SS/TRACK CURRENT TEMPERATURE (°C)
Shutdown Threshold Temperature
TEMPERATURE (°C)
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FUNCTIONS
(Pin On-Time Current Input. resistor from this one-shot timer current thereby switching frequency. VRNG (Pin Sense Voltage Limit Set. voltage this sets nominal sense voltage maximum output current from 0.5V resistive divider from INTVCC. nominal sense voltage defaults 95mV when this tied ground, 215mV when tied INTVCC. PGOOD (Pin Power Good Output. Open-drain logic output that pulled ground when output voltage between ±10% regulation point. output voltage must regulation least 120s before power good output pulled ground. (Pin Pulse Skip Mode Enable Pin. This provides pulse skip mode enable/disable control. Pulling this below 0.8V disables pulse skip mode operation forces continuous operation. Pulling this above 0.8V enables pulse skip mode operation. This also connected feedback resistor divider from secondary winding inductor regulate second output voltage. (Pin Error Amplifier Compensation Point Current Control Threshold. current comparator threshold increases with control voltage. voltage ranges from 2.6V with 1.2V corresponding zero sense voltage (zero current). (Pin Feedback Input. Connect through resistor divider network VOUT output voltage. RUN/SS (Pin RUN/Soft-Start Input. soft-start, capacitor ground this sets ramp rate output voltage (approximately 0.6s/F). Pulling this below 1.5V will shut down LTC3812-5, turn both external MOSFET switches reduce quiescent supply current 224A. SGND (Pin Signal Ground. small-signal components should connect this ground eventually connect PGND point. NDRV (Pin Drive Output External Pass Device Linear Regulator INTVCC. Connect gate external NMOS pass device pull-up resistor input voltage VIN.
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LTC3812-5 FUNCTIONS
EXTVCC (Pin 10): External Driver Supply Voltage. When this voltage exceeds 4.2V, internal switch connects this INTVCC through turns external MOSFET connected NDRV, that controller gate drive drawn from EXTVCC. INTVCC (Pin 11): Main Supply Driver Supply Pin. internal circuits bottom gate output driver powered from this pin. INTVCC should bypassed SGND PGND with (X5R better) capacitor close proximity LTC3812-5. (Pin 12): Bottom Gate Drive. drives gate bottom N-channel synchronous switch MOSFET. This swings from PGND INTVCC. PGND (Pin 13): Bottom Gate Return. This connects source pull-down MOSFET driver normally connected ground. (Pin 14): Switch Node Connection Inductor Bootstrap Capacitor. Voltage swing this from Schottky diode (external) voltage drop below ground VIN. (Pin 15): Gate Drive. drives gate N-channel synchronous switch MOSFET. driver draws power from BOOST returns pin, providing true floating drive MOSFET. BOOST (Pin 16): Gate Driver Supply. BOOST supplies power floating driver. BOOST should bypassed with (X5R better) 0.1F capacitor. additional fast recovery Schottky diode from INTVCC BOOST will create complete floating charge-pumped supply BOOST. Exposed (Pin 17): Ground. Exposed must soldered ground.
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LTC3812-5 FUNCTIONAL DIAGRAM
INTVCC EXTVCC NDRV INTVCC INTVCC MODE LOGIC 5.5V 0.8V
NDRV
0.8V
INTVCC
4.2V
INTVCC EXTVCC
270A
1.4A
5.5V
4.7V BOOST
100nA 2.4V (76pF) IION
TIMEOUT LOGIC FCNT
ICMP
IREV
SWITCH LOGIC
VOUT INTVCC PGND CVCC
SHDN
COUT
1.4V VRNG FOLDBACK 2.6V 0.7V
OVERTEMP SENSE
PGOOD RFB1
1.5V FAULT
1.5V
0.8V
RUN/SS
SHDN
0.72V
SGND 0.88V
RFB2
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LTC3812-5 OPERATION
Main Control Loop LTC3812-5 current mode controller DC/DC step-down converters. normal operation, MOSFET turned fixed interval determined one-shot timer (OST). When MOSFET turned off, bottom MOSFET turned until current comparator ICMP trips, restarting one-shot timer initiating next cycle. Inductor current determined sensing voltage between PGND pins using bottom MOSFET on-resistance. voltage sets comparator threshold corresponding inductor valley current. fast 25MHz error amplifier adjusts this voltage comparing feedback signal internal 0.8V reference voltage. load current increases, causes drop feedback voltage relative reference. voltage then rises until average inductor current again matches load current. operating frequency determined implicitly MOSFET on-time duty cycle required maintain regulation. one-shot timer generates time that proportional ideal duty cycle, thus holding frequency approximately constant with changes VIN. nominal frequency adjusted with external resistor RON. Pulling RUN/SS forces controller into shutdown state, turning both Forcing voltage above 1.5V will turn device.
PULSE SKIP MODE
Pulse Skip Mode LTC3812-5 operate modes selectable with pin-pulse skip mode forced continuous mode (see Figure Pulse skip mode selected when increased efficiency light loads desired (see Figure this mode, bottom MOSFET turned when inductor current reverses minimize efficiency loss reverse current flow gate charge switching. load currents, will drop below zero current level (1.2V) shutting both switches. Both switches will remain with output capacitor supplying load current until voltage rises above zero current
EFFICIENCY 0.01 LOAD
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PULSE SKIP
FORCED CONTINUOUS
Figure Efficiency Pulse Skip/Forced Continuous Modes
FORCED CONTINUOUS
DECREASING LOAD CURRENT
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Figure Comparison Inductor Current Waveforms Pulse Skip Mode Forced Continuous Operation
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LTC3812-5 OPERATION
level initiate another cycle. this mode, frequency proportional load current light loads. Pulse skip mode operation disabled comparator when brought below 0.8V, forcing continuous synchronous operation. Forced continuous mode less efficient resistive losses, advantage better transient response currents, approximately constant frequency operation, ability maintain regulation when sinking current. Fault Monitoring/Protection Constant on-time current mode architecture provides accurate cycle-by-cycle current limit protection-a feature that very important protecting high voltage power supply from output short-circuits. cycle-by-cycle current monitor guarantees that inductor current will never exceed value programmed VRNG pin. Foldback current limiting provides further protection output shorted ground. drops, buffered current threshold voltage ITHB pulled down clamped This reduces inductor valley current level one-sixth maximum value approaches Foldback current limiting disabled start-up. Overvoltage undervoltage comparators pull PGOOD output output feedback voltage exits ±10% window around regulation point after internal 120s power mask timer expires. Furthermore, overvoltage condition, turned turned immediately held until overvoltage condition clears. LTC3812-5 provides undervoltage lockout comparator INTVCC supply. INTVCC threshold 4.2V guarantee that MOSFETs have sufficient gate drive voltage before turning INTVCC under threshold, LTC3812-5 shut down drivers turned off. Strong Gate Drivers LTC3812-5 contains very impedance drivers capable supplying amps current slew large MOSFET gates quickly. This minimizes transition losses allows paralleling MOSFETs higher current applications. floating high side driver drives topside MOSFET side driver drives bottom side MOSFET (see Figure bottom side driver supplied directly from INTVCC pin. MOSFET drivers biased from floating bootstrap capacitor which normally recharged during each cycle through external diode from INTVCC when MOSFET turns off. pulse skip mode operation, where possible that bottom MOSFET will extended period time, internal timeout guarantees that bottom MOSFET turned least once every on-time period refresh bootstrap capacitor.
INTVCC
LTC3812-5
INTVCC
BOOST
VOUT
PGND
COUT
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Figure Floating Driver Supply Negative Return
IC/Driver Supply Power LTC3812-5's internal control circuitry bottom MOSFET drivers operate from supply voltage (INTVCC pin) range 4.2V 14V. LTC3812-5 integrated linear regulator controllers easily generate this IC/driver supply from either high voltage input from output voltage. best efficiency supply derived from input voltage during start-up then derived from lower voltage output soon output higher than 4.7V. Alternatively, supply derived from input continuously output <4.7V external supply appropriate range used. LTC3812-5 will automatically detect which mode being used operate properly.
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LTC3812-5 OPERATION
four possible operating modes generating this supply summarized follows (see Figure LTC3812-5 generates 5.5V start-up supply from small external SOT-23 NMOS acting linear regulator with drain connected gate controlled LTC3812-5's internal linear regulator controller through NDRV pin. soon output voltage reaches 4.7V, 5.5V IC/driver supply derived from output through internal dropout regulator optimize efficiency. output lost short, LTC3812-5 goes through repeated duty cycle soft-start cycles (with drivers shut between) attempt bring output without burning SOT-23 NMOS. This scheme eliminates long start-up times associated with conventional trickle charger using external NMOS quickly charge IC/driver supply capacitor (CINTVCC). Similar except that external NMOS used continuous IC/driver power instead just startup. NMOS sized proper dissipation
Mode MOSFET Start-Up Only
270A 270A
driver shutdown/restart VOUT 4.7V disabled. This scheme less efficient necessary VOUT 4.7V boost network desired. Trickle charge mode provides even simpler approach eliminating external NMOS. IC/driver supply capacitors charged through single high valued resistor connected input supply. When INTVCC voltage reaches turn-on threshold (automatically raised from 4.2V provide extra headroom start-up), drivers turn begin charging output capacitor. When output reaches 4.7V, IC/driver power derived from output. trickle-charge mode, supply capacitors must have sufficient capacitance such that they discharged below INTVCC threshold before output high enough take over else power supply will start. voltage supply available. simplest approach voltage supply (between 4.2V 14V) available connected directly IC/driver supply pins.
Mode MOSFET Continuous
NDRV INTVCC LTC3812-5
NDRV
5.5V
INTVCC LTC3812-5
5.5V
EXTVCC
VOUT 4.7V)
EXTVCC
Mode Trickle Charge Mode
Mode External Supply
NDRV INTVCC LTC3812-5
NDRV
5.5V
INTVCC LTC3812-5
4.2V
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EXTVCC
VOUT
EXTVCC
Figure Operating Modes IC/Driver Supply
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LTC3812-5 APPLICATIONS INFORMATION
basic LTC3812-5 application circuit shown first page this data sheet. External component selection primarily determined maximum input voltage load current begins with selection power MOSFET switches. LTC3812-5 uses on-resistance synchronous power MOSFET determining inductor current. desired amount ripple current operating frequency largely determines inductor value. Next, selected ability handle large current into converter COUT chosen with enough meet output voltage ripple transient specification. Finally, loop compensation components selected meet required transient/ phase margin specifications. MAXIMUM SENSE VOLTAGE VRNG Inductor current determined measuring voltage across sense resistance (the on-resistance bottom MOSFET) that appears between PGND pins. maximum sense voltage voltage applied VRNG equal approximately: VSENSE(MAX) 0.173VRNG 0.026 current mode control loop will allow inductor current valleys exceed VSENSE(MAX)/RSENSE. practice, should allow some margin variations LTC3812-5 external component values good guide selecting sense resistance RSENSE VSENSE(MAX) POWER MOSFET SELECTION LTC3812-5 requires external N-channel power MOSFETs, (main) switch bottom (synchronous) switch. Important parameters power MOSFETs breakdown voltage BVDSS, threshold voltage V(GS)TH, on-resistance RDS(ON), input capacitance maximum current IDS(MAX). Since bottom MOSFET used current sense element, particular attention must paid on-resistance. MOSFET on-resistance typically specified with maximum value RDS(ON)(MAX) 25°C. this case, additional margin required accommodate rise MOSFET on-resistance with temperature: RDS(ON)(MAX) RSENSE
term normalization factor (unity 25°C) accounting significant variation on-resistance with temperature (see Figure typically varies from 0.4%/°C 1.0%/°C depending particular MOSFET used.
NORMALIZED ON-RESISTANCE
external resistive divider from INTVCC used voltage VRNG between 0.5V resulting nominal sense voltages 60mV 320mV. Additionally, VRNG tied SGND INTVCC which case nominal sense voltage defaults 95mV 215mV, respectively.
JUNCTION TEMPERATURE (°C)
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Figure RDS(ON) Temperature
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LTC3812-5 APPLICATIONS INFORMATION
most important parameter high voltage applications breakdown voltage BVDSS. Both bottom MOSFETs will full input voltage plus additional ringing switch node across drain-to-source during off-time must chosen with appropriate breakdown specification. LTC3812-5 designed used with 4.5V gate drive supply (INTVCC pin) driving logic-level MOSFETs (VGS(MIN) 4.5V). maximum efficiency, on-resistance RDS(ON) input capacitance should minimized. RDS(ON) minimizes conduction losses input capacitance minimizes transition losses. MOSFET input capacitance combination several components taken from typical "gate charge" curve included most data sheets (Figure
MILLER EFFECT CMILLER QA)/VDS
voltage, adjusted different voltages multiplying ratio application curve specified values. estimate CMILLER term take change gate charge from points manufacturers data sheet divide stated voltage specified. CMILLER most important selection criteria determining transition loss term MOSFET directly specified MOSFET data sheets. CRSS specified sometimes definitions these parameters included. When controller operating continuous mode duty cycles bottom MOSFETs given Main Switch Duty Cycle VOUT VOUT
Synchronous Switch Duty Cycle
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power dissipation main synchronous MOSFETs maximum output current given PTOP VOUT (IMAX )RDS(ON) VIN2 (RDR )(CMILLER VTH(IL) VTH(IL) PBOT VOUT (IMAX
)RDS(0N)
Figure Gate Charge Characteristic
curve generated forcing constant input current into gate common source, current source loaded stage then plotting gate voltage versus time. initial slope effect gate-to-source gate-to-drain capacitance. flat portion curve result Miller multiplication effect drain-to-gate capacitance drain drops voltage across current source load. upper sloping line drain-to-gate accumulation capacitance gate-to-source capacitance. Miller charge (the increase coulombs horizontal axis from while curve flat) specified given drain
where temperature dependency RDS(ON), effective driver resistance (approximately VMILLER), drain potential change drain potential particular application. VTH(IL) data sheet specified typical gate threshold voltage specified power MOSFET data sheet specified
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LTC3812-5 APPLICATIONS INFORMATION
drain current. CMILLER calculated capacitance using gate charge curve from MOSFET data sheet technique described above. Both MOSFETs have losses while topside N-channel equation incudes additional term transition losses, which peak highest input voltage. high input voltage duty cycle applications that typical LTC3812-5, transition losses dominate loss term therefore using higher RDS(ON) device with lower CMILLER usually provides highest efficiency. synchronous MOSFET losses greatest high input voltage when switch duty factor during short-circuit when synchronous switch close 100% period. Since there transition loss term synchronous MOSFET, optimal efficiency obtained minimizing RDS(ON) using larger MOSFETs paralleling multiple MOSFETS. Multiple MOSFETs used parallel lower RDS(ON) meet current thermal requirements desired. LTC3812-5 contains large impedance drivers capable driving large gate capacitances without significantly slowing transition times. fact, when driving MOSFETs with very gate charge, sometimes helpful slow down drivers adding small gate resistors less) reduce noise caused fast transitions.
1000
OPERATING FREQUENCY choice operating frequency tradeoff between efficiency component size. frequency operation improves efficiency reducing MOSFET switching losses requires larger inductance and/or capacitance order maintain output ripple voltage. operating frequency LTC3812-5 applications determined implicitly one-shot timer that controls on-time MOSFET switch. on-time current voltage according
2.4V (76pF) IION
Tying resistor from yields on-time inversely proportional VIN. step-down converter, this results approximately constant frequency operation input supply varies: VOUT [Hz] 2.4V RON(76pF)
Figure shows relates switching frequency several common output voltages.
SWITCHING FREQUENCY (kHz)
VOUT
VOUT 3.3V
VOUT
1000
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Figure Switching Frequency
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LTC3812-5 APPLICATIONS INFORMATION
MINIMUM OFF-TIME DROPOUT OPERATION minimum off-time tOFF(MIN) smallest amount time that LTC3812-5 capable turning bottom MOSFET, tripping current comparator turning MOSFET back off. This time generally about 250ns. minimum off-time limit imposes maximum duty cycle tON/(tON tOFF(MIN)). maximum duty cycle reached, dropping input voltage example, then output will drop regulation. minimum input voltage avoid dropout VIN(MIN) VOUT tOFF(MIN) this requires large inductor. There tradeoff between component size, efficiency operating frequency. reasonable starting point choose ripple current that about IOUT(MAX). largest ripple current occurs highest VIN. guarantee that ripple current does exceed specified maximum, inductance should chosen according VOUT IL(MAX) VOUT VIN(MAX)
plot maximum duty cycle frequency shown Figure INDUCTOR SELECTION Given desired input output voltages, inductor value operating frequency determine ripple current:
Once value known, type inductor must selected. High efficiency converters generally cannot afford core loss found cost powdered iron cores, forcing more expensive ferrite, molypermalloy Kool cores. variety inductors designed high current, voltage applications available from manufacturers such Sumida, Panasonic, Coiltronics, Coilcraft Toko. SCHOTTKY DIODE SELECTION Schottky diode shown front page schematic conducts during dead time between conduction power MOSFET switches. intended prevent body diode bottom MOSFET from turning storing charge during dead time, which cause modest (about efficiency loss. diode rated about half fifth full load current since
Lower ripple current reduces core losses inductor, losses output capacitors output voltage ripple. Highest efficiency operation obtained frequency with small ripple current. However, achieving
SWITCHING FREQUENCY (MHz)
DROPOUT REGION
0.25 0.50 0.75 DUTY CYCLE (VOUT/VIN)
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Figure Maximum Switching Frequency Duty Cycle
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LTC3812-5 APPLICATIONS INFORMATION
only fraction duty cycle. order diode effective, inductance between bottom MOSFET must small possible, mandating that these components placed adjacently. diode omitted efficiency loss tolerable. INPUT CAPACITOR SELECTION continuous mode, drain current MOSFET approximately square wave duty cycle VOUT/VIN which must supplied input capacitor. prevent large input transients, input capacitor sized maximum current given ICIN(RMS) IO(MAX) VOUT
good approach combination aluminum electrolytics bulk capacitance ceramics current. current cannot handled aluminum capacitors alone, when used together, percentage current that will supplied aluminum capacitor reduced approximately: IRMS,ALUM (8fCRESR 100%
where RESR aluminum capacitor overall capacitance ceramic capacitors. Using aluminum electrolytic with ceramic also helps damp high ceramic, minimizing ringing. OUTPUT CAPACITOR SELECTION selection COUT primarily determined required minimize voltage ripple. output ripple (VOUT) approximately equal VOUT 8fCOUT
This formula maximum 2VOUT, where IRMS IO(MAX)/2. This simple worst-case condition commonly used design because even significant deviations offer much relief. Note that ripple current ratings from capacitor manufacturers often based only 2000 hours life. This makes advisable further derate capacitor choose capacitor rated higher temperature than required. Several capacitors also placed parallel meet size height requirements design. Because tantalum OS-CON capacitors available voltages above 30V, ceramics aluminum electrolytics must used regulators with input supplies above 30V. Ceramic capacitors have advantage very handle high current, ceramics with high voltage ratings 50V) available with more than microfarads capacitance. Furthermore, ceramics have high voltage coefficients which means that capacitance values decrease even more when used rated voltage. type ceramics recommended their lower voltage temperature coefficients. Another consideration when using ceramics their high which, properly damped, result excessive voltage stress power MOSFETs. Aluminum electrolytics have much higher bulk capacitance, they have higher lower current ratings.
Since increases with input voltage, output ripple highest maximum input voltage. also significant effect load transient response. Fast load transitions output will appear voltage across COUT until feedback loop LTC3812-5 change inductor current match load current value. Typically, once requirement satisfied capacitance adequate filtering required current rating. Manufacturers such Nichicon, Nippon Chemi-Con Sanyo should considered high performance throughhole capacitors. OS-CON (organic semiconductor dielectric) capacitor available from Sanyo lowest product size aluminum electrolytic somewhat higher price. additional ceramic capacitor parallel with OS-CON capacitors recommended reduce effect their lead inductance. surface mount applications, multiple capacitors placed parallel required meet ESR, current
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LTC3812-5 APPLICATIONS INFORMATION
handling load step requirements. tantalum, special polymer aluminum electrolytic capacitors available surface mount packages. Special polymer capacitors offer very have lower capacitance density than other types. Tantalum capacitors have highest capacitance density important only types that have been surge tested switching power supplies. Several excellent surge-tested choices TPSV KEMET T510 series. Aluminum electrolytic capacitors have significantly higher ESR, used cost-driven applications providing that consideration given ripple current ratings long term reliability. Other capacitor types include Panasonic Sanyo POSCAPs. OUTPUT VOLTAGE LTC3812-5 output voltage resistor divider according following formula: VOUT 0.8V RFB1 RFB2 reverse breakdown external diode, must greater than VIN(MAX). Another important consideration external diode reverse recovery reverse leakage, either which cause excessive reverse current flow full reverse voltage. reverse current times reverse voltage exceeds maximum allowable power dissipation, diode damaged. best results, ultrafast recovery diode such MMDL770T1. IC/MOSFET DRIVER SUPPLY (INTVCC) LTC3812-5 drivers supplied from INTVCC BOOST pins (see Figure which have absolute maximum voltage 14V. Since main supply voltage, typically much higher than separate supply driver power (INTVCC) must used. LTC3812-5 integrated bias supply control circuitry that allows IC/driver supply easily generated from and/or VOUT with minimal external components. There four ways this shown simplified schematics Figure explained following sections. Using Linear Regulator INTVCC Supply Mode small external SOT-23 MOSFET, controlled NDRV pin, used generate 5.5V start-up supply from VIN. small SOT-23 package used because NMOS continuously only during brief start-up period. soon output voltage reaches 4.7V, LTC3812-5 turns external NMOS LTC3812-5 regulates 5.5V supply from EXTVCC (connected VOUT VOUT derived boost network) through internal dropout regulator. this mode work properly, EXTVCC must range 4.7V EXTVCC 15V. VOUT 4.7V, charge pump extra winding used raise EXTVCC proper voltage, alternatively, Mode should used explained later this section. VOUT shorted otherwise goes below minimum 4.5V threshold, MOSFET connected turned back maintain 5.5V supply. However output cannot brought within timeout period,
external resistor divider connected output shown Functional Diagram, allowing remote voltage sensing. resultant feedback signal compared with internal precision 800mV voltage reference error amplifier. internal reference guaranteed tolerance less than ±1%. Tolerance feedback resistors will additional error output voltage. 0.1% resistors recommended. MOSFET DRIVER SUPPLY (CB, external bootstrap capacitor connected BOOST supplies gate drive voltage topside MOSFET. This capacitor charged through diode from INTVCC when switch node low. When MOSFET turns switch node rises BOOST rises approximately INTVCC. boost capacitor needs store about times gate charge required MOSFET. most applications 0.1F 0.47F dielectric capacitor adequate.
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LTC3812-5 APPLICATIONS INFORMATION
drivers turned prevent SOT-23 MOSFET from overheating. Soft-start cycles then attempted duty cycle intervals bring output back (see Figure This fault timeout operation enabled choosing choosing RNDRV such that resistor current INDRV greater than 270A using following formulas: RNDRV where (f)(QG(TOP) QG(BOTTOM)) threshold voltage MOSFET. value RNDRV also affects VIN(MIN) follows: VIN(MIN) VINTVCC(MIN) (40A) RNDRV where VINTVCC(MIN) normally 4.5V driving logic level MOSFETs. minimum enough, consider reducing RNDRV and/or using darlington instead NMOS reduce ~1.4V. When using RNDRV equal computed value, LTC3812-5 will enable duty cycle soft-start retries only when desired maximum power dissipation, PMOSFET(MAX), MOSFET exceeded leave drivers continuously otherwise. shutoff/restart times function RUN/SS capacitor value.
FAULT TIMEOUT ENABLED DRIVER POWER FROM VOUT RUN/SS DRIVER POWER FROM START-UP VOUT SHORT-CIRCUIT EVENT START-UP INTO SHORT CIRCUIT DRIVER POWER FROM EXTVCC THRESHOLD
external NMOS linear regulator should standard threshold type (i.e., logic level threshold). rate charge from 5.5V controlled LTC3812-5 approximately regardless size capacitor connected INTVCC pin. charging current this capacitor approximately: 5.5V INTVCC
PMOSFET(MAX) /ICC 270A
safe operating area (SOA) external NMOS should chosen that capacitor charging does damage NMOS. Excessive values capacitor unnecessary should avoided. Typically values work well. more design requirement this mode minimum soft-start capacitor value. fault timeout enabled when RUN/SS voltage greater than This gives power supply time bring output before starts timeout sequence. prevent timeout sequence from starting prematurely during start-up, minimum value necessary ensure that VRUN/SS until VEXTVCC 4.7V. ensure this, choose: COUT (2.3 10-6)/IOUT(MAX) Mode should used VOUT outside 4.7V EXTVCC operating range extra complexity charge pump extra inductor winding wanted
DRIVER THRESHOLD
ISS/TRACK 1.4A (SOURCE) ISS/TRACK 0.1A (SINK)
TG/BG
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Figure Fault Timeout Operation
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LTC3812-5 APPLICATIONS INFORMATION
boost this voltage above 4.7V. this mode, EXTVCC grounded NMOS chosen handle worstcase power dissipation: PMOSFET (VIN(MAX))[(f)(QG(TOP) QG(BOTTOM) 3mA] operate properly, fault timeout operation must disabled choosing RNDRV (VIN(MAX) 5.5V VT)/270A required RNDRV value results unacceptable value VIN(MIN) (see Equation fault timeout operation also disabled connecting 500k resistor from RUN/SS INTVCC. Using Trickle Charge Mode Trickle charge mode selected shorting NDRV INTVCC connecting EXTVCC VOUT. Trickle charge mode advantage requiring external MOSFET takes longer start slow charge CINTVCC through RPULLUP (tDELAY 0.77 RPULLUP CINTVCC) usually requires larger INTVCC capacitor value hold supply voltage during start-up. Once INTVCC voltage reaches trickle charge threshold drivers will turn start discharging CINTVCC rate determined driver current order ensure proper start-up, CINTVCC must chosen large enough that EXTVCC voltage reaches switchover threshold 4.7V before CINTVCC discharges below falling threshold This ensured CINTVCC Larger COUT IMAX VOUT(REG) this mode, INTVCC, EXTVCC NDRV must shorted together. INTVCC Supply EXTVCC Connection LTC3812-5 contains internal dropout regulator produce 5.5V INTVCC supply from EXTVCC voltage. This regulator turns when EXTVCC above 4.7V remains until EXTVCC drops below 4.45V. This allows IC/MOSFET power derived from output output derived boost network during normal operation from external NMOS from during start-up short-circuit. Using EXTVCC this results significant efficiency gains compared what would possible when deriving this power continuously from typically much higher voltage. EXTVCC connection also allows power supply configured trickle charge mode which starts with high-valued "bleed" resistor connected from INTVCC charge INTVCC capacitor. soon output rises above 4.7V internal EXTVCC regulator takes over before INTVCC capacitor discharges below threshold. When EXTVCC regulator active, EXTVCC supply 50mA RMS. apply more than EXTVCC pin. following list summarizes possible connections EXTVCC: EXTVCC grounded. This connection will require INTVCC powered continuously from external NMOS from resulting efficiency penalty high high input voltages. EXTVCC connected directly VOUT. This normal connection 4.7V VOUT provides highest efficiency. power supply will start using external NMOS bleed resistor until output supply available. EXTVCC connected output-derived boost network. VOUT 4.7V. voltage output boosted using charge pump flyback winding greater than 4.7V. EXTVCC connected INTVCC. This required connection EXTVCC INTVCC connected external supply where external supply 4.2V VEXT 14V.
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where gate drive current (f)(QG(TOP) QG(BOTTOM)) IMAX maximum inductor current selected VRNG. RPULLUP, value should fall following range ensure proper start-up: RPULLUP (VIN(MAX) 14V)/ICCSR RPULLUP (VIN(MIN) 9V)/IQ,SHUTDOWN Using External Supply Connected INTVCC external supply available between 4.2V 14V, supply connected directly INTVCC pins.
LTC3812-5 APPLICATIONS INFORMATION
Applications using large MOSFETs with high input voltage high frequency operation result large EXTVCC current. LTC3812-5 thermally enhanced package, maximum junction temperature will rarely exceeded, however, good design practice verify that maximum junction temperature rating current rating within maximum limits. Typically, most EXTVCC current consists MOSFET gates current. continuous mode operation, this EXTVCC current IEXTVCC f(QG(TOP) QG(BOTTOM)) 50mA junction temperature estimated from equations given Note Electrical Characteristics follows: IEXTVCC (VEXTVCC VINTVCC)(38°C/W) 125°C absolute maximum ratings exceeded, consider using external supply connected directly INTVCC pin. FEEDBACK LOOP/COMPENSATION Feedback Loop Types typical LTC3812-5 circuit, feedback loop consists modulator, output filter load, feedback amplifier with compensation network. these components affect loop behavior must accounted loop compensation. modulator output filter consists internal current comparator, output MOSFET drivers external MOSFETs, inductor output capacitor. Current mode control eliminates effect inductor moving inner loop, reducing first order system. From feedback loop point view, looks like linear voltage controlled current source from VOUT gain equal (IMAXROUT)/1.2V. fairly benign behavior typical loop compensation frequencies with significant phase shift appearing half switching frequency. external output capacitor load cause first order roll output ROUTCOUT pole frequency, with attendant phase shift. This roll what filters waveform, resulting desired output voltage. output capacitor also contributes zero COUTRESR frequency which adds back phase cancels first order roll off. far, response loop pretty well user's control. modulator fundamental piece LTC3812-5 design external output capacitor usually chosen based regulation load current requirements without considering loop response. feedback amplifier, other hand, gives handle with which adjust response. goal have 180° phase shift loop regulates), something less than 360° phase shift (preferably about 300°) point that loop gain falls 0dB, i.e., crossover frequency, with much gain possible frequencies below crossover frequency. Since modulator/output filter first order system with maximum phase shift frequencies below fSW/4) feedback amplifier adds another phase shift, some phase boost required crossover frequency achieve good phase margin. zero below crossover frequency, this zero provide enough phase boost achieve desired phase margin only requirement compensation will guarantee that gain below zero frequencies above fSW/4. zero above crossover frequency, feedback amplifier will probably required provide phase boost. most LTC3810 applications, Type compensation will provide enough phase boost; however some applications where high bandwidth required with ceramics lots bulk capacitance, Type compensation necessary provide additional phase boost. types compensation networks, "Type "Type shown Figures When component values chosen properly, these networks provide
VREF GAIN (dB) PHASE (DEG)
-6dB/OCT GAIN -6dB/OCT
FREQ PHASE -180 -270 -360
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Figure Type Schematic Transfer Function
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LTC3812-5 APPLICATIONS INFORMATION
GAIN (dB)
-6dB/OCT
GAIN
+6dB/OCT
-6dB/OCT FREQ
VREF
PHASE
-180 -270 -360
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Figure Type Schematic Transfer Function
"phase bump" crossover frequency. Type uses single pole-zero pair provide about phase boost while Type uses poles zeros provide 150° phase boost. Feedback Component Selection Selecting values typical Type Type loop nontrivial task. applications shown this data sheet show typical values, optimized power components shown. They should give acceptable performance with similar power components, even major power component changed significantly. Applications that require optimized transient response will require recalculation compensation values specifically circuit question. underlying mathematics complex, component values calculated straightforward manner know gain phase modulator crossover frequency. Modulator gain phase obtained three ways: measured directly from breadboard, appropriate parasitic values known, simulated generated from modulator transfer function. Measurement will give more accurate results, simulation transfer function often close enough give working system. measure modulator gain phase directly, wire breadboard with LTC3812-5 actual MOSFETs, inductor input output capacitors that final design will use. This breadboard should appropriate construction techniques high speed analog circuitry: bypass capacitors located close LTC3812-5, long wires connecting components,
appropriately sized ground returns, etc. Wire feedback amplifier with 0.1F feedback capacitor from 100k resistor from VOUT Choose bias resistor (RB) required desired output voltage. Disconnect from ground connect signal generator source output network analyzer inject test signal into loop. Measure gain phase from output node positive terminal output capacitor. Make sure analyzer's input coupled that voltages present both VOUT nodes don't corrupt measurements damage analyzer. breadboard measurement practical, SPICE simulation used generate approximate gain/phase curves. Plug expected capacitor, inductor MOSFET values into following SPICE deck generate plot VOUT/VITH with gain phase degrees. Refer your SPICE manual details generate this plot.
*3810 modulator gain/phase *2006 Linear Technology *this file simulates simplified model *the LTC3810 generating v(out)/v(ith) *bode plot .param rdson=.0135 ;MOSFET rdson .param Vrng=2 ;use INTVCC ground .param vsnsmax={0.173*Vrng-0.026} .param Imax={vsnsmax/rdson} .param DL=4 ;inductor ripple current *inductor current *output cout out2 270u ;capacitor value resr out2 0.018 ;capacitor *load Rout load resistor vstim stimulus 10meg .probe .end
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PHASE (DEG)
LTC3812-5 APPLICATIONS INFORMATION
Mathematical software such MATHCAD MATLAB also used generate plots using following transfer function modulator: H(s) VSENSE(MAX) RDS(ON) With gain/phase plot hand, loop crossover frequency chosen. Usually curves look something like Figure Choose crossover frequency about switching frequency maximum bandwidth. Although tempting beyond fSW/4, remember that significant phase shift occurs half switching frequency that isn't modeled above H(s) equation PSPICE code. Note gain (GAIN, phase (PHASE, degrees) this point. desired feedback amplifier gain will -GAIN make loop gain this frequency. calculate needed phase boost, assuming target phase margin: BOOST (PHASE 30°) required BOOST less than 60°, Type loop used successfully, saving external components. BOOST values greater than usually require Type loops satisfactory performance. Finally, choose convenient resistor value (10k usually good value). calculate remaining values: RESR COUT COUT constant used calculations) chosen crossover frequency 10(GAIN/20) (this converts GAIN absolute gain) TYPE Loop: BOOST
(R1) VOUT VREF TYPE Loop: BOOST tan2 (R1) VOUT VREF SPICE mathematical software used generate gain/phase plots compensated power supply sanity check component values before trying them actual hardware. software, following transfer function: T(s) A(s)H(s)
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PHASE (DEG)
GAIN (dB)
GAIN
PHASE
-180 FREQUENCY (Hz)
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Figure Transfer Function Buck Modulator
LTC3812-5 APPLICATIONS INFORMATION
where H(s) given equation A(s) depends compensation circuit used: Type RFB1 threshold forces continuous synchronous operation, allowing current reverse light loads maintaining high frequency operation. prevent forcing current back into main power supply, potentially boosting input supply dangerous voltage level, forced continuous mode operation disabled when RUN/SS voltage below 2.5V during soft-start tracking. During these periods, PGOOD signal forced low. addition providing logic input force continuous operation, provides mean maintain flyback winding output when primary operating pulse skip mode. secondary output VOUT2 normally shown Figure turns ratio transformer. However, controller goes into pulse skip mode halts switching light primary load current, then VOUT2 will droop. external resistor divider from VOUT2 sets minimum voltage VOUT2(MIN) below which continuous operation forced until VOUT2 risen above minimum. VOUT2(MIN) 0.8V
Table
Voltage: 0.75V Voltage: 0.85V Feedback Resistors CONDITION Forced Continuous Current Reversal Enabled Pulse Skip Mode Operation Current Reversal Regulating Secondary Winding
Type
(R1+
SPICE, replace VSTIM line previous PSPICE code with following code generate gain/phase plot V(out)/V(outin):
rfb1 outin 52.5k rfb2 eithx ithx laplace {0.8-v(vfb)} {1/(1+s/1000)} eith 210k outin ;delete this line Type 120p ;delete this line Type vstim outin dc=0 ac=1m
1N4148
PULSE SKIP MODE OPERATION determines whether bottom MOSFET remains when current reverses inductor. Tying this above 0.8V threshold enables pulse skip mode operation where bottom MOSFET turns when inductor current reverses. load current which current reverses discontinuous operation begins depends amplitude inductor ripple current will vary with changes VIN. Tying below 0.8V
LTC3812-5 SGND PGND
Figure Secondary Output Loop
VOUT2 COUT2 VOUT1 COUT
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LTC3812-5 APPLICATIONS INFORMATION
FAULT CONDITIONS: CURRENT LIMIT FOLDBACK maximum inductor current inherently limited current mode controller maximum sense voltage. LTC3812-5, maximum sense voltage controlled voltage VRNG pin. With valley current control, maximum sense voltage sense resistance determine maximum allowed inductor valley current. corresponding output current limit ILIMIT VSNS(MAX) RDS(ON)
RUN/SOFT-START FUNCTION RUN/SS multipurpose that provides softstart function means shut down LTC3812-5. Soft-start reduces input supply's surge current controlling ramp rate output voltage, eliminates output overshoot also used power supply sequencing. Pulling RUN/SS below 1.5V puts LTC3812-5 into quiescent current shutdown 224A). This driven directly from logic shown Figure Releasing RUN/SS allows internal 1.4A current source charge soft-start capacitor, CSS. When voltage RUN/SS reaches 1.5V, LTC3812-5 turns begins regulating output 1.5V. RUN/SS voltage increases from 1.5V 2.3V, output voltage raised from 100% regulated value. Current foldback, forced continuous mode fault timeout disabled during this soft-start phase PGOOD signal forced low. RUN/SS voltage continues charge until reaches internally clamped value RUN/SS starts delay before starting approximately: tDELAY,START 1.5V (1.1s/µF 1.4µA
current limit value should checked ensure that ILIMIT(MIN) IOUT(MAX). minimum value current limit generally occurs with largest highest ambient temperature, conditions that cause largest power loss converter. Note that important check self-consistency between assumed MOSFET junction temperature resulting value ILIMIT which heats MOSFET switches. Caution should used when setting current limit based upon RDS(ON) MOSFETs. maximum current limit determined minimum MOSFET on-resistance. Data sheets typically specify nominal maximum values RDS(ON), minimum. reasonable assumption that minimum RDS(ON) lies same percentage below typical value maximum lies above Consult MOSFET manufacturer further guidelines. further limit current event short-circuit ground, LTC3812-5 includes foldback current limiting. output falls more than 60%, then maximum sense voltage progressively lowered about tenth full value. aware also that when fault timeout enabled external NMOS regulator, over current limit cause output fall below minimum 4.5V threshold. This condition will cause linear regulator timeout/restart sequence described Linear Regulator Timeout section this condition persists.
plus additional delay, before output will reach regulated value tDELAY,REG 0.8V 0.6s/µF 1.4µA
start delay reduced using diode Figure
3.3V
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RUN/SS
RUN/SS
Figure RUN/SS Interfacing
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LTC3812-5 APPLICATIONS INFORMATION
EFFICIENCY CONSIDERATIONS percent efficiency switching regulator equal output power divided input power times 100%. often useful analyze individual losses determine what limiting efficiency which change would produce most improvement. Although dissipative elements circuit produce losses, four main sources account most losses LTC3812-5 circuits: losses. These arise from resistances MOSFETs, inductor board traces cause efficiency drop high output currents. continuous mode average output current flows through chopped between bottom MOSFETs. MOSFETs have approximately same RDS(ON), then resistance MOSFET simply summed with resistances board traces obtain loss. example, RDS(ON) 0.01 0.005, loss will range from 15mW 1.5W output current varies from 10A. Transition loss. This loss arises from brief amount time MOSFET spends saturated region during switch node transitions. depends upon input voltage, load current, driver strength MOSFET capacitance, among other factors. loss significant input voltages above estimated from second term PMAIN equation found Power MOSFET Selection section. When transition losses significant, efficiency improved lowering frequency and/or using MOSFET(s) with lower CRSS expense higher RDS(ON). INTVCC current. This MOSFET driver control currents. Control current typically about driver current calculated IGATE f(QG(TOP) QG(BOT)), where QG(TOP) QG(BOT) gate charges bottom MOSFETs. This loss proportional supply voltage that INTVCC derived from, i.e., external NMOS linear regulator, VOUT internal EXTVCC regulator, VEXT when external supply connected INTVCC. loss. input capacitor difficult filtering large input current regulator. must have very minimize loss sufficient capacitance prevent current from causing additional upstream losses fuses batteries. Other losses, including COUT loss, Schottky diode conduction loss during dead time inductor core loss generally account less than additional loss. When making adjustments improve efficiency, input current best indicator changes efficiency. make change input current decreases, then efficiency increased. there change input current, then there change efficiency. CHECKING TRANSIENT RESPONSE regulator loop response checked looking load transient response. Switching regulators take several cycles respond step load current. When load step occurs, VOUT immediately shifts amount equal ILOAD (ESR), where effective series resistance COUT. ILOAD also begins charge discharge COUT generating feedback error signal used regulator return VOUT steady-state value. During this recovery time, VOUT monitored overshoot ringing that would indicate stability problem. DESIGN EXAMPLE design example, take supply with following specifications: 60V, VOUT ±5%, IOUT(MAX) 250kHz. First, calculate timing resistor: 110k 2.4V 250kHz 76pF
choose inductor about ripple current maximum VIN: 7.6H 250kHz
With 7.7H inductor, ripple current will vary from 1.5A 2.4A (25% 40%) over input supply range. Next, choose bottom MOSFET switch. Since drain MOSFET will full supply voltage
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LTC3812-5 APPLICATIONS INFORMATION
(max) plus ringing, choose MOSFET. Si7850DP has: BVDSS RDS(ON) (max)/31m (nom), 0.007/°C, CMILLER (8.3nC 2.8nC)/30V 183pF VGS(MILLER) 3.8V, 22°C/W. This yields nominal sense voltage VSNS(NOM) 0.025 195mV guarantee proper current limit worst-case conditions, increase nominal VSNS least 320mV tying VRNG 2V). check current limit acceptable VSNS 320mV, assume junction temperature about 55°C above 70°C ambient (125°C 1.7): ILIMIT 320mV 2.4A 7.3A 0.031 EXTVCC pin. small SOT23 MOSFET such ZXMN10A07F used pass device fault timeout enabled. Choose RNDRV guarantee that fault timeout enabled when power dissipation exceeds 0.4W (max 70°C ambient): 250kHz 18nC 12mA RNDRV 0.4W 0.012A 112k 270µA
choose RNDRV 100k. chosen current rating about 85°C. output capacitors chosen 0.018 minimize output voltage changes inductor ripple current load steps. ripple voltage will only: VOUT(RIPPLE) IL(MAX) 2.4A 0.018 43mV However, load step will cause output change VOUT(STEP) ILOAD 0.018 108mV optional ceramic output capacitor included minimize effect output ripple. complete circuit shown Figure Board Layout Checklist When laying board follow suggested approaches. simple board layout requires dedicated ground plane layer. Also, higher currents, recommended multilayer board help with heat sinking power components. ground plane layer should have traces should close possible layer with power MOSFETs. Place CIN, COUT, MOSFETs, inductor compact area. help have some components bottom side board. immediate connect components ground plane including SGND PGND LTC3812-5. several bigger vias power components.
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double-check assumed MOSFET: PBOT 7.3A 0.031 2.6W 70°C 2.6W 22°C/W 127°C Verify that Si7850DP also good choice MOSFET checking power dissipation current limit maximum input voltage, assuming junction temperature 30°C above 70°C ambient (100°C 1.5):
PMAIN 7.3A (1.5 0.031 7.3A 250kHz 183pF 3.8V 3.8V 0.206W 1.32W 1.53W
70°C 1.53W 22°C/W 104°C junction temperature will significantly less nominal current, this analysis shows that careful attention heat sinking board will necessary this circuit. Since VOUT 4.7V, INTVCC voltage generated from VOUT with internal connecting VOUT
LTC3812-5 APPLICATIONS INFORMATION
compact plane switch node (SW) improve cooling MOSFETs keep down. planes VOUT maintain good voltage filtering keep power losses low. Flood unused areas layers with copper. Flooding with copper will reduce temperature rise power component. connect copper areas (VIN, VOUT, other rail your system). When laying printed circuit board, without ground plane, following checklist ensure proper operation controller. Segregate signal power grounds. small signal components should return SGND point which then tied PGND close source Place close controller possible, keeping PGND, traces short. Connect input capacitor(s) close power MOSFETs. This capacitor carries MOSFET current. Keep high dV/dt BOOST nodes away from sensitive small-signal nodes. Connect INTVCC decoupling capacitor CVCC closely INTVCC SGND pins. Connect driver boost capacitor closely BOOST pins. Connect bottom driver decoupling capacitor CINTVCC closely INTVCC PGND pins.
110k 100pF 100k PGOOD VRNG BOOST LTC3812-5 PGND RUN/SS SGND INTVCC EXTVCC NDRV 47pF RFB2 1.89k 200k SGND RFB1
RNDRV 100k ZXMN10A07F BAS19 CVCC CDRVCC 0.1F 0.1F
CIN1 100V
CIN2 100V PGND
150k
1000pF
PGOOD
Si7850DP
7.7H
VOUT COUT1 270F 6.3V COUT2 6.3V
Si7850DP B1100
PGND
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Figure Input Voltage 5V/6A
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LTC3812-5 TYPICAL APPLICATIONS
Input Voltage 5V/5A with Power from Supply Ceramic Output Capacitors
CIN1 100V CIN2 PGND
110k 100pF PGOOD VRNG BOOST LTC3812-5 PGND RUN/SS SGND INTVCC EXTVCC NDRV 200pF RFB2 1.89k 100k SGND RFB1 BAS19 CVCC CDRVCC 0.1F 0.1F
1000pF
PGOOD
Si7850DP
4.7H
VOUT
Si7850DP B1100 PGND
COUT1 6.3V
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Input Voltage 3.3V/5A with Fault Timeout Pulse Skip Disabled
71.5k 100pF PGOOD VRNG BOOST LTC3812-5 PGND RUN/SS SGND INTVCC EXTVCC NDRV 47pF RFB2 3.2k 200k SGND RFB1
RNDRV 250k ZVN4210G BAS19 CVCC CDRVCC 0.1F 0.1F
CIN1 100V
CIN2 100V PGND
1000pF
PGOOD
Si7850DP
4.7H
VOUT 3.3V COUT1 270F 6.3V COUT2 6.3V
Si7850DP B1100
PGND
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LTC3812-5 PACKAGE DESCRIPTION
Package 16-Lead Plastic TSSOP (4.4mm)
(Reference 05-08-1663)
Exposed Variation
4.90 5.10* (.193 .201) 2.74 (.108) 1514 1110
2.74 (.108)
6.60 ±0.10 4.50 ±0.10
NOTE
2.74 (.108) 0.45 ±0.05 1.05 ±0.10 0.65 2.74 6.40 (.108) (.252)
RECOMMENDED SOLDER LAYOUT
1.10 (.0433)
4.30 4.50* (.169 .177)
0.25
0.09 0.20 (.0035 .0079)
0.50 0.75 (.020 .030)
0.65 (.0256)
NOTE: CONTROLLING DIMENSION: MILLIMETERS MILLIMETERS DIMENSIONS (INCHES) DRAWING SCALE
0.195 0.30 (.0077 .0118)
0.05 0.15 (.002 .006)
FE16 (BA) TSSOP 0204
RECOMMENDED MINIMUM METAL SIZE EXPOSED ATTACHMENT *DIMENSIONS INCLUDE MOLD FLASH. MOLD FLASH SHALL EXCEED 0.150mm (.006") SIDE
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Information furnished Linear Technology Corporation believed accurate reliable. However, responsibility assumed use. Linear Technology Corporation makes representation that interconnection circuits described herein will infringe existing patent rights.
LTC3812-5 TYPICAL APPLICATION
Input Voltage 12V/5A with Trickle Charger Start-Up
CIN1 100V CIN2 100V PGND
261k 100pF PGOOD VRNG BOOST LTC3812-5 PGND RUN/SS SGND INTVCC EXTVCC NDRV 47pF RFB2 200k SGND RFB1
RNDRV 250k
BAS19 CVCC CDRVCC 0.1F Si7850DP 0.1F Si7850DP
1000pF
PGOOD
VOUT COUT1 270F COUT2
B1100
PGND
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RELATED PARTS
PART NUMBER 1074HV/LT1076HV LTC1735 LTC1778 LT1956 LT3010 LT3430/LT3431 LT3433 LTC3703 LT3800 LTC3810 LTC3810-5 LTC3835 LT3844 LT3845
DESCRIPTION Monolithic 5A/2A Step-Down DC/DC Converters Synchronous Step-Down DC/DC Controller RSENSESynchronous DC/DC Controller Monolithic 1.5A, 500kHz Step-Down Regulator 50mA, Linear Regulator Monolithic 200kHz/500kHz Step-Down Regulator Monolithic Step-Up/Step-Down DC/DC Converter 100V Synchronous DC/DC Controller Synchronous DC/DC Controller 100V Synchronous DC/DC Controller RSENSE Current Mode Controller Synchronous DC/DC Controller Non-Synchronous DC/DC Controller Synchronous DC/DC Controller
COMMENTS 60V, TO-220 Packages 3.5V 36V, 0.8V VOUT Current Mode, IOUT 36V, Fast Transient Response, Current Mode, IOUT 5.5V 60V, 2.5mA Supply Current, 16-Pin SSOP 1.275V VOUT 60V, Protection Diode Required, 8-Lead MSOP 5.5V 60V, Saturation Switch, 16-Pin SSOP 60V, 500mA Switch, Automatic Step-Up/Step-Down, 100V, 9.3V Gate Drive Supply 60V, 200kHz, 100V, Current Mode, RSENSE Required 60V, IOUT 20A, Large Gate Drivers VIN: 36V, VOUT: 0.8V 60V, 100kHz 600kHz, 60V, 100kHz 600kHz,
RSENSE trademark Linear Technology Corporation.
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Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, 95035-7417
(408) 432-1900 FAX: (408) 434-0507
0408 PRINTED
www.linear.com
LINEAR TECHNOLOGY CORPORATION 2007
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