®1681 controller simplifies design high power synchronous dual transis
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LT1681 Dual Transistor Synchronous Forward Controller DESCRIPTIO
®1681 controller simplifies design high power synchronous dual transistor forward DC/DC converters. part employs fixed frequency current mode control supports both isolated nonisolated topologies. drives external N-channel power MOSFETs operates with input voltages 72V. LT1681's operating frequency programmable synchronized 350kHz. Switch phase also controlled during synchronized operation accommodate multiple converter systems. Internal logic guarantees maximum duty cycle operation prevent transformer saturation. LT1681 incorporates soft-start feature that provides controlled increase supplied current during start-up after undervoltage lockout overvoltage/overcurrent event. part available 20-lead wide package support high voltage pin-to-pin clearance.
registered trademarks Linear Technology Corporation.
High Voltage: Operation Synchronizable Operating Frequency Output Switch Phase Multiple Controller Systems Fixed Frequency Operation 350kHz Adaptive Adjustable Blanking Synchronous Rectifier Driver Local Voltage Reference Undervoltage Lockout Protection with Hysteresis Input Overvoltage Protection Programmable Start Inhibit Transformer Primary Saturation Protection Optocoupler Feedback Support Soft-Start Control
APPLICATIO
Isolated Telecommunication Systems Personal Computers Peripherals Lead Acid Battery Backup Systems Automotive Heavy Equipment
TYPICAL APPLICATIO
4.7µH VIN+
36V-72V 5V/7A Synchronous Forward Converter (Half-Brick Footprint)
4.1µH MURS120T3 22µF 100V 1.5µF 100V VIN- 1.5µF 100V MURS120T3 0.025 1/2W
C2:SANYO 100MV22AX VITRAMON VJ1825Y155MXB KEMET T510X337KO10AS COILCRAFT DO1608C-472 PANASONIC ETQP6F4R1LF4 Q1,Q3:100V SILICONIX SUD40N10-25 Q5,Q6: SILICONIX Si4450 T1:COILTRONICS VP5-1200 Q10: SEMI MMBT3906LTI 73.2k 270k 0.25W CMPZ5248B
ZVN3310F
BAT54
BIAS MMBD914LT1 0.1µF 100V
330pF 0.047µF
BIAS
BAS21 VBST BLKSENS BSTREF SENSE TMAX PGND LT1681 OVLO THERM SYNC SGND SHDN 5VREF FSET 52.3k 150pF 4.7nF 0.01µF
CMPZ5248B
0.1µF 68µF
1.24k
3.01k
LTC1693-2 VCC1 VCC2 OUT2 OUT1 GND2 GND1
VOUT IOUT VOUT+
0.25W
100V 100V 0.25W
MBR0540T1
330µF
VOUT-
FZT690 4.7µF CMPZ5242B
0.22µF
1681 TA01
1681f
LT1681
ABSOLUTE MAXIMUM RATINGS
(Note
PACKAGE/ORDER INFORMATION
VIEW SHDN OVLO THERM SGND 5VREF FSET SYNC PACKAGE 20-LEAD PLASTIC TJMAX 125°C, 85°C/ VBST BSTREF BLKSENS PWRGND IMAX SENSE
Supply Voltages Power Supply (VCC) 0.3V Topside Supply (VBST) VBSTREF 0.3V VBSTREF (VBST(MAX) 90V) Topside Reference (VBSTREF) 0.6V Input Voltages SHDN 0.3V 0.3V Other Inputs 0.3V 5VREF 0.3V Maximum Currents 5VREF 85mA 10mA FSET Other Inputs Operating Ambient Temperature Range LT1681E (Note 40°C 85°C LT1681I 40°C 85°C Storage Temperature Range 65°C 150°C Lead Temperature (Soldering, sec). 300°C
ORDER PART NUMBER LT1681ESW LT1681ISW
Consult Marketing parts specified with wider operating temperature ranges.
ELECTRICAL CHARACTERISTICS
denotes specifications which apply over full operating temperature range, otherwise specifications 25°C. VBST 12V, VBSTREF VREF 1.25V, 1000pF.
SYMBOL PARAMETER Operating Supply Voltage Range Active Supply Current Active Supply Current Standby Supply Current IBST VSHDN VCCUVLO Active Supply Current Standby Supply Current Shutdown Rising Threshold Shutdown Threshold Hysteresis Soft-Start Charge Current Soft-Start Reset Threshold Undervoltage Lockout Threshold Undervoltage Lockout Hysteresis VBSTUVLO Boost Undervoltage Lockout (VBST-BSTREF) Boost UVLO Hysteresis Falling Edge Rising Edge Falling Edge Rising Edge
CONDITIONS
1200 1.35 8.60 8.95
UNITS
Supply Protection (Note
VSHDN 1.35V, VSHDN 0.3V Logic High (Note VSHDN 0.3V
1.15 0.25 1.25 8.40 8.75 0.35
1681f
LT1681
ELECTRICAL CHARACTERISTICS
denotes specifications which apply over full operating temperature range, otherwise specifications 25°C. VBST 12V, VBSTREF VREF 1.25V, 1000pF.
SYMBOL V5VREF I5VREFSC R5VREF Error ISENSE VSENSE IBLKSENS tMIN IMAX Sense IIMAX VIMAX VTHERM/ VOVLO fOSC IFSET VSYNC fSYNC Input Bias Current IMAX Threshold (Rising Edge) IMAX Threshold Hysteresis IMAX Output Switch Disable Delay Threshold (Rising Edge) Threshold Hysteresis Fault Delay Output Disable Oscillator Frequency, Free Frequency Programming Error, Free FSET Input Bias Current SYNC Logic High Input Threshold SYNC Logic Input Threshold SYNC Frequency Maximum SYNC Pulse Width (Logic High Logic Low) fOSC Oscillator Free-Run Frequency 50mV Overdrive Measured FSET fOSC 500kHz (Note FSET Charging, VFSET Positive-Going Edge Negative-Going Edge
PARAMETER Reference Voltage Short-Circuit Current Output Impedance Error Amplifier Reference Voltage Feedback Input Current Error Amplifier Voltage Gain Error Amplifier Current Limit Zero Current Output Voltage Maximum Output Voltage Gain Bandwidth Product Amplifier Gain Input Bias Current Current Limit Threshold Current Sense Switch Delay
CONDITIONS (I5VREF IVC) 20mA
4.85 4.80
5.10 5.15
UNITS
External Reference
Source, (I5VREF IVC) 20mA Measured Feedback
1.242 1.225
1.250
1.258 1.265
VREF Source Sink
(Note
Current Sense Blanking
Measured SENSE
VBLKSENS Blanking Input Threshold Blanking Input Bias Current Switch Minimum Time VBLKSENS VBG, Measured Output
Measured IMAX Input Measured IMAX Input Measured Output
THERM OVLO Fault Detectors
1.25
Oscillator Synchronization Decoder fOSC/2 1/fOSC
1681f
LT1681
ELECTRICAL CHARACTERISTICS
denotes specifications which apply over full operating temperature range, otherwise specifications 25°C. VBST 12V, VBSTREF VREF 1.25V, 1000pF.
SYMBOL tTGr/f tBGr/f tSGr/f tSG-BG tTG-BG PARAMETER Voltage Voltage Rise/Fall Times Voltage Voltage Rise/Fall Times Voltage Voltage Rise/Fall Times Enable Time Enable Time 90%/90% On/Off Thresholds On/Off Thresholds
CONDITIONS
11.5 11.5 11.5
UNITS
Output Drivers
90%/90%
90%/90%
Note Absolute Maximum Ratings those values beyond which life device impaired. Note Supply current specification does include external gate charge currents. Actual supply currents will higher vary with operating frequency, operating voltages type external switch elements used. Applications Information.
Note Guaranteed tested. Note LT1681E guaranteed meet performance specifications from 70°C. Specifications over 40°C 85°C operating temperature range assured design, characterization correlation with statistical process controls. guaranteed performance specifications over -40°C 85°C range, LT1681I available.
TYPICAL PERFOR CHARACTERISTICS
Supply Current Temperature
SUPPLY CURRENT (mA)
SUPPLY CURRENT (mA)
SUPPLY CURRENT (nA)
TEMPERATURE (°C)
1681
Supply Current SHDN Voltage
1100 25°C
Supply Current Supply Voltage
25°C
SHDN VOLTAGE (mV)
1681
SUPPLY VOLTAGE
1681
1681f
LT1681 TYPICAL PERFOR CHARACTERISTICS
IBST Boost Supply Current Temperature
IBST BOOST SUPPLY CURRENT (mA)
UVLO SUPPLY CURRENT (mA)
SUPPLY CURRENT (µA)
TEMPERATURE (°C)
5VREF Voltage Temperature
5VREF SHORT-CIRCUIT CURRENT LIMIT (mA)
5.10
5.05
ERROR REFERENCE
5VREF VOLTAGE
5.00
4.95
4.90
TEMPERATURE (°C)
Short-Circuit Current Limit Temperature
SHORT-CIRCUIT CURRENT LIMIT (mA)
SOFT-START OUTPUT CURRENT (µA)
SOFT-START OUTPUT CURRENT (µA)
TEMPERATURE (°C)
1681 1681 1681
Supply Current SHDN Voltage
25°C
UVLO Supply Current Temperature
SHDN CURRENT
1681
TEMPERATURE (°C)
1681
5VREF Short-Circuit Current Limit Temperature
1.260
Error Reference Temperature
1.255
1.250
1.245
TEMPERATURE (°C)
1681
1.240
TEMPERATURE (°C)
1681
Soft-Start Output Current Temperature
Soft-Start Output Current Soft-Start Voltage
25°C
TEMPERATURE (°C)
1681
SOFT-START VOLTAGE (mV)
1681
1681f
LT1681 TYPICAL PERFOR CHARACTERISTICS
Soft-Start Output Current Soft-Start Voltage
CURRENT SENSE BANDWIDTH (MHz)
1681
SOFT-START OUTPUT CURRENT (µA)
SOFT-START VOLTAGE
CTIO
SHDN (Pin Shutdown Pin. voltages exceeding positive-going threshold 1.25V enables LT1681. 150mV input hysteresis resists mode switching instability. SHDN controlled either logic-level input with analog signal. This shutdown feature typically used input supply undervoltage protection. resistor divider from converter input supply SHDN monitors that supply control system power-up sequencing, etc. internal functions disabled during shutdown. OVLO (Pin Overvoltage Shutdown Sense. Typically connected input supply through resistor divider. voltage exceeds 1.25V, LT1681 switching function disabled protect boosted circuitry from exceeding absolute maximum voltage. 40mV input hysteresis resists mode switching instability. Exceeding OVLO threshold also triggers soft-start reset, resulting graceful recovery from input transient event. THERM (Pin System Thermal Shutdown. Auxiliary shutdown that typically used system thermal protection. voltage exceeds 1.25V, LT1681 switching function disabled. 40mV input hysteresis resists mode switching instability. Exceeding THERM threshold also triggers soft-start reset, resulting graceful recovery. SGND (Pin Signal Ground Reference. Careful board layout techniques must used prevent corruption signal ground reference. High current switching paths must oriented converter ground plane such that currents to/from switches affect integrity LT1681 signal ground reference. 5VREF (Pin Local Reference. Allows connection external loads 20mA Typically bypassed with ceramic capacitor SGND. Reference output current limit protected typical value 45mA. load reference exceeds current limit value, LT1681 switching function disabled soft-start function reset. FSET (Pin Oscillator Timing Pin. Connect resistor (RFSET) from 5VREF this capacitor (CFSET) from this ground. LT1681 oscillator operates monitoring voltage CFSET charged RFSET. When voltage FSET reaches 2.5V, oscillator rapidly discharges capacitor with average current 0.8mA. Once
1681f
Current Sense Amplifier Bandwidth Temperature
25°C
TEMPERATURE (°C)
1681
LT1681
CTIO
voltage reduced 1.5V, becomes high impedance charging cycle repeats. oscillator operates twice switching frequency controller. Oscillator frequency fOSC approximated relation:
fOSC
FSET FSET RFSET
SYNC (Pin Oscillator Synchronization Input with TTL-Level Compatible Input. SYNC input signal desired synchronized operating frequency) controls both internal oscillator (running twice SYNC frequency) output switch phase. synchronization function desired, this shorted ground. LT1681 internal oscillator drives toggle flip-flop that assures duty cycle operation during oscillator freerun. oscillator, therefore, runs twice operating frequency converter. SYNC input decoder incorporates frequency doubling circuit oscillator synchronization, resetting internal oscillator both rising falling edges input signal. SYNC input decoder also differentiates transition phase forces toggle flip-flop phase-lock with SYNC input. transition logic high SYNC input signal corresponds initiation switching cycle (primary switches turning pending current control) transition logic forces primary switch state. such, maximum operating duty cycle equal duty cycle SYNC signal. SYNC input therefore used reduce maximum duty cycle converter reducing duty cycle SYNC input. (Pin Soft-Start. Connect capacitor (CSS) from this ground. output voltage LT1681 error amplifier corresponds peak current sense amplifier output detected before resetting switch outputs. soft-start circuit forces error amplifier output zero sense
current start-up. 10µA current forced from this onto external capacitor. voltage ramps does LT1681 internally sensed current limit. This effectively forces internal current limit ramp from zero, allowing overall converter current slowly increase until normal output regulation achieved. This function reduces output overshoot converter start-up. soft-start function incorporates 1VBE "dead zone" such that zero current condition maintained until rises 1VBE above ground. voltage reset start-up condition during shutdown, undervoltage lockout overvoltage overcurrent events, yielding graceful converter output recovery from these events. (Pin Error Amplifier Inverting Input. Typically connected resistor divider from output compensation components pin. converter output voltage feedback node. Input bias current ~50nA forces high event open-feedback path condition. error amplifier internally referenced 1.25V. Values VOUT feedback resistor (RFB1) ground resistor (RFB2) calculated program converter output voltage (VOUT) following relation: VOUT 1.25 (RFB1 RFB2)/RFB2 (Pin 10): Error Amplifier Output. LT1681 error amplifier impedance output inverting gain stage. amplifier ample current source capability allow easy integration isolation optocouplers that require bias currents 10mA. External loading reduces external current sourcing capacity 5VREF same amount load pin. error amplifier typically configured using feedback network realize integrator circuit. This circuit creates dominant pole converter regulation feedback loop. Integrator characteristics dominated value capacitor connected from feedback resistor connected pin. Specific integrator characteristics configured optimize transient response.
1681f
LT1681
CTIO
error amplifier also configured transimpedance amplifier secondary-side controller applications. (See Applications Information section configuration compensation details) SENSE (Pin 11): Current Sense Amplifier (CSA) Noninverting Input. Current monitored ground referenced current sense resistor, typically series with source bottom-side switch FET. Internal limit circuitry provides maximum peak value 150mV across sense resistor during normal operation. IMAX (Pin 12): Primary Current Runaway Protection. IMAX used detect primary-side switch currents shuts down primary switches current runaway condition detected. IMAX function disabled during current sense blanking interval. typically connected primary bottom-side switch source monitors switch current ground-referenced current sense resistor. voltage exceeds 360mV, LT1681 switching function disabled 130ns. Exceeding IMAX threshold also triggers soft-start reset, resulting graceful recovery from current runaway event. single-sense resistor systems, this shorted SENSE protection during blanking interval shorted SGND used. (Pin 13): Synchronous Switch Output Driver. This connected directly gate synchronous switch small FETs used (CGATE 5000pF), however, gate drive buffer recommended peak efficiencies. output synchronized out-of-phase with output. control timing output causes transition "lead" primary switch path during turnon 150ns. (Pin 14): Local Power Supply Input. Bypass with capacitor least times greater than C5VREF PGND. LT1681 incorporates undervoltage lockout that disables switching functions below 8.4V. LT1681 supports operational power supply voltages from (20V absolute maximum). PWRGND (Pin 15): Output Driver Ground Reference. Connect through impedance trace decoupling capacitor. (Pin 16): Bottom-Side Primary Switch/Forward Switch Output Driver. connected directly gate primary bottom-side forward switches small FETs used (CGATE total 5000pF), however, gate drive buffer recommended peak efficiencies. output enabled start each oscillator cycle phase with timed "lag" output during turn-on "lead" output during turn-off. These delays force concentration transitional losses onto bottom-side primary switch. BLKSENS (Pin 17): Blanking Sense Input. current sense function (via SENSE pin) disabled while BLKSENS below BLKSENS typically connected gate bottom-side primary switch MOSFET. BSTREF (Pin 18): VBST Supply Reference. Typically connects source topside external power switch. (Pin 19): Topside (Boosted) Primary Output Driver. connected directly gate primary topside switch small FETs used (CGATE 5000pF), however, gate drive buffer recommended peak efficiencies. VBST (Pin 20): Topside Primary Driver Bootstrapped Supply. This "boosted" supply rail referenced BSTREF pin. Supply voltage maintained bootstrap capacitor tied from VBST boosted supply reference (BSTREF) pin. charge capacitor refreshed each switch cycle through Schottky diode connected from supply (cathode) VBST (anode). bootstrap capacitor (CBOOST) must least times greater than total load capacitance pin. capacitor range 0.1µF generally adequate most applications. bootstrap diode must have reversebreakdown voltage greater than converter VIN. LT1681 supports operational VBST supply voltages (absolute maximum) referenced ground. Undervoltage lockout disables topside switch until VBST-BSTREF 7.0V start-up protection topside switch.
1681f
VBST BSTREF LOGIC
FSET
PHASE DETECT
SENSE 5VREF
IMAX
ERROR 350mV
1.25V
SHDN
(<8V)
ILIM 10µA
1.25V
REFERENCE GENERATOR 1.25V
THERM
5VREF
1.25V
225mV
1681
OVLO
SGND
PWRGND
BLKSENS
LT1681
BLOCK DIAGRA
SYNC
1681f
LT1681
APPLICATIO ATIO
Overview
LT1681 high voltage, high current synchronous regulator controller, optimized with dual transistor forward topologies. uses constant frequency, current mode architecture with internal logic that prevents operation over duty cycle. unique synchronization scheme allows system clock synchronized operational frequency 350kHz, along with phase control easy integration multicontroller systems. local precision supply available external support circuitry loaded 20mA. Internal fault detection circuitry disables switching when variety system faults detected such input supply overvoltage undervoltage faults, excessive system temperature, transformer primary-side saturation local supply overcurrent conditions. LT1681 current limit soft-start feature that gradually increases current drive capability converter system yield smooth start-up with minimal overshoot. soft-start circuitry also used smooth recoveries from system fault conditions. External switches employed switch elements, hearty switch drivers allow implementation high current designs. adaptive blanking scheme built into LT1681 allows correct current-sense blanking regardless switch size even while using external switch drive buffers. LT1681 employs voltage output error amplifier, providing superior integrator linearity allowing easy high bandwidth integration optocoupler feedback fully isolated solutions. Theory Operation (See Block Diagram) LT1681 senses output voltage associated converter pin. difference between voltage this internal 1.25V reference amplified generate error voltage pin, which used threshold current sense comparator. current sense comparator gets information from SENSE pin, which monitors voltage drop across external current sense resistor. When detected switch current increases level corresponding error
voltage pin, switches disabled until next switch cycle. During normal operation, LT1681 internal oscillator runs twice switching frequency. oscillator output toggles flip-flop, generating duty cycle pulse that used internally system clock When output this flip-flop transitions high, primary switches enabled. primary-side switches stay enabled until transformer primary current, sensed SENSE pin, connected ground-referenced resistor series with bottom-side switch FET, sufficient trip current sense comparator and, turn, reset latch. When latch resets, primary switches disabled synchronous switch enabled. adaptive blanking circuit senses bottomside gate voltage BLKSENS prevents current sensing until fully enabled, preventing false triggering turn-on transition glitch. current comparator threshold obtained when flip-flop output transitions low, latch bypassed primary switches disabled until next flipflop output transition, forcing maximum switch duty cycle less than 50%. System Fault Detection-The General Fault Condition (GFC) LT1681 contains circuitry detecting internal system faults. Detection fault triggers "general fault condition" GFC. When detected, LT1681 disables switching discharges soft-start capacitor. When subsides, LT1681 initiates startup cycle soft-start circuitry assure graceful recovery. Recovery from gated soft-start capacitor discharge. capacitor must discharged threshold 225mV before concluded. zero output current threshold typically transistor VBE, 0.7V, latching until 225mV threshold achieved assures zero output current state obtained event short-duration fault. also triggered during system state change event, such entering shutdown mode, prevent mode transition abnormalities.
1681f
LT1681
APPLICATIO ATIO
Events that trigger are: Exceeding current limit 5VREF
Detecting undervoltage condition Detecting undervoltage condition 5VREF Pulling SHDN below shutdown threshold Exceeding IMAX threshold Exceeding 1.25V fault detector threshold either OVLO THERM pins OVLO THERM pins used directly trigger GFC. either these pins used, they disabled connecting SGND. intention OLVO allow monitoring input supply protect from overvoltage condition. Monitoring system temperature (THERM) possible through resistor divider using thermistor resistor divider component. 5VREF provide precision supply required these applications. When these fault detection circuits disabled during shutdown UVLO conditions, reduction OVLO THERM input impedance ground will occur. prevent excessive input currents, impedance pull-up devices must used these pins. Undervoltage Lockout LT1681 maintains current operational mode when undervoltage condition detected supply pin, when below undervoltage lockout (UVLO) threshold. During UVLO condition pin, LT1681 disables internal functions with exception shutdown UVLO circuitry. external 5VREF supply also disabled during this condition. Disabling switching control circuity reduces LT1681 supply current 1mA, simplifying integration trickle charging systems that employ output feedback supply generation. function high side switch output (TG) also gated UVLO circuitry monitoring bootstrap supply (VBST-BSTREF). Switching disabled until voltage across bootstrap supply greater than 7.4V. This helps prevent possibility forcing high side switch into linear operational region, potentially
LT1681
1681
Figure Nonisolated Error Configuration
Another common error amplifier configuration optocoupler fully isolated converters with secondary-side control (see Figure such system, dominant pole feedback loop created secondary-side controller, error amplifier needs only
1681f
causing excessive power dissipation inadequate gate drive during start-up. Error Amplifier Configurations converter output voltage information back LT1681 onto where transformed into output current control voltage error amplifier. error amplifier generally configured integrator used create dominant pole main converter feedback loop. LT1681 error amplifier true high gain voltage amplifier. amplifier noninverting input internally referenced 1.25V; inverting input output pin. Because both frequency gain integrator frequency characteristics controlled with external components, this amplifier allows greater flexibility precision compared with transconductance error amplifier. nonisolated converter configuration where resistor divider used program desired output voltage, error amplifier configured simple active integrator, forming system dominant pole (see Figure Placing capacitor CERR from will single-pole crossover frequency (2RFBCERR)-1. Additional poles zeros added increasing complexity network.
VOUT CERR 1.25V
LT1681
APPLICATIO ATIO
translate optocoupler information. bandwidths optocoupler amplifier should high possible simplify system compensation. This high bandwidth operation accomplished using error amplifier transimpedance amplifier, with optocoupler transistor emitter providing feedback information directly into pin. resistor from ground provides bias condition optocoupler. Connecting optocoupler transistor collector local 5VREF supply reduces Miller capacitance effects maximizes bandwidth optocoupler. Higher optocoupler current also means higher bandwidth, 5VREF supply provide collector currents 10mA.
VOUT SENSE 5VREF
OSCILLATOR FREQUENCY (kHz)
1.25V LT1681
1681
Figure Optocoupler High Configuration
Oscillator Frequency Programming Synchronization LT1681 internal oscillator runs twice system switching frequency. oscillator output toggles flipflop, generating duty cycle pulse that used internally system clock Free-run frequency internal oscillator programmed timing network connected FSET pin. pull-up resistor RFSET, connected from 5VREF FSET, provides current charge timing capacitor CFSET connected from FSET ground. oscillator operates allowing RFSET charge CFSET 2.5V which point RFSET pulled back toward ground 2.5k resistor internal LT1681. When voltage across CFSET pulled down 1.5V, FSET becomes high impedance, once again allowing RFSET charge CFSET.
Figure plot oscillator frequency CFSET RFSET. Typical values 300kHz operation (150kHz system frequency) CFSET 150pF RFSET 51k.
TIMING RESISTOR 330pF 200pF 150pF 100pF
1681
Figure Oscillator Frequency Timing Components
relatively fast fall time oscillator waveform, FSET held 1.5V threshold internal lowimpedance clamp reduce undershoot error. this externally forced reason, external current limiting required prevent damage LT1681. Continuous source current from FSET should exceed 1mA. Putting resistor series with impedance pull-down device will assure proper function protect from damage. Oscillator Synchronization Synchronization LT1681 system clock accomplished driving level logic pulse train desired system switching frequency into SYNC pin. order assure proper synchronization, each phase synchronization signal must less then oscillator free-run cycle. SYNC input pulse controls phasing well frequency controller switching. SYNC circuit functions forcing phase oscillator output flip-flop match phase SYNC pulse prematurely ending oscillator charge cycle each transition edge. SYNC low-to-high transition, LT1681 starts switch-on cycle minimum switch-off period forced during SYNC logic period. Because SYNC logic period corresponds directly
1681f
LT1681
APPLICATIO ATIO
minimum time, converter maximum duty cycle forced using SYNC input. example, duty cycle SYNC pulse forces maximum duty cycle operation converter. Because logic pulse width exceeds logic high pulse width duty cycle operation, oscillator free-run cycle time must programmed exceed logic duration.
2.5V FSET 1.5V SYNC SYSTEM CLOCK (INTERNAL)
1681
Figure Oscillator/SYNC Waveforms
also possible LT1681 SYNC-only mode disabling oscillator completely. Connecting resistor divider from 5VREF FSET pin, forcing voltage within charge range 1.5V 2.5V, will allow oscillator follow SYNC input exclusively with provision free-run. Setting values force voltage close possible recommended.
5VREF LT1681 FSET 100pF
1681
Figure Oscillator Connection Sync-Only Mode Operation
Shutdown LT1681 SHDN will support CMOS logic signals also analog inputs. SHDN turn-on (rising) threshold 1.25V with 150mV hysteresis. common SHDN undervoltage detection input supply. Driving SHDN with resistor divider connected from input supply ground will prevent switching until desired input supply voltage achieved.
LT1681 enters ultralow current shutdown mode when SHDN below 350mV. During this mode, total supply current drops typical value less than 1µA. When SHDN rises above 350mV, will draw increasing amounts supply current until just before 1.25V turn-on threshold achieved, when typical supply current reaches 60µA. shutdown function disabled connecting SHDN VCC. This internally clamped 2.5V through series input resistance therefore draw almost when tied directly supply. This additional current minimized making connection through external series resistor (100k typically used). Soft-Start LT1681 current control (VC) limits sensed current zero voltages less than 1.4V through full current limit 3.2V, yielding 1.8V over full regulation range. voltage internally forced less than equal 0.7V. such, "dead zone" between 0.7V, where zero sensed current condition maintained. voltages above 0.7V, sensed current limit threshold rise needed maintained current limit value. Once rises maximum value less 0.7V, 2.5V, circuit effect. sources typical current 10µA. Placing capacitor (CSS) from ground will cause voltage ramp controlled rate, allowing graceful increase maximum converter output current during start-up condition. start-up delay time full available current limit (sec) LT1681 internally pulls below zero current threshold during fault condition assure graceful recovery. circuit also acts fault control latch assure full-range recovery from short duration fault. Once fault condition detected, LT1681 will suspend switching until discharged approximately 225mV.
1681f
LT1681
APPLICATIO ATIO
Layout Considerations-Grounding
LT1681 typically used high current converter designs that involve substantial switching transients. switch drivers designed drive large capacitances and, such, generate significant transient currents. Careful consideration must made regarding input local power-supply bypassing avoid corrupting ground references used error amplifier current sense circuitry. Effective grounding two-transistor synchronous forward topology where LT1681 used inherently difficult. situation complicated further number bypass elements that must considered. Typically, high current paths transients from input supply local drive supplies must kept isolated from SGND, which sensitive circuits such error reference current sense circuits, well local 5VREF supply, referred. virtue topologies used LT1681 applications, large currents from primary switches, well switch drive transients,
LT1681
5VREF
SGND
Figure High Current Transient Return Paths
pass through sense resistor ground. This defines ground connection sense resistor reference point both SGND PGND. nonisolated applications where SGND output reference, have condition where every bypass capacitor converter referenced same point. Effective grounding achieved considering return current paths from sense resistor each respective bypass capacitor. Don't tempted small traces separate grounds. power ground plane important always high-power converters, bypass elements must oriented such that transient currents return paths mix. Care must taken keep these transients away from SGND reference. effective approach 2-layer ground plane, reserving entire layer SGND. 5VREF non-isolated converter output bypasses then directly connected SGND plane.
VBST VBST BSTREF PGND
1681
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4.7µH 4.8µH MMBT3906LT1 VOUT+ 1.5µF 100V
VIN+
1.5µF 100V 0.25W 100V 1000pF 100V 0.25W Q14, FDS6680A 1.5µF 100V MURS120T3 MURS120T3 MMBT3906LT1 0.030 1/2W FDS6680A 2.2nF 250V VOUT- MBR0530
330µF
TYPICAL APPLICATIO
4.22k 3.01k 0.1µF
1681
VIN-
ZVN3310F BAT54 FZT690B 4.7µF
0.25W
0.22µF MMBZ5240BLT1
330pF
MURATA ERIE GHM3045X7R222K-GC VITRAMON VJ1825Y155MXB 330µF KEMET T510X337K010AS 330µF 6.3V KEMET T520D337M006AS ISO1: FAIRCHILD MOC207 COILCRAFT DO1608C-472 PANASONIC ETQPAF4R8HFA COILCRAFT DO1608C-105 SILICONIX Si4486EY Q14,Q15: FAIRCHILD FDS6680A MIDCOM 31267R COILTRONICS CTX02-14675 (FUNCTIONAL INSULATION) MIDCOM 31322R (BASIC INSULATION) MIDCOM 31264R (FUNCTIONAL INSULATION) MIDCOM 31323R (BASIC INSULATION) BAT54
1000pF 0.1µF BAS21 0.1µF 4700pF 3300pF 82pF 4700pF 52.3k 0.1µF BAS21 BAT54 BAS21 0.022µF 3.3k
73.2k
267k 0.25W
220pF
SYNC
ISNS ISNSGND VCOMP OPTODRV LTC1698 OVPIN MARGIN VAUX PGND PWRGD ICOMP 1.24k
MMBZ5245LT1 5VREF
VBST BLKSENS BSTREF SENSE IMAX PGND LT1681SW OVLO THERM SYNC SGND SHDN 5VREF FSET
MMBZ5248B-7
ISO1 5VREF MOC207
0.1µF
68µF
MMBT3906LT1
1.24k
LT1681
Figure 36V-72V 5V/10A Isolated Synchronous Forward Converter
1681f
3.3µH VSEC MMBT3906LT1 VOUT+ 1000pF 100V MMBT3906LT1 0.025 1/2W 1000pF 100V 0.25W Q15, FDS6680A 1.5µF 100V 2.35µH
LT1681
1.5µF 100V MURS120T3 MURS120T3
TYPICAL APPLICATIO
VOUT- MBR0530 ZVN3310F BAT54 4.7µF FZT690B 0.25W
0.22µF MMBZ5240BLT1
330pF
VIN- MURATA ERIE GHM3045X7R222K-GC VITRAMON VJ1825Y155MXB 330µF KEMET T510X337K010AS 330µF 6.3V KEMET T520D337M006AS C26: TPSE686M020R0150 ISO1: FAIRCHILD MOC207 COILCRAFT DO1608C-332 PULSE P1977 PLANAR INDUCTOR COILCRAFT DO1608C-105 SILICONIX Si4486EY Q14,Q15,Q17: FAIRCHILD FDS6680A FAIRCHILD NDT410EL Q12: ZETEX ZVN3310F Q13: ZETEX FZT690 PULSE P1976 PLANAR TRANSFORMER (FUNCTIONAL INSULATION) PULSE PA-0191 (BASIC INSULATION) MIDCOM 31264R (FUNCTIONAL INSULATION) MIDCOM 31323R (BASIC INSULATION) BAT54 1000pF 0.1µF 100V BAT54 BAS21 0.1µF 4700pF 3300pF 82pF 4700pF 0.1µF BAS21 0.022µF
73.2k
220pF SYNC OPTODRV LTC1698 ISNS ISNSGND VCOMP OVPIN MARGIN VAUX PGND PWRGD ICOMP 2.43k 3.01k 1.78k TRIM 0.33µF
267k 0.25W
BAS21
MMBZ5245LT1 5VREF 52.3k
MMBZ5248B-7
VBST BLKSENS BSTREF SENSE IMAX PGND LT1681SW OVLO THERM SYNC SGND SHDN 5VREF FSET
ISO1 5VREF MOC207
0.1µF
MMBT3906LT1
1.24k
1698
68µF
1.24k
Figure 36V-72V 3.3V/20A Isolated Synchronous Forward Converter
0.25W
VIN+
1.5µF 100V
FDS6680A 2200pF 250V
330µF
1681f
3.3µH VSEC MMBT3906LT1 VOUT+ 1000pF 100V MMBT3906LT1 0.025 1/2W VOUT- 1000pF 100V 0.25W Q15, FDS6680A 1.5µF 100V 2.35µH
VIN+
1.5µF 100V 0.25W 1.5µF 100V MURS120T3 MURS120T3
330µF
FDS6680A 2200pF 250V
TYPICAL APPLICATIO
MBR0530 0.25W
5VREF BAT54 0.25W MMBZ5248LT1 330pF 4.7µF FZT690
1.5k 0.25W ZVN3310F
1.5k 0.25W
0.22µF MMBZ5240BLT1
NDT410EL
MMBT3904LT1
MMBD914LT1
MURATA ERIE GHM3045X7R222K-GC VITRAMON VJ1825Y155MXB 330µF KEMET T510X337K010AS 330µF 6.3V KEMET T520D337M006AS C26: TPSE686M020R0150 ISO1: FAIRCHILD MOC207 COILCRAFT DO1608C-332 PULSE P1977 PLANAR INDUCTOR COILCRAFT DO1608C-105 SILICONIX Si4486EY Q14,Q15,Q17: FAIRCHILD FDS6680A FAIRCHILD NDT410EL Q12: ZETEX ZVN3310F Q13: ZETEX FZT690 PULSE P1976 PLANAR TRANSFORMER (FUNCTIONAL INSULATION) PULSE PA-0191 (BASIC INSULATION) MIDCOM 31264R (FUNCTIONAL INSULATION) MIDCOM 31323R (BASIC INSULATION) VOUT+ 1000pF
4.7µF BAT54 0.1µF 100V BAS21 BAS21 0.1µF 4700pF 3300pF 82pF 4700pF 0.1µF BAT54 BAS21
0.25W 0.022µF 2.43k 3.01k SENSE+
73.2k
LT1006S8 ISNS ISNSGND VCOMP SYNC OPTODRV LTC1698 OVPIN MARGIN VAUX PGND PWRGD ICOMP 3.01k
267k 0.25W
220pF
MMBZ5245LT1 5VREF 52.3k
VBST BLKSENS BSTREF SENSE IMAX PGND LT1681SW OVLO THERM SYNC SGND 5VREF FSET SHDN
1.78k
3.01k
3.01k
SENSE- TRIM 0.33µF
ISO1 5VREF MOC207
0.1µF
MMBT3906LT1
1.24k
3.01k
1698
68µF
1.24k
0.25W
Figure 36V-72V 3.3V/20A Isolated Synchronous Forward Converter with Fast Start Differential Sense
LT1681
VIN-
1681f
LT1681
TYPICAL APPLICATIO
LT1681/LTC1698 36V-72V 5V/10A Module (See Figure Application Schematic) LT1681/LTC1698 Isolated 5V/10A Converter Efficiency Load Current
EFFICIENCY
LT1681/LTC1698 36V-72V 3.3V/20A Module (See Figure Application Schematic)
EFFICIENCY
CURRENT
1681 TA04
LT1681/LTC1698 Isolated 3.3V/20A Converter Efficiency Load Current
CURRENT
1681 TA05
1681f
LT1681
PACKAGE DESCRIPTION
Package 20-Lead Plastic Small Outline (Wide .300 Inch)
(Reference 05-08-1620)
0.291 0.299** (7.391 7.595) 0.010 0.029 (0.254 0.737)
0.009 0.013 (0.229 0.330)
NOTE 0.016 0.050 (0.406 1.270)
NOTE: IDENT, NOTCH CAVITIES BOTTOM PACKAGES MANUFACTURING OPTIONS. PART SUPPLIED WITH WITHOUT OPTIONS *DIMENSION DOES INCLUDE MOLD FLASH. MOLD FLASH SHALL EXCEED 0.006" (0.152mm) SIDE **DIMENSION DOES INCLUDE INTERLEAD FLASH. INTERLEAD FLASH SHALL EXCEED 0.010" (0.254mm) SIDE
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.
0.496 0.512* (12.598 13.005)
NOTE
0.394 0.419 (10.007 10.643)
0.093 0.104 (2.362 2.642)
0.037 0.045 (0.940 1.143)
0.050 (1.270) 0.014 0.019 (0.356 0.482)
0.004 0.012 (0.102 0.305)
(WIDE) 1098
1681f
LT1681 RELATED PARTS
PART NUMBER LT1158 LT1160 LT1162 LT1336 LT1339 LTC®1530 LTC1622 LTC1628-PG LTC1628-SYNC LT1680 LTC1698 DESCRIPTION Half-Bridge N-Channel MOSFET Driver Half-Bridge N-Channel MOSFET Driver Dual Half-Bridge N-Channel MOSFET Driver Half-Bridge N-Channel MOSFET Driver High Power Synchronous DC/DC Controller High Power Step-Down Switching Regulator Controller 550kHz Step-Down Controller
COMMENTS Current Limit Protection, 100% Duty Cycle Input Supply, Shoot-Through 60V, Good Full-Bridge Applications Smooth Operation High Duty Cycle (95% 100%) Dual N-Channel MOSFET Controller Excellent 8-Pin MSOP; Synchronizable; Soft-Start; Current Mode Efficiency; Sense Resistor; 16-Pin SSOP Power Good Output; Minimum Input/Output Capacitors; 3.5V Synchronizable 150kHz 300kHz, High Side Current Sense, Input with LT1681, Isolated Power Supplies, Contains Voltage Margining, Optocoupler Driver, Synchronization Circuit with Primary Side Output; 0.925V VOUT Output; 8.4; 1.3V VOUT 3.5V Burst Mode® Operation; 16-Pin Narrow SSOP; 3.5V Mobile VID; 0.925V VOUT 3.5V Current Mode; 550kHz; Very Small Solution Size Efficiency, 550kHz, 2.65V 8.5V, 0.8V VOUT VIN, Synchronizable 750kHz GN16-Pin, 0.8V Reference Current Mode; 550kHz; Small 16-Pin SSOP, 9.8V 3.5V 36V, Power Good Output, 300kHz Operation Power Supply Design, Adaptive Direct Sense Minimizes COUT, 36V, 300kHz Package, VOUT 0.6V 1.75V 5-Bit Mobile VID, Active Voltage Positioning IMVP2, VOUT 0.6V 1.75V, Active Voltage Positioning IMVP2,
LTC1625/LTC1775 RSENSE Current Mode Synchronous Step-Down Controller Dual, 2-Phase Synchronous Step-Down Controller Dual, 2-Phase Synchronous Step-Down Controller High Power DC/DC Current Mode Step-Up Controller Secondary Synchronous Rectifier Controller
LTC1709-7 LTC1709-8 LTC1735 LTC1736 LTC1772 LTC1773 LTC1778 LTC1874 LTC1876 LTC1922-1 LTC1929 LTC3714 LTC3716
High Efficiency, 2-Phase Synchronous Step-Down Controller with 5-Bit High Efficiency, 2-Phase Synchronous Step-Down Controller High Efficiency, Synchronous Step-Down Controller High Efficiency, Synchronous Step-Down Controller with 5-Bit ThinSOTStep-Down Controller Synchronous Step-Down Controller Wide Operating Range, RSENSE Step-Down Controller Dual, Step-Down Controller 2-Phase, Dual Synchronous Step-Down Controller with Step-Up Regulator Synchronous Phase Modulated Full-Bridge Controller 2-Phase Synchronous Controller Intel Compatible, Wide Operating Range, RSENSE Step-Down Controller with Internal High Efficiency, 2-Phase Synchronous Step-Down Controller with 5-Bit Mobile
RSENSE ThinSOT trademarks Linear Technology Corporation. Burst Mode registered trademark Linear Technolgy Corporation.
1681f
Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, 95035-7417
(408)432-1900 FAX: (408) 434-0507 www.linear-tech.com
LT/TP 0302 PRINTED
LINEAR TECHNOLOGY CORPORATION 2001
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