LT3430/LT3430-1 High Voltage, 200kHz/100kHz Step-Down Switching Regula
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LT3430/LT3430-1 High Voltage, 200kHz/100kHz Step-Down Switching Regulators DESCRIPTION
LT®3430/LT3430-1 monolithic buck switching regulators that accept input voltages 60V. high efficiency switch included along with necessary oscillator, control logic circuitry. current mode architecture provides fast transient response excellent loop stability. Special design techniques high voltage process achieve high efficiency over wide input range. Efficiency maintained over wide output current range using output bias circuitry utilizing supply boost capacitor saturate power switch. Patented circuitry* maintains peak switch current over full duty cycle range. shutdown reduces supply current 30µA SYNC externally synchronized with logic level input from 228kHz 700kHz LT3430 from 125kHz 250kHz LT3430-1. LT3430/LT3430-1 available thermally enhanced 16-pin TSSOP package.
Lare registered trademarks Linear Technology Corporation. Patent 6498466
Wide Input Range: 5.5V Peak Switch Current over Duty Cycles Constant Switching Frequency: 200kHz (LT3430) 100kHz (LT3430-1) Switch Resistance Current Mode Effective Supply Current: 2.5mA Shutdown Current: 30µA 1.2V Feedback Reference Voltage Easily Synchronizable Cycle-by-Cycle Current Limiting Small, 16-Pin Thermally Enhanced TSSOP Package
APPLICATIONS
Industrial Automotive Power Supplies Portable Computers Battery Chargers Distributed Power Systems
TYPICAL APPLICATION
Buck Converter
MMSD914TI
Efficiency Load Current
VOUT
4.7µF 100V LT3430** SHDN SYNC
30BQ060
EFFICIENCY
5.5V*
BOOST
0.68µF 22µH VOUT
15.4k 4.99k
BIAS
100µF SOLID TANTALUM
LT3430-1 68µH LT3430 =27µH
220pF
LOAD CURRENT
3430 TA02
3.3k 0.022µF
3430 TA01
*FOR INPUT VOLTAGES BELOW 7.5V, SOME RESTRICTIONS APPLY LT3430-1 CIRCUIT APPLICATIONS INFORMATION SECTION
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LT3430/LT3430-1 ABSOLUTE MAXIMUM RATINGS
(Note
PACKAGE/ORDER INFORMATION
VIEW BOOST SHDN SYNC BIAS
Input Voltage (VIN) BOOST Above (Note BOOST Voltage SYNC Voltage SHDN Voltage BIAS Voltage Voltage/Current 3.5V/2mA Operating Junction Temperature Range LT3430EFE (Notes -40°C 125°C LT3430IFE (Notes -40°C 125°C Storage Temperature Range -65°C 150°C Lead Temperature (Soldering, sec) 300°C
PACKAGE 16-LEAD PLASTIC TSSOP TJMAX 125°C, 45°C/W, 10°C/W EXPOSED (PIN GND, MUST SOLDERED
ORDER PART NUMBER LT3430EFE LT3430IFE LT3430EFE-1 LT3430IFE-1
PART MARKING 3430EFE 3430IFE 3430EFE-1 3430IFE-1
Order Options Tape Reel: Lead Free: #PBF Lead Free Tape Reel: #TRPBF Lead Free Part Marking: http://www.linear.com/leadfree/ Consult Marketing parts specified with wider operating temperature ranges.
ELECTRICAL CHARACTERISTICS
PARAMETER Reference Voltage (VREF) Input Bias Current Error Voltage Gain Error Switch Source Current Sink Current Switching Threshold High Clamp Switch Current Limit (Note
denotes specifications which apply over full operating temperature range, otherwise specifications 25°C. 15V, 1.5V, SHDN BOOST Open Circuit, Open Circuit, unless otherwise noted.
CONDITIONS 5.5V
1.204 1.195
1.219 -0.2 2200
1.234 1.243 -1.5 3300 4200
UNITS µMho µMho
(VC) ±10µA
1650 1000
1.4V Duty Cycle SHDN Open, Boost -40°C 25°C 125°C (Note 2.5A, Boost (Note
0.14 0.18
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Switch Resistance Maximum Switch Duty Cycle (LT3430)
LT3430/LT3430-1 ELECTRICAL CHARACTERISTICS
PARAMETER Maximum Switch Duty Cycle (LT3430-1)
denotes specifications which apply over full operating temperature range, otherwise specifications 25°C. 15V, 1.5V, SHDN BOOST Open Circuit, Open Circuit, unless otherwise noted.
CONDITIONS 0.05
UNITS
Switch Frequency (LT3430) Switch Frequency (LT3430-1)
Give
0.15 2.53 0.58 0.60
Line Regulation Shifting Threshold Minimum Input Voltage Minimum Boost Voltage Boost Current (Note Input Supply Current (IVIN) Bias Supply Current (IBIAS) Shutdown Supply Current Lockout Threshold Shutdown Threshold Minimum SYNC Amplitude SYNC Frequency Range (LT3430) SYNC Frequency Range (LT3430-1) SYNC Input Resistance
5.5V 10kHz (Note (Note 2.5A Boost 0.75A Boost 2.5A (Note VBIAS (Note VBIAS SHDN 60V, Open Open Open, Shutting Down Open, Starting
0.15 0.25
2.42 0.37 0.42
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 Gain measured with swing equal 200mV above clamp level 200mV below upper clamp level. Note Minimum input voltage measured directly, guaranteed other tests. defined voltage where internal bias lines still regulated that reference voltage oscillator remain constant. Actual minimum input voltage maintain regulated output will depend upon output voltage load current. Applications Information. Note This minimum voltage across boost capacitor needed guarantee full saturation internal power switch. Note Boost current current flowing into BOOST with held above input voltage. flows only during switch time. Note Input supply current quiescent current drawn input when BIAS held with switching disabled. Bias supply current current drawn BIAS when BIAS held Total input referred supply current calculated summing input supply current (IVIN) with fraction bias supply current (IBIAS): ITOTAL IVIN (IBIAS)(VOUT/VIN) With 15V, VOUT IVIN 1.4mA, IBIAS 2.9mA, ITOTAL 2.4mA.
Note Switch resistance calculated dividing voltage forced current (3A). Typical Performance Characteristics graph switch voltage other currents. Note LT3430EFE/LT3430EFE-1 guaranteed meet performance specifications from 125°C junction temperature. Specifications over -40°C 125°C operating junction temperature range assured design, characterization correlation with statistical process controls. LT3430IFE/LT3430IFE-1 guaranteed over full -40°C 125°C operating junction temperature range. Note Peak Switch Current Limit Junction Temperature graph Typical Performance Characteristics section. 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 maximum operational Boost-SW voltage limited thermal load current constraints. `Boost Pin' `Thermal Calculations' Applications Information section.
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LT3430/LT3430-1 TYPICAL PERFORMANCE CHARACTERISTICS
Switch Peak Current Limit
25°C 1.229 SWITCH PEAK CURRENT TYPICAL FEEDBACK VOLTAGE VOLTAGE 1.219 CURRENT 1.214 1.209 1.204 JUNCTION TEMPERATURE (°C) JUNCTION TEMPERATURE (°C) CURRENT (µA) 1.224 CURRENT (µA) 1.234
Voltage Current
SHDN Bias Current
CURRENT REQUIRED FORCE SHUTDOWN (FLOWS PIN). AFTER SHUTDOWN, CURRENT DROPS
GUARANTEED MINIMUM
2.38V STANDBY THRESHOLD (CURRENT FLOWS PIN)
DUTY CYCLE
3430
3430
3430
Lockout Shutdown Threshold
INPUT SUPPLY CURRENT (µA) SHDN VOLTAGE START-UP SHUTDOWN LOCKOUT
Shutdown Supply Current
VSHDN 25°C INPUT SUPPLY CURRENT (µA) INPUT VOLTAGE
3430
Shutdown Supply Current
SHUTDOWN VOLTAGE
3430
25°C
JUNCTION TEMPERATURE (°C)
3430
Error Amplifier Transconductance
2500 3000
Error Amplifier Transconductance
PHASE SWITICHING FREQUENCY (kHz) CURRENT (µA)
Frequency Foldback
25°C
TRANSCONDUCTANCE (µmho)
2000 GAIN (µMho)
2500 GAIN
PHASE (DEG)
1500
2000
CURRENT 3430 3430-1 SWITCHING FREQUENCY
1000
1500
ROUT 200k
COUT 12pF
1000
ERROR AMPLIFIER EQUIVALENT CIRCUIT
RLOAD 25°C 100k FREQUENCY (Hz)
3430
3430
JUNCTION TEMPERATURE (°C)
3430
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LT3430/LT3430-1 TYPICAL PERFORMANCE CHARACTERISTICS
Switching Frequency
INPUT VOLTAGE FREQUENCY (kHz) (LT3430)
Minimum Input Voltage with Output
25°C BOOST CURRENT (mA)
BOOST Current
25°C
MINIMUM INPUT VOLTAGE START
MINIMUM INPUT VOLTAGE
JUNCTION TEMPERATURE (°C)
3430
LOAD CURRENT
3430
SWITCH CURRENT
3430
Shutdown Threshold
THRESHOLD VOLTAGE JUNCTION TEMPERATURE (°C) SWITCH VOLTAGE (mV)
Switch Voltage Drop
125°C
25°C
-40°C
SWITCH CURRENT
3430
3430
Switch Peak Current Limit
SWITCH PEAK CURRENT LIMIT SWITCH MINIMUM TIME (ns)
Switch Minimum Time Temperature
JUNCTION TEMPERATURE (°C)
3430
JUNCTION TEMPERATURE (°C)
3430
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LT3430/LT3430-1 FUNCTIONS
(Pins 17): connections reference regulated output, load regulation will suffer "ground" load same voltage pins This condition will occur when load current other currents flow through metal paths between pins load ground. Keep paths between pins load ground short ground plane when possible. package exposed that fused pins. (Pin should soldered copper ground plane under device reduce thermal resistance. (See Applications Information-Layout Considerations.) (Pins switch emitter on-chip power switch. This driven input voltage during switch time. Inductor current drives switch voltage negative during switch time. Negative voltage clamped with external catch diode. Maximum negative switch voltage allowed -0.8V. (Pins This collector on-chip power switch. powers internal control circuitry when voltage BIAS present. High dI/dt edges occur this during switch turn off. Keep path short from through input bypass capacitor, through catch diode back trace inductance this path creates voltage spikes switch off, adding voltage across internal NPN. BOOST (Pin BOOST used provide drive voltage, higher than input voltage, internal bipolar power switch. Without this added voltage, typical switch voltage loss would about 1.5V. additional BOOST voltage allows switch saturate voltage loss approximates that structure. (Pins 13): Connection. BIAS (Pin 10): BIAS used improve efficiency when operating higher input voltages light load current. Connecting this regulated output voltage forces most internal circuitry draw operating current from output voltage rather than input supply. This architecture increases efficiency especially when input voltage much higher than output. Minimum output voltage setting this mode operation (Pin 11): output error amplifier input peak switch current comparator. normally used frequency compensation, also serve current clamp control loop override. sits about 0.9V light loads 2.1V maximum load. driven ground shut regulator, driven high, current must limited 4mA. (Pin 12): feedback used output voltage using external voltage divider that generates 1.22V desired output voltage. Three additional functions performed pin. When voltage drops below 0.6V, switch current limit reduced external SYNC function disabled. Below 0.8V, switching frequency also reduced. Feedback Functions Applications Information details. SYNC (Pin 14): SYNC used synchronize internal oscillator external signal. directly logic compatible driven with signal between duty cycle. synchronizing range 125kHz 250kHz LT3430-1 228kHz 700kHz LT3430. Synchronizing Applications Information details. SHDN (Pin 15): SHDN used turn regulator reduce input drain current microamperes. This thresholds: 2.38V disable switching second 0.4V force complete micropower shutdown. 2.38V threshold functions accurate undervoltage lockout (UVLO); sometimes used prevent regulator from delivering power until input voltage reached predetermined level. SHDN functions required, either left open allow internal bias current lift default high state) forced high level exceed
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LT3430/LT3430-1 BLOCK DIAGRAM
LT3430/LT3430-1 constant frequency, current mode buck converters. This means that there internal clock feedback loops that control duty cycle power switch. addition normal error amplifier, there current sense amplifier that monitors switch current cycle-by-cycle basis. switch cycle starts with oscillator pulse which sets flip-flop turn switch When switch current reaches level inverting input comparator, flip-flop reset switch turns off. Output voltage control obtained using output error amplifier switch current trip point. This technique means that error amplifier commands current delivered output rather than voltage. voltage system will have phase shift resonant frequency inductor output capacitor, then abrupt 180° shift will occur. current system will have phase shift much lower frequency, will have additional shift until well beyond resonant frequency. This makes
RLIMIT RSENSE
much easier frequency compensate feedback loop also gives much quicker transient response. Most circuitry LT3430/LT3430-1 operates from internal 2.9V bias line. bias regulator normally draws power from regulator input pin, BIAS connected external voltage equal higher than bias power will drawn from external source (typically regulated output voltage). This will improve efficiency BIAS voltage lower than regulator input voltage. High switch efficiency attained using BOOST provide voltage switch driver which higher than input voltage, allowing switch saturated. This boosted voltage generated with external capacitor diode. comparators connected shutdown pin. 2.38V threshold undervoltage lockout second 0.4V threshold complete shutdown.
BIAS
SLOPE COMP SYNC ANTISLOPE COMP SHUTDOWN COMPARATOR 200kHz: LT3430 100kHz: LT3430-1 OSCILLATOR
0.4V 5.5µA SHDN
LOCKOUT COMPARATOR FOLDBACK CURRENT LIMIT CLAMP FREQUENCY FOLDBACK
VC(MAX) CLAMP
2.38V
Figure LT3430/LT3430-1 Block Diagram
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CURRENT COMPARATOR BOOST FLIP-FLOP DRIVER CIRCUITRY POWER SWITCH ERROR AMPLIFIER 2000µMho 1.22V
3430
2.9V BIAS REGULATOR
INTERNAL
LT3430/LT3430-1 APPLICATIONS INFORMATION
FEEDBACK FUNCTIONS feedback (FB) LT3430/LT3430-1 used output voltage provide several overload protection features. first part this section deals with selecting resistors output voltage second part talks about foldback frequency current limiting created pin. Please read both parts before committing final design. suggested value LT3430 output divider resistor (see Figure from ground (R2) less, formula shown below. LT3430-1, choose resistors that Thevinin resistance divider feedback 7.5k. output voltage error caused ignoring input bias current less than 0.25% with table standard values shown Table common output voltages. Please read following divider resistors increased above suggested values. R2(VOUT 1.22) 1.22
(NEAREST 7.32 8.45 15.4 46.4 18.7 20.5 30.9 73.2 ERROR OUTPUT DISCREET RESISTOR STEPS +0.32 -0.43 -0.30 -0.27 +0.54 -0.40 -0.20 +0.37
average current through diode inductor equal short-circuit current limit switch (typically LT3430/LT3430-1, folding back less than 2A). Minimum switch time limitations would prevent switcher from attaining sufficiently duty cycle switching frequency were maintained 200kHz (100kHz LT3430-1), frequency reduced about (3:1 LT3430-1) when feedback voltage drops below 0.8V (see Frequency Foldback graph). This does affect operation with normal load conditions; simply sees gear shift switching frequency during start-up output voltage rises. addition lower switching frequency, LT3430/ LT3430-1 also operate lower switch current limit when feedback voltage drops below 0.6V. Figure performs this function clamping voltage less than normal 2.1V upper clamp level. This foldback current limit greatly reduces power dissipation diode inductor during short-circuit conditions. External synchronization also disabled prevent interference with foldback operation. Again, nearly transparent user under normal load conditions. only loads that affected current source loads which maintain full load current with output voltage less than final value. these rare situations feedback clamped above 0.6V with external diode defeat foldback current limit. Caution: clamping feedback means that frequency shifting will also defeated, combination high input voltage dead shorted output cause LT3430/LT3430-1 lose control current limit. internal circuitry which forces reduced switching frequency also causes current flow feedback when output voltage low. equivalent circuitry shown Figure completely during normal operation. falls below 0.8V, begins conduct current LT3430 reduces frequency rate approximately 1.4kHz/µA. ensure adequate frequency foldback (under worst-case short-circuit conditions), external divider Thevinin resistance (RTHEV) must enough pull 115µA with 0.44V (RTHEV 3.8k)(LT3430-1 RTHEV 7.5k). result that reductions frequency current limit affected output voltage divider impedance. Cau34301fa
Table *LT3430, **LT3430-1
OUTPUT VOLTAGE 3.3* 3.3** 12** 4.99 4.99 4.99 4.12 12.7 12.1 8.25
More Than Just Voltage Feedback feedback used more than just output voltage sensing. also reduces switching frequency current limit when output voltage very (see Frequency Foldback graph Typical Performance Characteristics). This done control power dissipation both external diode inductor during short-circuit conditions. shorted output requires switching regulator operate very duty cycles,
LT3430/LT3430-1 APPLICATIONS INFORMATION
LT3430 FREQUENCY SHIFTING 1.4V ERROR AMPLIFIER OUTPUT
SYNC CIRCUIT
1.2V BUFFER
3430
Figure Frequency Current Limit Foldback
tion should used resistors increased beyond suggested values short-circuit conditions occur with high input voltage. High frequency pickup will increase protection accorded frequency current foldback will decrease. Choosing Inductor most applications, output inductor will fall into range 47µH (10µH 100µH LT3430-1). Lower values chosen reduce physical size inductor. Higher values allow more output current because they reduce peak current seen LT3430/LT3430-1 switch, which limit. Higher values also reduce output ripple voltage. When choosing inductor will need consider output ripple voltage, maximum load current, peak inductor current fault current inductor. addition, other factors such core copper losses, allowable component height, EMI, saturation cost should also considered. following procedure suggested handling these somewhat complicated conflicting requirements. Output Ripple Voltage Figure shows comparison output ripple voltage LT3430/LT3430-1 using either tantalum ceramic output capacitor. seen from Figure that output ripple voltage significantly reduced using
20mV/DIV
VOUT USING 100µF CERAMIC OUTPUT CAPACITOR
20mV/DIV
VOUT USING 100µF 0.08 TANTALUM OUTPUT CAPACITOR
VOUT 22µH
2µs/DIV
3430
Figure LT3430 Output Ripple Voltage Waveforms. Ceramic Tantalum Output Capacitors
ceramic output capacitor; significant decrease output ripple voltage very ceramic capacitors. Output ripple voltage determined ripple current (ILP-P) through inductor high frequency impedance output capacitor. high frequencies, impedance tantalum capacitor dominated effective series resistance (ESR). Tantalum Output Capacitor typical method reducing output ripple voltage when using tantalum output capacitor increase inductor value reduce ripple current inductor). following equations will help choosing required
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LT3430/LT3430-1 APPLICATIONS INFORMATION
inductor value achieve desirable output ripple voltage level. output ripple voltage less importance, subsequent suggestions Peak Inductor Fault Current will additionally help selection inductor value. Peak-to-peak output ripple voltage triwave (created peak-to-peak ripple current (ILP-P) times ESR) square wave (created parasitic inductance (ESL) ripple current slew rate). Capacitive reactance assumed small compared ESL. VRIPPLE (ILP-P )(ESR) (ESL) where: equivalent series resistance output capacitor equivalent series inductance output capacitor dI/dt slew rate inductor ripple current VIN/L Peak-to-peak ripple current (ILP-P) through inductor into output capacitor typically chosen between maximum load current. approximated (VOUT )(VIN VOUT ILP-P (VIN f)(L) Example: with 40V, VOUT 22µH, 0.080 10nH, output ripple voltage approximated follows: IP-P Ceramic Output Capacitor alternative further reduce output ripple voltage reduce output capacitor using ceramic capacitor. Although this reduction removes useful zero overall loop response, this zero replaced inserting resistor (RC) series with compensation capacitor (See Ceramic Capacitors Applications Information.) Peak Inductor Current Fault Current ensure that inductor will saturate, peak inductor current should calculated knowing maximum load current. appropriate inductor should then chosen. addition, decision should made whether inductor must withstand continuous fault conditions. maximum load current instance, inductor survive continuous overload condition. Dead shorts will actually more gentle inductor because LT3430/LT3430-1 have frequency current limit foldback.
Table
VENDOR/ PART Sumida CDRH104R-150 CDRH104R-220 CDRH104R-330 CDRH124-220 CDRH124-330 CDRH127-330 CDRH127-470 CEI122-220 Coiltronics UP3B-330 0.069 0.108 0.120 0.046 0.130 0.050 0.073 0.093 0.066 0.097 0.065 0.100 0.085 VALUE (µH) (Amps) (Ohms) HEIGHT (mm)
(40)(22 10-6 )(200
(5)(40
0.99
VRIPPLE (0.99 )(0.08) (1.8 0.079 0.018 97mVP-P reduce output ripple voltage further requires increase inductor value with trade-off being physically larger inductor with possibility increased component height cost.
UP3B-470 UP4B-680 Coilcraft DO3316P-153 DO5022p-683
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LT3430/LT3430-1 APPLICATIONS INFORMATION
Peak switch inductor current significantly higher than output current, especially with smaller inductors lighter loads, don't omit this step. Powdered iron cores forgiving because they saturate softly, whereas ferrite cores saturate abruptly. Other core materials fall somewhere between. following formula assumes continuous mode operation, errs only slightly high side discontinuous mode, used conditions. IPEAK IOUT Decide design tolerate "open" core geometry like barrel, which have high magnetic field radiation, whether needs closed core like toroid prevent problems. This tough decision because rods barrels temptingly cheap small there helpful guidelines calculate when magnetic field radiation will problem. Additional Considerations After making initial choice, consider additional factors such core losses second sourcing, etc. experts Linear Technology's Applications department feel uncertain about final choice. They have experience with wide range inductor types tell about latest developments profile, surface mounting, etc. Maximum Output Load Current Maximum load current buck converter limited maximum switch current rating (IP). current rating LT3430/LT3430-1 Unlike most current mode converters, LT3430/LT3430-1 maximum switch current limit does fall high duty cycles. Most current mode converters suffer drop peak switch current duty cycles above 50%. This effects slope compensation required prevent subharmonic oscillations current mode converters. (For detailed analysis, Application Note 19.) LT3430/LT3430-1 able maintain peak switch current limit over full duty cycle range using patented circuitry* cancel effects slope compensation peak switch current without affecting frequency compensation provides. Maximum load current would equal maximum switch current infinitely large inductor, with finite inductor size, maximum load current reduced onehalf peak-to-peak inductor current (ILP-P). following formula assumes continuous mode operation, implying that term right less than one-half IOUT(MAX) Continuous Mode
(VOUT )(VIN VOUT ILP-P IOUT (2)(VIN f)(L)
ILP-P
0.52)(12 0.52) 2(15 6)(200 )(12
VOUT 12V, VF(D1) 0.52V, 200kHz 15µH: IOUT (MAX)
2.5A
Note that there less load current available higher input voltage because inductor ripple current increases. 24V, duty cycle same conditions: IOUT (MAX)
0.71 2.29A
0.52)(24 0.52) 2(15 6)(200 )(24
calculate actual peak switch current with given conditions, use: ISW(PEAK) IOUT ILP-P (VOUT )(VIN VOUT IOUT 2(L)( f)(VIN
Patent 6,498,466
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LT3430/LT3430-1 APPLICATIONS INFORMATION
Reduced Inductor Value Discontinuous Mode smallest inductor value most importance converter design, order reduce inductor size/cost, discontinuous mode yield smallest inductor solution. maximum output load current discontinuous mode, however, must calculated defined later this section. Discontinuous mode entered when output load current less than one-half inductor ripple current (ILP-P). this mode, inductor current falls zero before next switch turn (see Figure Buck converters will discontinuous mode output load current given IOUT Discontinuous Mode (VOUT )(VIN VOUT (2)(VIN )(f)(L) IOUT(MAX) (200 3)(4.7 -6)(15) Discontinuous 0.52)(15 0.52) Mode IOUT(MAX) 1.21A Discontinuous Mode What been shown here that high inductor ripple current discontinuous mode operation tolerated, small inductor values used. higher output load current required, inductor value must increased. IOUT(MAX) longer meets discontinuous mode criteria, IOUT(MAX) equation continuous mode; LT3430/LT3430-1 designed operate well both modes operation, allowing large range inductor values used. Short-Circuit Considerations LT3430/LT3430-1 current mode controllers. They node voltage input current comparator which turns output switch cycle-by-cycle basis this peak current reached. internal clamp node, nominally then acts output switch peak current limit. This action becomes switch current limit specification. maximum available output power then determined switch current limit. potential controllability problem could occur under short-circuit conditions. power supply output short circuited, feedback amplifier responds output voltage raising control voltage, peak current limit value. Ideally, output switch would turned then turned current exceeded value indicated However, there finite response time involved both current comparator turnoff output switch. These result minimum time tON(MIN). When combined with large ratio diode forward voltage plus inductor voltage drop, potential exists loss control. Expressed mathematically requirement maintain control
inductor value buck converter usually chosen large enough keep inductor ripple current (ILP-P) low; this done minimize output ripple voltage maximize output load current. case large inductor values, seen equation above, discontinuous mode will associated with "light loads." When choosing small inductor values, however, discontinuous mode will occur much higher output load currents. limit smallest inductor value that chosen LT3430/LT3430-1 peak switch current (IP) maximum output load current required, given IOUT(MAX) Discontinuous Mode (2)(ILP-P 2(VOUT )(VIN VOUT ILP-P
Example: 15V, VOUT 0.52V, 200kHz 4.7µH.
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LT3430/LT3430-1 APPLICATIONS INFORMATION
where: switching frequency switch minimum time diode forward voltage Input voltage inductor voltage drop this condition observed, current will limited IPK, will cycle-by-cycle ratchet some higher value. Using nominal LT3430/LT3430-1 clock frequencies 200KHz/100kHz, 0.7V, maximum maintain control would approximately 90ns LT3430 180ns LT3430-1, unacceptably short times. solution this dilemma slow down oscillator when voltage abnormally thereby indicating some sort short-circuit condition. Oscillator frequency unaffected until voltage drops about normal value. Below this point oscillator frequency decreases roughly linearly down limit about 40kHz. (30kHz LT3430-1) This lower oscillator frequency during short-circuit conditions then maintain control with effective minimum time. recommended that [VIN/(VOUT VF)] ratios soft-start circuit should used LT3430 control output capacitor charge rate during start-up during recovery from output short circuit, thereby adding additional control over peak inductor current. Buck Converter with Adjustable Soft-Start later this data sheet. OUTPUT CAPACITOR output capacitor normally chosen effective series resistance (ESR), because this what determines output ripple voltage. takes volume, physically smaller capacitors have high ESR. range typical LT3430 applications 0.05 0.2. typical output capacitor type TPS, 100µF 10V, with guaranteed less than (The LT3430-1 will typically these capacitors parallel). This size surface mount solid tantalum capacitor. capacitors specially constructed tested ESR, they give lowest given volume. value microfarads particularly critical, values
Table Surface Mount Solid Tantalum Capacitor Ripple Current
Case Size TPS, Sprague 593D Case Size TPS, Sprague 593D Case Size (typ) (typ) (Max., Ripple Current
from 22µF greater than 500µF work well, cannot cheat mother nature ESR. find tiny 22µF solid tantalum capacitor, will have high ESR, output ripple voltage will terrible. Table shows some typical solid tantalum surface mount capacitors. Many engineers have heard that solid tantalum capacitors prone failure they undergo high surge currents. This historically true, type capacitors specially tested surge capability, surge ruggedness critical issue with output capacitor. Solid tantalum capacitors fail during very high turn-on surges, which occur output regulators. High discharge surges, such when regulator output dead shorted, harm capacitors. Unlike input capacitor, ripple current output capacitor normally enough that ripple current rating issue. current waveform triangular with typical value 250mARMS. formula calculate this Output capacitor ripple current (RMS): IRIPPLE(RMS) 0.29(VOUT )(VIN VOUT (L)( f)(VIN)
Ceramic Capacitors Higher value, lower cost ceramic capacitors becoming available. They generally chosen their good high frequency operation, small size very (effective series resistance). Their reduces output ripple voltage also removes useful zero loop frequency response, common tantalum capacitors. compensate this, resistor placed series with compensation capacitor Care must taken however, since this resistor sets high
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LT3430/LT3430-1 APPLICATIONS INFORMATION
frequency gain error amplifier, including gain switching frequency. gain error amplifier high enough switching frequency, output ripple voltage (although smaller ceramic output capacitor) still affect proper operation regulator. filter capacitor parallel with RC/CC network suggested control possible ripple pin. "All Ceramic" solution possible LT3430/LT3430-1 choosing correct compensation components given application. Example: 40V, VOUT LT3430 stabilized, provide good transient response maintain very output ripple voltage using following component values: (refer first page this data sheet component references) 4.7µF, 3.3k, 22nF, 220pF COUT 100µF. Application Note further detail techniques proper loop compensation. INPUT CAPACITOR Step-down regulators draw current from input supply pulses. rise fall times these pulses very fast. input capacitor required reduce voltage ripple this causes input LT3430/LT3430-1 force switching current into tight local loop, thereby minimizing EMI. ripple current calculated from: IRIPPLE(RMS) IOUT VOUT (VIN VOUT VIN2 Ceramic capacitors ideal input bypassing. 200kHz (100kHz) switching frequency, energy storage requirement input capacitor suggests that values range 4.7µF 20µF (10µF 47µF) suitable most applications. operation required close minimum input required output LT3430, larger value required. This prevent excessive ripple causing dips below minimum operating voltage resulting erratic operation. Depending LT3430/LT3430-1 circuit powered need check input voltage transients. input voltage transients caused input voltage steps connecting LT3430/LT3430-1 converter already powered source such wall adapter. sudden application input voltage will cause large surge current input leads that will store energy parasitic inductance leads. This energy will cause input voltage swing above level input power source exceed maximum voltage rating input capacitor LT3430/LT3430-1. easiest suppress input voltage transients small aluminum electrolytic capacitor parallel with input capacitor. selected capacitor needs have right amount order critically dampen resonant circuit formed input lead inductance input capacitor. typical values will fall range capacitance will fall range 50µF. tantalum capacitors used, values 22µF 470µF range generally needed minimize meet ripple current surge ratings. Care should taken ensure ripple surge ratings exceeded. Kemet T495 series surge rated. recommends derating capacitor operating voltage high surge applications. CATCH DIODE Highest efficiency operation requires Schottky type diode. switching losses minimized forward voltage drop, behavior benign lack significant reverse recovery time. so-called "ultrafast" recovery diodes generally recommended. When operating continuous mode, reverse recovery time exhibited "ultrafast" diodes will result slingshot type effect. power internal switch will ramp current into diode attempt recover. Then, when diode finally turned off, some tens nanoseconds later, node voltage ramps extremely high dV/dt, perhaps even 10V/ns With real world lead inductances, node easily overshoot rail. This result
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LT3430/LT3430-1 APPLICATIONS INFORMATION
poor behavior overshoot severe enough, damage itself. suggested catch diode (D1) International Rectifier 30BQ060 Schottky. rated average forward current reverse voltage. Typical forward voltage 0.52V diode conducts current only during switch time. Peak reverse voltage equal regulator input voltage. Average forward current normal operation calculated from: ID(AVG) IOUT (VIN VOUT less demanding conditions, this will improve circuit operation efficiency. Under input voltage load conditions, higher value capacitor will reduce discharge ripple improve start-up operation. LT3430-1 1.5µF boost capacitor recommended. SHUTDOWN FUNCTION UNDERVOLTAGE LOCKOUT Figure shows undervoltage lockout (UVLO) LT3430/LT3430-1. Typically, UVLO used situations where input supply current limited, relatively high source resistance. switching regulator draws constant power from source, source current increases source voltage drops. This looks like negative resistance load source cause source current limit latch under source voltage conditions. UVLO prevents regulator from operating source voltages where these problems might occur. Threshold voltage lockout about 2.38V. 5.5µA bias current flows this threshold. internally generated current used force default high state shutdown left open. When shutdown current issue, error this current minimized making less. shutdown current issue, raised 100k, error initial bias current changes with temperature should considered. 100k (25k suggested) (VIN 2.38V 2.38V LO(5.5µA)
This formula will yield values higher than with maximum load current BOOST most 3430 applications, boost components 0.68µF capacitor MMSD914TI diode. anode typically connected regulated output voltage generate voltage approximately VOUT above drive output stage. However, output stage discharges boost capacitor during time switch. output driver requires least headroom throughout this period keep switch fully saturated. output voltage less than 3.3V, recommended that alternate boost supply used. output voltages greater than recommended place zener diode (D4; page series with Boost diode Boost-to-SW voltage between This minimizes power loss within improving maximum ambient temperature operation. addition, minimizes Boost current overshoot during power switch turn reduce noise within regulator loop. output voltages greater than standard demoboard output, location provided. 0.68µF boost capacitor recommended most LT3430 applications. Almost type film ceramic capacitor suitable, should ensure fully recharged during time switch. LT3430 capacitor value derived from conditions 4800ns time, 75mA boost current 0.7V discharge ripple. boost capacitor value could reduced under
Minimum input voltage Keep connections from resistors shutdown short make sure that interplane surface capacitance switching nodes minimized. high resistor values used, shutdown should bypassed with 1000pF capacitor prevent coupling problems from switch node. hysteresis desired undervoltage lockout point, resistor added output node. Resistor values calculated from:
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LT3430/LT3430-1 APPLICATIONS INFORMATION
LT3430/LTC3430-1 INPUT SHDN 2.38V
STANDBY
OUTPUT
5.5µA
TOTAL SHUTDOWN 0.4V
3430
Figure Undervoltage Lockout
2.38( V/VOUT
(RHI VOUT
2.38 .5µA
suggested Input voltage which switching stops input voltage descends trip level Hysteresis input voltage level Example: output voltage switching stop input voltage drops below should restart unless input rises back 13.5V. therefore 1.5V 12V. 25k. 2.38(1.5/5 2.38 25k(5.5µA
(10.41) 116k 2.24 116k (5/1.5)
equal initial operating frequency 700kHz. This means that minimum practical sync frequency equal worst-case high self-oscillating frequency (228kHz), typical operating frequency 200kHz. Caution should used when synchronizing above 265kHz because higher sync frequencies amplitude internal slope compensation used prevent subharmonic switching reduced. This type subharmonic switching only occurs input voltages less than twice output voltage. Higher inductor values will tend eliminate this problem. Frequency Compensation section discussion entirely different cause subharmonic switching before assuming that cause insufficient slope compensation. Application Note more details theory slope compensation. LT3430-1 synchronizing range from 125kHz 250kHz (slope compensation loss occurs above 133kHz). power-up, when being clamped (see Figure Q2), sync function disabled. This allows frequency foldback operate shorted output condition. During normal operation, switching frequency controlled internal oscillator until reaches 0.6V, after which SYNC becomes operational. synchronization required, this should connected ground.
SYNCHRONIZING SYNC input must pass from logic level low, through maximum synchronization threshold with duty cycle between 90%. input driven directly from logic level output. LT3430 synchronizing range
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LT3430/LT3430-1 APPLICATIONS INFORMATION
LAYOUT CONSIDERATIONS with high frequency switchers, when considering layout, care must taken order achieve optimal electrical, thermal noise performance. maximum efficiency, switch rise fall times typically nanosecond range. prevent noise both radiated conducted, high speed switching current path, shown Figure must kept short possible. This implemented suggested layout Figure ShortenLT3430/ LT3430-1
this path will also reduce parasitic trace inductance approximately 25nH/inch. switch off, this parasitic inductance produces flyback spike across LT3430/ LT3430-1 switch. When operating higher currents input voltages, with poor layout, this spike generate voltages across LT3430/LT3430-1 that exceed absolute maximum rating. ground plane should always used under switcher circuitry prevent interplane coupling overall noise.
HIGH FREQUENCY CIRCULATING PATH
LOAD
3430
LT3430/ LT3430-1 BOOST
PINS SHORTED TOGETHER. PINS ALSO SHORTED TOGETHER (USING AVAILABLE SPACE UNDERNEATH DEVICE BETWEEN PINS PLANE) MINIMIZE LT3430/LT3430-1 C3-D1 LOOP
Figure High Speed Switching Path
CONNECT GROUND PLANE
SYNC VOUT SOLDER EXPOSED (PIN ENTIRE COPPER GROUND PLANE UNDERNEATH DEVICE. NOTE: BOOST BIAS COPPER TRACES SEPARATE LAYER FROM GROUND PLANE
SHDN
KELVIN SENSE VOUT
LT3430/ LT3430-1 BOOST BIAS
KEEP COMPONENTS AWAY FROM HIGH FREQUENCY, HIGH CURRENT COMPONENTS
3430
PLACE FEEDTHROUGH AROUND GROUND PINS CORNERS) GOOD THERMAL CONDUCTIVITY
Figure Suggested Layout
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LT3430/LT3430-1 APPLICATIONS INFORMATION
components should kept away possible from switch boost nodes. LT3430/ LT3430-1 pinout been designed this. ground these components should separated from switch current path. Failure will result poor stability subharmonic like oscillation. Board layout also significant effect thermal resistance. Pins GND, should soldered continuous copper ground plane under LT3430/ LT3430-1 die. package exposed (Pin which best thermal path heat package. Soldering exposed copper ground plane under device will reduce temperature increase power capability LT3430/LT3430-1. Adding multiple solder filled feedthroughs under around four corner pins ground plane will also help. Similar treatment catch diode coil terminations will reduce additional heating effects. PARASITIC RESONANCE Resonance "ringing" sometimes seen switch node (see Figure Very high frequency ringing following switch rise time caused switch/diode/input capacitor lead inductance diode capacitance. Schottky diodes have very high junction capacitance that ring many cycles when excited high frequency. total lead length input capacitor, diode switch path inch, inductance will approximately 25nH. switch off, this will produce spike across output device addition input voltage. higher currents this spike order higher with poor layout, potentially exceeding absolute switch voltage. path around switch, catch diode input capacitor must kept short possible ensure reliable operation. When looking this, >100MHz oscilloscope must used, waveforms should observed leads package. This switch spike will also cause node below ground. LT3430/LT3430-1 have special circuitry inside which mitigates this problem, negative voltages over 0.8V lasting longer than 10ns should avoided. Note that 100MHz oscilloscopes barely fast enough details falling edge overshoot Figure second, much lower frequency ringing seen during switch time load current enough allow inductor current fall zero during part switch time (see Figure Switch diode capacitance resonate with inductor form damped ringing 1MHz 10MHz. This ringing harmful regulator been shown contribute significantly EMI. attempt damp with resistive snubber will degrade efficiency.
LT3430
10mV/DIV RISE FALL
SWITCH NODE VOLTAGE
2V/DIV 0.2A/DIV INDUCTOR CURRENT IOUT 0.1A
50ns/DIV
VOUT 22µH
3430
1µs/DIV
3430
Figure Switch Node Resonance
Figure Discontinuous Mode Ringing
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LT3430/LT3430-1 APPLICATIONS INFORMATION
THERMAL CALCULATIONS Power dissipation LT3430/LT3430-1 chip comes from four sources: switch loss, switch loss, boost circuit current, input quiescent current. following formulas show calculate each these losses. These formulas assume continuous mode operation, they should used calculating efficiency light load currents. Switch loss: (IOUT (VOUT tEFF (1/2)(IOUT )(VIN)(
Thermal resistance LT3430/LT3430-1 package influenced presence internal backside planes. TSSOP (Exposed Pad) Package: With full plane under TSSOP package, thermal resistance will about 45°C/W. calculate temperature, proper thermal resistance number desired package worst-case ambient temperature: PTOT) When estimating ambient, remember nearby catch diode inductor will also dissipating power: PDIODE )(VIN VOUT )(ILOAD
(Note: Switching losses less LT3430-1 operating only 100kHz) Boost current loss: VOUT (IOUT /36) PBOOST Quiescent current loss: (0.0015) VOUT (0.003 Switch resistance 0.15) tEFF Effective switch current/voltage overlap time tIf) (VIN/1.2)ns (VIN/1.1)ns (IOUT/0.2)ns Switch frequency Example: with 40V, VOUT IOUT
Forward voltage diode (assume 0.52V PDIODE (0.52)(40 5)(2) 0.91W
PINDUCTOR (ILOAD)2(RIND) RIND Inductor resistance (assume 0.1) PINDUCTOR (2)2(0.1) 0.4W Only portion temperature rise external inductor diode coupled junction LT3430. Based empirical measurements, thermal effect LT3430 junction temperature power dissipation external inductor catch diode calculated
(0.15)(2)2
2)(2)(40) 0.08 0.72 0.8W
TJ(LT3430) (PDIODE PINDUCTOR)(5°C/W) Using example calculations LT3430 dissipation, LT3430 temperature will estimated PTOT) (PDIODE PINDUCTOR)] With TSSOP package 45°C/W), ambient temperature 50°C: 0.92) 1.31) 98°C temperature peak certain combinations VIN, VOUT load current. While higher gives greater switch losses, quiescent catch diode losses,
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PBOOST
40(0.0015) 5(0.003) 0.08W
(5)2 0.04W
Total power dissipation given PTOT PBOOST 0.8W 0.04W 0.08W 0.92W
LT3430/LT3430-1 APPLICATIONS INFORMATION
lower generate greater losses switch losses. general, maximum minimum levels should checked with maximum typical load current calculation LT3430/LT3430-1 temperature. more accurate temperature required, measurement SYNC resistance GND) used. SYNC resistance measured forcing voltage greater than 0.5V monitoring current over temperature oven. This should done with minimal device power (low switching 0V)) order calibrate SYNC resistance with ambient (oven) temperature. Note: Some internal power dissipation BOOST voltage, transferred outside reduce junction temperature, increasing voltage drop path boost diode (see Figure This reduction junction temperature inside will allow higher ambient temperature operation given conditions. BOOST circuitry dissipates power given PDISS BOOST VOUT (ISW section, value designed 0.7V droop VDROOP. Hence, output voltage would still allow minimum 3.3V boost function using capacitor calculated. target output voltage required, however, excess placed across boost capacitor which required boost function still dissipates additional power. What required voltage drop path achieve minimal power dissipation while still maintaining minimum boost voltage across zener, placed series with (see Figure drops voltage Example BOOST power dissipation input output conversion given PBOOST
zener placed series with then power dissipation becomes PBOOST 0.167
Typically (the boost voltage across capacitor equals VOUT. This because diodes considered almost equal, where: VOUT VFD2 (-VFD1) VOUT Hence equation used boost circuitry power dissipation given previous Thermal Calculations section stated PDISS(BOOST) VOUT (ISW VOUT
package with thermal resistance 45°C/W, ambient temperature savings would T(ambient) sav-
BOOST LT3430/ LT3430-1 SHDN SYNC BIAS
VOUT
Here seen that boost power dissipation increases square VOUT. possible, however, reduce below VOUT save power dissipation increasing voltage drop path Care should taken that does fall below minimum 3.3V boost voltage required full saturation internal power switch. output voltages approximately During switch turn will fall boost capacitor dicharged BOOST pin. previous BOOST
3430
Figure BOOST Pin, Diode Selection
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LT3430/LT3430-1 APPLICATIONS INFORMATION
ings 0.233W 45°C/W 11°C. zener should sized excess 0.233W operaton. tolerances zener should considered ensure minimum exceeds 3.3V VDROOP. Input Voltage Operating Frequency Considerations absolute maximum input supply voltage LT3430/ LT3430-1 specified 60V. This based solely internal semiconductor junction breakdown effects. internal power dissipation, actual maximum achievable particular application less than this. detailed theoretical basis estimating internal power loss given section, Thermal Considerations. Note that switching loss proportional both operating frequency output current. majority switching loss also proportional square input voltage. example, while combination 40V, VOUT fOSC 200kHz easily achievable, simultaneously raising fOSC 700kHz possible. Nevertheless, input voltage transients usually accommodated, assuming resulting increase internal dissipation insufficient time duration raise temperature significantly. second consideration controllability. potential limitation occurs with high step-down ratio VOUT, this requires correspondingly narrow minimum switch time. approximate expression this (assuming continuous mode operation) given follows: fOSC summary: aware that simultaneous requirements high VIN, high IOUT high fOSC achievable practice internal dissipation. Thermal Considerations section offers basis estimate internal power. questionable cases prototype supply should built exercised verify acceptable operation. simultaneous requirements high VIN, VOUT high fOSC result unacceptably short minimum switch time. Cycle skipping and/or odd/even cycle behavior will result although correct output voltage usually maintained. LT3430-1 100kHz switching frequency will allow higher VIN/VOUT ratios without pulse skipping. FREQUENCY COMPENSATION Before starting theoretical analysis frequency response, following should remembered-the worse board layout, more difficult circuit will stabilize. This true almost high frequency analog circuits, read Layout Considerations section first. Common layout errors that appear stability problems distant placement input decoupling capacitor and/or catch diode, connecting compensation ground track carrying significant switch current. addition, theoretical analysis considers only first order non-ideal component behavior. these reasons, important that final stability check made with production layout components. LT3430/LT3430-1 current mode control. This alleviates many phase shift problems associated with inductor. basic regulator loop shown Figure LT3430/LT3430-1 considered blocks, error amplifier power stage. Figure shows overall loop response. pin, frequency compensation components used are: 3.3k, 0.022µF 220pF. output capacitor used 100µF, tantalum capacitor with typical 100m. LT3430-1 uses these capacitors parallel. tantalum output capacitor provides useful zero loop frequency response maintaining
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where: input voltage VOUT output voltage Schottky diode forward drop fOSC switching frequency potential controllability problem arises LT3430/ LT3430-1 called upon produce time shorter than able produce. Feedback loop action will lower then reduce control voltage point where some sort cycle-skipping odd/even cycle behavior exhibited.
LT3430/LT3430-1 APPLICATIONS INFORMATION
stability. This ESR, however, contributes significantly ripple voltage output (see Output Ripple Voltage Applications Information section). possible reduce capacitor size output ripple voltage replacing tantalum output capacitor with ceramic output capacitor because very ESR. zero provided tantalum output capacitor must reinserted back into loop. Alternatively, there cases where, even with tantalum output capacitor, additional zero required loop increase phase margin improved transient response. zero added into loop placing resistor (RC) series with compensation capacitor, placing capacitor (CFB) between output pin. When using maximum value limitations. First, combination output capacitor stop loop rolling altogether. Second, loop gain rolled sufficiently switching frequency, output ripple will perturb enough cause unstable duty cycle switching similar subharmonic oscillations. needed, additional capacitor (CF) added across RC/CC network from ground further suppress ripple voltage. With tantalum output capacitor, LT3430/LT3430-1 already includes resistor (RC) filter capacitor (CF) (see Figures compensate
LT3430/LTC3430-1 CURRENT MODE POWER STAGE 2mho ERROR AMPLIFIER 2000µmho 200k
3430
loop over entire range allow stable pulse skipping high VIN-to-VOUT ratios 10). ceramic output capacitor still used with simple adjustment resistor stable operation (see Ceramic Capacitors section stabilizing LT3430). additional phase margin required, capacitor (CFB) inserted between output care must taken high output voltage applications. Sudden shorts output create unacceptably large negative transients pin. VIN-to-VOUT ratios higher loop bandwidths possible readjusting frequency compensation components pin. When checking loop stability, circuit should operated over application's full voltage, current temperature range. Proper loop compensation obtained empirical methods described Application Notes CONVERTER WITH BACKUP OUTPUT REGULATOR systems with primary backup supply, example, battery powered device with wall adapter input, output LT3430/LT3430-1 held backup supply with LT3430/LT3430-1 input disconnected. this condition, will source current into pin. SHDN held ground, only shut down current 30µA will pulled from second supply. With SHDN floating,
GAIN PHASE
3430
OUTPUT TANTALUM CERAMIC RLOAD GAIN (dB)
PHASE (DEG)
Figure Model Loop Response
1.22V
100k FREQUENCY (Hz) 3.3k VOUT 22nF ILOAD 220pF COUT 100µF, 10V,
Figure Overall Loop Response
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LT3430/LT3430-1 APPLICATIONS INFORMATION
LT3430/LT3430-1 will consume their quiescent operating current 1.5mA. will also source current other components connected input line. this load greater than 10mA input could shorted ground, series Schottky diode must added, shown Figure With these safeguards, output held voltages absolute maximum rating. BUCK CONVERTER WITH ADJUSTABLE SOFT-START Large capacitive loads high input voltages cause high input currents start-up. Figure shows circuit that limits dv/dt output start-up, controlling capacitor charge rate. buck converter typical configuration with addition output starts rise, turns regulating switch current maintain constant dv/dt output. Output rise time controlled current through defined Q1's VBE. Once output regulation, turns circuit operates normally. transient protection base (R4)(C )(VOUT Rise Time Using values shown Figure 10-9 Rise Time
ramp linear rise times order 100ms possible. Since circuit voltage controlled, ramp rate unaffected load characteristics maximum output current unchanged. Variants this circuit used sequencing multiple regulator outputs.
MMSD914TI
30BQ060 REMOVABLE INPUT SHDN SYNC 4.7µF
BOOST LT3430 BIAS
0.68µF 33µH 15.4k 220pF
3430
ALTERNATE SUPPLY
30BQ060
4.99k
100µF
3.3k 0.022µF
Figure Dual Source Supply with 25µA Reverse Leakage
MMSD914TI 0.68µF
BOOST INPUT 4.7µF
BIAS
33µH
LT3430 SHDN SYNC
30BQ060 100µF B250A
15.4k
OUTPUT
4.99k 220pF CDRH104R-220M 15nF
3430
3.3k 0.022µF
Figure Buck Converter with Adjustable Soft-Start
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LT3430/LT3430-1 APPLICATIONS INFORMATION
DUAL OUTPUT SEPIC CONVERTER circuit Figure generates both positive negative outputs with single piece magnetics. inductors shown actually just windings standard Coiltronics inductor. topology output standard buck converter. topology would simple flyback winding coupled buck converter were present. creates SEPIC (single-ended primary inductance converter) topology which improves regulation reduces ripple current Without voltage swing compared would vary relative loading coupling losses. provides impedance path maintain equal voltage swing L1B, improving regulation. flyback converter, during switch time, converter's energy stored only, since current flows L1B. switch off, energy transferred magnetic coupling into L1B, powering rail. pulls positive during switch time, causing current flow, energy build switch off, energy stored both supply rail. This reduces current changes current waveform from square triangular. details this circuit, including maximum output currents, Design Note 100. POSITIVE-TO-NEGATIVE CONVERTER circuit Figure positive-to-negative topology using grounded inductor. differs from standard approach chip derives feedback signal because LT3430/LT3430-1 accepts only positive feedback signals. ground must tied regulated negative output. resistor divider then provides proper feedback voltage chip. following equation used calculate maximum load current positive-to-negative converter: (VIN )(VOUT (VOUT )(VIN 0.15) )(f)(L) (VOUT 0.15)(VOUT
IMAX
Maximum rated switch current Minimum input voltage VOUT Output voltage Catch diode forward voltage 0.15 Switch voltage drop Example: with VIN(MIN) 5.5V, VOUT 12V, 10µH, 0.52V, IMAX 0.6A.
MMSD914TI 0.68µF BOOST 7.5V LT3430 SHDN SYNC 3.3k 0.022µF SINGLE CORE WITH WINDINGS COILTRONICS #CTX25-4A LOAD ZERO, OPTIONAL PRELOAD USED IMPROVE LOAD REGULATION 30BQ060 100µF TANT 220pF 4.99k 15.4k L1A* 25µH VOUT
4.7µF 100V CERAMIC
100µF TANT
L1B*
100µF TANT
VOUT
3430
Figure Dual Output SEPIC Converter
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LT3430/LT3430-1 APPLICATIONS INFORMATION
MMSD914TI 0.68µF LT3430 30BQ060 36.5k 30BQ015 100µF TANT OUTPUT** -12V, 0.5A
3430
INPUT 5.5V
BOOST
10µH
Minimum inductor continuous mode: (VIN )(VOUT LMIN 2(f)(VIN VOUT IOUT -12V converter using LT3430/LT34301 with peak switch current catch diode 0.52V: ICONT (40)2 (3)2 1.148A 4(40 12)(40 0.52)
4.7µF 100V
4.12k
INCREASE HIGHER CURRENT APPLICATIONS. APPLICATIONS INFORMATION MAXIMUM LOAD CURRENT DEPENDS MINIMUM INPUT VOLTAGE INDUCTOR SIZE. APPLICATIONS INFORMATION
Figure Positive-to-Negative Converter
INDUCTOR VALUE criteria choosing inductor typically based ensuring that peak switch current rating exceeded. This gives lowest value inductance that used, some cases (lower output load currents) give value that creates unnecessarily high output ripple voltage. difficulty calculating minimum inductor size needed that must first decide whether switcher will continuous discontinuous mode critical point where switch current reaches first step following formula calculate load current above which switcher must continuous mode. your load current less than this, discontinuous mode formula calculate minimum inductor needed. load current higher, continuous mode formula. Output current where continuous mode needed: ICONT (VIN 4(VIN VOUT )(VIN VOUT
load current 0.5A, this says that discontinuous mode used minimum inductor needed found from: 2(12)(0.5) LMIN 6.7µH (200 )(3)2 practice, inductor should increased about over calculated minimum handle losses variations value. This suggests minimum inductor 10µH this application. Ripple Current Input Output Capacitors Positive-to-negative converters have high ripple current input capacitor. long capacitor lifetime, value this current must less than high frequency ripple current rating capacitor. following formula will give approximate value ripple current. This formula assumes continuous mode large inductor value. Small inductors will give somewhat higher ripple current, especially discontinuous mode. exact formulas very complex appear Application Note pages purposes here have simply added fudge factor (ff). value about higher load currents 15µH. increases about smaller inductors lower load currents. Capacitor IRMS (ff)(IOUT output capacitor ripple current positive-tonegative converter similar that typical buck regulator-it triangular waveform with peak-to-peak
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Minimum inductor discontinuous mode: )(IOUT LMIN (f)(IP
LT3430/LT3430-1 APPLICATIONS INFORMATION
value equal peak-to-peak triangular waveform inductor. output ripple design Figure places input capacitor between regulated negative output. This placement input capacitor significantly reduces size required output capacitor (versus placing input capacitor between ground). peak-to-peak ripple current both inductor output capacitor (assuming continuous mode) IP-P VOUT VOUT chosen capacitor (see Output Ripple Voltage Applications Information). Diode Current Average diode current equal load current. Peak diode current will considerably higher. Peak diode current: Continuous Mode (VIN )(VOUT IOUT 2(L)(f)(VIN VOUT Discontinuous Mode 2(IOUT )(VOUT (L)(f)
Duty Cycle ICOUT (RMS) IP-P
output ripple voltage this configuration typical buck regulator based predominantly inductor's triangular peak-to-peak ripple current
Keep mind that during start-up output overloads, average diode current much higher than with normal loads. Care should used diodes rated less than used, especially continuous overload conditions must tolerated.
TYPICAL APPLICATION
3.3V, Buck Converter
MMSD914TI 5.5V* BOOST 4.7µF 100V LT3430-1 SHDN SYNC 3.3k 0.022µF
3430
1.5µF 30BQ060 68µH VOUT 3.3V
20.5k 12.1k
BIAS
100µF SOLID TANTALUM PARALLEL
220pF
*FOR INPUT VOLTAGES BELOW 7.5V, SOME RESTRICTIONS APPLY
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LT3430/LT3430-1 PACKAGE DESCRIPTION
Package 16-Lead Plastic TSSOP (4.4mm)
(Reference 05-08-1663)
Exposed Variation
3.58 (.141) 4.90 5.10* (.193 .201) 3.58 (.141) 1514 1110 6.60 ±0.10 4.50 ±0.10
NOTE
2.94 (.116) 0.45 ±0.05 1.05 ±0.10 0.65 2.94 6.40 (.116) (.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 (BB) 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.
LT3430/LT3430-1 RELATED PARTS
PART NUMBER LT1074/LT1074HV LT1076/LT1076HV LT1676 LT1765 LT1766 LT1767 LT1776 LT1940 LT1956 LT1976 LT3010 LTC3407 LTC3412 LTC3414 LT3430/LT3431 LT3433 DESCRIPTION 4.4A (IOUT), 100kHz, High Efficiency Step-Down DC/DC Converters 1.6A (IOUT), 100kHz, High Efficiency Step-Down DC/DC Converters 60V, 440mA (IOUT), 100kHz, High Efficiency Step-Down DC/DC Converter 25V, 2.75A (IOUT), 1.25MHz, High Efficiency Step-Down DC/DC Converter 60V, 1.2A (IOUT), 200kHz, High Efficiency Step-Down DC/DC Converter 25V, 1.2A (IOUT), 1.25MHz, High Efficiency Step-Down DC/DC Converter 40V, 550mA (IOUT), 200kHz, High Efficiency Step-Down DC/DC Converter 25V, Dual 1.2A (IOUT), 1.1MHz, High Efficiency Step-Down DC/DC Converter 60V, 1.2A (IOUT), 500kHz, High Efficiency Step-Down DC/DC Converter 60V, 1.2A (IOUT), 200kHz, High Efficiency Step-Down DC/DC Converter with Burst Mode® Operation 80V, 50mA Noise Linear Regulator Dual 600mA (IOUT), 1.5MHz, Synchronous Step-Down DC/DC Converter 2.5A (IOUT), 4MHz, Synchronous Step-Down DC/DC Converter (IOUT), 4MHz, Synchronous Step-Down DC/DC Converter 60V, 2.75A (IOUT), 200kHz/500kHz, High Efficiency Step-Down DC/DC Converters 60V, 400mA (IOUT), 200kHz, High Efficiency Step-Up Step-Down DC/DC Converter with Burst Mode Operation COMMENTS VIN: 7.3V 45V/64V, VOUT(MIN): 2.21V, 8.5mA, ISD: 10µA DD-5/7, TO220-5/7 VIN: 7.3V 45V/64V, VOUT(MIN): 2.21V, 8.5mA, ISD: 10µA DD-5/7, TO220-5/7 VIN: 7.4V 60V, VOUT(MIN): 1.24V, 3.2mA, ISD: 2.5µA, VIN: 25V, VOUT(MIN): 1.20V, 1mA, ISD: 15µA, TSSOP16E VIN: 5.5V 60V, VOUT(MIN): 1.20V, 2.5mA, ISD: 25µA, TSSOP16/E VIN: 25V, VOUT(MIN): 1.20V, 1mA, ISD: 6µA, MS8/E VIN: 7.4V 40V, VOUT(MIN): 1.24V, 3.2mA, ISD: 30µA, VIN: 25V, VOUT(MIN): 1.2V, 3.8mA, ISD: <1µA, TSSOP16E VIN: 5.5V 60V, VOUT(MIN): 1.20V, 2.5mA, ISD: 25µA, TSSOP16/E VIN: 3.3V 60V, VOUT(MIN): 1.20V, 100µA, ISD: <1µA, TSSOP16/E VIN: 1.5V 80V, VOUT(MIN): 1.28V, 30µA, ISD: <1µA, MSE8 VIN: 2.5V 5.5V, VOUT(MIN): 0.6V, 40µA, ISD: <1µA, MS10E VIN: 2.5V 5.5V, VOUT(MIN): 0.8V, 60µA, ISD: <1µA, TSSOP16E VIN: 2.3V 5.5V, VOUT(MIN): 0.8V, 64µA, ISD: <1µA, TSSOP20E VIN: 5.5V 60V, VOUT(MIN): 1.20V, 2.5mA, ISD: 30µA, TSSOP16E VIN: 60V, VOUT(MIN): 3.3V 20V, 100µA, ISD: <1µA, TSSOP16E
LTC3727/LTC3727-1 36V, 500kHz, High Efficiency Step-Down DC/DC Controllers Burst Mode registered trademark Linear Technology Corporation.
VIN: 36V, VOUT(MIN): 0.8V, 670µA, ISD: 20µA, QFN-32, SSOP-28
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Linear Technology Corporation
(408) 432-1900 FAX: (408) 434-0507
0107 PRINTED
1630 McCarthy Blvd., Milpitas, 95035-7417
www.linear.com
LINEAR TECHNOLOGY CORPORATION 2006
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