LT3757/LT3757A Boost, Flyback, SEPIC and Inverting Controller Features Description Wide Input Voltage Range: 2.9V to 40V n Positive or Negative Output Voltage Programming with a Single Feedback Pin n Current Mode Control Provides Excellent Transient Response n Programmable Operating Frequency (100kHz to 1MHz) with One External Resistor n Synchronizable to an External Clock n Low Shutdown Current < 1µA n Internal 7.2V Low Dropout Voltage Regulator n Programmable Input Undervoltage Lockout with Hysteresis n Programmable Soft-Start n Small 10-Lead DFN (3mm × 3mm) and Thermally Enhanced 10-Pin MSOP Packages The LT®3757/LT3757A are wide input range, current mode, DC/DC controllers which are capable of generating either positive or negative output voltages. They can be configured as either a boost, flyback, SEPIC or inverting converter. The LT3757/LT3757A drive a low side external N-channel power MOSFET from an internal regulated 7.2V supply. The fixed frequency, current-mode architecture results in stable operation over a wide range of supply and output voltages. n The operating frequency of LT3757/LT3757A can be set with an external resistor over a 100kHz to 1MHz range, and can be synchronized to an external clock using the SYNC pin. A low minimum operating supply voltage of 2.9V, and a low shutdown quiescent current of less than 1µA, make the LT3757/LT3757A ideally suited for batteryoperated systems. Applications Automotive and Industrial Boost, Flyback, SEPIC and Inverting Converters n Telecom Power Supplies n Portable Electronic Equipment n The LT3757/LT3757A feature soft-start and frequency foldback functions to limit inductor current during start-up and output short-circuit. The LT3757A has improved load transient performance compared to the LT3757. L, LT, LTC, LTM, Linear Technology, the Linear logo and Burst Mode are registered trademarks and No RSENSE and ThinSOT are trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners. Typical Application High Efficiency Boost Converter 10µF 25V X5R Efficiency 100 200k VIN 90 10µH SHDN/UVLO 43.2k SYNC LT3757 226k + FBX GND INTVCC VC 22k 0.1µF GATE SENSE RT SS 41.2k 300kHz VOUT 24V 2A 6.8nF 47µF 35V ×2 0.01Ω VIN = 8V VIN = 16V 70 60 50 16.2k 4.7µF 10V X5R 80 EFFICIENCY (%) VIN 8V TO 16V 10µF 25V X5R 40 30 0.001 0.1 1 0.01 OUTPUT CURRENT (A) 10 3757 TA01b 3757 TA01a 3757afd 1 LT3757/LT3757A Absolute Maximum Ratings (Note 1) VIN, SHDN/UVLO (Note 6)..........................................40V INTVCC.....................................................VIN + 0.3V, 20V GATE......................................................... INTVCC + 0.3V SYNC...........................................................................8V VC , SS..........................................................................3V RT.............................................................................1.5V SENSE.....................................................................±0.3V FBX.................................................................. –6V to 6V Operating Temperature Range (Notes 2, 8) LT3757E/LT3757AE............................ –40°C to 125°C LT3757I/LT3757AI.............................. –40°C to 125°C LT3757H/LT3757AH............................ –40°C to 150°C LT3757MP/LT3757AMP...................... –55°C to 150°C Storage Temperature Range DFN..................................................... –65°C to 125°C MSOP................................................. –65°C to 150°C Lead Temperature (Soldering, 10 sec) MSOP................................................................ 300°C Pin Configuration TOP VIEW TOP VIEW VC 1 10 VIN FBX 2 9 SHDN/UVLO SS 3 RT 4 7 GATE SYNC 5 6 SENSE 11 VC FBX SS RT SYNC 8 INTVCC DD PACKAGE 10-LEAD (3mm × 3mm) PLASTIC DFN TJMAX = 125°C, θJA = 43°C/W EXPOSED PAD (PIN 11) IS GND, MUST BE SOLDERED TO PCB 1 2 3 4 5 11 10 9 8 7 6 VIN SHDN/UVLO INTVCC GATE SENSE MSE PACKAGE 10-LEAD PLASTIC MSOP TJMAX = 150°C, θJA = 40°C/W EXPOSED PAD (PIN 11) IS GND, MUST BE SOLDERED TO PCB Order Information LEAD FREE FINISH TAPE AND REEL PART MARKING* PACKAGE DESCRIPTION TEMPERATURE RANGE LT3757EDD#PBF LT3757EDD#TRPBF LDYW 10-Lead (3mm × 3mm) Plastic DFN –40°C to 125°C LT3757IDD#PBF LT3757IDD#TRPBF LDYW 10-Lead (3mm × 3mm) Plastic DFN –40°C to 125°C LT3757EMSE#PBF LT3757EMSE#TRPBF LTDYX 10-Lead (3mm × 3mm) Plastic MSOP –40°C to 125°C LT3757IMSE#PBF LT3757IMSE#TRPBF LTDYX 10-Lead (3mm × 3mm) Plastic MSOP –40°C to 125°C LT3757HMSE#PBF LT3757HMSE#TRPBF LTDYX 10-Lead (3mm × 3mm) Plastic MSOP –40°C to 150°C LT3757MPMSE#PBF LT3757MPMSE#TRPBF LTDYX 10-Lead (3mm × 3mm) Plastic MSOP –55°C to 150°C LT3757AEDD#PBF LT3757AEDD#TRPBF LGGR 10-Lead (3mm × 3mm) Plastic DFN –40°C to 125°C LT3757AIDD#PBF LT3757AIDD#TRPBF LGGR 10-Lead (3mm × 3mm) Plastic DFN –40°C to 125°C LT3757AEMSE#PBF LT3757AEMSE#TRPBF LTGGM 10-Lead (3mm × 3mm) Plastic MSOP –40°C to 125°C LT3757AIMSE#PBF LT3757AIMSE#TRPBF LTGGM 10-Lead (3mm × 3mm) Plastic MSOP –40°C to 125°C LT3757AHMSE#PBF LT3757AHMSE#TRPBF LTGGM 10-Lead (3mm × 3mm) Plastic MSOP –40°C to 150°C LT3757AMPMSE#PBF LT3757AMPMSE#TRPBF LTGGM 10-Lead (3mm × 3mm) Plastic MSOP –55°C to 150°C Consult LTC Marketing for parts specified with wider operating temperature ranges. *The temperature grade is identified by a label on the shipping container. For more information on lead free part marking, go to: http://www.linear.com/leadfree/ For more information on tape and reel specifications, go to: http://www.linear.com/tapeandreel/ 3757afd 2 LT3757/LT3757A Electrical Characteristics The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. VIN = 24V, SHDN/UVLO = 24V, SENSE = 0V, unless otherwise noted. PARAMETER CONDITIONS MIN VIN Operating Range TYP 2.9 MAX UNITS 40 V VIN Shutdown IQ SHDN/UVLO = 0V SHDN/UVLO = 1.15V 0.1 1 6 µA µA VIN Operating IQ VC = 0.3V, RT = 41.2k 1.6 2.2 mA VIN Operating IQ with Internal LDO Disabled VC = 0.3V, RT = 41.2k, INTVCC = 7.5V 280 400 µA 110 120 mV SENSE Current Limit Threshold SENSE Input Bias Current l 100 Current Out of Pin –65 µA Error Amplifier FBX Regulation Voltage (VFBX(REG)) VFBX > 0V (Note 3) VFBX < 0V (Note 3) FBX Overvoltage Lockout VFBX > 0V (Note 4) VFBX < 0V (Note 4) FBX Pin Input Current VFBX = 1.6V (Note 3) VFBX = –0.8V (Note 3) Transconductance gm (∆IVC /∆VFBX) (Note 3) VC Output Impedance (Note 3) VFBX Line Regulation [∆VFBX /(∆VIN • VFBX(REG))] VFBX > 0V, 2.9V < VIN < 40V (Notes 3, 7) VFBX < 0V, 2.9V < VIN < 40V (Notes 3, 7) l l 1.569 –0.816 1.6 –0.80 1.631 –0.784 V V 6 7 8 11 10 14 % % 70 100 10 nA nA –10 230 µS 5 0.002 0.0025 VC Current Mode Gain (∆VVC /∆VSENSE) MΩ 0.056 0.05 %/V %/V 5.5 V/V VC Source Current VFBX = 0V, VC = 1.5V –15 µA VC Sink Current VFBX = 1.7V VFBX = –0.85V 12 11 µA µA Oscillator Switching Frequency RT = 41.2k to GND, VFBX = 1.6V RT = 140k to GND, VFBX = 1.6V RT = 10.5k to GND, VFBX = 1.6V RT Voltage VFBX = 1.6V 270 300 100 1000 330 kHz kHz kHz 1.2 V Minimum Off-Time 220 ns Minimum On-Time 220 ns SYNC Input Low 0.4 SYNC Input High SS Pull-Up Current 1.5 SS = 0V, Current Out of Pin V V –10 µA Low Dropout Regulator INTVCC Regulation Voltage INTVCC Undervoltage Lockout Threshold l Falling INTVCC UVLO Hysteresis INTVCC Overvoltage Lockout Threshold INTVCC Current Limit VIN = 40V VIN = 15V INTVCC Load Regulation (∆VINTVCC / VINTVCC) 0 < IINTVCC < 20mA, VIN = 8V INTVCC Line Regulation ∆VINTVCC /(VINTVCC • ∆VIN) 8V < VIN < 40V 7 7.2 7.4 V 2.6 2.7 0.1 2.8 V V 16 17.5 30 40 95 –0.9 –0.5 0.008 V 55 mA mA % 0.03 %/V Dropout Voltage (VIN – VINTVCC) VIN = 6V, IINTVCC = 20mA 400 mV INTVCC Current in Shutdown SHDN/UVLO = 0V, INTVCC = 8V 16 µA 3757afd 3 LT3757/LT3757A Electrical Characteristics The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. VIN = 24V, SHDN/UVLO = 24V, SENSE = 0V, unless otherwise noted. PARAMETER CONDITIONS MIN TYP MAX INTVCC Voltage to Bypass Internal LDO UNITS 7.5 V 1.27 V 0.4 V 2 2.5 µA 100 nA Logic Inputs SHDN/UVLO Threshold Voltage Falling VIN = INTVCC = 8V 1.17 SHDN/UVLO Input Low Voltage I(VIN) Drops Below 1µA SHDN/UVLO Pin Bias Current Low SHDN/UVLO = 1.15V SHDN/UVLO Pin Bias Current High SHDN/UVLO = 1.30V 10 t r Gate Driver Output Rise Time CL = 3300pF (Note 5), INTVCC = 7.5V 22 ns t f Gate Driver Output Fall Time CL = 3300pF (Note 5), INTVCC = 7.5V 20 ns l 1.22 1.7 Gate Driver 0.05 Gate VOL INTVCC –0.05 Gate VOH Note 1: Stresses beyond those listed under Absolute Maximum Ratings may cause permanent damage to the device. Exposure to any Absolute Maximum Rating condition for extended periods may affect device reliability and lifetime. Note 2: The LT3757E/LT3757AE are guaranteed to meet performance specifications from the 0°C to 125°C junction temperature. Specifications over the –40°C to 125°C operating junction temperature range are assured by design, characterization and correlation with statistical process controls. The LT3757I/LT3757AI are guaranteed over the full –40°C to 125°C operating junction temperature range. The LT3757H/LT3757AH are guaranteed over the full –40°C to 150°C operating junction temperature range. High junction temperatures degrade operating lifetimes. Operating lifetime is derated at junction temperatures greater than 125°C. The LT3757MP/LT3757AMP are 100% tested and guaranteed over the full –55°C to 150°C operating junction temperature range. Positive Feedback Voltage vs Temperature, VIN TA = 25°C, unless otherwise noted. Negative Feedback Voltage vs Temperature, VIN Quiescent Current vs Temperature, VIN VIN = 40V 1600 VIN = 24V 1595 VIN = 8V 1590 1585 1580 –75 –50 –25 VIN = INTVCC = 2.9V SHDN/UVLO = 1.33V 0 25 50 75 100 125 150 TEMPERATURE (°C) 3757 G01 –790 1.8 VIN = INTVCC = 2.9V SHDN/UVLO = 1.33V –792 –794 VIN = 8V –796 –798 –800 VIN = 40V VIN = 24V QUIESCENT CURRENT (mA) –788 REGULATED FEEDBACK VOLTAGE (mV) 1605 V Note 3: The LT3757/LT3757A are tested in a feedback loop which servos VFBX to the reference voltages (1.6V and –0.8V) with the VC pin forced to 1.3V. Note 4: FBX overvoltage lockout is measured at VFBX(OVERVOLTAGE) relative to regulated VFBX(REG). Note 5: Rise and fall times are measured at 10% and 90% levels. Note 6: For VIN below 6V, the SHDN/UVLO pin must not exceed VIN. Note 7: SHDN/UVLO = 1.33V when VIN = 2.9V. Note 8: The LT3757/LT3757A include overtemperature protection that is intended to protect the device during momentary overload conditions. Junction temperature will exceed the maximum operating junction temperature when overtemperature protection is active. Continuous operation above the specified maximum operating junction temperature may impair device reliability. Typical Performance Characteristics REGULATED FEEDBACK VOLTAGE (mV) V 1.7 VIN = 40V 1.6 1.5 –802 –804 –75 –50 –25 VIN = 24V VIN = INTVCC = 2.9V 0 25 50 75 100 125 150 TEMPERATURE (°C) 3757 G02 1.4 –75 –50 –25 0 25 50 75 100 125 150 TEMPERATURE (°C) 3757 G03 3757afd 4 LT3757/LT3757A Typical Performance Characteristics Dynamic Quiescent Current vs Switching Frequency 35 TA = 25°C, unless otherwise noted. Normalized Switching Frequency vs FBX RT vs Switching Frequency 1000 CL = 3300pF 120 NORMALIZED FREQUENCY (%) 30 20 RT (kΩ) IQ(mA) 25 100 15 10 5 0 10 0 100 200 300 400 500 600 700 800 900 1000 SWITCHING FREQUENCY (KHz) 20 –0.4 0 0.4 0.8 FBX VOLTAGE (V) 1.2 1.6 3757 G06 SENSE Current Limit Threshold vs Duty Cycle 120 115 115 110 310 300 290 SENSE THRESHOLD (mV) 320 SENSE THRESHOLD (mV) SWITCHING FREQUENCY (kHz) 40 SENSE Current Limit Threshold vs Temperature RT = 41.2K 110 105 105 100 280 270 –75 –50 –25 100 –75 –50 –25 0 25 50 75 100 125 150 TEMPERATURE (°C) 3757 G07 SHDN/UVLO RISING 1.22 SHDN/UVLO FALLING 1.20 1.18 –75 –50 –25 0 25 50 75 100 125 150 TEMPERATURE (°C) 3757 G10 20 40 2.4 30 2.2 20 100 80 2.0 1.8 10 0 40 60 DUTY CYCLE (%) SHDN/UVLO Hysteresis Current vs Temperature ISHDN/ UVLO (µA) SHDN/UVLO CURRENT (µA) 1.26 0 3757 G09 SHDN/UVLO Current vs Voltage 1.28 1.24 95 0 25 50 75 100 125 150 TEMPERATURE (°C) 3757 G08 SHDN/UVLO Threshold vs Temperature SHDN/UVLO VOLTAGE (V) 60 3757 G05 Switching Frequency vs Temperature 330 80 0 –0.8 0 100 200 300 400 500 600 700 800 900 1000 SWITCHING FREQUENCY (KHz) 3757 G04 100 0 10 20 30 SHDN/UVLO VOLTAGE (V) 40 3757 G11 1.6 –75 –50 –25 0 25 50 75 100 125 150 TEMPERATURE (°C) 3757 G12 3757afd 5 LT3757/LT3757A Typical Performance Characteristics INTVCC Minimum Output Current vs VIN INTVCC vs Temperature 90 TJ = 150°C 80 INTVCC CURRENT (mA) INTVCC (V) 7.3 7.2 7.1 INTVCC Load Regulation 7.3 VIN = 8V 7.2 70 INTVCC VOLTAGE (V) 7.4 TA = 25°C, unless otherwise noted. 60 50 INTVCC = 6V 40 INTVCC = 4.5V 30 20 7.1 7 6.9 10 7.0 –75 –50 –25 0 0 25 50 75 100 125 150 TEMPERATURE (°C) 0 5 10 3757 G13 15 20 25 VIN (V) 30 35 6.8 40 700 7.30 90 150°C 7.15 125°C 0 5 10 15 20 25 VIN (V) 30 35 40 60 400 25°C 300 0°C 200 –55°C 0 0 5 3757 G16 30 10 15 20 0 0 5 10 15 20 25 30 CL (nF) 3757 G17 3757 G18 FBX Frequency Foldback Waveforms During Overcurrent Typical Start-Up Waveforms VIN = 12V VIN = 12V VOUT 10V/DIV RISE TIME 20 TIME (ns) 40 10 INTVCC LOAD (mA) CL = 3300pF 25 FALL TIME RISE TIME 50 20 Gate Drive Rise and Fall Time vs INTVCC 30 70 75°C 100 7.10 70 INTVCC = 7.2V 80 TIME (ns) DROPOUT VOLTAGE (mV) INTVCC VOLTAGE (V) 7.20 60 Gate Drive Rise and Fall Time vs CL VIN = 6V 500 40 50 30 20 INTVCC LOAD (mA) 3757 G15 600 7.25 10 3757 G14 INTVCC Dropout Voltage vs Current, Temperature INTVCC Line Regulation 0 VSW 20V/DIV VOUT 5V/DIV FALL TIME 15 5 0 IL1A + IL1B 5A/DIV IL1A + IL1B 5A/DIV 10 2ms/DIV PAGE 31 CIRCUIT 3 6 9 12 3757 G20 50µs/DIV 3757 G21 PAGE 31 CIRCUIT 15 INTVCC (V) 3757 G19 3757afd 6 LT3757/LT3757A Pin Functions VC (Pin 1): Error Amplifier Compensation Pin. Used to stabilize the voltage loop with an external RC network. FBX (Pin 2): Positive and Negative Feedback Pin. Receives the feedback voltage from the external resistor divider across the output. Also modulates the frequency during start-up and fault conditions when FBX is close to GND. SS (Pin 3): Soft-Start Pin. This pin modulates compensation pin voltage (VC ) clamp. The soft-start interval is set with an external capacitor. The pin has a 10µA (typical) pull-up current source to an internal 2.5V rail. The soft-start pin is reset to GND by an undervoltage condition at SHDN/ UVLO, an INTVCC undervoltage or overvoltage condition or an internal thermal lockout. RT (Pin 4): Switching Frequency Adjustment Pin. Set the frequency using a resistor to GND. Do not leave this pin open. SYNC (Pin 5): Frequency Synchronization Pin. Used to synchronize the switching frequency to an outside clock. If this feature is used, an RT resistor should be chosen to program a switching frequency 20% slower than the SYNC pulse frequency. Tie the SYNC pin to GND if this feature is not used. SYNC is ignored when FBX is close to GND. SENSE (Pin 6): The Current Sense Input for the Control Loop. Kelvin connect this pin to the positive terminal of the switch current sense resistor in the source of the N-channel MOSFET. The negative terminal of the current sense resistor should be connected to GND plane close to the IC. GATE (Pin 7): N-Channel MOSFET Gate Driver Output. Switches between INTVCC and GND. Driven to GND when IC is shut down, during thermal lockout or when INTVCC is above or below the OV or UV thresholds, respectively. INTVCC (Pin 8): Regulated Supply for Internal Loads and Gate Driver. Supplied from VIN and regulated to 7.2V (typical). INTVCC must be bypassed with a minimum of 4.7µF capacitor placed close to pin. INTVCC can be connected directly to VIN, if VIN is less than 17.5V. INTVCC can also be connected to a power supply whose voltage is higher than 7.5V, and lower than VIN, provided that supply does not exceed 17.5V. SHDN/UVLO (Pin 9): Shutdown and Undervoltage Detect Pin. An accurate 1.22V (nominal) falling threshold with externally programmable hysteresis detects when power is okay to enable switching. Rising hysteresis is generated by the external resistor divider and an accurate internal 2µA pull-down current. An undervoltage condition resets sort-start. Tie to 0.4V, or less, to disable the device and reduce VIN quiescent current below 1µA. VIN (Pin 10): Input Supply Pin. Must be locally bypassed with a 0.22µF, or larger, capacitor placed close to the pin. Exposed Pad (Pin 11): Ground. This pin also serves as the negative terminal of the current sense resistor. The Exposed Pad must be soldered directly to the local ground plane. 3757afd 7 LT3757/LT3757A Block Diagram CDC L1 R4 + R3 D1 VOUT • VIN CIN L2 • 9 IS1 2µA 2.5V 2.5V IS3 IS2 10µA VC 1 CC2 A10 CC1 1.72V –0.88V – + – + A12 G6 VC FBX 2 –0.8V D2 7.2V LDO INTVCC 8 + – DRIVER – +A7 G5 R S GATE G2 O VISENSE RAMP GENERATOR – +A3 G1 + + – D3 SS – + + A5 – 108mV SENSE 6 GND RSENSE 100kHz-1MHz OSCILLATOR A4 Q1 FREQ PROG RT SYNC 5 CSS M1 11 1.25V FREQ FOLDBACK 3 7 PWM COMPARATOR RAMP 1.25V CVCC 2.7V UP 2.6V DOWN SLOPE FREQUENCY FOLDBACK R5 8k CURRENT LIMIT A6 + A2 – COUT1 VIN SR1 + A1 – 1.6V A8 TSD 165˚C Q2 FBX G3 A11 17.5V + A9 – UVLO Q3 RC COUT2 1.22V INTERNAL REGULATOR AND UVLO G4 R2 + R1 10 SHDN/UVLO – + FBX 4 3757 F01 RT Figure 1. LT3757 Block Diagram Working as a SEPIC Converter 3757afd 8 LT3757/LT3757A Applications Information Main Control Loop The LT3757 uses a fixed frequency, current mode control scheme to provide excellent line and load regulation. Operation can be best understood by referring to the Block Diagram in Figure 1. The start of each oscillator cycle sets the SR latch (SR1) and turns on the external power MOSFET switch M1 through driver G2. The switch current flows through the external current sensing resistor RSENSE and generates a voltage proportional to the switch current. This current sense voltage VISENSE (amplified by A5) is added to a stabilizing slope compensation ramp and the resulting sum (SLOPE) is fed into the positive terminal of the PWM comparator A7. When SLOPE exceeds the level at the negative input of A7 (VC pin), SR1 is reset, turning off the power switch. The level at the negative input of A7 is set by the error amplifier A1 (or A2) and is an amplified version of the difference between the feedback voltage (FBX pin) and the reference voltage (1.6V or –0.8V, depending on the configuration). In this manner, the error amplifier sets the correct peak switch current level to keep the output in regulation. The LT3757 has a switch current limit function. The current sense voltage is input to the current limit comparator A6. If the SENSE pin voltage is higher than the sense current limit threshold VSENSE(MAX) (110mV, typical), A6 will reset SR1 and turn off M1 immediately. The LT3757 is capable of generating either positive or negative output voltage with a single FBX pin. It can be configured as a boost, flyback or SEPIC converter to generate positive output voltage, or as an inverting converter to generate negative output voltage. When configured as a SEPIC converter, as shown in Figure 1, the FBX pin is pulled up to the internal bias voltage of 1.6V by a voltage divider (R1 and R2) connected from VOUT to GND. Comparator A2 becomes inactive and comparator A1 performs the inverting amplification from FBX to VC . When the LT3757 is in an inverting configuration, the FBX pin is pulled down to –0.8V by a voltage divider connected from VOUT to GND. Comparator A1 becomes inactive and comparator A2 performs the noninverting amplification from FBX to VC . The LT3757 has overvoltage protection functions to protect the converter from excessive output voltage overshoot during start-up or recovery from a short-circuit condition. An overvoltage comparator A11 (with 20mV hysteresis) senses when the FBX pin voltage exceeds the positive regulated voltage (1.6V) by 8% and provides a reset pulse. Similarly, an overvoltage comparator A12 (with 10mV hysteresis) senses when the FBX pin voltage exceeds the negative regulated voltage (–0.8V) by 11% and provides a reset pulse. Both reset pulses are sent to the main RS latch (SR1) through G6 and G5. The power MOSFET switch M1 is actively held off for the duration of an output overvoltage condition. Programming Turn-On and Turn-Off Thresholds with the SHDN/UVLO Pin The SHDN/UVLO pin controls whether the LT3757 is enabled or is in shutdown state. A micropower 1.22V reference, a comparator A10 and a controllable current source IS1 allow the user to accurately program the supply voltage at which the IC turns on and off. The falling value can be accurately set by the resistor dividers R3 and R4. When SHDN/UVLO is above 0.7V, and below the 1.22V threshold, the small pull-down current source IS1 (typical 2µA) is active. The purpose of this current is to allow the user to program the rising hysteresis. The Block Diagram of the comparator and the external resistors is shown in Figure 1. The typical falling threshold voltage and rising threshold voltage can be calculated by the following equations: (R3 + R4) R4 V = 2µA • R3+ V VIN,RISING IN,FALLING VVIN,FALLING = 1.22 • For applications where the SHDN/UVLO pin is only used as a logic input, the SHDN/UVLO pin can be connected directly to the input voltage VIN for always-on operation. 3757afd 9 LT3757/LT3757A Applications Information INTVCC Regulator Bypassing and Operation An internal, low dropout (LDO) voltage regulator produces the 7.2V INTVCC supply which powers the gate driver, as shown in Figure 1. If a low input voltage operation is expected (e.g., supplying power from a lithium-ion battery or a 3.3V logic supply), low threshold MOSFETs should be used. The LT3757 contains an undervoltage lockout comparator A8 and an overvoltage lockout comparator A9 for the INTVCC supply. The INTVCC undervoltage (UV) threshold is 2.7V (typical), with 100mV hysteresis, to ensure that the MOSFETs have sufficient gate drive voltage before turning on. The logic circuitry within the LT3757 is also powered from the internal INTVCC supply. The INTVCC overvoltage (OV) threshold is set to be 17.5V (typical) to protect the gate of the power MOSFET. When INTVCC is below the UV threshold, or above the OV threshold, the GATE pin will be forced to GND and the soft-start operation will be triggered. The INTVCC regulator must be bypassed to ground immediately adjacent to the IC pins with a minimum of 4.7µF ceramic capacitor. Good bypassing is necessary to supply the high transient currents required by the MOSFET gate driver. In an actual application, most of the IC supply current is used to drive the gate capacitance of the power MOSFET. The on-chip power dissipation can be a significant concern when a large power MOSFET is being driven at a high frequency and the VIN voltage is high. It is important to limit the power dissipation through selection of MOSFET and/ or operating frequency so the LT3757 does not exceed its maximum junction temperature rating. The junction temperature TJ can be estimated using the following equations: The LT3757 uses packages with an Exposed Pad for enhanced thermal conduction. With proper soldering to the Exposed Pad on the underside of the package and a full copper plane underneath the device, thermal resistance (θJA) will be about 43°C/W for the DD package and 40°C/W for the MSE package. For an ambient board temperature of TA = 70°C and maximum junction temperature of 125°C, the maximum IDRIVE (IDRIVE(MAX)) of the DD package can be calculated as: IDRIVE(MAX) = The LT3757 has an internal INTVCC IDRIVE current limit function to protect the IC from excessive on-chip power dissipation. The IDRIVE current limit decreases as the VIN increases (see the INTVCC Minimum Output Current vs VIN graph in the Typical Performance Characteristics section). If IDRIVE reaches the current limit, INTVCC voltage will fall and may trigger the soft-start. Based on the preceding equation and the INTVCC Minimum Output Current vs VIN graph, the user can calculate the maximum MOSFET gate charge the LT3757 can drive at a given VIN and switch frequency. A plot of the maximum QG vs VIN at different frequencies to guarantee a minimum 4.5V INTVCC is shown in Figure 2. As illustrated in Figure 2, a trade-off between the operating frequency and the size of the power MOSFET may be needed in order to maintain a reliable IC junction temperature. 300 250 300kHz TJ = TA + PIC • θJA θJA = junction-to-ambient thermal resistance PIC= IC power consumption = VIN • (IQ + IDRIVE) IQ = VIN operation IQ = 1.6mA IDRIVE = average gate drive current = f • QG f = switching frequency QG = power MOSFET total gate charge 10 200 QG (nC) TA = ambient temperature (TJ − TA ) 1.28W −IQ = − 1.6mA (θ JA • VIN ) VIN 150 100 1MHz 50 0 0 5 10 15 20 VIN (V) 25 30 35 40 3757 F02 Figure 2. Recommended Maximum QG vs VIN at Different Frequencies to Ensure INTVCC Higher Than 4.5V 3757afd LT3757/LT3757A Applications Information Prior to lowering the operating frequency, however, be sure to check with power MOSFET manufacturers for their most recent low QG, low RDS(ON) devices. Power MOSFET manufacturing technologies are continually improving, with newer and better performance devices being introduced almost yearly. An effective approach to reduce the power consumption of the internal LDO for gate drive is to tie the INTVCC pin to an external voltage source high enough to turn off the internal LDO regulator. If the input voltage VIN does not exceed the absolute maximum rating of both the power MOSFET gate-source voltage (VGS) and the INTVCC overvoltage lockout threshold voltage (17.5V), the INTVCC pin can be shorted directly to the VIN pin. In this condition, the internal LDO will be turned off and the gate driver will be powered directly from the input voltage, VIN. With the INTVCC pin shorted to VIN, however, a small current (around 16µA) will load the INTVCC in shutdown mode. For applications that require the lowest shutdown mode input supply current, do not connect the INTVCC pin to VIN. In SEPIC or flyback applications, the INTVCC pin can be connected to the output voltage VOUT through a blocking diode, as shown in Figure 3, if VOUT meets the following conditions: 1. VOUT < VIN (pin voltage) 2. VOUT < 17.5V 3. VOUT < maximum VGS rating of power MOSFET A resistor RVCC can be connected, as shown in Figure 3, to limit the inrush current from VOUT. Regardless of whether or not the INTVCC pin is connected to an external voltage source, it is always necessary to have the driver circuitry bypassed with a 4.7µF low ESR ceramic capacitor to ground immediately adjacent to the INTVCC and GND pins. LT3757 DVCC INTVCC RVCC Operating Frequency and Synchronization The choice of operating frequency may be determined by on-chip power dissipation, otherwise it is a trade-off between efficiency and component size. Low frequency operation improves efficiency by reducing gate drive current and MOSFET and diode switching losses. However, lower frequency operation requires a physically larger inductor. Switching frequency also has implications for loop compensation. The LT3757 uses a constant-frequency architecture that can be programmed over a 100kHz to 1000kHz range with a single external resistor from the RT pin to ground, as shown in Figure 1. The RT pin must have an external resistor to GND for proper operation of the LT3757. A table for selecting the value of RT for a given operating frequency is shown in Table 1. Table 1. Timing Resistor (RT ) Value OSCILLATOR FREQUENCY (kHz) RT (kΩ) 100 140 200 63.4 300 41.2 400 30.9 500 24.3 600 19.6 700 16.5 800 14 900 12.1 1000 10.5 The operating frequency of the LT3757 can be synchronized to an external clock source. By providing a digital clock signal into the SYNC pin, the LT3757 will operate at the SYNC clock frequency. If this feature is used, an RT resistor should be chosen to program a switching frequency 20% slower than SYNC pulse frequency. The SYNC pulse should have a minimum pulse width of 200ns. Tie the SYNC pin to GND if this feature is not used. VOUT CVCC 4.7µF GND 3757 F03 Figure 3. Connecting INTVCC to VOUT 3757afd 11 LT3757/LT3757A Applications Information Duty Cycle Consideration Soft-Start Switching duty cycle is a key variable defining converter operation. As such, its limits must be considered. Minimum on-time is the smallest time duration that the LT3757 is capable of turning on the power MOSFET. This time is generally about 220ns (typical) (see Minimum On-Time in the Electrical Characteristics table). In each switching cycle, the LT3757 keeps the power switch off for at least 220ns (typical) (see Minimum Off-Time in the Electrical Characteristics table). The LT3757 contains several features to limit peak switch currents and output voltage (VOUT) overshoot during start-up or recovery from a fault condition. The primary purpose of these features is to prevent damage to external components or the load. The minimum on-time and minimum off-time and the switching frequency define the minimum and maximum switching duty cycles a converter is able to generate: Minimum duty cycle = minimum on-time • frequency Maximum duty cycle = 1 – (minimum off-time • frequency) Programming the Output Voltage The output voltage (VOUT) is set by a resistor divider, as shown in Figure 1. The positive and negative VOUT are set by the following equations: R2 VOUT,POSITIVE = 1.6V • 1+ R1 R2 VOUT,NEGATIVE = –0.8V • 1+ R1 The resistors R1 and R2 are typically chosen so that the error caused by the current flowing into the FBX pin during normal operation is less than 1% (this translates to a maximum value of R1 at about 158k). In the applications where VOUT is pulled up by an external positive power supply, the FBX pin is also pulled up through the R2 and R1 network. Make sure the FBX does not exceed its absolute maximum rating (6V). The R5, D2, and D3 in Figure 1 provide a resistive clamp in the positive direction. To ensure FBX is lower than 6V, choose sufficiently large R1 and R2 to meet the following condition: High peak switch currents during start-up may occur in switching regulators. Since VOUT is far from its final value, the feedback loop is saturated and the regulator tries to charge the output capacitor as quickly as possible, resulting in large peak currents. A large surge current may cause inductor saturation or power switch failure. The LT3757 addresses this mechanism with the SS pin. As shown in Figure 1, the SS pin reduces the power MOSFET current by pulling down the VC pin through Q2. In this way the SS allows the output capacitor to charge gradually toward its final value while limiting the start-up peak currents. The typical start-up waveforms are shown in the Typical Performance Characteristics section. The inductor current IL slewing rate is limited by the soft-start function. Besides start-up, soft-start can also be triggered by the following faults: 1. INTVCC > 17.5V 2. INTVCC < 2.6V 3. Thermal lockout Any of these three faults will cause the LT3757 to stop switching immediately. The SS pin will be discharged by Q3. When all faults are cleared and the SS pin has been discharged below 0.2V, a 10µA current source IS2 starts charging the SS pin, initiating a soft-start operation. The soft-start interval is set by the soft-start capacitor selection according to the equation: TSS = CSS • 1.25V 10µA R2 R2 6V • 1+ + 3.5V • > VOUT(MAX) R1 8kΩ where VOUT(MAX) is the maximum VOUT that is pulled up by an external power supply. 3757afd 12 LT3757/LT3757A Applications Information FBX Frequency Foldback When VOUT is very low during start-up or a short-circuit fault on the output, the switching regulator must operate at low duty cycles to maintain the power switch current within the current limit range, since the inductor current decay rate is very low during switch off time. The minimum on-time limitation may prevent the switcher from attaining a sufficiently low duty cycle at the programmed switching frequency. So, the switch current will keep increasing through each switch cycle, exceeding the programmed current limit. To prevent the switch peak currents from exceeding the programmed value, the LT3757 contains a frequency foldback function to reduce the switching frequency when the FBX voltage is low (see the Normalized Switching Frequency vs FBX graph in the Typical Performance Characteristics section). The typical frequency foldback waveforms are shown in the Typical Performance Characteristics section. The frequency foldback function prevents IL from exceeding the programmed limits because of the minimum on-time. During frequency foldback, external clock synchronization is disabled to prevent interference with frequency reducing operation. Thermal Lockout If LT3757 die temperature reaches 165°C (typical), the part will go into thermal lockout. The power switch will be turned off. A soft-start operation will be triggered. The part will be enabled again when the die temperature has dropped by 5°C (nominal). The optimum values depend on the converter topology, the component values and the operating conditions (including the input voltage, load current, etc.). To compensate the feedback loop of the LT3757/LT3757A, a series resistorcapacitor network is usually connected from the VC pin to GND. Figure 1 shows the typical VC compensation network. For most applications, the capacitor should be in the range of 470pF to 22nF, and the resistor should be in the range of 5k to 50k. A small capacitor is often connected in parallel with the RC compensation network to attenuate the VC voltage ripple induced from the output voltage ripple through the internal error amplifier. The parallel capacitor usually ranges in value from 10pF to 100pF. A practical approach to design the compensation network is to start with one of the circuits in this data sheet that is similar to your application, and tune the compensation network to optimize the performance. Stability should then be checked across all operating conditions, including load current, input voltage and temperature. SENSE Pin Programming For control and protection, the LT3757 measures the power MOSFET current by using a sense resistor (RSENSE) between GND and the MOSFET source. Figure 4 shows a typical waveform of the sense voltage (VSENSE) across the sense resistor. It is important to use Kelvin traces between the SENSE pin and RSENSE, and to place the IC GND as close as possible to the GND terminal of the RSENSE for proper operation. VSENSE ∆VSENSE = χ • VSENSE(MAX) Loop Compensation Loop compensation determines the stability and transient performance. The LT3757/LT3757A use current mode control to regulate the output which simplifies loop compensation. The LT3757A improves the no-load to heavy load transient response, when compared to the LT3757. New internal circuits ensure that the transient from not switching to switching at high current can be made in a few cycles. VSENSE(MAX) VSENSE(PEAK) t DTS TS 3757 F04 Figure 4. The Sense Voltage During a Switching Cycle 3757afd 13 LT3757/LT3757A Applications Information Due to the current limit function of the SENSE pin, RSENSE should be selected to guarantee that the peak current sense voltage VSENSE(PEAK) during steady state normal operation is lower than the SENSE current limit threshold (see the Electrical Characteristics table). Given a 20% margin, VSENSE(PEAK) is set to be 80mV. Then, the maximum switch ripple current percentage can be calculated using the following equation: c= ∆VSENSE 80mV − 0.5 • ∆VSENSE c is used in subsequent design examples to calculate inductor value. ∆VSENSE is the ripple voltage across RSENSE. The LT3757 switching controller incorporates 100ns timing interval to blank the ringing on the current sense signal immediately after M1 is turned on. This ringing is caused by the parasitic inductance and capacitance of the PCB trace, the sense resistor, the diode, and the MOSFET. The 100ns timing interval is adequate for most of the LT3757 applications. In the applications that have very large and long ringing on the current sense signal, a small RC filter can be added to filter out the excess ringing. Figure 5 shows the RC filter on SENSE pin. It is usually sufficient to choose 22Ω for RFLT and 2.2nF to 10nF for CFLT. Keep RFLT’s resistance low. Remember that there is 65µA (typical) flowing out of the SENSE pin. Adding RFLT will affect the SENSE current limit threshold: VSENSE_ILIM = 108mV – 65µA • RFLT LT3757 The LT3757 can be configured as different topologies. The first topology to be analyzed will be the boost converter, followed by the flyback, SEPIC and inverting converters. Boost Converter: Switch Duty Cycle and Frequency The LT3757 can be configured as a boost converter for the applications where the converter output voltage is higher than the input voltage. Remember that boost converters are not short-circuit protected. Under a shorted output condition, the inductor current is limited only by the input supply capability. For applications requiring a step-up converter that is short-circuit protected, please refer to the Applications Information section covering SEPIC converters. The conversion ratio as a function of duty cycle is RFLT VOUT 1 = VIN 1− D in continuous conduction mode (CCM). For a boost converter operating in CCM, the duty cycle of the main switch can be calculated based on the output voltage (VOUT) and the input voltage (VIN). The maximum duty cycle (DMAX) occurs when the converter has the minimum input voltage: M1 GATE Application Circuits DMAX = VOUT − VIN(MIN) VOUT Discontinuous conduction mode (DCM) provides higher conversion ratios at a given frequency at the cost of reduced efficiencies and higher switching currents. SENSE GND CFLT RSENSE 3757 F05 Figure 5. The RC Filter on SENSE Pin 3757afd 14 LT3757/LT3757A Applications Information Boost Converter: Inductor and Sense Resistor Selection For the boost topology, the maximum average inductor current is: Set the sense voltage at IL(PEAK) to be the minimum of the SENSE current limit threshold with a 20% margin. The sense resistor value can then be calculated to be: 80mV 1 IL(MAX) =IO(MAX) • 1− DMAX Then, the ripple current can be calculated by: Boost Converter: Power MOSFET Selection ∆IL = c •IL(MAX) = c •IO(MAX) • 1 1− DMAX The constant c in the preceding equation represents the percentage peak-to-peak ripple current in the inductor, relative to IL(MAX). The inductor ripple current has a direct effect on the choice of the inductor value. Choosing smaller values of ∆IL requires large inductances and reduces the current loop gain (the converter will approach voltage mode). Accepting larger values of ∆IL provides fast transient response and allows the use of low inductances, but results in higher input current ripple and greater core losses. It is recommended that c fall within the range of 0.2 to 0.6. Given an operating input voltage range, and having chosen the operating frequency and ripple current in the inductor, the inductor value of the boost converter can be determined using the following equation: L= VIN(MIN) ∆IL • f • DMAX The peak and RMS inductor current are: c IL(PEAK) =IL(MAX) • 1+ 2 IL(RMS) =IL(MAX) • 1+ 2 c 12 Based on these equations, the user should choose the inductors having sufficient saturation and RMS current ratings. RSENSE = IL(PEAK) Important parameters for the power MOSFET include the drain-source voltage rating (VDS), the threshold voltage (VGS(TH)), the on-resistance (RDS(ON)), the gate to source and gate to drain charges (QGS and QGD), the maximum drain current (ID(MAX)) and the MOSFET’s thermal resistances (RθJC and RθJA). The power MOSFET will see full output voltage, plus a diode forward voltage, and any additional ringing across its drain-to-source during its off-time. It is recommended to choose a MOSFET whose BVDSS is higher than VOUT by a safety margin (a 10V safety margin is usually sufficient). The power dissipated by the MOSFET in a boost converter is: PFET = I2L(MAX) • RDS(ON) • DMAX + 2 • V2OUT • IL(MAX) • CRSS • f /1A The first term in the preceding equation represents the conduction losses in the device, and the second term, the switching loss. CRSS is the reverse transfer capacitance, which is usually specified in the MOSFET characteristics. For maximum efficiency, RDS(ON) and CRSS should be minimized. From a known power dissipated in the power MOSFET, its junction temperature can be obtained using the following equation: TJ = TA + PFET • θJA = TA + PFET • (θJC + θCA) TJ must not exceed the MOSFET maximum junction temperature rating. It is recommended to measure the MOSFET temperature in steady state to ensure that absolute maximum ratings are not exceeded. 3757afd 15 LT3757/LT3757A Applications Information Boost Converter: Output Diode Selection To maximize efficiency, a fast switching diode with low forward drop and low reverse leakage is desirable. The peak reverse voltage that the diode must withstand is equal to the regulator output voltage plus any additional ringing across its anode-to-cathode during the on-time. The average forward current in normal operation is equal to the output current, and the peak current is equal to: c ID(PEAK) =IL(PEAK) = 1+ •IL(MAX) 2 It is recommended that the peak repetitive reverse voltage rating VRRM is higher than VOUT by a safety margin (a 10V safety margin is usually sufficient). The power dissipated by the diode is: PD = IO(MAX) • VD and the diode junction temperature is: TJ = TA + PD • RθJA The RθJA to be used in this equation normally includes the RθJC for the device plus the thermal resistance from the board to the ambient temperature in the enclosure. TJ must not exceed the diode maximum junction temperature rating. these three parameters (ESR, ESL and bulk C) on the output voltage ripple waveform for a typical boost converter is illustrated in Figure 6. tON ∆VCOUT VOUT (AC) ∆VESR RINGING DUE TO TOTAL INDUCTANCE (BOARD + CAP) 3757 F05 Figure 6. The Output Ripple Waveform of a Boost Converter The choice of component(s) begins with the maximum acceptable ripple voltage (expressed as a percentage of the output voltage), and how this ripple should be divided between the ESR step ∆VESR and the charging/discharging ∆VCOUT. For the purpose of simplicity, we will choose 2% for the maximum output ripple, to be divided equally between ∆VESR and ∆VCOUT. This percentage ripple will change, depending on the requirements of the application, and the following equations can easily be modified. For a 1% contribution to the total ripple voltage, the ESR of the output capacitor can be determined using the following equation: Boost Converter: Output Capacitor Selection Contributions of ESR (equivalent series resistance), ESL (equivalent series inductance) and the bulk capacitance must be considered when choosing the correct output capacitors for a given output ripple voltage. The effect of tOFF ESRCOUT ≤ 0.01• VOUT ID(PEAK) 3757afd 16 LT3757/LT3757A Applications Information For the bulk C component, which also contributes 1% to the total ripple: COUT ≥ IO(MAX) 0.01• VOUT • f The output capacitor in a boost regulator experiences high RMS ripple currents, as shown in Figure 6. The RMS ripple current rating of the output capacitor can be determined using the following equation: IRMS(COUT) ≥IO(MAX) • DMAX 1− DMAX Multiple capacitors are often paralleled to meet ESR requirements. Typically, once the ESR requirement is satisfied, the capacitance is adequate for filtering and has the required RMS current rating. Additional ceramic capacitors in parallel are commonly used to reduce the effect of parasitic inductance in the output capacitor, which reduces high frequency switching noise on the converter output. Boost Converter: Input Capacitor Selection The input capacitor of a boost converter is less critical than the output capacitor, due to the fact that the inductor is in series with the input, and the input current waveform is continuous. The input voltage source impedance determines the size of the input capacitor, which is typically in the range of 10µF to 100µF. A low ESR capacitor is recommended, although it is not as critical as for the output capacitor. Flyback Converter Applications The LT3757 can be configured as a flyback converter for the applications where the converters have multiple outputs, high output voltages or isolated outputs. Figure 7 shows a simplified flyback converter. The flyback converter has a very low parts count for multiple outputs, and with prudent selection of turns ratio, can have high output/input voltage conversion ratios with a desirable duty cycle. However, it has low efficiency due to the high peak currents, high peak voltages and consequent power loss. The flyback converter is commonly used for an output power of less than 50W. The flyback converter can be designed to operate either in continuous or discontinuous mode. Compared to continuous mode, discontinuous mode has the advantage of smaller transformer inductances and easy loop compensation, and the disadvantage of higher peak-to-average current and lower efficiency. In the high output voltage applications, the flyback converters can be designed to operate in discontinuous mode to avoid using large transformers. VIN SUGGESTED RCD SNUBBER + CIN D NP:NS – VSN + CSN RSN LS LP ID + + COUT – DSN ISW LT3757 The RMS input capacitor ripple current for a boost converter is: GATE SENSE M + VDS – RSENSE IRMS(CIN) = 0.3 • ∆IL GND 3757 F06 Figure 7. A Simplified Flyback Converter 3757afd 17 LT3757/LT3757A Applications Information Flyback Converter: Switch Duty Cycle and Turns Ratio The flyback converter conversion ratio in the continuous mode operation is: VOUT NS D = • VIN NP 1− D where NS/NP is the second to primary turns ratio. Figure 8 shows the waveforms of the flyback converter in discontinuous mode operation. During each switching period TS, three subintervals occur: DTS, D2TS, D3TS. During DTS, M is on, and D is reverse-biased. During D2TS, M is off, and LS is conducting current. Both LP and LS currents are zero during D3TS. The flyback converter conversion ratio in the discontinuous mode operation is: VOUT NS D = • VIN NP D2 According to the preceding equations, the user has relative freedom in selecting the switch duty cycle or turns ratio to suit a given application. The selections of the duty cycle and the turns ratio are somewhat iterative processes, due to the number of variables involved. The user can choose either a duty cycle or a turns ratio as the start point. The following trade-offs should be considered when selecting the switch duty cycle or turns ratio, to optimize the converter performance. A higher duty cycle affects the flyback converter in the following aspects: • Lower MOSFET RMS current ISW(RMS), but higher MOSFET VDS peak voltage • Lower diode peak reverse voltage, but higher diode RMS current ID(RMS) • Higher transformer turns ratio (NP/NS) The choice, 1 D = D + D2 3 (for discontinuous mode operation with a given D3) gives the power MOSFET the lowest power stress (the product of RMS current and peak voltage). However, in the high output voltage applications, a higher duty cycle may be adopted to limit the large peak reverse voltage of the diode. The choice, VDS ISW 2 D = D + D2 3 ISW(MAX) ID ID(MAX) DTS D2TS TS t D3TS 3757 F07 (for discontinuous mode operation with a given D3) gives the diode the lowest power stress (the product of RMS current and peak voltage). An extreme high or low duty cycle results in high power stress on the MOSFET or diode, and reduces efficiency. It is recommended to choose a duty cycle, D, between 20% and 80%. Figure 8. Waveforms of the Flyback Converter in Discontinuous Mode Operation 3757afd 18 LT3757/LT3757A Applications Information Flyback Converter: Transformer Design for Discontinuous Mode Operation According to Figure 8, the primary and secondary peak currents are: The transformer design for discontinuous mode of operation is chosen as presented here. According to Figure 8, the minimum D3 (D3MIN) occurs when the converter has the minimum VIN and the maximum output power (POUT). Choose D3MIN to be equal to or higher than 10% to guarantee the converter is always in discontinuous mode operation (choosing higher D3 allows the use of low inductances, but results in a higher switch peak current). ILP(PEAK) = ISW(PEAK) = 2 • ILP(MAX) ILS(PEAK) = ID(PEAK) = 2 • ILS(MAX) The primary and second inductor values of the flyback converter transformer can be determined using the following equations: LP = The user can choose a DMAX as the start point. Then, the maximum average primary currents can be calculated by the following equation: ILP(MAX) =ISW(MAX) = POUT(MAX) DMAX • VIN(MIN) • h where h is the converter efficiency. If the flyback converter has multiple outputs, POUT(MAX) is the sum of all the output power. The maximum average secondary current is: ILS(MAX) =ID(MAX) = IOUT(MAX) where: D2 D2 = 1 – DMAX – D3 the primary and secondary RMS currents are: ILP(RMS) = 2 •ILP(MAX) • DMAX 3 D2 I = 2 •ILS(MAX) • LS(RMS) 3 LS = D 2MAX • V 2IN(MIN) • h 2 • POUT(MAX) • f D2 2 • (VOUT + VD ) 2 • I OUT(MAX) • f The primary to second turns ratio is: L NP = P LS NS Flyback Converter: Snubber Design Transformer leakage inductance (on either the primary or secondary) causes a voltage spike to occur after the MOSFET turn-off. This is increasingly prominent at higher load currents, where more stored energy must be dissipated. In some cases a snubber circuit will be required to avoid overvoltage breakdown at the MOSFET’s drain node. There are different snubber circuits, and Application Note 19 is a good reference on snubber design. An RCD snubber is shown in Figure 7. The snubber resistor value (RSN) can be calculated by the following equation: RSN = 2 • V 2SN − VSN • VOUT • NP NS I2SW(PEAK) • L LK • f 3757afd 19 LT3757/LT3757A Applications Information where VSN is the snubber capacitor voltage. A smaller VSN results in a larger snubber loss. A reasonable VSN is 2 to 2.5 times of: VOUT • NP NS LLK is the leakage inductance of the primary winding, which is usually specified in the transformer characteristics. LLK can be obtained by measuring the primary inductance with the secondary windings shorted. The snubber capacitor value (CCN) can be determined using the following equation: CCN = VSN ∆VSN • RCN • f where ∆VSN is the voltage ripple across CCN. A reasonable ∆VSN is 5% to 10% of VSN. The reverse voltage rating of DSN should be higher than the sum of VSN and VIN(MAX). Flyback Converter: Sense Resistor Selection In a flyback converter, when the power switch is turned on, the current flowing through the sense resistor (ISENSE) is: ISENSE = ILP Set the sense voltage at ILP(PEAK) to be the minimum of the SENSE current limit threshold with a 20% margin. The sense resistor value can then be calculated to be: RSENSE = 80mV ILP(PEAK) Flyback Converter: Power MOSFET Selection For the flyback configuration, the MOSFET is selected with a VDC rating high enough to handle the maximum VIN, the reflected secondary voltage and the voltage spike due to the leakage inductance. Approximate the required MOSFET VDC rating using: BVDSS > VDS(PEAK) where: VDS(PEAK) = VIN(MAX) + VSN The power dissipated by the MOSFET in a flyback converter is: PFET = I2M(RMS) • RDS(ON) + 2 • V2DS(PEAK) • IL(MAX) • CRSS • f /1A The first term in this equation represents the conduction losses in the device, and the second term, the switching loss. CRSS is the reverse transfer capacitance, which is usually specified in the MOSFET characteristics. From a known power dissipated in the power MOSFET, its junction temperature can be obtained using the following equation: TJ = TA + PFET • θJA = TA + PFET • (θJC + θCA) TJ must not exceed the MOSFET maximum junction temperature rating. It is recommended to measure the MOSFET temperature in steady state to ensure that absolute maximum ratings are not exceeded. 3757afd 20 LT3757/LT3757A Applications Information Flyback Converter: Output Diode Selection Flyback Converter: Input Capacitor Selection The output diode in a flyback converter is subject to large RMS current and peak reverse voltage stresses. A fast switching diode with a low forward drop and a low reverse leakage is desired. Schottky diodes are recommended if the output voltage is below 100V. The input capacitor in a flyback converter is subject to a large RMS current due to the discontinuous primary current. To prevent large voltage transients, use a low ESR input capacitor sized for the maximum RMS current. The RMS ripple current rating of the input capacitors in discontinuous operation can be determined using the following equation: Approximate the required peak repetitive reverse voltage rating VRRM using: N VRRM > S • VIN(MAX) + VOUT NP IRMS(CIN),DISCONTINUOUS ≥ POUT(MAX) VIN(MIN) • h • 4 − (3 • DMAX ) 3 • DMAX The power dissipated by the diode is: SEPIC Converter Applications PD = IO(MAX) • VD The LT3757 can be configured as a SEPIC (single-ended primary inductance converter), as shown in Figure 1. This topology allows for the input to be higher, equal, or lower than the desired output voltage. The conversion ratio as a function of duty cycle is: and the diode junction temperature is: TJ = TA + PD • RθJA The RθJA to be used in this equation normally includes the RθJC for the device, plus the thermal resistance from the board to the ambient temperature in the enclosure. TJ must not exceed the diode maximum junction temperature rating. Flyback Converter: Output Capacitor Selection The output capacitor of the flyback converter has a similar operation condition as that of the boost converter. Refer to the Boost Converter: Output Capacitor Selection section for the calculation of COUT and ESRCOUT. The RMS ripple current rating of the output capacitors in discontinuous operation can be determined using the following equation: IRMS(COUT),DISCONTINUOUS ≥ IO(MAX) • 4 − (3 • D2) 3 • D2 VOUT + VD D = VIN 1− D in continuous conduction mode (CCM). In a SEPIC converter, no DC path exists between the input and output. This is an advantage over the boost converter for applications requiring the output to be disconnected from the input source when the circuit is in shutdown. Compared to the flyback converter, the SEPIC converter has the advantage that both the power MOSFET and the output diode voltages are clamped by the capacitors (CIN, CDC and COUT), therefore, there is less voltage ringing across the power MOSFET and the output diodes. The SEPIC converter requires much smaller input capacitors than those of the flyback converter. This is due to the fact that, in the SEPIC converter, the inductor L1 is in series with the input, and the ripple current flowing through the input capacitor is continuous. 3757afd 21 LT3757/LT3757A Applications Information SEPIC Converter: Switch Duty Cycle and Frequency For a SEPIC converter operating in CCM, the duty cycle of the main switch can be calculated based on the output voltage (VOUT), the input voltage (VIN) and the diode forward voltage (VD). The maximum duty cycle (DMAX) occurs when the converter has the minimum input voltage: DMAX = VOUT + VD VIN(MIN) + VOUT + VD As shown in Figure 1, the SEPIC converter contains two inductors: L1 and L2. L1 and L2 can be independent, but can also be wound on the same core, since identical voltages are applied to L1 and L2 throughout the switching cycle. For the SEPIC topology, the current through L1 is the converter input current. Based on the fact that, ideally, the output power is equal to the input power, the maximum average inductor currents of L1 and L2 are: DMAX 1− DMAX I L2(MAX) = I O(MAX) ISW ISW(MAX) =IL1(MAX) +IL2(MAX) =IO(MAX) • 1 1− DMAX and the peak switch current is: SEPIC Converter: Inductor and Sense Resistor Selection I L1(MAX) = IIN(MAX) = I O(MAX) • In a SEPIC converter, the switch current is equal to IL1 + IL2 when the power switch is on, therefore, the maximum average switch current is defined as: 1 c ISW(PEAK) = 1+ •IO(MAX) • 2 1− DMAX The constant c in the preceding equations represents the percentage peak-to-peak ripple current in the switch, relative to ISW(MAX), as shown in Figure 9. Then, the switch ripple current ∆ISW can be calculated by: ∆ISW = c • ISW(MAX) The inductor ripple currents ∆IL1 and ∆IL2 are identical: ∆IL1 = ∆IL2 = 0.5 • ∆ISW The inductor ripple current has a direct effect on the choice of the inductor value. Choosing smaller values of ∆IL requires large inductances and reduces the current loop gain (the converter will approach voltage mode). Accepting larger values of ∆IL allows the use of low inductances, but results in higher input current ripple and greater core losses. It is recommended that c falls in the range of 0.2 to 0.4. ∆ISW = χ • ISW(MAX) ISW(MAX) t DTS TS 3757 F08 Figure 9. The Switch Current Waveform of the SEPIC Converter 3757afd 22 LT3757/LT3757A Applications Information Given an operating input voltage range, and having chosen the operating frequency and ripple current in the inductor, the inductor value (L1 and L2 are independent) of the SEPIC converter can be determined using the following equation: L1= L2 = VIN(MIN) 0.5 • ∆ISW • f • DMAX For most SEPIC applications, the equal inductor values will fall in the range of 1µH to 100µH. By making L1 = L2, and winding them on the same core, the value of inductance in the preceding equation is replaced by 2L, due to mutual inductance: L= VIN(MIN) ∆ISW • f Based on the preceding equations, the user should choose the inductors having sufficient saturation and RMS current ratings. In a SEPIC converter, when the power switch is turned on, the current flowing through the sense resistor (ISENSE) is the switch current. Set the sense voltage at ISENSE(PEAK) to be the minimum of the SENSE current limit threshold with a 20% margin. The sense resistor value can then be calculated to be: RSENSE = 80 mV ISW(PEAK) SEPIC Converter: Power MOSFET Selection • DMAX This maintains the same ripple current and energy storage in the inductors. The peak inductor currents are: IL1(PEAK) = IL1(MAX) + 0.5 • ∆IL1 For the SEPIC configuration, choose a MOSFET with a VDC rating higher than the sum of the output voltage and input voltage by a safety margin (a 10V safety margin is usually sufficient). IL2(PEAK) = IL2(MAX) + 0.5 • ∆IL2 The power dissipated by the MOSFET in a SEPIC converter is: The RMS inductor currents are: PFET = I2SW(MAX) • RDS(ON) • DMAX c 2L1 I =I • 1+ L1(RMS) L1(MAX) 12 where: cL1 = ∆IL1 IL1(MAX) IL2(RMS) =IL2(MAX) • 1+ The first term in this equation represents the conduction losses in the device, and the second term, the switching loss. CRSS is the reverse transfer capacitance, which is usually specified in the MOSFET characteristics. For maximum efficiency, RDS(ON) and CRSS should be minimized. From a known power dissipated in the power MOSFET, its junction temperature can be obtained using the following equation: TJ = TA + PFET • θJA = TA + PFET • (θJC + θCA) where: cL2 = c 2L2 12 + 2 • (VIN(MIN) + VOUT)2 • IL(MAX) • CRSS • f /1A ∆IL2 IL2 (MAX) TJ must not exceed the MOSFET maximum junction temperature rating. It is recommended to measure the MOSFET temperature in steady state to ensure that absolute maximum ratings are not exceeded. 3757afd 23 LT3757/LT3757A Applications Information SEPIC Converter: Output Diode Selection To maximize efficiency, a fast switching diode with a low forward drop and low reverse leakage is desirable. The average forward current in normal operation is equal to the output current, and the peak current is equal to: 1 c ID(PEAK) = 1+ •IO(MAX) • 2 1− DMAX CDC has nearly a rectangular current waveform. During the switch off-time, the current through CDC is IIN, while approximately –IO flows during the on-time. The RMS rating of the coupling capacitor is determined by the following equation: VOUT + VD IRMS(CDC) > IO(MAX) • VIN(MIN) It is recommended that the peak repetitive reverse voltage rating VRRM is higher than VOUT + VIN(MAX) by a safety margin (a 10V safety margin is usually sufficient). A low ESR and ESL, X5R or X7R ceramic capacitor works well for CDC. The power dissipated by the diode is: Inverting Converter Applications PD = IO(MAX) • VD The LT3757 can be configured as a dual-inductor inverting topology, as shown in Figure 10. The VOUT to VIN ratio is: and the diode junction temperature is: TJ = TA + PD • RθJA The RθJA used in this equation normally includes the RθJC for the device, plus the thermal resistance from the board, to the ambient temperature in the enclosure. TJ must not exceed the diode maximum junction temperature rating. SEPIC Converter: Output and Input Capacitor Selection The selections of the output and input capacitors of the SEPIC converter are similar to those of the boost converter. Please refer to the Boost Converter, Output Capacitor Selection and Boost Converter, Input Capacitor Selection sections. SEPIC Converter: Selecting the DC Coupling Capacitor VOUT − VD D =− VIN 1− D in continuous conduction mode (CCM). L1 VIN + CIN CDC + – LT3757 GATE M1 L2 – D1 COUT + SENSE RSENSE GND VOUT + 3757 F09 Figure 10. A Simplified Inverting Converter The DC voltage rating of the DC coupling capacitor (CDC, as shown in Figure 1) should be larger than the maximum input voltage: VCDC > VIN(MAX) 3757afd 24 LT3757/LT3757A Applications Information Inverting Converter: Switch Duty Cycle and Frequency For an inverting converter operating in CCM, the duty cycle of the main switch can be calculated based on the negative output voltage (VOUT) and the input voltage (VIN). The maximum duty cycle (DMAX) occurs when the converter has the minimum input voltage: DMAX = VOUT − VD VOUT − VD − VIN(MIN) Inverting Converter: Inductor, Sense Resistor, Power MOSFET, Output Diode and Input Capacitor Selections After specifying the maximum output ripple, the user can select the output capacitors according to the preceding equation. The ESR can be minimized by using high quality X5R or X7R dielectric ceramic capacitors. In many applications, ceramic capacitors are sufficient to limit the output voltage ripple. The RMS ripple current rating of the output capacitor needs to be greater than: IRMS(COUT) > 0.3 • ∆IL2 Inverting Converter: Selecting the DC Coupling Capacitor The selections of the inductor, sense resistor, power MOSFET, output diode and input capacitor of an inverting converter are similar to those of the SEPIC converter. Please refer to the corresponding SEPIC converter sections. The DC voltage rating of the DC coupling capacitor (CDC, as shown in Figure 10) should be larger than the maximum input voltage minus the output voltage (negative voltage): Inverting Converter: Output Capacitor Selection CDC has nearly a rectangular current waveform. During the switch off-time, the current through CDC is IIN, while approximately –IO flows during the on-time. The RMS rating of the coupling capacitor is determined by the following equation: DMAX IRMS(CDC) >IO(MAX) • 1− DMAX The inverting converter requires much smaller output capacitors than those of the boost, flyback and SEPIC converters for similar output ripples. This is due to the fact that, in the inverting converter, the inductor L2 is in series with the output, and the ripple current flowing through the output capacitors are continuous. The output ripple voltage is produced by the ripple current of L2 flowing through the ESR and bulk capacitance of the output capacitor: 1 ∆VOUT(P – P) = ∆IL2 • ESRCOUT + 8 • f • COUT VCDC > VIN(MAX) – VOUT A low ESR and ESL, X5R or X7R ceramic capacitor works well for CDC. 3757afd 25 LT3757/LT3757A Applications Information should be kept as tight as possible to reduce inductive ringing: Board Layout The high speed operation of the LT3757 demands careful attention to board layout and component placement. The Exposed Pad of the package is the only GND terminal of the IC, and is important for thermal management of the IC. Therefore, it is crucial to achieve a good electrical and thermal contact between the Exposed Pad and the ground plane of the board. For the LT3757 to deliver its full output power, it is imperative that a good thermal path be provided to dissipate the heat generated within the package. It is recommended that multiple vias in the printed circuit board be used to conduct heat away from the IC and into a copper plane with as much area as possible. • In boost configuration, the high di/dt loop contains the output capacitor, the sensing resistor, the power MOSFET and the Schottky diode. • In flyback configuration, the high di/dt primary loop contains the input capacitor, the primary winding, the power MOSFET and the sensing resistor. The high di/ dt secondary loop contains the output capacitor, the secondary winding and the output diode. • In SEPIC configuration, the high di/dt loop contains the power MOSFET, sense resistor, output capacitor, Schottky diode and the coupling capacitor. To prevent radiation and high frequency resonance problems, proper layout of the components connected to the IC is essential, especially the power paths with higher di/ dt. The following high di/dt loops of different topologies • In inverting configuration, the high di/dt loop contains power MOSFET, sense resistor, Schottky diode and the coupling capacitor. CIN CC2 L1 R3 RC R1 1 R2 2 RSS 3 8 RT 4 7 5 6 LT3757 R4 CC1 VIN 10 9 CVCC 1 2 RS VIAS TO GROUND PLANE COUT2 8 M1 7 3 6 4 5 COUT1 D1 VOUT 3757 F10 Figure 11. 8V to 16V Input, 24V/2A Output Boost Converter Suggested Layout 3757afd 26 LT3757/LT3757A Applications Information Check the stress on the power MOSFET by measuring its drain-to-source voltage directly across the device terminals (reference the ground of a single scope probe directly to the source pad on the PC board). Beware of inductive ringing, which can exceed the maximum specified voltage rating of the MOSFET. If this ringing cannot be avoided, and exceeds the maximum rating of the device, either choose a higher voltage device or specify an avalancherated power MOSFET. The small-signal components should be placed away from high frequency switching nodes. For optimum load regulation and true remote sensing, the top of the output voltage sensing resistor divider should connect independently to the top of the output capacitor (Kelvin connection), staying away from any high dV/dt traces. Place the divider resistors near the LT3757 in order to keep the high impedance FBX node short. Figure 11 shows the suggested layout of the 8V to 16V Input, 24V/2A Output Boost Converter. Recommended Component Manufacturers Some of the recommended component manufacturers are listed in Table 2. Table 2. Recommended Component Manufacturers VENDOR AVX COMPONENTS WEB ADDRESS Capacitors avx.com Inductors, Transformers bhelectronics.com Coilcraft Inductors coilcraft.com Cooper Bussmann Inductors bussmann.com BH Electronics Diodes, Inc Diodes diodes.com MOSFETs fairchildsemi.com Diodes generalsemiconductor.com International Rectifier MOSFETs, Diodes irf.com IRC Sense Resistors irctt.com Capacitors kemet.com Fairchild General Semiconductor Kemet Magnetics Inc Toroid Cores mag-inc.com Microsemi Diodes microsemi.com Murata-Erie Inductors, Capacitors murata.co.jp Nichicon Capacitors nichicon.com On Semiconductor Diodes onsemi.com Panasonic Capacitors panasonic.com Sanyo Capacitors sanyo.co.jp Sumida Inductors sumida.com Taiyo Yuden Capacitors t-yuden.com TDK Capacitors, Inductors component.tdk.com Thermalloy Heat Sinks aavidthermalloy.com Tokin Capacitors nec-tokinamerica.com Toko Inductors tokoam.com United Chemi-Con Capacitors chemi-con.com Vishay/Dale Resistors vishay.com Vishay/Siliconix MOSFETs vishay.com Vishay/Sprague Capacitors vishay.com Würth Elektronik Inductors we-online.com Small-Signal Discretes zetex.com Zetex 3757afd 27 LT3757/LT3757A Typical Applications 3.3V Input, 5V/10A Output Boost Converter L1 0.5µH CIN 22µF 6.3V ×2 Efficiency vs Output Current VIN INTVCC 49.9k CVCC 4.7µF 10V X5R SHDN/UVLO 34k LT3757 GATE SYNC SS 41.2k 300kHz D1 M1 FBX RT 100 34k 1% 22Ω SENSE VOUT 5V 10A + COUT1 150µF 6.3V ×4 GND VC 0.004Ω 1W 6.8k 0.1µF 22nF CIN: TAIYO YUDEN JMK325BJ226MM COUT1: PANASONIC EEFUEOJ151R COUT2: TAIYO YUDEN JMK325BJ226MM D1: MBRB2515L L1: VISHAY SILICONIX IHLP-5050FD-01 M1: VISHAY SILICONIX SI4448DY 80 70 60 50 40 COUT2 22µF 6.3V X5R ×4 15.8k 1% 2.2nF 90 EFFICIENCY (%) VIN 3.3V 30 20 0.001 0.01 0.1 1 OUTPUT CURRENT (A) 10 3757 TA02b 3757 TA02a Boost Preregulator for Automotive Stop-Start/Idle VIN 3V TO 36V L1 3.3µH 10µF 50V X5R ×2 GATE 1M SYNC M1 SENSE SHDN/UVLO 698k D1 LT3757A OUTPUT POWER = 10W 8mΩ RT VIN SS FBX VC INTVCC 16.2k + C1 10µF 50V ×4 10k 0.1µF 10nF L1: COILTRONIX DR127-3R3 M1: VISHAY SILICONIX Si7848BDP D1: VISHAY SILICONIX 50SQ04FN C1: KEMET T495X106K050A Transient VIN and VOUT Waveforms 10µF 50V X5R GND 75k 41.2k 300kHz VOUT 9VMIN 2A 0V VOUT 5V/DIV VIN 5V/DIV 10ms/DIV 3757 TA03b 4.7µF 3757 TA03a 3757afd 28 LT3757/LT3757A Typical Applications 8V to 16V Input, 24V/2A Output Boost Converter VIN 8V TO 16V CIN 10µF 25V X5R R3 200k L1 10µH VIN SHDN/UVLO R4 43.2k D1 LT3757 SYNC GATE M1 R2 226k 1% SENSE RT SS VC RT 41.2k 300kHz CC2 100pF GND FBX INTVCC RC 22k CSS 0.1µF VOUT 24V 2A CC1 6.8nF CVCC 4.7µF 10V X5R RS 0.01Ω 1W CIN, COUT2: MURATA GRM31CR61E106KA12 COUT1: KEMET T495X476K035AS D1: ON SEMI MBRS340T3G L1: VISHAY SILICONIX IHLP-5050FD-01 10µH M1: VISHAY SILICONIX Si4840BDP R1 16.2k 1% + COUT1 47µF 35V ×4 COUT2 10µF 25V X5R 3757 TA04a Load Step Response at VIN = 12V Efficiency vs Output Current 100 90 EFFICIENCY (%) 80 70 VOUT 500mV/DIV (AC) VIN = 8V VIN = 16V IOUT 1.6A 1A/DIV 0.4A 60 50 40 30 0.001 500µs/DIV 0.1 1 0.01 OUTPUT CURRENT (A) 3757 TA04c 10 3757 TA04b 3757afd 29 LT3757/LT3757A Typical Applications High Voltage Flyback Power Supply DANGER! HIGH VOLTAGE OPERATION BY HIGH VOLTAGE TRAINED PERSONNEL ONLY VIN 5V TO 12V T1 1:10 CIN 47µF 16V ×4 • 105k SHDN/UVLO 46.4k VIN INTVCC SYNC LT3757 22Ω CVCC 47µF 25V X5R 140k 100kHz GND VSW M1 1.50M 1% 1M 1% COUT 68nF ×2 22Ω SENSE FBX 10nF 6.8k 0.1µF VOUT 350V 10mA 1M 1% 220pF GATE RT SS VC • D1 0.02Ω 16.2k 1% 22nF 100pF 3757 TA05a CIN : MURATA GRM32ER61C476K COUT : TDK C3225X7R2J683K D1: VISHAY SILICONIX GSD2004S DUAL DIODE CONNECTED IN SERIES M1: VISHAY SILICONIX Si7850DP T1: TDK DCT15EFD-U44S003 Start-Up Waveforms Switching Waveforms VOUT 5V/DIV (AC) VSW 20V/DIV VOUT 100V/DIV 2ms/DIV 3757 TA05b 5µs/DIV 3757 TA05c 3757afd 30 LT3757/LT3757A Typical Applications 5.5V to 36V Input, 12V/2A Output SEPIC Converter VIN 5.5V TO 36V CIN1 4.7µF 50V ×2 CIN2 4.7µF 50V ×2 + 105k VIN L1A • IL1A SHDN/UVLO 46.4k SYNC LT3757A CDC 4.7µF 50V, X5R, ×2 GATE M1 IL1B SENSE RT SS 10k 6.8nF 0.1µF VOUT 12V 2A L1B • 0.01Ω 1W FBX GND INTVCC VC 41.2k 300kHz D1 VSW 105k 1% + 4.7µF 10V X5R COUT1 47µF 20V ×4 COUT2 10µF 25V X5R 15.8k 1% 3757 TA06a CIN1, CDC: TAIYO YUDEN UMK316BJ475KL CIN2: KEMET T495X475K050AS COUT1: KEMET T495X476K020AS COUT2: TAIYO YUDEN TMK432BJ106MM D1: ON SEMI MBRS360T3G L1A, L1B: COILTRONICS DRQ127-4R7 (*COUPLED INDUCTORS) M1: VISHAY SILICONIX Si7460DP Efficiency vs Output Current Load Step Waveforms 100 VIN = 12V 90 VIN = 8V EFFICIENCY (%) 80 70 VOUT 2V/DIV AC-COUPLED VIN = 16V 60 IOUT 2A 2A/DIV 0A 50 40 500µs/DIV 30 20 0.001 0.1 1 0.01 OUTPUT CURRENT (A) 3757 TA06c 10 3757 TA06b Start-Up Waveforms Frequency Foldback Waveforms When Output Short-Circuits VIN = 12V VIN = 12V VOUT 10V/DIV VSW 20V/DIV VOUT 5V/DIV IL1A + IL1B 5A/DIV IL1A + IL1B 5A/DIV 2ms/DIV 3757 TA06d 50µs/DIV 3757 TA06e 3757afd 31 LT3757/LT3757A Typical Applications 5V to 12V Input, ±12V/0.4A Output SEPIC Converter VIN 5V TO 12V + CIN2 47µF 16V CIN1 1µF 16V, X5R 105k • VIN SHDN/UVLO 46.4k LT3757 SYNC T1 1,2,3,4 CDC1 4.7µF 16V, X5R M1 GATE CDC2 4.7µF 16V X5R SENSE RT FBX D1 5 0.02Ω COUT2 4.7µF 16V, X5R ×3 D2 GND INTVCC 6 • 30.9k 400kHz COUT2 4.7µF 16V, X5R ×3 158Ω 1% • SS VC 1.05k 1% 22k 0.1µF 100pF CVCC 4.7µF 10V X5R 6.8nF VOUT1 12V 0.4A GND VOUT2 –12V 0.4A 3757 TA07 D1, D2: MBRS140T3 T1: COILTRONICS VP1-0076 (*PRIMARY = 4 WINDINGS IN PARALLEL) M1: SILICONIX/VISHAY Si4840BDY Nonisolated Inverting SLIC Supply VP5-0155 (PRIMARY = 3 WINDINGS IN PARALLEL) VIN 5V TO 16V D1 DFLS160 CIN 22µF 25V, X5R ×2 GND R2 105k C2 10µF 50V X5R VIN SHDN/UVLO R1 46.4k LT3757 SYNC GATE T1 1,2,3 • 4 • M1 Si7850DP D2 DFLS160 SENSE RT FBX 5 SS 63.4k 200kHz 9.1k 0.1µF 10nF 100pF • GND INTVCC VC CVCC 4.7µF 10V, X5R C4 22µF 25V X5R COUT 3.3µF 100V VOUT1 –24V 200mA D3 DFLS160 0.012Ω 0.5W 6 • 15.8k C3 22µF 25V X5R C5 22µF 25V X5R 464k VOUT1 –72V 200mA 3757 TA08 3757afd 32 LT3757/LT3757A Package Description DD Package 10-Lead Plastic DFN (3mm × 3mm) (Reference LTC DWG # 05-08-1699 Rev C) 0.70 ±0.05 3.55 ±0.05 1.65 ±0.05 2.15 ±0.05 (2 SIDES) PACKAGE OUTLINE 0.25 ± 0.05 0.50 BSC 2.38 ±0.05 (2 SIDES) RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS 3.00 ±0.10 (4 SIDES) R = 0.125 TYP 6 0.40 ± 0.10 10 1.65 ± 0.10 (2 SIDES) PIN 1 NOTCH R = 0.20 OR 0.35 × 45° CHAMFER PIN 1 TOP MARK (SEE NOTE 6) 0.200 REF 0.75 ±0.05 0.00 – 0.05 5 1 (DD) DFN REV C 0310 0.25 ± 0.05 0.50 BSC 2.38 ±0.10 (2 SIDES) BOTTOM VIEW—EXPOSED PAD NOTE: 1. DRAWING TO BE MADE A JEDEC PACKAGE OUTLINE M0-229 VARIATION OF (WEED-2). CHECK THE LTC WEBSITE DATA SHEET FOR CURRENT STATUS OF VARIATION ASSIGNMENT 2. DRAWING NOT TO SCALE 3. ALL DIMENSIONS ARE IN MILLIMETERS 4. DIMENSIONS OF EXPOSED PAD ON BOTTOM OF PACKAGE DO NOT INCLUDE MOLD FLASH. MOLD FLASH, IF PRESENT, SHALL NOT EXCEED 0.15mm ON ANY SIDE 5. EXPOSED PAD SHALL BE SOLDER PLATED 6. SHADED AREA IS ONLY A REFERENCE FOR PIN 1 LOCATION ON THE TOP AND BOTTOM OF PACKAGE 3757afd 33 LT3757/LT3757A Package Description MSE Package 10-Lead Plastic MSOP, Exposed Die Pad (Reference LTC DWG # 05-08-1664 Rev H) BOTTOM VIEW OF EXPOSED PAD OPTION 1.88 ±0.102 (.074 ±.004) 5.23 (.206) MIN 1 0.889 ±0.127 (.035 ±.005) 1.68 ±0.102 (.066 ±.004) 0.05 REF 10 0.305 ± 0.038 (.0120 ±.0015) TYP RECOMMENDED SOLDER PAD LAYOUT 3.00 ±0.102 (.118 ±.004) (NOTE 3) DETAIL “B” CORNER TAIL IS PART OF DETAIL “B” THE LEADFRAME FEATURE. FOR REFERENCE ONLY NO MEASUREMENT PURPOSE 10 9 8 7 6 DETAIL “A” 0° – 6° TYP 1 2 3 4 5 GAUGE PLANE 0.53 ±0.152 (.021 ±.006) DETAIL “A” 0.18 (.007) 0.497 ±0.076 (.0196 ±.003) REF 3.00 ±0.102 (.118 ±.004) (NOTE 4) 4.90 ±0.152 (.193 ±.006) 0.254 (.010) 0.29 REF 1.68 (.066) 3.20 – 3.45 (.126 – .136) 0.50 (.0197) BSC 1.88 (.074) SEATING PLANE 1.10 (.043) MAX 0.17 – 0.27 (.007 – .011) TYP 0.50 (.0197) NOTE: BSC 1. DIMENSIONS IN MILLIMETER/(INCH) 2. DRAWING NOT TO SCALE 3. DIMENSION DOES NOT INCLUDE MOLD FLASH, PROTRUSIONS OR GATE BURRS. MOLD FLASH, PROTRUSIONS OR GATE BURRS SHALL NOT EXCEED 0.152mm (.006") PER SIDE 4. DIMENSION DOES NOT INCLUDE INTERLEAD FLASH OR PROTRUSIONS. INTERLEAD FLASH OR PROTRUSIONS SHALL NOT EXCEED 0.152mm (.006") PER SIDE 5. LEAD COPLANARITY (BOTTOM OF LEADS AFTER FORMING) SHALL BE 0.102mm (.004") MAX 6. EXPOSED PAD DIMENSION DOES INCLUDE MOLD FLASH. MOLD FLASH ON E-PAD SHALL NOT EXCEED 0.254mm (.010") PER SIDE. 0.86 (.034) REF 0.1016 ±0.0508 (.004 ±.002) MSOP (MSE) 0911 REV H 3757afd 34 LT3757/LT3757A Revision History (Revision history begins at Rev B) REV DATE DESCRIPTION PAGE NUMBER B 3/10 Deleted Bullet from Features and Last Line of Description Updated Entire Page to Add H-Grade and Military Grade Updated Electrical Characteristics Notes and Typical Performance Characteristics for H-Grade and Military Grade C D 5/11 07/12 1 2 4 to 6 Revised TA04a and Replaced TA04c in Typical Applications 30 Updated Related Parts 36 Revised MP-grade temperature range in Absolute Maximum Ratings and Order Information sections 2 Revised Note 2 4 Revised formula in Applications Information 19 Updated Typical Application drawing TA04a values 30 Revised Typical Application title TA06 32 Added LT3757A version Throughout Updated Block Diagram 8 Updated Programming the Output Voltage section 12 Updated Loop Compensation section 13 Added an application circuit in the Typical Applications section 28 Updated the schematic and Load Step Waveforms in the Typical Applications section 31 3757afd Information furnished by Linear Technology Corporation is believed to be accurate and reliable. However, no responsibility is assumed for its use. Linear Technology Corporation makes no representation that the interconnection of its circuits as described herein will not infringe on existing patent rights. 35 LT3757/LT3757A Typical Application High Efficiency Inverting Power Supply Efficiency vs Output Current R2 105k VIN L1 SHDN/UVLO R1 46.4k LT3757 SYNC GATE M1 Si7848BDP SENSE RT SS VC 41.2k 300kHz GND 9.1k 0.1µF CDC 47µF 25V, X5R • 100 L2 10nF 0.006Ω 1W FBX INTVCC CVCC 4.7µF 10V X5R 90 VOUT –5V 3A to 5A 84.5k D1 MBRD835L 16k L1, L2: COILTRONICS DRQ127-3R3 (*COUPLED INDUCTORS) 80 EFFICIENCY (%) CIN 47µF 16V X5R • VIN 5V TO 15V COUT 100µF 6.3V, X5R ×2 3757 TA09a VIN = 5V 70 VIN = 16V 60 50 40 30 20 10 0.001 0.1 1 0.01 OUTPUT CURRENT (A) 10 3757 TA09b Related Parts PART NUMBER DESCRIPTION COMMENTS LT3758A Boost, Flyback, SEPIC and Inverting Controller 5.5V ≤ VIN ≤ 100V, Current Mode Control, 100kHz to 1MHz Programmable Operation Frequency, 3mm × 3mm DFN-10 and MSOP-10E Packages LT3759 Boost, SEPIC and Inverting Controller 1.6V ≤ VIN ≤ 42V, Current Mode Control, 100kHz to 1MHz Programmable Operation Frequency, MSOP-12E Packages LT3957A Boost, Flyback, SEPIC and Inverting Controller with 5A, 40V Switch 3V ≤ VIN ≤ 40V, Current Mode Control, 100kHz to 1MHz Programmable Operation Frequency, 5mm × 6mm QFN Package LT3958 Boost, Flyback, SEPIC and Inverting Controller with 3.3A, 84V Switch 5V ≤ VIN ≤ 80V, Current Mode Control, 100kHz to 1MHz Programmable Operation Frequency, 5mm × 6mm QFN Package LT3573/LT3574/ LT3575 40V Isolated Flyback Converters Monolithic No-Opto Flybacks with Integrated 1.25A/0.65A/2.5A Switch LT3511/LT3512 100V Isolated Flyback Converters Monolithic No-Opto Flybacks with Integrated 240mA/420mA Switch LT3798 Offline Isolated No Opto-Coupler Flyback Controller with Active PFC VIN and VOUT Limited Only by External Components, MSOP-16 Package LT3799/LT3799-1 Offline Isolated Flyback LED Controllers with Active PFC VIN and VOUT Limited Only by External Components, MSOP-16 Package 3757afd 36 Linear Technology Corporation LT 0712 REV D • PRINTED IN USA 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com LINEAR TECHNOLOGY CORPORATION 2008