LT3957 Boost, Flyback, SEPIC and Inverting Converter with 5A, 40V Switch DESCRIPTION FEATURES n n n n n n n n n n n Wide Input Voltage Range: 3V to 40V Single Feedback Pin for Positive or Negative Output Voltage Internal 5A/40V Power Switch Current Mode Control Provides Excellent Transient Response Programmable Operating Frequency (100kHz to 1MHz) with One External Resistor Synchronizable to an External Clock Low Shutdown Current < 1μA Internal 5.
LT3957 ABSOLUTE MAXIMUM RATINGS PIN CONFIGURATION (Note 1) VC FBX SS RT SYNC NC TOP VIEW NC VIN, EN/UVLO (Note 5), SW ......................................40V INTVCC ......................................................VIN + 0.3V, 8V SYNC ..........................................................................8V VC, SS .........................................................................3V RT ...............................................................................................
LT3957 ELECTRICAL CHARACTERISTICS The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA ≈ TJ = 25°C. VIN = 24V, EN/UVLO = 24V, SENSE2 = 0V, unless otherwise noted. PARAMETER CONDITIONS MIN VIN Operating Range TYP 3 MAX UNITS 40 V VIN Shutdown IQ EN/UVLO = 0V EN/UVLO = 1.15V 0.1 1 6 μA μA VIN Operating IQ VC = 0.3V, RT = 41.2k 1.7 2.3 mA VIN Operating IQ with Internal LDO Disabled VC = 0.3V, RT = 41.2k, INTVCC = 5.
LT3957 ELECTRICAL CHARACTERISTICS The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA ≈ TJ = 25°C. VIN = 24V, EN/UVLO = 24V, SENSE2 = 0V, unless otherwise noted. PARAMETER CONDITIONS MIN TYP MAX INTVCC Current Limit VIN = 40V VIN = 15V 32 40 95 55 INTVCC Load Regulation (ΔVINTVCC / VINTVCC) 0 < IINTVCC < 20mA, VIN = 8V –1 –0.5 UNITS mA mA % INTVCC Line Regulation (ΔVINTVCC / [ΔVIN • VINTVCC]) 6V < VIN < 40V 0.
LT3957 TYPICAL PERFORMANCE CHARACTERISTICS Dynamic Quiescent Current vs Switching Frequency TA ≈ TJ = 25°C, unless otherwise noted. Normalized Switching Frequency vs FBX RT vs Switching Frequency 120 1000 12 NORMALIZED FREQUENCY (%) 10 RT (kΩ) IQ(mA) 8 6 100 4 2 10 0 100 200 300 400 500 600 700 800 900 1000 SWITCHING FREQUENCY (kHz) 40 20 310 305 300 295 290 285 0 0.4 0.8 FBX VOLTAGE (V) 1.2 1.6 SW Pin Current Limit vs Duty Cycle 6.6 6.6 6.4 6.4 SW PIN CURRENT LIMIT (A) RT = 41.
LT3957 TYPICAL PERFORMANCE CHARACTERISTICS INTVCC Minimum Output Current Limit vs VIN INTVCC vs Temperature 90 5.4 5.2 5.1 VIN = 6V 5.2 70 INTVCC VOLTAGE (V) INTVCC CURRENT (mA) 5.3 INTVCC Load Regulation 5.3 TJ = 125°C INTVCC = 3V 80 INTVCC (V) TA ≈ TJ = 25°C, unless otherwise noted. 60 50 40 30 20 5.1 5.0 4.9 10 5.0 –50 50 25 0 75 TEMPERATURE (°C) –25 100 0 125 1 10 VIN (V) 4.8 100 0 40 30 20 INTVCC LOAD (mA) 3957 G14 3957 G13 700 5.30 VIN = 5V 50 125°C 45 5.
LT3957 PIN FUNCTIONS NC (Pins 1, 2, 10, 35, 36): No Internal Connection. Leave these pins open or connect them to the adjacent pins. SENSE2 (Pin 3): The Current Sense Input for the Control Loop. Connect this pin to SENSE1 pin directly or through a low pass filter (connect this pin to SENSE1 pin through a resistor, and to SGND through a capacitor). SGND (Pins 4, 23, 24, Exposed Pad Pin 37): Signal Ground. All small-signal components should connect to this ground.
LT3957 BLOCK DIAGRAM CDC L1 R4 R3 L2 EN/UVLO A10 IS1 2μA IS2 10μA – + VIN 27 COUT • SW 8, 9, 20, 21, 38 1.22V INTERNAL REGULATOR AND UVLO CURRENT LIMIT UVLO M2 5.2V LDO A8 Q3 2.5V VOUT CIN 25 2.5V D1 • VIN 28 G4 INTVCC 2.7V 1.72V – + CVCC IS3 A11 TLO 165˚C DRIVER G6 –0.88V – + A12 VC Q2 1.6V SR1 – +A7 G5 R G2 O M1 S 6 PWM COMPARATOR + A1 – RSENSE VISENSE SLOPE – A6 + –0.8V RAMP + A2 – 1.28V RAMP GENERATOR – +A3 1.
LT3957 APPLICATIONS INFORMATION Main Control Loop The LT3957 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 internal power MOSFET switch M1 through driver G2. The switch current flows through the internal current sensing resistor RSENSE and generates a voltage proportional to the switch current.
LT3957 APPLICATIONS INFORMATION INTVCC Regulator Bypassing and Operation An internal, low dropout (LDO) voltage regulator produces the 5.2V INTVCC supply which powers the gate driver, as shown in Figure 1. The LT3957 contains an undervoltage lockout comparator A8 for the INTVCC supply. The INTVCC undervoltage (UV) threshold is 2.7V (typical), with 0.1V hysteresis, to ensure that the internal MOSFET has sufficient gate drive voltage before turning on.
LT3957 APPLICATIONS INFORMATION Operating Frequency and Synchronization Duty Cycle Consideration The choice of operating frequency may be determined by on-chip power dissipation (a low switching frequency may be required to ensure IC junction temperature does not exceed 125°C), 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.
LT3957 APPLICATIONS INFORMATION Soft-Start FBX Frequency Foldback The LT3957 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.
LT3957 APPLICATIONS INFORMATION The Internal Power Switch Current On-Chip Power Dissipation and Thermal Lockout (TLO) For control and protection, the LT3957 measures the internal power MOSFET current by using a sense resistor (RSENSE) between GND and the MOSFET source. Figure 3 shows a typical waveform of the internal switch current (ISW).
LT3957 APPLICATIONS INFORMATION APPLICATION CIRCUITS The LT3957 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 LT3957 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.
LT3957 APPLICATIONS INFORMATION The power dissipated by the diode is: tON tOFF $VCOUT PD = IO(MAX) • VD where VD is diode’s forward voltage drop, 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.
LT3957 APPLICATIONS INFORMATION FLYBACK CONVERTER APPLICATIONS The LT3957 can be configured as a flyback converter for the applications where the converters have multiple outputs, high output voltages or isolated outputs. Due to the 40V rating of the internal power switch, LT3797 should be used in low input voltage flyback converters. Figure 6 shows a simplified flyback converter.
LT3957 APPLICATIONS INFORMATION According to the Absolute Maximum Ratings table, the SW voltage Absolute Maximum value is 40V. Therefore, the maximum primary to secondary turns ratio (for both the continuous and the discontinuous operation) should be. NP 40V − VIN(MAX) ≤ NS k • VOUT According to the preceding equations, the user has relative freedom in selecting the switch duty cycle or turns ratio to suit a given application.
LT3957 APPLICATIONS INFORMATION In some cases a snubber circuit will be required to avoid overvoltage breakdown at the MOSFET’s drain node. There are different snubber circuits (such as RC snubber, RCD snubber, Zener clamp, etc.), and Application Note 19 is a good reference on snubber design. An RC snubber circuit can be connected between SW and GND to damp the ringing on SW pins. The snubber resistor values should be close to the impedance of the parasitic resonance.
LT3957 APPLICATIONS INFORMATION SEPIC CONVERTER APPLICATIONS The LT3957 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: VOUT + VD D = VIN 1− D in continuous conduction mode (CCM). In a SEPIC converter, no DC path exists between the input and output.
LT3957 APPLICATIONS INFORMATION 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) ΔISW • ƒ • DMAX This maintains the same ripple current and energy storage in the inductors. The peak inductor currents are: IL1(PEAK) = IL1(MAX) + 0.5 • ΔIL1 IL2(PEAK) = IL2(MAX) + 0.5 • ΔIL2 The maximum RMS inductor currents are approximately equal to the maximum average inductor currents.
LT3957 APPLICATIONS INFORMATION Inverting Converter: Switch Duty Cycle and Frequency Inverting Converter: Selecting the DC Coupling Capacitor 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).
LT3957 APPLICATIONS INFORMATION • In flyback configuration, the high di/dt primary loop contains the input capacitor, the primary winding, the internal power MOSFET. 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 internal power MOSFET, output capacitor, Schottky diode and the coupling capacitor.
LT3957 APPLICATIONS INFORMATION Recommended Component Manufacturers Some of the recommended component manufacturers are listed in Table 2. Table 2. Recommended Component Manufacturers VENDOR COMPONENTS WEB ADDRESS Capacitors avx.com Inductors, Transformers bhelectronics.com Coilcraft Inductors coilcraft.com Cooper Bussmann AVX BH Electronics Inductors bussmann.com Diodes, Inc Diodes diodes.com General Semiconductor Diodes generalsemiconductor. com International Rectifier Diodes irf.
LT3957 TYPICAL APPLICATIONS 4.5V to 16V Input, 24V Output Boost Converter VIN 4.5V TO 16V L1 10μH CIN 10μF 25V X5R R3 200k VIN D1 GND EN/UVLO R4 95.3k COUT 10μF 50V X5R s2 SW VOUT 24V 600mA LT3957 SGND SENSE1 SYNC SENSE2 R2 226k FBX RT RT 41.2k 300kHz SS VC CSS 0.33μF R1 15.8k INTVCC RC 6.8k CC 22nF CVCC 4.
LT3957 TYPICAL APPLICATIONS 5V to 16V Input, 12V Output SEPIC Converter CDC 4.7μF, 25V X5R D1 VOUT 12V COUT 1A 22μF 16V X5R s2 • L1A VIN 5V TO 16V CIN 4.7μF 25V X5R VIN 200k L1B SW GND EN/UVLO 82.5k • LT3957 SGND SENSE1 SYNC SENSE2 105k FBX RT 41.2k 300kHz SS INTVCC VC 0.47μF 10k 10nF 15.8k CVCC 4.
LT3957 TYPICAL APPLICATIONS 5V to 16V Input, –12V Output Inverting Converter CDC 4.7μF, 50V X7R • L1B CIN 4.7μF 25V X5R VIN 200k D1 SW GND EN/UVLO 82.5k VOUT –12V COUT 1A 22μF 16V X5R s2 • L1A VIN 5V TO 16V LT3957 SGND SENSE1 SYNC SENSE2 105k FBX RT 41.2k 300kHz SS INTVCC VC 0.47μF 7.5k CVCC 4.
LT3957 PACKAGE DESCRIPTION UHE Package Variation: UHE28MA 36-Lead Plastic QFN (5mm × 6mm) (Reference LTC DWG # 05-08-1836 Rev B) 28 27 25 24 23 21 20 0.70 p0.05 30 31 5.50 p 0.05 4.10 p 0.05 1.50 REF 16 3.00 p 0.05 3.00 p 0.05 32 33 17 1.53 p 0.05 1.88 p 0.05 0.12 p 0.05 15 14 PACKAGE OUTLINE 13 0.48 p 0.05 34 12 35 36 1 2 3 4 6 0.50 BSC 8 9 0.25 p0.05 10 2.00 REF 5.10 p 0.05 6.50 p 0.
LT3957 TYPICAL APPLICATIONS 4V to 6V Input, 180V Output Flyback Converter DANGER! HIGH VOLTAGE! T1 1:10 VIN 4V TO 6V D1 VOUT 180V 15mA • CIN 100μF 6.3V s2 • COUT 68nF s2 D2 220pF 22Ω 75k SW GND EN/UVLO FBX VIN 37.4k LT3957 1.80M SENSE1 22Ω SGND 15.8k SENSE2 SYNC 10nF VC INTVCC RT SS 140k 100kHz 0.1μF 10k 100pF 4.
