LT3958 High Input Voltage, Boost, Flyback, SEPIC and Inverting Converter Description Features n n n n n n n n n n n Wide Input Voltage Range: 5V to 80V Single Feedback Pin for Positive or Negative Output Voltage Internal 3.3A/84V Power Switch Current Mode Control Provides Excellent Transient Response Programmable Operating Frequency (100kHz to 1MHz) with One External Resistor Synchronizeable to an External Clock Low Shutdown Current < 1µA Internal 7.
LT3958 VC FBX SS RT SYNC TOP VIEW 36 35 34 33 32 31 30 NC 1 28 INTVCC 27 VIN NC 2 SENSE2 3 37 SGND SGND 4 25 EN/UVLO 24 SGND 23 SGND SENSE1 6 38 SW SW 8 SW 9 21 SW 20 SW NC 10 GND GND GND GND GND 12 13 14 15 16 17 GND VIN, EN/UVLO (Note 5)...............................................80V SW.............................................................................84V INTVCC..................................................... VIN + 0.3V, 15V SYNC.....................................
LT3958 Electrical Characteristics The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. VIN = 24V, EN/UVLO = 24V, SENSE2 = 0V, unless otherwise noted. PARAMETER CONDITIONS MIN VIN Operating Range TYP 5 MAX UNITS 80 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 1.6 2.2 mA VIN Operating IQ with Internal LDO Disabled VC = 0.3V, RT = 41.2k, INTVCC = 7.
LT3958 Electrical Characteristics The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. VIN = 24V, EN/UVLO = 24V, SENSE2 = 0V, unless otherwise noted. PARAMETER CONDITIONS MIN TYP MAX INTVCC Current Limit VIN = 80V VIN = 20V 19 24 50 29 INTVCC Load Regulation (∆VINTVCC / VINTVCC) 0 < IINTVCC < 10mA, VIN = 8V –1 –0.4 INTVCC Line Regulation (∆VINTVCC / [∆VIN • VINTVCC]) 8V < VIN < 80V 0.005 UNITS mA mA % 0.
LT3958 Typical Performance Characteristics –792 1.602 1.600 VIN = 24V 1.598 VIN = 8V 1.596 1.594 VIN = INTVCC = 5V 1.592 1.590 –50 –25 50 25 0 75 TEMPERATURE (°C) 100 Negative Feedback Voltage vs Temperature, VIN 1.
LT3958 Typical Performance Characteristics EN/UVLO Threshold vs Temperature 50 2.4 40 EN/UVLO RISING 1.24 EN/UVLO FALLING 1.22 2.2 IEN/UVLO (µA) EN/UVLO CURRENT (µA) 1.26 30 20 1.18 –50 50 25 0 75 TEMPERATURE (°C) –25 100 0 125 0 INTVCC vs Temperature VIN = 8V 7.1 INTVCC VOLTAGE (V) 7.2 INTVCC VOLTAGE (V) INTVCC (V) 7.30 7.1 7 6.9 7.0 –50 50 25 0 75 TEMPERATURE (°C) –25 100 6.
LT3958 Typical Performance Characteristics SEPIC Typical Start-Up Waveforms TA = 25°C, unless otherwise noted. SEPIC FBX Frequency Foldback Waveforms During Overcurrent VIN = 24V VIN = 24V VOUT 10V/DIV VOUT 5V/DIV VSW 20V/DIV IL1A + IL1B 2A/DIV IL1A + IL1B 2A/DIV 5ms/DIV 3958 G19 SEE TYPICAL APPLICATION: 10V TO 60V INPUT, 12V OUTPUT SEPIC CONVERTER 50µs/DIV 3958 G20 SEE TYPICAL APPLICATION: 10V TO 60V INPUT, 12V OUTPUT SEPIC CONVERTER 3958fa For more information www.linear.
LT3958 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.
LT3958 Block Diagram CDC L1 R4 R3 A10 IS1 2µA IS2 10µA – + 1.72V –0.88V – + A9 – + –0.8V A12 VC – +A7 28 + – CVCC DRIVER R S G2 O FBX 30 VC 32 SS M1 6 PWM COMPARATOR RSENSE VISENSE RAMP GENERATOR – +A3 – A6 + + + 34 SYNC A5 + – A4 CC2 RC CSS 12, 13, 14, 15, 16, 17 3 SENSE2 Q1 FREQ PROG 33 RT SGND 4, 23, 24, 37 R2 R1 GND 48mV SENSE SENSE1 100kHz-1MHz OSCILLATOR G1 FREQUENCY 1.2V FOLDBACK INTVCC 3.75V G5 – VOUT INTVCC RAMP 1.
LT3958 Applications Information Main Control Loop The LT3958 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.
LT3958 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. The LT3958 contains an undervoltage lockout comparator A8 and an overvoltage lockout comparator A9 for the INTVCC supply. The INTVCC undervoltage (UV) threshold is 3.75V (typical), with 0.15V hysteresis, to ensure that the internal MOSFET has sufficient gate drive voltage before turning on.
LT3958 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.
LT3958 Applications Information Soft-Start FBX Frequency Foldback The LT3958 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.
LT3958 Applications Information The Internal Power Switch Current On-Chip Power Dissipation and Thermal Lockout (TLO) For control and protection, the LT3958 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). The on-chip power dissipation of LT3958 can be estimated using the following equation: ISW ∆ISW ISW(PEAK) t DTS TS 3958 F03 Figure 3.
LT3958 Applications Information Application Circuits The LT3958 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 LT3958 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.
LT3958 Applications Information The power dissipated by the diode is: tON ∆VCOUT PD = IO(MAX) • VD where VD is diode’s forward voltage drop, and the diode junction temperature is: VOUT (AC) ∆VESR TJ = TA + PD • RθJA 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.
LT3958 Applications Information Flyback Converter Applications The LT3958 can be configured as a flyback converter for the applications where the converters have multiple outputs, high output voltages or isolated outputs. Figure 6 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.
LT3958 Applications Information 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.
LT3958 Applications Information The snubber resistor value (RSN) can be calculated by the following equation: RSN = 2 • N V 2 SN − VSN • VOUT • P NS I2 SW(PEAK) • L LK • ƒ 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.
LT3958 Applications Information 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.
LT3958 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) ∆ISW • ƒ • DMAX This maintains the same ripple current and energy storage in the inductors. The peak inductor currents are: 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.
LT3958 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).
LT3958 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.
LT3958 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.
LT3958 Typical Applications 10V to 40V Input, 48V Output Boost Converter L1 33µH CIN 4.7µF 50V X5R R3 392k VIN D1 GND EN/UVLO R4 53.6k COUT 4.7µF 50V X5R ×2 SW Efficiency vs Output Current VOUT 48V 0.5A 100 LT3958 SGND SENSE1 SYNC SENSE2 R2 464k FBX RT SS RT 41.2k 300kHz CSS 0.33µF R1 15.8k INTVCC VC RC 10k CC 10nF CVCC 4.
LT3958 Typical Applications 10V to 60V Input, 12V Output SEPIC Converter L1A D1 VOUT 12V COUT 1A 22µF 16V X5R ×2 • VIN 10V TO 60V CDC 2.2µF, 100V X7R, ×2 CIN 2.2µF 100V X5R VIN 392k SW GND EN/UVLO 66.5k • L1B LT3958 SGND SENSE1 SYNC SENSE2 105k FBX RT 41.2k 300kHz INTVCC VC SS 0.47µF 10k 10nF 15.8k CVCC 4.
LT3958 Typical Applications 10V to 60V Input, –12V Output Inverting Converter CDC 2.2µF, 100V X7R, ×2 • L1B CIN 2.2µF 100V X5R VIN 392k D1 SW GND EN/UVLO 66.5k VOUT –12V COUT 1A 22µF 16V X5R ×2 • L1A VIN 10V TO 60V LT3958 SGND SENSE1 SYNC SENSE2 105k FBX RT 41.2k 300kHz INTVCC VC SS 0.47µF 10k 10nF 7.5k CVCC 4.
LT3958 Package Description Please refer to http://www.linear.com/product/LT3958#packaging for the most recent package drawings. UHE Package Variation: UHE36(28)MA 36(28)-Lead Plastic QFN (5mm × 6mm) (Reference LTC DWG # 05-08-1836 Rev D) 28 27 25 24 23 21 20 0.70 ±0.05 30 1.88 ± 0.05 31 5.50 ± 0.05 4.10 ± 0.05 1.50 REF 3.00 ± 0.05 32 33 16 3.00 ± 0.05 0.12 ± 0.05 34 17 1.53 ± 0.05 15 14 PACKAGE OUTLINE 13 0.48 ± 0.05 12 35 36 1 2 3 4 6 0.50 BSC 8 9 0.25 ±0.05 10 2.00 REF 5.
LT3958 Revision History REV DATE DESCRIPTION A 09/17 Clarification for Boost, Flyback and SEPIC Maximum Output Current Capability sections PAGE NUMBER 15, 18, 20 3958fa 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. For more information www.
LT3958 typical applications 5V Output Nonisolated Flyback Power Supply T1 3:1 VIN 10V TO 40V 0.1µF 50V CIN 4.7µF 50V X5R D1 • 1.8k 1W Efficiency vs Output Current VOUT 5V 2A 90 COUT 100µF 6.3V • 80 VIN EFFICIENCY (%) DSN 200k SW GND EN/UVLO 32.4k LT3958 SGND SENSE1 SYNC SENSE2 63.4k 200kHz 0.47µF 10k 70 65 55 50 INTVCC VC 75 60 34k 1% FBX RT SS VIN = 24V 85 15.8k 1% 4.7µF 10V X5R 0 0.5 1 1.