Datasheet

ManualsBrandsLINEAR TECHNOLOGY ManualsPMICsPMIC - DC/DC voltage regulator Boost, Cuk, Flyback, SEPIC QFN 36

LT3957A

3957afa

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 5.2V Low Dropout Voltage Regulator

Programmable Input Undervoltage Lockout with

Hysteresis

Programmable Soft-Start

Thermally Enhanced QFN (5mm × 6mm) Package

TYPICAL APPLICATION

DESCRIPTION

Boost, Flyback, SEPIC and

Inverting Converter

with 5A, 40V Switch

The LT

3957A is a wide input range, current mode DC/DC

converter which is capable of generating either positive or

negative output voltages. It can be configured as either a

boost, flyback, SEPIC or inverting converter. It features

an internal low side N-channel power MOSFET rated for

40V at 5A and driven from an internal regulated 5.2V

supply. The fixed frequency, current-mode architecture

results in stable operation over a wide range of supply

and output voltages.

The operating frequency of LT3957A 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 minimum operating supply voltage of 3V, and a low

shutdown quiescent current of less than 1µA, make the

LT3957A ideally suited for battery-powered systems.

The LT3957A features soft-start and frequency foldback

functions to limit inductor current during start-up. The

LT3957A has improved load transient performance com-

pared to the LT3957.

L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks and

No R

SENSE

and ThinSOT are trademarks of Linear Technology Corporation. All other trademarks

are the property of their respective owners. Protected by U.S. Patents, including 7825665.

High Efficiency Output Boost Converter

FEATURES

APPLICATIONS

Automotive

Telecom

Industrial

Efficiency vs Output Current

SENSE2

SENSE1

LT3957A

10µH

GND

FBX

RT SS

INTV

EN/UVLO

SYNC

SGND

95.3k

3957A TA01a

200k

41.2k

300kHz

4.7µF

0.33µF

6.8k

22nF

226k

15.8k

10µF

OUT

24V

600mA

4.5V TO 16V

10µF

OUTPUT CURRENT (mA)

EFFICIENCY (%)

100

100 200 300

400 500

3957A TA01b

600

= 12V

Summary of content (28 pages)

PAGE 1
LT3957A 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.
PAGE 2
LT3957A 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 ............................................................................1.5V SENSE1, SGND...
PAGE 3
LT3957A 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.
PAGE 4
LT3957A 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.
PAGE 5
LT3957A 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 1000 12 120 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.
PAGE 6
LT3957A 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 0 50 25 0 75 TEMPERATURE (°C) –25 100 4.8 1 125 10 VIN (V) 100 0 40 30 20 INTVCC LOAD (mA) 3957A G14 3957A G13 5.30 700 VIN = 5V 50 125°C 45 5.
PAGE 7
LT3957A 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.
PAGE 8
LT3957A BLOCK DIAGRAM CDC L1 R4 R3 L2 EN/UVLO A10 IS1 2μA IS2 10μA – + 27 VIN COUT t 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 t 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 A6 VISENSE SLOPE – + –0.8V 1.28V RAMP GENERATOR – +A3 1.
PAGE 9
LT3957A APPLICATIONS INFORMATION Main Control Loop The LT3957A 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.
PAGE 10
LT3957A APPLICATIONS INFORMATION 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. When INTVCC is below the UV threshold, the internal power switch will be turned off and the soft-start operation will be triggered. The logic circuitry within the LT3957A is also powered from the internal INTVCC supply. The INTVCC regulator must be bypassed to SGND immediately adjacent to the IC pins with a minimum of 4.
PAGE 11
LT3957A APPLICATIONS INFORMATION The operating frequency of the LT3957A can be synchronized to an external clock source. By providing a digital clock signal into the SYNC pin, the LT3957A will operate at the SYNC clock frequency. The LT3957A detects the rising edge of each clock cycle. If this feature is used, an RT resistor should be chosen to program a switching frequency 20% slower than SYNC pulse frequency. It is recommended that the SYNC pin has a minimum pulse width of 200ns.
PAGE 12
LT3957A APPLICATIONS INFORMATION 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 may keep increasing through each switch cycle, exceeding the programmed current limit.
PAGE 13
LT3957A APPLICATIONS INFORMATION On-Chip Power Dissipation and Thermal Lockout (TLO) The on-chip power dissipation of LT3957A can be estimated using the following equation: PIC ≈ I2SW • D • RDS(ON) + V2PEAK • ISW • ƒ • 200pF/A + VIN • (1.6mA + ƒ • 10nC) where RDS(ON) is the internal switch on-resistance which can be obtained from the Typical Performance Characteristics section. VSW(PEAK) is the peak switch off-state voltage.
PAGE 14
LT3957A APPLICATIONS INFORMATION ΔISW increases output current capability, but requires large inductances and reduces the current loop gain (the converter will approach voltage mode). Accepting larger values of ΔISW provides fast transient response and allows the use of low inductances, but results in higher input current ripple and greater core losses, and reduces output current capability.
PAGE 15
LT3957A APPLICATIONS INFORMATION 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.
PAGE 16
LT3957A APPLICATIONS INFORMATION It is recommended to choose a duty cycle between 20% and 80%. VSW Flyback Converter: Maximum Output Current Capability and Transformer Design ISW The maximum output current capability and transformer design for continuous conduction mode (CCM) is chosen as presented here. ISW(MAX) The maximum duty cycle (DMAX) occurs when the converter has the minimum VIN: ID ID(MAX) DTS D2TS t D3TS TS 3957A F07 Figure 7.
PAGE 17
LT3957A APPLICATIONS INFORMATION The primary winding peak current is the switch current limit (typical 5.9A). The primary and secondary maximum RMS currents are: ILP(RMS) ≈ ILS(RMS) ≈ POUT(MAX) DMAX • VIN(MIN) • η IOUT(MAX) 1−DMAX where η is the converter efficiency. Based on the preceding equations, the user should design/ choose the transformer having sufficient saturation and RMS current ratings.
PAGE 18
LT3957A APPLICATIONS INFORMATION The RMS ripple current rating of the output capacitors in continuous operation can be determined using the following equation: IRMS(COUT),CONTINUOUS ≈ IO(MAX) • DMAX 1−DMAX Flyback Converter: Input Capacitor Selection 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.
PAGE 19
LT3957A APPLICATIONS INFORMATION but results in higher input current ripple and greater core losses and reduces output current capability. 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) • DMAX 0.5 • ΔISW • ƒ For most SEPIC applications, the equal inductor values will fall in the range of 1μH to 100μH.
PAGE 20
LT3957A APPLICATIONS INFORMATION CDC L1 VIN + L2 – + – CIN COUT SW LT3957A GND VOUT + D1 + 3757A F08 Figure 8. A Simplified Inverting Converter 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.
PAGE 21
LT3957A APPLICATIONS INFORMATION IC is essential, especially the power paths with higher di/dt. The following high di/dt loops of different topologies should be kept as tight as possible to reduce inductive ringing: • In boost configuration, the high di/dt loop contains the output capacitor, the internal power MOSFET and the Schottky diode. • In flyback configuration, the high di/dt primary loop contains the input capacitor, the primary winding, the internal power MOSFET.
PAGE 22
LT3957A 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.
PAGE 23
LT3957A TYPICAL APPLICATIONS 4.5V to 16V Input, 24V Output Boost Converter L1 10μH CIN 10μF 25V X5R R3 200k VIN D1 COUT 10μF 50V X5R w2 SW GND EN/UVLO R4 95.3k VOUT 24V 600mA LT3957A 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.
PAGE 24
LT3957A TYPICAL APPLICATIONS 5V to 16V Input, 12V Output SEPIC Converter CDC 4.7μF, 25V X5R D1 VOUT 12V COUT 1A 22μF 16V X5R w2 t L1A VIN 5V TO 16V CIN 4.7μF 25V X5R VIN 200k L1B SW GND EN/UVLO 82.5k t LT3957A SGND SENSE1 SYNC SENSE2 105k FBX RT 41.2k 300kHz SS INTVCC VC 0.47μF 10k 10nF 15.8k CVCC 4.
PAGE 25
LT3957A TYPICAL APPLICATIONS 5V to 16V Input, –12V Output Inverting Converter CDC 4.7μF, 50V X7R t L1B CIN 4.7μF 25V X5R VIN 200k D1 SW GND EN/UVLO 82.5k VOUT –12V COUT 1A 22μF 16V X5R w2 t L1A VIN 5V TO 16V LT3957A SGND SENSE1 SYNC SENSE2 105k FBX RT 41.2k 300kHz SS INTVCC VC 0.47μF 7.5k CVCC 4.
PAGE 26
LT3957A PACKAGE DESCRIPTION Please refer to http://www.linear.com/designtools/packaging/ for the most recent package drawings. UHE Package Variation: UHE36(28)MA 36(28)-Lead Plastic QFN (5mm w 6mm) (Reference LTC DWG # 05-08-1836 Rev D) 28 27 25 24 23 21 20 0.70 t0.05 17 30 31 5.50 t 0.05 4.10 t 0.05 1.50 REF 1.53 t 0.05 1.88 t 0.05 3.00 t 0.05 32 33 16 3.00 t 0.05 15 0.12 t 0.05 14 PACKAGE OUTLINE 13 0.48 t 0.05 34 12 35 36 1 2 3 4 6 0.50 BSC 8 9 0.25 t0.05 10 2.00 REF 5.
PAGE 27
LT3957A REVISION HISTORY REV DATE DESCRIPTION A 10/12 VC Low Side Clamp Voltage line item added PAGE NUMBER 3 3957afa 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.
PAGE 28
LT3957A TYPICAL APPLICATIONS 4V to 6V Input, 180V Output Flyback Converter DANGER! HIGH VOLTAGE! T1 1:10 VIN 4V TO 6V D1 VOUT 180V 15mA t CIN 100μF 6.3V w2 t COUT 68nF w2 D2 22Ω 75k VIN 220pF SW GND EN/UVLO FBX 37.4k LT3957A 1.80M SENSE1 22Ω SGND 15.8k SENSE2 SYNC 10nF VC INTVCC RT SS 140k 100kHz 0.1μF 10k 100pF 4.