Datasheet

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LTC3721-1

sn37211 37211fs

APPLICATIO S

FEATURES

TYPICAL APPLICATIO

DESCRIPTIO

The LTC

3721-1 push-pull PWM controller provides all of

the control and protection functions necessary for com-

pact and highly efficient, isolated power converters. High

integration minimizes external component count, while

preserving design flexibility.

The robust push-pull output stages switch at half the

oscillator frequency. Dead-time is independently pro-

grammed with an external resistor. A UVLO program input

provides precise system turn-on and turn off voltages. The

LTC3721-1 features peak current mode control with pro-

grammable slope compensation and leading edge

blanking.

The LTC3721-1 features extremely low operating and

start-up currents and reliable short-circuit and

overtemperature protection. The LTC3721-1 is offered in

16-pin SSOP and (4mm × 4mm) QFN packages.

■

Telecommunications, Infrastructure Power Systems

■

Distributed Power Architectures

■

Server Power Supplies

■

High Density Power Modules

, LTC and LT are registered trademarks of Linear Technology Corporation.

■

High Efficiency Push-Pull PWM

■

1.5A Sink, 1A Source Output Drivers

■

Adjustable Push-Pull Dead-Time

■

Adjustable System Undervoltage Lockout and

Hysteresis

■

Adjustable Leading Edge Blanking

■

Low Start-Up and Quiescent Currents

■

Current Mode Operation

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Single Resistor Slope Compensation

■

UVLO and 25mA Shunt Regulator

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Programmable Fixed Frequency Operation to 1MHz

■

Soft-Start, Cycle-by-Cycle Current Limiting and

Hiccup Mode Short-Circuit Protection

■

5V, 15mA Low Dropout Regulator

■

16-Pin SSOP and (4mm × 4mm) QFN Packages

Push-Pull PWM Controller

OUT

TOP

GND-F

LT1431

GND-S

COLL R

REF

COMP R

MID

37211 TA01

FROM

AUXILIARY

WINDING

REF

DRVA

DRVB

CSV

DPRG

LTC3721-1

REF

COMP

UVLO

LEB

FB GND

Isolated Push-Pull Converter

Summary of content (16 pages)

PAGE 1
LTC3721-1 Push-Pull PWM Controller U FEATURES DESCRIPTIO ■ The LTC®3721-1 push-pull PWM controller provides all of the control and protection functions necessary for compact and highly efficient, isolated power converters. High integration minimizes external component count, while preserving design flexibility. ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ High Efficiency Push-Pull PWM 1.
PAGE 2
LTC3721-1 U W W W ABSOLUTE AXI U RATI GS (Note 1) VCC to GND (Low Impedance Source) .......– 0.3V to 10V (Chip Self-Regulates at 10.3V) UVLO to GND ............................................. – 0.3V to VCC All Other Pins to GND (Low Impedance Source) .........................– 0.3V to 5.5V VCC (Current Fed) ................................................. 40mA VREF Output Current ............................... Self-Regulated Operating Temperature (Notes 5,6) LTC3721-1 ...........................
PAGE 3
LTC3721-1 ELECTRICAL CHARACTERISTICS The ● denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. VCC = 9.5V unless otherwise noted. SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS Pulse Width Modulator ROS Ramp Offset Voltage Measured on COMP, CS = 0V 0.65 IRMP Ramp Discharge Current CS = 1V, COMP = 0V, CT = 4V 50 mA ISLP Slope Compensation Current Measured on CS, CT = 1V CT = 2.
PAGE 4
LTC3721-1 ELECTRICAL CHARACTERISTICS The ● denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. VCC = 9.5V unless otherwise noted. SYMBOL CONDITIONS MIN TYP MAX UNITS SSR Soft-Start Reset Threshold PARAMETER Measured on SS 0.7 0.4 0.1 V FLT Fault Reset Threshold Measured on SS 4.5 4.2 3.5 V Note 5: The LTC3721–1 is guaranteed to meet performance specifications from 0°C to 85°C.
PAGE 5
LTC3721-1 U W TYPICAL PERFOR A CE CHARACTERISTICS PHASE (DEG) 100 80 60 40 20 0 190 275 180 250 170 225 160 200 –180 DELAY (ns) 200k PREBIAS 150 140 175 150 130 125 –270 120 100 –360 110 75 10 100 1k 10k 100k FREQUENCY (Hz) 1M 100 –55 10M 50 –25 5 35 65 TEMPERATURE (°C) 372311 G07 95 125 10.5 80 10.4 FB Input Voltage vs Temperature 1.204 40 CT = 1V 30 1.203 10.3 FB VOLTAGE (V) SHUNT VOLTAGE (V) CT = 2.25V 50 10.2 10.1 10.0 0 –55 9.
PAGE 6
LTC3721-1 U U PI DESCRIPTIO S (GN Package/UF Package) VREF (Pin 1/Pin 15): Output of the 5.0V Reference. VREF is capable of supplying up to 18mA to external circuitry. VREF should be decoupled to GND with a 1µF ceramic capacitor. DRVB (Pin 4/Pin 1): High Speed 1.5A Sink, 1A Source Totem Pole MOSFET Driver. Connect to gate of external push-pull MOSFET with as short a PCB trace as practical to preserve drive signal integrity.
PAGE 7
LTC3721-1 U U PI DESCRIPTIO S (GN Package/UF Package) FB (Pin 13/Pin 12): Error Amplifier Inverting Input. This is the voltage feedback input for the LTC3721-1. The nominal regulation voltage at FB is 1.2V. SS (Pin 14/Pin 13): Soft-Start/Restart Delay Circuitry Timing Capacitor. A capacitor from SS to GND provides a controlled ramp of the current command. During overload conditions, SS is discharged to ground initiating a softstart cycle. SS charging current is approximately 13µA.
PAGE 8
RLEB CS SS COMP FB – + 1.2V BLANK 600mV 650mV + – 300mV – + – + 50k UVLO 5V – + – + VCC 14.9k VCC GOOD S R SLOPE COMPENSATOR FAULT LOGIC Q 5V VREF REF GOOD REF, LDO PULSE WIDTH MODULATOR 1.2V 13µA VREF + – SYSTEM UVLO 10µA PULSE-BY-PULSE CURRENT LIMIT SHUTDOWN CURRENT LIMIT ERROR AMPLIFIER 10.25V “ON” 6V “OFF” VCC UVLO VCC S R OSCILLATOR CT GND DPRG Q LTC3721-1 Block Diagram T Q Q 1.5A SINK 1A SOURCE 1.
PAGE 9
LTC3721-1 U OPERATIO Please refer to the detailed Block Diagram for this discussion. The LTC3721-1 is a PWM push-pull controller that operates with pulse-by-pulse peak current mode control. It is best suited for moderate to high power isolated power systems where small size and high efficiency are required. The push-pull topology delivers excellent transformer utilization and requires only two low side power MOSFET switches.
PAGE 10
LTC3721-1 U OPERATIO VCC should be bypassed with a 0.1µF to 1µF multilayer ceramic capacitor to decouple the fast transient currents demanded by the output drivers and a bulk tantalum or electrolytic capacitor to hold up the VCC supply before the bootstrap winding, or an auxiliary regulator circuit takes over. CHOLDUP = (ICC + IDRIVE) • tDELAY/3.8V (minimum UVLO hysteresis) Regulated bias supplies as low as 7V can be utilized to provide bias to the LTC3721-1.
PAGE 11
LTC3721-1 U OPERATIO CT = 1/(14.8k • FOSC) Note that this is the frequency seen on CT. The output drivers switch at 1/2 of this frequency. Also note that higher switching frequency and added driver dead-time via DPRG will reduce the maximum duty cycle. The LTC3721-1 derives a compensating slope current from the oscillator ramp waveform and sources this current out of CS.
PAGE 12
LTC3721-1 U OPERATIO Leading Edge Blanking The LTC3721-1 provides programmable leading edge blanking to prevent nuisance tripping of the current sense circuitry. Leading edge blanking relieves the filtering requirements for the CS pin, greatly improving the response to real overcurrent conditions. It also allows the use of a ground referenced current sense resistor or transformer(s), further simplifying the design.
PAGE 13
LTC3721-1 U OPERATIO The duty cycle of these square waves is guaranteed to never exceed 50% by the LTC3721-1. In steady state operation, the duty ratio is given by: D = VOUT/(2 • VIN • N) To calculate the transformer turns ratio, first determine the minimum input voltage (VIN(MIN)) and the maximum duty ratio (D(MAX)) of the controller IC. This will be the worst case condition. An example is provided below: VIN = 32V to 75V, use VIN(MIN) = 30V to account for system voltage drops.
PAGE 14
LTC3721-1 U PACKAGE DESCRIPTIO GN Package 16-Lead Plastic SSOP (Narrow .150 Inch) (Reference LTC DWG # 05-08-1641) .189 – .196* (4.801 – 4.978) .045 ±.005 16 15 14 13 12 11 10 9 .254 MIN .009 (0.229) REF .150 – .165 .229 – .244 (5.817 – 6.198) .0165 ± .0015 .150 – .157** (3.810 – 3.988) .0250 BSC RECOMMENDED SOLDER PAD LAYOUT 1 .015 ± .004 × 45° (0.38 ± 0.10) .007 – .0098 (0.178 – 0.249) .0532 – .0688 (1.35 – 1.75) 2 3 4 5 6 7 8 .004 – .0098 (0.102 – 0.249) 0° – 8° TYP .016 – .050 (0.
PAGE 15
LTC3721-1 U PACKAGE DESCRIPTIO UF Package 16-Lead Plastic QFN (4mm × 4mm) (Reference LTC DWG # 05-08-1692) 0.72 ±0.05 4.35 ± 0.05 2.15 ± 0.05 2.90 ± 0.05 (4 SIDES) PACKAGE OUTLINE 0.30 ±0.05 0.65 BSC RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS BOTTOM VIEW—EXPOSED PAD 4.00 ± 0.10 (4 SIDES) R = 0.115 TYP 0.75 ± 0.05 PIN 1 TOP MARK (NOTE 6) 0.55 ± 0.20 15 16 1 2.15 ± 0.10 (4-SIDES) 2 (UF) QFN 1103 0.200 REF 0.00 – 0.05 0.30 ± 0.05 0.65 BSC NOTE: 1.
PAGE 16
LTC3721-1 U TYPICAL APPLICATIO +VS 12 +VOUT MBRB20200CT • 1nF 200V 5.1Ω 1/2W C2, C3, C4, C5 4x33µF 16V 12VIN 1nF 200V C6 56µF 35V 11 3 4 6 +VIN + 160Ω 2W • 5.1Ω 1/2W L1 22µH 4 C8 0.1µF 100V C64 180pF 200V 1 + MBRB20200CT –VIN C7 56µF 35V 8 • R1 0.01Ω 1.5W 5V R2 0.01Ω 1.5W EFFICIENCY (%) 4.7Ω 1/4W 330pF FMMT718 VIN –VOUT 10.6VIN 13.2VIN 91 73.2k 48V/3.65A C9 39µF 100V 92 L2 22µH 7 4.7Ω 1/4W + T2 0.59mH PULSE P0353 –VS 9 • 2xSi7370DP 1.