SC4812 データシートの表示（PDF） - Semtech Corporation

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SC4812

Current Mode PWM Controller with Integrated Start-up Circuit and Op Amp

Semtech Corporation 

SC4812

POWER MANAGEMENT

Application Information

Introduction

The SC4812 is a 8 pin peak current mode controlled

PWM controller for isolated or non-isolated DC-DC

switching power supplies. It features an internal startup

regulator, programmable external soft start and

programmable cycle -by-cycle current limit It operates in

a fixed frequency at 260KHz.

The SC4812 can be applied in a flyback or forward

topology with the input voltage ranging from 36V to 75V.

Startup Regulator

The internal startup regulator of the SC4812 makes its

initial startup without a lossy startup resistor or external

startup circuitry to save cost and board space. As input

voltage exceeds 12V, it provides regulated 7.4V to VCC

which is with the maximum turn-on voltage 6.7V.

Soft-Start/Shutdown and VCC Lockout

During power startup, the output voltage has to ramp up

in a controlled manner to avoid overshoot. The SC4812

internal non-inverting terminal to the error amplifier is

connected to the soft-start pin, which forces the internal

2.5V reference to gradually ramp up during power

startup. In a non-isolated application, as the internal error

amplifier is used, output voltage of a converter ramps

up with close loop startup fashion smoothly. In case of

an isolated application in which the internal error amplifier

may not be used, isolated voltage feedback is through

an opto-coupler and connected to the COMP pin. During

power startup, the COMP pin will follow the soft-start pin

voltage through an internal diode.

Soft-start operation begins when SS/SHDN ramps above

0.7V. When soft-start completed, SS/SHDN is regulated

to 2.50V, the internal voltage reference. Pull SS/SHDN

below 0.5V to disable the controller.

Undervoltage lockout shuts down the controller when VCC

is less than 5.5V. The internal startup regulator and the

reference remain on during shutdown.

Current-Sense

The function of the CS pin is to limit the peak current

through the MOSFET. Current is sensed at CS as a voltage

across a sense resistor between the source of the

MOSFET and GND. An RC filter is recommended to

connect CS to the sense resistor to reduce effect of the

MOSFET turn-on leading edge spike and noise. Select

the current-sense resistor with the following equation:

RS = 0.465V/(1.2xILIM_PRI)

where I is the maximum peak primary-side current.

, LIM_PRI

When VCS > 465mV, the power MOSFET turns off after

70nS propagation delay from the switch current reaches

the trip point to the driver.

PWM Comparator and Slope Compensation

The SC4812 runs fixed 260 KHz frequency by an internal

oscillator. At the beginning of each switching cycle, the

OUT pin switches the MOSFET on. the OUT pin switches

off after the 80% maximum duty cycle has been reached,

regardless of the feedback.

Since an artificial slope is required for current-mode

operation when duty cycle is larger than 50%, the SC4812

uses an internal ramp generator for slope compensation.

The internal ramp signal is reset at the beginning of each

cycle and slews at 26mV/µS.

The PWM comparator compares the instantaneous

primary peak current to the feedback error signal through

the opto-coupler, the internal offset and slope

compensation and determine when to turn off the

MOSFET. In steady state operation, the MOSFET turns

off when:

ILIM_PRI · RS > VOPTO – VOFFSET - VSLOPE

where ILIM_PRI is the current on the primary side through

the MOSFET. VOFFSET is 1.5V internal offset voltage and

VSLOPE is the artificial ramp starting at with slew rate 26mV/

µS.

When selecting an inductor in a forward-converter or

magnetizing inductance in a flyback-converter, the

following condition must be met to avoid control-loop sub-

harmonic oscillations:

NS

NP

×

K

×

RS

L

×

VO

= 26mV / µS

where K = 0.5 TO 1, AND NS and NP are the turns of the

main transformer on secondary and primary side

respectively. L is the output inductor on secondary side

in a forward-converter or magnetizing inductance on

secondary side in a flyback-converter.

 2007 Semtech Corp.

7

www.semtech.com

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