APU3037 데이터 시트보기 (PDF) - Advanced Power Electronics Corp

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APU3037

8-PIN SYNCHRONOUS PWM CONTROLLER

Advanced Power Electronics Corp 

APU3037 / APU3037A

BLOCK DIAGRAM

Vcc

Bias

3V

0.2V

Generator

1.25V

3V

20uA

SS/SD 8

POR

1.25V

25K

Fb 1

25K

Comp 7

4.0V

Vc

0.2V

POR

64uA Max

3.5V

Oscillator

Ct

Error Amp

Error Comp

S

Q

R

Reset Dom

0.5V

FbLo Comp

POR

6 Vc

5 HDrv

2 Vcc

3 LDrv

4 Gnd

Figure 2 - Simplified block diagram of the APU3037.

THEORY OF OPERATION

Introduction

The APU3037 is a fixed frequency, voltage mode syn-

chronous controller and consists of a precision refer-

ence voltage, an error amplifier, an internal oscillator, a

PWM comparator, 0.5A peak gate driver, soft-start and

shutdown circuits (see Block Diagram).

The output voltage of the synchronous converter is set

and controlled by the output of the error amplifier; this is

the amplified error signal from the sensed output voltage

and the reference voltage.

This voltage is compared to a fixed frequency linear

sawtooth ramp and generates fixed frequency pulses of

variable duty-cycle, which drives the two N-channel ex-

ternal MOSFETs.The timing of the IC is provided through

an internal oscillator circuit which uses on-chip capaci-

tor to set the oscillation frequency to 200 KHz (400 KHz

for “A” version).

Soft-Start

The APU3037 has a programmable soft-start to control

the output voltage rise and limit the current surge at the

start-up. To ensure correct start-up, the soft-start se-

quence initiates when the Vc and Vcc rise above their

threshold (3.3V and 4.2V respectively) and generates

the Power On Reset (POR) signal. Soft-start function

operates by sourcing an internal current to charge an

external capacitor to about 3V. Initially, the soft-start func-

tion clamps the E/A’s output of the PWM converter. As

the charging voltage of the external capacitor ramps up,

the PWM signals increase from zero to the point the

feedback loop takes control.

Short-Circuit Protection

The outputs are protected against the short-circuit. The

APU3037 protects the circuit for shorted output by sens-

ing the output voltage (through the external resistor di-

vider). The APU3037 shuts down the PWM signals, when

the Fb voltage drops below 0.6V (0.4V for APU3037A).

The APU3037 also protects the output from over-voltaging

when the control FET is shorted. This is done by turning

on the sync FET with the maximum duty cycle.

Under-Voltage Lockout

The under-voltage lockout circuit assures that the

MOSFET driver outputs remain in the off state whenever

the supply voltage drops below set parameters. Lockout

occurs if Vc and Vcc fall below 3.3V and 4.2V respec-

tively. Normal operation resumes once Vc and Vcc rise

above the set values.

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