IRU3138CS datasheet(5/18 Pages) IRF

5 / 18 page

IRU3138

Rev. 1.0

01/29/04

www.irf.com

THEORY OF OPERATION

Introduction

The IRU3138 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, 1A peak gate driver, soft-start and

shutdown circuits (see Block Diagram). The output volt-

age 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 refer-

ence voltage. This voltage is compared to a fixed fre-

quency linear sawtooth ramp and generates fixed fre-

quency pulses of variable duty-cycle, which drives the

two N-channel external MOSFETs.The timing of the IC

is provided through an internal oscillator circuit which

uses on-chip capacitor. The oscillation frequency is pro-

grammable between 200KHz to 400KHz by using an

external resistor. Figure 4A shows switching frequency

vs. external resistor.

Soft-Start

The IRU3138 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 (3.5V and 4.25V 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 and

disables the short circuit protection. During the power

up, the output starts at zero and voltage at Fb is below

0.4V. The feedback UVLO is disabled during this time

by injecting a current (64

mA) into the Fb. This generates

a voltage about 1.6V (64

mA325K) across the negative

input of E/A and positive input of the feedback UVLO

comparator (see Fig3).

Figure 3 - Soft-start circuit for IRU3138.

The magnitude of this current is inversely proportional to

the voltage at soft-start pin.

The 20

mA current source starts to charge up the exter-

nal capacitor. In the mean time, the soft-start voltage

ramps up, the current flowing into Fb pin starts to de-

crease linearly and so does the voltage at the positive

pin of feedback UVLO comparator and the voltage nega-

tive input of E/A.

When the soft-start capacitor is around 1V, the current

flowing into the Fb pin is approximately 32

mA. The volt-

age at the positive input of the E/A is approximately:

The E/A will start to operate and the output voltage starts

to increase. As the soft-start capacitor voltage contin-

ues to go up, the current flowing into the Fb pin will keep

decreasing. Because the voltage at pin of E/A is regu-

lated to reference voltage 0.8V, the voltage at the Fb is:

The feedback voltage increases linearly as the injecting

current goes down. The injecting current drops to zero

when soft-start voltage is around 2V and the output volt-

age goes into steady state.

As shown in Figure 4, the positive pin of feedback UVLO

comparator is always higher than 0.4V, therefore, feed-

back UVLO is not functional during soft-start.

Figure 4 - Theoretical operational waveforms

during soft-start.

mA325K = 0.8V

VFB = 0.8-25K

3(Injected Current)

20uA

64uA

Max

POR

Error Amp

64uA

325K=1.6V

When SS=0

POR

0.4V

Feeback

UVLO Comp

SS/SD

Comp

25K

0.8V

25K

HDrv

LDrv

Soft-Start

Voltage

Voltage at negative input

of Error Amp and Feedback

UVLO comparator

Voltage at Fb pin

Current flowing

into Fb pin

64uA

0uA

0.8V

≅1.6V

0.8V

≅2V

≅1V

Output of UVLO

POR

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