LT3572

7

3572fa

OPERATION

Switching Regulator

The LT3572 uses a constant frequency, current mode,

control scheme to provide excellent line and load regulation

for the output drivers. Operation can be best understood

by referring to the Block Diagram in Figure 1. A pulse

from the oscillator sets the RS flip-flop, A4, and turns on

the internal NPN bipolar power switch, Q1. Current in Q1

and the external inductor, L1, begins to increase. When

this current exceeds a level determined by the voltage at

the output of the error amplifier A1, comparator A2 resets

A4, turning Q1 off. The current in L1 flows through the

external Schottky diode D1 and begins to decrease. The

cycle begins again at the next pulse from the oscillator.

In this way, the voltage at the output of the error amplifier

controls the current through the indictor to the output. The

soft-start capacitor, C2, clamps the output of the error

amplifier causing the current limit to slowly increase. This

helps reduce overshoot on the output and helps minimize

inrush current on the input.

Output Drivers

The function of the driver section is to level shift the

drivers operate in an H-bridge fashion, where the OUTA

and OUTB pins are the same polarity as the PWMA and

PWMB pins respectively and the OUTA and OUTB are

inverted from PWMA and PWMB respectively. The OUT

pins will be high impedance until the FB pin is within

95% of its regulated voltage. The OUT pins will follow

PWMA and PWMB as long as FB stays within 85% of the

regulated voltage. If FB drops below 85%, the OUT pins

will go high impedance.