LM2612 datasheet(13/19 Pages) NSC

13 / 19 page

Circuit Operation (Continued)

PWM Operation

The LM2612 can be set to current-mode PWM operation by

connecting the SYNC/MODE pin to VDD. While in PWM

(Pulse Width Modulation) mode, the output voltage is regu-

lated by switching at a constant frequency and then modu-

lating the energy per cycle to control power to the load.

Energy per cycle is set by modulating the PFET switch

on-time pulse-width to control the peak inductor current. This

is done by controlling the PFET switch using a flip-flop driven

by an oscillator and a comparator that compares a ramp

from the current-sense amplifier with an error signal from a

voltage-feedback error amplifier. At the beginning of each

cycle, the oscillator sets the flip-flop and turns on the PFET

switch, causing the inductor current to ramp up. When the

current sense signal ramps past the error amplifier signal,

the PWM comparator resets the flip-flop and turns off the

PFET switch, ending the first part of the cycle. The NFET

synchronous rectifier turns on until the next clock pulse or

the inductor current ramps to zero. If an increase in load

pulls the output voltage down, the error amplifier output

increases, which allows the inductor current to ramp higher

before the comparator turns off the PFET switch. This in-

creases the average current sent to the output and adjusts

for the increase in the load.

Before going to the PWM comparator, the current sense

signal is summed with a slope compensation ramp from the

oscillator for stability of the current feedback loop. During the

second part of the cycle, a zero crossing detector turns off

the NFET synchronous rectifier if the inductor current ramps

to zero.

20007101

FIGURE 2. Simplified Functional Diagram

LM2612 Folha de dados(PDF) 13 Page - National Semiconductor (TI)