EAOUT

EANEG

1.5V

5k

REF

SOFT

START

SGND

PVIN

VDD

EN

PGND

SYNC/

MODE

FB

ZERO

CROSSING

DETECTOR

MOSFET

CONTROL

LOGIC

SHUTDOWN

CONTROL

OSCILLATOR

AND MODE

CONTROL

SW

PFM

COMP.

OVP

COMP.

PWM

COMP.

CURRENT

SENSE

6

LM2619

SNVS212B – NOVEMBER 2002 – REVISED MAY 2013

www.ti.com

The output voltage is regulated by modulating the PFET switch on time to control the average current sent to the

load. The effect is identical to sending a duty-cycle modulated rectangular wave formed by the switch and

synchronous rectifier at SW to a low-pass filter formed by the inductor and output filter capacitor. The output

voltage is equal to the average voltage at the SW pin.

Figure 21. Simplified Functional Diagram

PWM OPERATION

While in PWM (Pulse Width Modulation) mode, the output voltage is regulated by switching at a constant

frequency and then modulating 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 comparing

the signal from the current-sense amplifier with a slope compensated error signal from the voltage-feedback error

amplifier. At the beginning of each cycle, the clock 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 turns off the

PFET switch and turns on the NFET synchronous rectifier, ending the first part of the cycle. 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. This increases the average current sent to the output and

adjusts for the increase in the load.

Before going to the PWM comparator, the error 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. The minimum on time of the PFET

in PWM mode is about 200ns.

8

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