TIME (200 ns/DIV)

200 mA/DIV

2V/DIV

10 mV/DIV

AC Coupled

L

L

LM3674

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SNVS405E – DECEMBER 2005 – REVISED SEPTEMBER 2006

The part uses an internal reference voltage of 0.5V. It is recommended to keep the part in shutdown until the

input voltage is 2.7V or higher.

CIRCUIT OPERATION

During the first portion of each switching cycle, the control block in the LM3674 turns on the internal PFET

switch. This allows current to flow from the input through the inductor to the output filter capacitor and load. The

inductor limits the current to a ramp with a slope of

(1)

by storing energy in a magnetic field. During the second portion of each cycle, the controller turns the PFET

switch off, blocking current flow from the input, and then turns the NFET synchronous rectifier on. The inductor

draws current from ground through the NFET to the output filter capacitor and load, which ramps the inductor

current down with a slope of

(2)

The output filter stores charge when the inductor current is high, and releases it when the inductor current is low,

smoothing the voltage across the load.

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 the SW pin 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.

PWM OPERATION

During PWM ( Pulse Width Modulation) operation the converter operates as a voltage-mode controller with input

voltage feed forward. This allows the converter to achieve excellent load and line regulation. The DC gain of the

power stage is proportional to the input voltage. To eliminate this dependence, feed forward inversely

proportional to the input voltage is introduced.

While in PWM 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. At the beginning of each clock cycle the PFET switch is turned

on and the inductor current ramps up until the comparator trips and the control logic turns off the switch.

The current limit comparator can also turn off the switch in case the current limit of the PFET is exceeded. Then

the NFET switch is turned on and the inductor current ramps down. The next cycle is initiated by the clock

turning off the NFET and turning on the PFET.

Internal Synchronous Rectification

While in PWM mode, the LM3674 uses an internal NFET as a synchronous rectifier to reduce rectifier forward

voltage drop and associated power loss. Synchronous rectification provides a significant improvement in

efficiency whenever the output voltage is relatively low compared to the voltage drop across an ordinary rectifier

diode.

Copyright © 2005–2006, Texas Instruments Incorporated

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Product Folder Links: LM3674