Under-

Voltage

Lockout

7.2V

Bias Voltages

Switch

Driver

Cboot Voltage

Detect

R

S

Q

Switch

Logic

Thermal

Shutdown

_

+

1.26V

Reference

Soft-Start

Shutdown

Detect

Slope

Compensation

_

+

500kHz

Oscillator

_

+

_

+

Set / Blankout

One-Shot

Short-Circuit

Detect

PWM

Comparator

Error

Amplifier

Over-voltage

Detect

CB

DR

SW

GND

FB

COMP

LM3477

www.ti.com

SNVS141J – MAY 2004 – REVISED JULY 2004

Functional Block Diagram

Functional Description

GENERAL DESCRIPTION

The LM3477/A is a switching regulator controller for topologies incorporating a high side switch. The most

common of these topologies is the step-down, or buck, converter. Other topologies such as the inverting (buck-

boost) and inverse SEPIC (zeta) converters can be realized. This datasheet will focus on buck converter

applications.

The LM3477/A employs current mode control architecture. Among the many benefits of this architecture are

superior line and load regulation, cycle-by-cycle current limiting, and simple loop compensation. The LM3477/A

features a patented adjustable slope compensation scheme to enable flexible inductor selection. The LM3477/A

has a combination of features that allow its use in a wide variety of applications. The input voltage can range

from 2.97V to 35V, with the output voltage being positive or negative depending on the topology. The current

limit can be scaled to safely drive a wide range of loads. An internal soft-start is provided to limit initial in-rush

current. Output over voltage and input under voltage protection ensure safe operation of the LM3477/A.

REGIONS OF OPERATION

Pulse width modulation (PWM) is the normal mode of operation. In PWM, the output voltage is well regulated

and has a ripple frequency equal to the switching frequency (500kHz). In low load conditions, the part operates in

hysteretic mode. In this mode, the output voltage is regulated between a high and low value that results in a

higher ripple magnitude and lower ripple frequency than in PWM mode (see OVER VOLTAGE PROTECTION

section).

Copyright © 2004, Texas Instruments Incorporated

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