LM3477AMM datasheet(10/23 Pages) NSC

10 / 23 page

Functional Block Diagram

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Functional Description

General Description

The LM3477/A is a switching regulator controller for topolo-

gies 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 pat-

ented adjustable slope compensation scheme to enable flex-

ible 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 opera-

tion. 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 VOLT-

AGE PROTECTION section).

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FIGURE 1. Operating Regions of the LM3477/A

LM3477AMM Datasheet(PDF) 10 Page - National Semiconductor (TI)