LM25010 datasheet(8/17 Pages) NSC

8 / 17 page

Functional Description

The LM25010 Step-Down Switching Regulator features all

the functions needed to implement a low cost, efficient buck

DC-DC converter capable of supplying in excess of 1A to the

load. This high voltage regulator integrates an N-Channel

buck switch, with an easy to implement constant on-time

controller. It is available in the thermally enhanced LLP-10

and TSSOP-14EP packages. The regulator compares the

feedback voltage to a 2.5V reference to control the buck

switch, and provides a switch on-time which varies inversely

with VIN. This feature results in the operating frequency

remaining relatively constant with load and input voltage

variations. The switching frequency can range from less than

100 kHz to 1.0 MHz. The regulator requires no loop com-

pensation resulting in very fast load transient response. The

valley current limit circuit holds the buck switch off until the

free-wheeling inductor current falls below the current limit

threshold, nominally set at 1.25A.

The LM25010 can be applied in numerous applications to

efficiently step-down higher DC voltages. Features include:

Thermal shutdown, VCC under-voltage lock-out, gate drive

under-voltage lock-out, and maximum duty cycle limit.

Control Circuit Overview

The LM25010 employs a control scheme based on a com-

parator and a one-shot on-timer, with the output voltage

feedback (FB) compared to an internal reference (2.5V). If

the FB voltage is below the reference the buck switch is

turned on for a time period determined by the input voltage

and a programming resistor (R

ON). Following the on-time the

switch remains off for a fixed 260 ns off-time, or until the FB

voltage falls below the reference, whichever is longer. The

buck switch then turns on for another on-time period. Refer-

ring to the Block Diagram, the output voltage is set by R1

and R2. The regulated output voltage is calculated as fol-

lows:

OUT = 2.5V x (R1 + R2) / R2

(1)

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FIGURE 1. Startup Sequence

LM25010 Datasheet(PDF) 8 Page - National Semiconductor (TI)