Functional Description

The LM5010A 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 80V

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 com-

pares 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

compensation 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 LM5010A can be applied in numerous applications to

efficiently step down higher DC voltages. This regulator is

well suited for 48V telecom applications, as well as the 42V

automotive power bus. Features include: Thermal shutdown,

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

out, and maximum duty cycle limit.

Control Circuit Overview

The LM5010A 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

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:

V

(1)

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FIGURE 1. Startup Sequence

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