LM34919 datasheet(8/18 Pages) NSC

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FIGURE 1. Start Up Sequence

Functional Description

The LM34919 Step Down Switching Regulator features all the

functions needed to implement a low cost, efficient buck bias

power converter capable of supplying at least 0.6A to the load.

This high voltage regulator contains an N-Channel buck

switch, is easy to implement, and is available in micro SMD

package. The regulator’s operation is based on a constant on-

time control scheme, where the on-time is determined by

IN. This feature allows the operating frequency to remain

relatively constant with load and input voltage variations. The

feedback control requires no loop compensation resulting in

very fast load transient response. The valley current limit de-

tection circuit, internally set at 0.64A, holds the buck switch

off until the high current level subsides. This scheme protects

against excessively high current if the output is short-circuited

when V

IN is high.

The LM34919 can be applied in numerous applications to ef-

ficiently regulate down higher voltages. Additional features

include: Thermal shutdown, V

CC under-voltage lockout, gate

drive under-voltage lockout, and maximum duty cycle limiter.

Control Circuit Overview

The LM34919 buck DC-DC regulator employs a control

scheme based on a comparator and a one-shot on-timer, with

the output voltage feedback (FB) compared to an internal ref-

erence (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 until the FB voltage falls below

the reference but not less than the minimum off-time. The

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

cally, during start-up, or when the load current increases

suddenly, the off-times are at the minimum. Once regulation

is established, the off-times are longer.

When in regulation, the LM34919 operates in continuous con-

duction mode at heavy load currents and discontinuous con-

duction mode at light load currents. In continuous conduction

mode current always flows through the inductor, never reach-

ing zero during the off-time. In this mode the operating fre-

quency remains relatively constant with load and line

variations. The minimum load current for continuous conduc-

LM34919 Datasheet(PDF) 8 Page - National Semiconductor (TI)

LM34919 Datasheet(HTML) 8 Page - National Semiconductor (TI)

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