30004434

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

V

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

The LM34919 can be applied in numerous applications to ef-

ficiently regulate down higher voltages. Additional features

include: Thermal shutdown, V

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-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-

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