UVLO

SW Pin

Inductor

Current

SS Pin

t2

7.0V

2.5V

t1

LM34917A

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SNOSAX7C – DECEMBER 2007 – REVISED MAY 2008

Figure 3. Startup Sequence

Functional Description

The LM34917A Step Down Switching Regulator features all the functions needed to implement a low cost,

efficient buck bias power converter capable of supplying at least 1.25A to the load. This high voltage regulator

contains an N-Channel buck switch, is easy to implement, and is available in the micro SMD package. The

regulator’s operation is based on a constant on-time control scheme where the on-time is inversely proportional

to the input voltage. This feature results in the operating frequency remaining relatively constant with load and

input voltage variations. The feedback control scheme requires no loop compensation resulting in very fast load

transient response. The valley current limit scheme protects against excessively high currents if the output is

inductor the valley current limit threshold changes with input and output voltages, and the on-time is reduced by

when the input voltage exceeds 34.8V. The LM34917A can be applied in numerous applications to efficiently

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

Control Circuit Overview

The LM34917A 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 reference (2.5V). If the FB voltage is below

the reference the buck switch is switched on for a time period determined by the input voltage and a

reference, but for a time not less than the minimum off-time forced by the LM34917A. The buck switch is then

switched on for another on-time period.

Copyright © 2007–2008, Texas Instruments Incorporated

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