LM34917A Datasheet, PDF (8/16 Pages) National Semiconductor (TI) – Ultra Small 33V, 1.25A Constant On-Time Buck Switching Regulator with Intelligent Current Limit

FIGURE 1. Startup Sequence
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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 con-
stant 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 tran-
sient response. The valley current limit scheme protects
against excessively high currents if the output is short circuit-
ed when V_IN is high. To aid in controlling excessive switch
current due to a possible saturating inductor the valley current
limit threshold changes with input and output voltages, and
the on-time is reduced by approximately 50% when current
limit is detected. An over-voltage detection at V_IN stops the
circuit's switching when the input voltage exceeds 34.8V. The
LM34917A can be applied in numerous applications to effi-
ciently regulate down higher voltages. Additional features
include: Thermal shutdown, V_CC under-voltage lock-out, gate
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 ref-
erence (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 programming resistor (R_ON). Following
the on-time the switch remains off until the FB voltage falls
below the 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.
When in regulation, the LM34917A operates in continuous
conduction mode at heavy load currents and discontinuous
conduction mode at light load currents. In continuous con-
duction mode the inductor’s current is always greater than
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