Datasheet

UVLO
V
IN
V
CC
SW Pin
Inductor
Current
SS Pin
t2
V
OUT
7.0V
2.5V
t1
LM34914
SNVS453B –MAY 2006–REVISED MARCH 2013 www.ti.com
Figure 8. Startup Sequence
Functional Description
The LM34914 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 thermally enhanced 3mm x 3mm WSON-
10 package. The regulator’s operation is based on a constant on-time control scheme where the on-time is
determined by V
IN
. 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
short circuited 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.The LM34914 can be applied in numerous applications to
efficiently 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 LM34914 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 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 forced by the LM34914. The buck switch is then turned on for another on-time
period.
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