LM34923 Datasheet, PDF (12/26 Pages) National Semiconductor (TI) – EVAL evaluation board provides the design engineer with a fully

LM34923
SNVS695A – MARCH 2011 – REVISED FEBRUARY 2013
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C3 must be located as close as possible to the VCC and RTN pins. In applications with a relatively high input
voltage, power dissipation in the bias regulator is a concern. An auxiliary voltage of between 7.5V and 10V can
be diode connected to the VCC pin to shut off the V_CC regulator, thereby reducing internal power dissipation. The
current required into the VCC pin depends on the voltage applied to VCC and the switching frequency. See the
graph “I_CC vs. Externally Applied V_CC.” Internally a diode connects VCC to VIN requiring that the auxiliary voltage
be less than V_IN.
The turn-on sequence is shown in Figure 31. During the initial delay (t1) VCC ramps up at a rate determined by
its current limit and C3 while internal circuitry stabilizes. When V_CC reaches the upper threshold of its under-
voltage lock-out, the buck switch is enabled. The inductor current increases to the current limit threshold (I_LIM)
and during t2 V_OUT increases as the output capacitor charges up. When V_OUT reaches the intended voltage the
average inductor current decreases (t3) to the nominal load current (I_O).
V_IN
t1
UVLO
V_CC
Vin
SW Pin
0V
I _LIM
Inductor
Current
I_O
t2
t3
V_OUT
Figure 31. Startup Sequence
Regulation Comparator
The feedback voltage at FB is compared to an internal 2.5V reference. In normal operation (the output voltage is
regulated), an on-time period is initiated when the voltage at FB falls below 2.5V. The buck switch stays on for
the on-time, causing the FB voltage to rise above 2.5V. After the on-time period, the buck switch stays off until
the FB voltage again falls below 2.5V. During start-up, the FB voltage will be below 2.5V at the end of each on-
time, resulting in the minimum off-time of 260 ns.
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