Datasheet

LT1933
12
1933fe
APPLICATIONS INFORMATION
cases the discharged output capacitor will present a load
to the switcher which will allow it to start. The plots show
the worst-case situation where V
IN
is ramping very slowly.
For lower start-up voltage, the boost diode can be tied to
V
IN
; however, this restricts the input range to one-half of
the absolute maximum rating of the BOOST pin.
At light loads, the inductor current becomes discontinu-
ous and the effective duty cycle can be very high. This
reduces the minimum input voltage to approximately
300mV above V
OUT
. At higher load currents, the inductor
current is continuous and the duty cycle is limited by the
maximum duty cycle of the LT1933, requiring a higher
input voltage to maintain regulation.
Soft-Start
The SHDN pin can be used to soft-start the LT1933, reducing
the maximum input current during start up. The SHDN pin
is driven through an external RC fi lter to create a voltage
ramp at this pin. Figure 4 shows the start up waveforms
with and without the soft-start circuit. By choosing a large
RC time constant, the peak start up current can be reduced
to the current that is required to regulate the output, with
no overshoot. Choose the value of the resistor so that it
can supply 60µA when the SHDN pin reaches 2.3V.
Figure 4. To Soft-Start the LT1933, Add a Resistor and Capacitor to
the SHDN Pin. V
INI
= 12V, V
OUT
= 3.3V, C
OUT
= 22μF, R
LOAD
= 10Ω
RUN
5V/DIV
V
OUT
5V/DIV
I
IN
100mA/DIV
RUN
SHDN
GND
1933 F04a
50µs/DIV
RUN
5V/DIV
V
OUT
5V/DIV
I
IN
100mA/DIV
RUN
15k
0.1µF
SHDN
GND
1933 F04b
0.5ms/DIV
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