Datasheet

14
LT3503
3503f
APPLICATIO S I FOR ATIO
WUU
U
Figure 8. To Soft-Start the LT3503, Add a Resistor and Capacitor to the SHDN Pin. V
IN
= 12V, V
OUT
= 3.3V, C
OUT
= 10µF, R
LOAD
= 5Ω
Soft-Start
The SHDN pin can be used to soft-start the LT3503,
reducing the maximum input current during start-up. The
SHDN pin is driven through an external RC filter to create
a voltage ramp at this pin. Figure 8 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 20µA when the SHDN
pin reaches 2.3V.
Shorted and Reversed Input Protection
If the inductor is chosen so that it won’t saturate exces-
sively, an LT3503 buck regulator will tolerate a shorted
output. There is another situation to consider in systems
where the output will be held high when the input to the
LT3503 is absent. This may occur in battery charging
applications or in battery backup systems where a battery
or some other supply is diode OR-ed with the LT3503’s
output. If the V
IN
pin is allowed to float and the SHDN pin
is held high (either by a logic signal or because it is tied to
V
IN
), then the LT3503’s internal circuitry will pull its
quiescent current through its SW pin. This is fine if your
system can tolerate a few mA in this state. If you ground
the SHDN pin, the SW pin current will drop to essentially
zero. However, if the V
IN
pin is grounded while the output
is held high, then parasitic diodes inside the LT3503 can
pull large currents from the output through the SW pin and
the V
IN
pin. Figure 9 shows a circuit that will run only when
the input voltage is present and that protects against a
shorted or reversed input.
RUN
V
SW
5V/DIV
V
IN
= 12V
V
OUT
= 3.3V
L = 3.3µH
C
OUT
= 10µF
V
OUT
2V/DIV
10µs/DIV
I
L
1A/DIV
V
SW
5V/DIV
V
IN
= 12V
V
OUT
= 3.3V
L = 3.3µH
C
OUT
= 10µF
V
OUT
2V/DIV
10µs/DIV
I
L
1A/DIV
SHDN
GND
RUN
15k
0.068µF
SHDN
GND
3503 F08b
3503 F08a