Datasheet

LT3502/LT3502A
15
3502fd
applications inForMation
Figure 6
(6a) LT3502A Typical Minimum Input Voltage, V
OUT
= 3.3V (6b) LT3502A Typical Minimum Input Voltage, V
OUT
= 5V
(6c) LT3502 Typical Minimum Input Voltage, V
OUT
= 3.3V (6d) LT3502 Typical Minimum Input Voltage, V
OUT
= 5V
Soft-Start
The SHDN pin can be used to soft start the LT3502/LT3502A,
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 7 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 80µA when the SHDN
pin reaches 2V.
Short and Reverse Protection
If the inductor is chosen so that it won’t saturate excessively,
the LT3502/LT3502A will tolerate a shorted output. When
operating in short-circuit condition, the LT3502/LT3502A
will reduce their frequency until the valley current is
650mA (Figure 8a). There is another situation to consider
in systems where the output will be held high when the
input to the LT3502/LT3502A 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 LT3502/LT3502A’s output. If the V
IN
pin is allowed to
float and the SHDN pin is held high (either by a logic signal
LOAD CURRENT (A)
0.001
7
6
5
4
3
2
1
0
3502 G19
0.01
0.1 1
V
IN
(V)
RUN
START
LOAD CURRENT (A)
0.001
4
V
IN
(V)
6
8
0.01 0.1 1
3502 G20
2
3
5
7
1
0
START
RUN
LOAD CURRENT (A)
0.001
7
6
5
4
3
2
1
0
3502 G21
0.01
0.1 1
V
IN
(V)
RUN
START
LOAD CURRENT (A)
0.001
4
V
IN
(V)
6
8
0.01 0.1 1
3502 G22
2
3
5
7
1
0
START
RUN