LT3502 LINER [Linear Technology], LT3502 Datasheet - Page 16

• Manufacturer Part Number: LT3502
• Manufacturer: LINER [Linear Technology]

LT3502/LT3502A
APPLICATIONS INFORMATION
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 fi lter 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
16
0.1μF
RUN
50k
Figure 7. To Soft Start the LT3502A, Add a Resistor and Capacitor to the SHDN Pin
RUN
SHDN
SHDN
GND
GND
3502 F07a
3502 F07b
500mA/DIV
500mA/DIV
10V/DIV
10V/DIV
2V/DIV
2V/DIV
V
V
V
V
OUT
OUT
SW
SW
I
I
L
L
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
fl oat and the SHDN pin is held high (either by a logic signal
or because it is tied to V
Figure 8a. The LT3502A Reduces its Frequency to Below 500kHz
to Protect Against Shorted Output with 40V Input
V
V
L = 6.8μH
C
V
V
L = 6.8μH
C
IN
OUT
OUT
IN
OUT
OUT
500mA/DIV
= 12V
= 12V
= 3.3V
= 10μF
= 3.3V
= 10μF
10V/DIV
V
SW
I
L
V
V
L = 6.8μH
C
IN
OUT
OUT
= 40V
= 0V
= 10μF
50μs/DIV
5μs/DIV
IN
2μs/DIV
), then the LT3502/LT3502A’s
3502 F07
IN
pin is allowed to
3502 F08a
3502fc