LT3502/LT3502A

16

3502fc

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 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

float and the SHDN pin is held high (either by a logic signal

Figure 7. To Soft Start the LT3502A, Add a Resistor and Capacitor to the SHDN Pin

Figure 8a. The LT3502A Reduces its Frequency to Below 500kHz

to Protect Against Shorted Output with 40V Input

10V/DIV

2μs/DIV

L = 6.8μH

3502 F08a

500mA/DIV

RUN

10V/DIV

L = 6.8μH

2V/DIV

5μs/DIV

500mA/DIV

10V/DIV

L = 6.8μH

2V/DIV

50μs/DIV

3502 F07

500mA/DIV

SHDN

GND

3502 F07a

RUN

50k

0.1μF

SHDN

GND

3502 F07b