6

LT1935

1935f

Soft Start

The SHDN pin can be used to soft start the LT1935,

reducing the maximum input current during start up. The

SHDN pin is driven through an external RC filter to create

a ramp at this pin. Figure 6 shows the start-up waveforms

with and without the soft start circuit. Without soft start,

the input current peaks at ~3A. With soft start, the peak

current is reduced to 1A. 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 100µA when the SHDN pin reaches 1.8V.

VOUT

100mV/DIV

ILOAD

200mA/DIV

50µs/DIV

1935 F05

84.5k

10k

FB

68pF

OUT

20µF

Figure 5. Transient Response with a 68pF Phase-Lead Capacitor

Figure 7. Suggested Layout

R1

R2

GND

C3

C2

L1

D1

C1

SHDN

1935 F03

+

RUN

5V/DIV

VOUT

2V/DIV

IIN

1A/DIV

20µs/DIV

1935 F06a

RUN

5V/DIV

VOUT

2V/DIV

IIN

1A/DIV

200µs/DIV

1935 F06b

10k

RUN

RUN

0.22µF

SHDN

GND

SHDN

GND

Figure 6. Adding a Resistor and Capacitor to the SHDN Pin

Layout Hints

The high speed operation of the LT1935 demands careful

attention to board layout. You will not get advertised

performance with careless layout. Figure 7 shows the

recommended component placement. Make the ground

pin copper area large. This helps to lower the die

temperature.