LT1935
APPLICATIONS INFORMATION
Soft Start
VOUT
100mV/DIV
ILOAD
200mA/DIV
50µs/DIV
OUT
84.5k
FB
10k
68pF
20µF
1935 F05
Figure 5. Transient Response with a 68pF Phase-Lead Capacitor
RUN
5V/DIV
VOUT
2V/DIV
IIN
1A/DIV
20µs/DIV
RUN
SHDN
GND
Figure 6. Adding a Resistor and Capacitor to the SHDN Pin
Reduces the Peak Input Current During Start-Up. V
IN
= 3.3V,
V
OUT
= 5V, C2 = 20µF, Output Load = 10Ω.
D1
V
OUT
L1
C1
+
C2
R2
SHDN
GND
R1
C3
1935 F03
Figure 7. Suggested Layout
1935f
6
U
W
U
U
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.
RUN
5V/DIV
VOUT
2V/DIV
IIN
1A/DIV
1935 F06a
200µs/DIV
1935 F06b
10k
RUN
0.22µF
SHDN
GND
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.
+
V
IN
LINEAR [ LINEAR INTEGRATED SYSTEMS ]
