Datasheet

6
LT1935
1935f
APPLICATIONS INFORMATION
WUU
U
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.
V
OUT
100mV/DIV
I
LOAD
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
V
OUT
V
IN
SHDN
1935 F03
+
+
RUN
5V/DIV
V
OUT
2V/DIV
I
IN
1A/DIV
20µs/DIV
1935 F06a
RUN
5V/DIV
V
OUT
2V/DIV
I
IN
1A/DIV
200µs/DIV
1935 F06b
10k
RUNRUN
0.22µF
SHDN
GND
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.
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.
Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.