LT1935 데이터 시트보기 (PDF) - Linear Technology
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LT1935 Datasheet PDF : 8 Pages
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LT1935
APPLICATIONS INFORMATION
VOUT
100mV/DIV
ILOAD
200mA/DIV
50µs/DIV
OUT
84.5k
68pF
1935 F05
FB
10k
20µF
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.
Figure 5. Transient Response with a 68pF Phase-Lead Capacitor
RUN
5V/DIV
VOUT
2V/DIV
IIN
1A/DIV
20µs/DIV
RUN SHDN
GND
1935 F06a
RUN
5V/DIV
VOUT
2V/DIV
IIN
1A/DIV
10k
RUN
0.22µF
200µs/DIV
SHDN
GND
1935 F06b
Figure 6. Adding a Resistor and Capacitor to the SHDN Pin
Reduces the Peak Input Current During Start-Up. VIN = 3.3V,
VOUT = 5V, C2 = 20µF, Output Load = 10Ω.
Layout Hints
L1
D1
C1
The high speed operation of the LT1935 demands careful
attention to board layout. You will not get advertised
VOUT
VIN
performance with careless layout. Figure 7 shows the
+
recommended component placement. Make the ground
C2
SHDN
pin copper area large. This helps to lower the die
R2
temperature.
GND
R1
C3
1935 F03
Figure 7. Suggested Layout
1935f
6
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