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ThinSOT Switching Regulator Controls Inrush Current
Design Note 365
Jesus Rosales
Introduction
Inrush current can be troublesome, especially when
regulators are operated from batteries or a currentlimited source. In such cases, excessive inrush current
can cause the input source to collapse and prevent the
converter from ever starting up. Inrush current can also
trip input line fuses or cause the output to overshoot.
There are many ways to deal with this problem, but most
of them are difficult to implement or have undesirable
side effects, often making the cure worse than the cause.
IIN
0.2A/DIV
VOUT
5V/DIV
50μs/DIV
Figure 2. Input Current and Output Voltage at Turn-On
Without CSS Capacitor for Figure 1 Circuit
A Simple Solution
Figure 1 shows a solution to this problem using the
LT®3467 in a 10V-16V input to 12V/300mA output SEPIC
converter. The LT3467 ThinSOT™ switching regulator
integrates a soft-start function with a built-in 42V,
1.1A switch. Figures 2 and 3 illustrate the effect that
the LT3467’s soft-start feature has on inrush currents.
IIN
0.2A/DIV
VOUT
5V/DIV
2ms/DIV
Figure 2 shows the start-up waveforms for the Figure 1
circuit with the SS feature disabled (by removing CSS
from the SS pin). The inrush current in this case is
limited only by the maximum duty cycle or maximum
switch current of the LT3467. The top trace shows input
current and the bottom trace shows output voltage. It
takes only about 100μs for the output to go from 0V
to 12V, requiring an input current of about 0.85A peak,
thus exceeding the nominal steady-state level.
VIN
10V TO 16V
Figure 3 shows the same waveforms with a 0.1μF
capacitor connected to the SS pin. Now, both input
current and output voltage rampup in a controlled
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16V
1μF
B0530W
t
CSS
0.1μF
6
1
VIN
SW
4
SHDN
LT3467
3
5
FB
SS
GND
VOUT
12V
300mA
115k
10pF
t
L1B
10μH
CLS62-100
13.3k
2
GND
DN365 F01
Figure 1. 10VIN-16VIN to 12V at 300mA SEPIC Converter
06/05/365_conv
DN365 F03
Figure 3. Input Current and Output Voltage at Turn-On
with CSS Capacitor for Figure 1 Circuit
L1
10μH
CLS62-100
2.2μF
25V
DN365 F02
4.7μF
16V
fashion and settle comfortably into their respective
steady-state levels. By using only one small capacitor
between the SS pin and ground, inrush current has
been eliminated. Figure 4 shows an efficiency curve
for the SEPIC converter of Figure 1.
Figure 5 shows a boost converter that provides 12V at
300mA from a 5V input source. Figures 6 and 7 show
the input and output currents, with and without the
soft-start feature enabled. Figure 6 shows that without
soft-start, even though the output voltage follows the
input by conducting through the inductor and diode,
there is an inrush of current which exceeds 2A at the
point where the converter begins to switch. Figure 7
shows the improved start-up results with soft-start
enabled. Figure 8 shows the efficiency of the circuit.
IIN
0.5A/DIV
VOUT
5V/DIV
50μs/DIV
Figure 6. Input Current and Output Voltage at Turn-On
Without CSS Capacitor for Figure 5 Circuit
IIN
0.5A/DIV
VOUT
5V/DIV
82
2ms/DIV
81
EFFICIENCY (%)
DN365 F06
DN365 F07
Figure 7. Input Current and Output Voltage at Turn-On
with CSS Capacitor for Figure 5 Circuit
80
90
79
89
88
77
100
50
200
250
150
LOAD CURRENT (mA)
EFFICIENCY (%)
78
300
87
86
85
DN365 F04
84
Figure 4. SEPIC Converter Efficiency for Figure 1 Circuit
83
82
10μH
CR43-100 B0530W
VIN
5V
2.2μF
4
5
6
1
VIN
SW
SHDN
LT3467
SS
CSS
0.1μF
GND
VOUT
12V
300mA
3
4.7μF
16V
13.3k
2
DN365 F07
Figure 5. 5VIN to 12V at 300mA BOOST Converter
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100
150
250
200
LOAD CURRENT (mA)
300
DN365 F08
Figure 8. BOOST Converter Efficiency
for Figure 5 Circuit
115k
10pF
FB
50
Conclusion
Countless hours can be spent looking for ways to control
inrush current or just getting the output to come up. The
LT3467 provides a simple solution. The addition of a
single, tiny, low cost capacitor can make the difference
between a smooth design cycle or a laborious one.
For applications help,
call (408) 432-1900
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