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SC4539
Applications Information (continued)
must endure high voltage for an extended period of
time.
D
1
V IN
V OUT
Soft-Start
The soft-start mode reduces in-rush current by utilizing
I IN
V OUT I OUT
V IN
the external compensation network. As the error amplifier
slowly charges the COMP node voltage, the duty cycle of
the boost switch ramps from 0% to its final value once in
regulation. The gradual increase of the duty cycle slowly
charges the output capacitor and limits in-rush current
during start up. Soft-start is implemented only when the
input power is cycled.
D is the duty cycle for continuous operation. Effi ciency (η)
can be approximated by using the curves provided in the
Typical Characteristics section. Table 1 lists inductors that
have been proven to work with SC4539.
Table 1 -- Recommended Inductors
Thermal Shutdown
A thermal shutdown system is included for protection in
the event the junction temperature exceeds 155°C. In
thermal shutdown, the on-chip power switch is disabled.
Switching and sinking resumes when the temperature
drops by 20°C.
Current Limit
Part Number
Coilcraft
LPS4018-223ML
Murata
LQH43CN150K03
Murata
LQH32CN150K53
Value
(μH)
22
15
15
DCR
(Ω)
0.360
0.320
0.580
Rated
Current (A)
0.70
0.570
0.300
Toler-
ance
±20%
±10%
±10%
Dimensions
(L x W x H)
(mm)
3.9 x 3.9 x 1.7
4.5 x 3.2 x 2.6
3.2 x 2.5 x 1.55
The power switch of the boost converter is protected by an
internal current limit function. The switch is opened when
the current exceeds the maximum switch current value.
Inductor Selection
The inductor value should be within the range of 4.7μH to
22μH. The DCR needs to be considered when selecting
the inductor to ensure optimum effi ciency. The largest
inductor package that can be tolerated in the circuit area
should be used since the DCR generally decreases with
Capacitor Selection
The input capacitor should be at least 2.2μF. A larger
capacitor will reduce the voltage ripple on the input. The
output capacitor values can range from 0.22μF to 1μF. The
compensation capacitor value should be 47nF. Capacitors
of X5R type material or better can be used for any of the
capacitors. See Table 2 for the recommended capacitors.
Table 2 -- Recommended Capacitors
increasing package size.
Part Number
Value
(μF)
Rated
Voltage (V)
Type
Case Size
The saturation current of the inductor should be much
Input Capacitor
higher than the peak current of the internal boost switch
to ensure that the inductor never enters saturation during
Murata
GRM188C70J225KE20
2.2
6.3
X7S
0603
normal operation of the part. The equation to calculate
the peak inductor current is
Output Capacitor
I L(Peak)
I IN
I L
2
Murata
GRM21BR71H105KA12L
Compensation Capacitor
Taiyo Yuden
EMK105BJ473KV-F
1.0
0.047
50
16
X7R
X7R
0805
0402
where
I L
V IN D
L f osc
11
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