sc4540 Semtech Corporation, sc4540 Datasheet - Page 11

• Manufacturer Part Number: sc4540
• Description: High Ei Ciency Boost Converter For Up To 10 Leds
• Manufacturer: Semtech Corporation

Applications Information (continued)
must withstand high voltage for an extended period of
time.
Soft-Start
The soft-start mode reduces in-rush current by utilizing
the external compensation network. As the error amplifi er
slowly charges the COMP node voltage, the duty cycle of
the boost switch ramps from 0% to its fi nal value when 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
power is cycled on the part.
Thermal Shutdown
A thermal shutdown mode 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
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 22μH to
68μH. The DCR needs to be considered when selecting
the inductor to ensure optimum effi ciency. The largest
inductor package that can be accommodated in the circuit
area should be used since the DCR generally decreases
with increasing package size.
The saturation current of the inductor should be much
higher than the peak current of the internal boost switch
to ensure that the inductor never enters saturation during
normal operation of the part. The equation to calculate
the peak inductor current is:
I
L(Peak)
I
IN
2
I
L
where
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 SC4540.
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 recommended capacitors.
Coilcraft
LPS6235
Coilcraft
LPS4018-223ML
Input Capacitor
Murata
GRM219R61E225KA12
Output Capacitor
Murata
GRM21BR71H105KA12L
Compensation Capacitor
Taiyo Yuden
EMK105BJ473KV-F
Part Number
Part Number
I
D
IN
I
L
1
V
Table 2 — Recommended Capacitors
L
Table 1 — Recommended Inductors
V
OUT
V
IN
V
OUT
IN
f
V
osc
Value
D
I
(μH)
IN
OUT
56
22
0.360
Value
0.047
DCR
0.28
(Ω)
(μF)
2.2
1.0
Current (A)
Rated
Voltage (V)
0.70
1.1
Rated
25
50
16
Toler-
±20%
±20%
ance
Type
X5R
X7R
X7R
SC4540
6.2 x 6.2 x 3.5
3.9 x 3.9 x 1.7
Dimensions
(L x W x H)
(mm)
Case Size
0805
0805
0402
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