SC4539ULTRT Semtech, SC4539ULTRT Datasheet - Page 11

• Manufacturer Part Number: SC4539ULTRT
• Description: TVS
• Manufacturer: Semtech

Specifications of SC4539ULTRT

• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

Applications Information (continued)
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 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.
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
The power switch of the boost is protected by an internal
current limit function. The switch is opened when the
current exceeds the maximum current switch value.
Inductor Selection
The inductor value should be within the range of 4.7μH to
22μH. The DCR needs to be considered when picking the
inductor to ensure optimum effi ciency. The largest induc-
tor package that can be tolerated 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
where
I
D
L(Peak)
I
L
1
L
V
V
I
IN
IN
V
OUT
f
IN
osc
D
2
I
L
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.
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 capacitors that can be used.
Coilcraft
LPS4018-223ML
Murata
LQH43CN150K03
Murata
LQH32CN150K53
Input Capacitor
Murata
GRM188C70J225KE20
Output Capacitor
Murata
GRM21BR71H105KA12L
Compensation Capacitor
Taiyo Yuden
EMK105BJ473KV-F
Part Number
Part Number
I
IN
V
OUT
Table 2 -- Recommended Capacitors
Table 1 -- Recommended Inductors
V
Value
I
IN
(μH)
OUT
22
15
15
0.360
0.320
0.580
Value
0.047
DCR
(Ω)
(μF)
2.2
1.0
Current (A)
Rated
0.570
0.300
Voltage (V)
0.70
Rated
6.3
50
16
Toler-
±20%
±10%
±10%
ance
Type
X7R
X7R
X7S
SC4539
3.2 x 2.5 x 1.55
3.9 x 3.9 x 1.7
4.5 x 3.2 x 2.6
Dimensions
(L x W x H)
(mm)
Case Size
0603
0805
0402
11