Datasheet

Application information STLED25
20/30 Doc ID 022323 Rev 3
7 Application information
7.1 External passive components
7.1.1 Inductor selection
The inductor is the key passive component for switching converters.
For the STLED25, an inductance of 4.7 µH is recommended. It is very important to select
the right inductor according to the maximum current the inductor can handle to avoid
saturation. The peak current for the step-up can be calculated with the following formula:
Equation 2
where:
V
OUT
: step-up output voltage. The boost control loop keeps it 135 mV higher than the
maximum among VD1-D5
I
OUT
: total output current
V
IN
: input voltage of STLED25
fs: switching frequency. Use the minimum value of 2.25 MHz for the worst case
η: efficiency of step-up converter. It is 0.87 at 75 mA load
7.1.2 Input and output capacitor selection
It is recommended to use ceramic capacitors with low ESR as input and output capacitors in
order to filter any disturbance present in the input line and to maintain stable operation of the
step-up converter. A minimum real capacitance value of 1 µF must be guaranteed for C
IN
and 4.2 µF C
OUT
in all conditions.
LfsV2
)VINV(VIN
VIN
IV
I
OUT
MINOUTMIN
MIN
OUTOUT
BOOSTPEAK
×××
−×
+
×η
×
=
−
Table 7. List of recommended inductors
Manufacturer Part number
Inductance
[µH]
Saturation current [A] Size [mm]
Samsung CIG21K4R7SCE 4.7 0.6 0805
Murata LQM21PN4R7 4.7 0.6 0805