STLED25 Datasheet, PDF (20/30 Pages) STMicroelectronics – 5 channels step-up white LED driver

Application information
7
Application information
STLED25
7.1
7.1.1
External passive components
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
I_{PEAK −BOOST}
= ^{VOUT × IOUT}
η × VIN _MIN
+ ^{VIN MIN × (VOUT − VIN MIN )}
2 × V_OUT × fs × L
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
Table 7. List of recommended inductors
Manufacturer
Part number
Samsung
Murata
CIG21K4R7SCE
LQM21PN4R7
Inductance
[µH]
4.7
4.7
Saturation current [A]
0.6
0.6
Size [mm]
0805
0805
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.
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Doc ID 022323 Rev 3