SM8122A Datasheet, PDF (7/13 Pages) Nippon Precision Circuits Inc – White LED Driver IC

SM8122A
Selecting the Inductor (L)
The inductor DC resistance affects the power efficiency, therefore a low DC resistance inductor is recom-
mended. Note also that the peak inductor current I_peak should not exceed the inductor maximum current rating.
In pulsed current mode control, the peak inductor current I_peak is given by the following equation.
I_peak = (V_IN × T_ON) / L
For example, if the input voltage V_IN is 3.6V, the inductance L is 4.7µH, and the SW-Tr ON time T_ON is 2MHz
× 85% = 0.425µs, then the peak inductor current I_peak is (3.6 × 0.425 × 10^-6) / (4.7 × 10^-6) = 0.326A = 326mA.
Selecting the Capacitors (C_IN, C_OUT)
The recommended capacitances for use with the SM8122A are 4.7µF ceramic input capacitor C_IN and 1.0µF
ceramic output capacitor C_OUT. The capacitor ESR ratings affect the ripple voltage, therefore capacitors with
low ESR rating are recommended. The input capacitor should be mounted close to the SM8122A IC. Note that
the capacitor voltage ratings should be selected to provide sufficient margin for the applied input and output
voltages.
For example, if a lithium-ion battery (2.5 to 4.5V) is connected to the input and 3 white LEDs connected in
series at the output draw 20mA, then the maximum input voltage is 4.5V and the maximum output voltage is
(4.0V × 3 LEDs) + 0.5V = 12.5V. Therefore, the input capacitor should have a voltage rating of 6V, and the
output capacitor should have a voltage rating of 16V.
Selecting the Rectifier Schottky Barrier Diode (SBD)
The rectifier schottky barrier diode forward-direction voltage drop affects the power efficiency, therefore a
Schottky barrier diode with low forward-direction voltage drop is recommended. Note that the diode should be
selected to provide sufficient margin for the rated current and reverse-direction withstand voltage.
Board Layout Notes
The following precautions should be followed for stable device operation.
I The inductor L and Schottky barrier diode SBD should be connected close to the pin SW using thick, short
circuit wiring.
I The input capacitor C_IN should be mounted close to the IC.
I The IC supply voltage V_DD wiring and inductor supply wiring should be isolated, reducing any common
impedances.
I The ground wiring should be connected at a single point, reducing any common impedances.
SBD
L
SW
VOUT
LED
COUT
VIN
VDD
CIN
CE
FB
R1
VSS
NIPPON PRECISION CIRCUITS INC.—7