SD42509 Datasheet, PDF (4/7 Pages) Silan Microelectronics Joint-stock – 1A WHITE LED DRIVER

SD42509
COMPONENTS SELECTION
1． Input Capacitor Selection
The input capacitor provides the pulse current when the power MOSFET is on, and charge the capacitor when
the power MOSFET is off, thus to keep the stability of the input voltage. The input capacitor is recommended to
be more than 10μF, which can reduce the peak current drawn from input source and the switch noise. The input
capacitor should be near to the input pin when in real routing.
2． Output Capacitor Selection
Parallel connecting a capacitor between the two ends of LED can reduce the output voltage ripple accordingly
reduce the ripple current of LED, while this capacitor will not effect the operating frequency and efficiency, but the
start time will be longer by reducing the rising speed of the voltage on LED. The larger the output capacitor is, the
smaller the current ripple on LED is. It is recommend use the capacitor of 4.7μF or larger.
3． Inductance Selection
The inductance is used to keep the output current constant, the bigger the inductance is, the smaller the output
current ripple is; while the bigger the physical size is, the bigger the series-wound resistor is. The selected
effective current (RMS current rating)of inductance current should be bigger than the maximum output current,
and the saturation current should be 30% bigger than maximum output current. In order to improve the efficiency,
the series-wound resistor (DCR) of inductance should be smaller than 0.2Ω.
The relation between inductance and ripple:
L
=
V_OUT
*
V_IN
V_IN
− V_OUT
* f * ΔI
Where: L: Inductance value
F: Oscillator frequency
ΔI is ripple current
When select inductance, you should consider the combination of various factors to select suitable inductance.
When output current is 1A, 47μH inductance is recommended.
4． Diode Selection
SD42509 is a non-synchronous step-down adjuster, so the diode should provide continuous current when the
power MOSFET is off. Because the forward voltage of Schottky diode is small, and the reverse continuous
current time is short, so it is usually used for continuous current. During the power MOSFET is conducting, the
diode will withstand high voltage, so the reverse voltage of selected diode should be bigger than the input voltage.
The average current through the diode is I_D:
I_D = (1− D) * I_LED ILED is the current of LED
When the input voltage is high with a small duty, ID increases, so the selected maximum continuous current
diode should be bigger than the output current.
HANGZHOU SILAN MICROELECTRONICS CO.,LTD
Http://www.silan.com.cn
REV:1.4 2009.02.26
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