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AIC - Applications for White LED Driver

Numéro de référence AN027
Description Applications for White LED Driver
Fabricant AIC 
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AN027 fiche technique
AN027
Applications for White LED Driver in Parallel vs. Series
Introduction
For the past few years there has been a huge
revolution in color LCD displayers for hand-held
devices, such as cellular phones, PDAs and DSCs.
White Light Emitting Diode (LED) Module, a new
technology for LCD displayers, plays an important role
in the applications powered by a single-cell Li-ion
battery. Due to the operating voltage limit (3.3V~4.2V)
of single-cell Li-ion battery, it cannot drive white LEDs,
which require a forward voltage at 3.6V.
Therefore, we need a boost circuit, which is powered
by a single-cell Li-ion battery, for white LEDs.
Generally in mobile phone applications, white LEDs
are usually used in two modelsseries and parallel.
This application note points out several differences
between these two models, and then gives
corresponding solutions.
The Model of White LEDs Driver
Parallel ModelCharge Pump Converter
For driving LEDs, fig.1 shows a parallel model
solution by using AIC1845 as a charge pump
converter, which needs only a few components to
achieve a regulated voltage. Several advantage and
disadvantage characteristics of this solution are as
follows:
Advantage
(1) Three ceramic capacitors are employed to
regulate voltage for driving LEDs with low profile
package, which is suitable for miniature
equipments.
(2) No coil, which may result in noise, is needed to
boost voltage for the charge pump converter. Due
to the advantage, EMI may not happen to mobile
phone application.
Disadvantage
(1) Disproportionate brightness may result from
different forward voltage of each LED, even
though constant voltage provided.
(2) Compare to solutions in serial with same input
voltage, parallel solution has relatively low
efficiency because of the regulation of output.
1 -Cell
Li-ion
Battery
CIN
2.2µF
1 VOUT
2
GND
3 SHDN
R1 680
6
C+
5
VIN
4
C-
COUT
2.2µF
CFLY
0.22µF
5VOUT
LED1 LED2 LED3 LED4
AIC1845
Fig.1 Parallel White LEDs Driver
October, 2003
1

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