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Maxim > Design Support > Technical Documents > Application Notes > Analog Switches and Multiplexers > APP 3624
Keywords: mobile phones, clamshell phones, white LEDs, keyboard illumination
APPLICATION NOTE 3624
Driving White LEDs in Clamshell Phones
Sep 23, 2005
Abstract: A switching-converter IC—designed as a white-LED driver—and an analog switch control three
forms of LED illumination in a mobile (clamshell) phone.
This circuit multiplexes the drive to three sets of LEDs found inside clamshell phones. These phones
usually have an external display that is illuminated when an incoming telephone call occurs. Once the
clamshell is opened, the external display illumination is switched off and the main internal display and
keyboard illumination are activated.
The circuit in Figure 1 is based around the MAX1848 (IC1) white LED driver. This provides a regulated
current through a chain of LEDs via a current-sensing resistor, R
SENSE
. The threshold of the current-
sense circuit is equal to 0.075 x V
CTRL
, the voltage on the CTRL pin, so the current through the LED
chain can be dynamically changed by varying the voltage on the CTRL pin. Multiple chains of LEDs can
be stacked in parallel and the respective current through the chains can be altered by varying the
resistor value in each chain.
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