White LEDs are widely used in LCD backlighting for mobile phones, PDAs and digital cameras. Torex has an application control solution for this type of LED – using a DC/DC converter or a charge pump system. This series of articles introduces the brightness control system necessary for LED backlighting and the use of white LEDs as a camera flash.
This application example uses the XC9103 series of step-up DC/DC converters. By changing the illumination time of white LEDs, the brightness can be controlled. The PWM signal input to the CE pin can be used to control the illumination time by increasing or decreasing the duty cycle of the PWM signal. The white LED can be brightened or darkened. The frequency of the PWM signal input to the CE pin is controllable between 60 Hz and 100 Hz.
Tr1 and a resistor can instantaneously increase the current (IF) through the white LED. This circuit is best suited for backlights or flashlights for cell phones and digital cameras. When TR1 is turned on, the current (IF) through the white LED is instantaneously increased to brighten the white LED. When TR1 is turned off, the white LED returns to the original brightness, and the brightness at this time is determined by the resistance of the feedback resistor 2.
Figure 1 circuit diagram
White LED forward current is
Note: The limit value of the forward pulse current can be found in the manufacturer's technical data sheet.
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