Ambient LED light circuit diagram driver

Download: Ambient Light Adaptive LED Driver Texas Instruments   http://www.ti.com/lit/ug/snvu113/snvu113.pdf

The ambient LED light circuit diagram adaptive LED driver automatically adjusts display brightness for changing ambient light conditions, providing a more viewable display with better contrast. The design drives five series connected LEDs from an input voltage of 4.5V to 12V. The LED current is nominally 100 mA and the output voltage is 15 to 20V.

The heart of the design is National’s PowerWise® LM3423 LED driver which has all the features needed to implement a boost converter with output current regulation and PWM dimming of the current. The LM3423 achieves output current regulation by sensing and appropriately adjusting the voltage across a resistor connected in series with the LED string.

Dimming or brightening the backlight of a display in response to ambient light condition can greatly increase the life of batteries in portable devices, and improves the visibility of the display under different viewing conditions. In this design a photodiode circuit is used to measure ambient light and provide a signal for dimming the LEDs. PWM dimming is the preferred method of dimming because it allows the light’s color temperature to remain constant irrespective of its intensity. PWM dimming is done by keeping the amplitude of the LED current constant, but periodically turning the LED driver on and off, thus varying the LEDs’ average current and light intensity. The switching is at done at a low frequency, but one which is high enough for the human eye to sense only changes in the light’s average intensity. Linear dimming, where the amplitude of the current in the LEDs is modulated, is not suitable because the LEDs’ light color temperature is sensitive to the current’s amplitude.

Features

■ 4.5-12V battery input voltage range

■ 100 mA Constant current LED at 15-20V

■ High switching frequency

■ High efficiency (85%)

■ PWM LED dimming up to 25kHz

■ LED brightness limited to less than 50% of maximum at

■ LED brightness proportional to ambient light