Ambient Light Adaptive LED Driver

Automatically adjust display brightness for changing ambient light conditions to provide a more viewable display with better contrast. The LED driver’s PWM dimming dims or brightens the display backlight in response to ambient light conditions, increasing battery life. A photodiode circuit measures ambient light and provides a signal for dimming the LEDs.

Source: National Semiconductor
11/10/2008

The ambient light adaptive LED driver automatically adjusts display brightness for changing ambient light conditions, providing a more viewable display with better contrast. The design drives three series-connected LEDs from an input voltage of 4.5V to 5.5V. The LED current is nominally 100 mA and the output voltage is 10V to 15V (the forward drop of 3 LEDs).

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 pulse-width modulation (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. Both high-side and low-side dimming can be implemented with this design. For high-side dimming, a MOSFET is placed between the positive output of the boost converter and the anode of the top-most LED of the string. For low-side dimming, the MOSFET is placed between ground and the cathode of the bottom LED of the 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 improve 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 allows the light’s color temperature to remain constant irrespective of its intensity. PWM dimming keeps the amplitude of the LED current constant, but periodically turns the LEDs on and off using the series-connected MOSFET, thus varying the LEDs’ average current and light intensity. The switching is at done at a low frequency, but at a level 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.

The dimming range of the average LED current is from 20 percent to 100 percent of its nominal value as ambient light conditions increase from dark to bright. To reduce power consumption and prolong battery life, the output current is clamped to no more than 50 percent of the nominal value when the input (battery) voltage falls below a predetermined level.

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