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 conditions 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 MOSFETs, thus varying the LEDs’ average current and light intensity. The switching is 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.