This is an Automatic LCD Panel Backlight Control circuit. This is a simple low cost implementation of an LED controller. It can compensate for aging effects and the temperature typically found in high power LEDs. This circuit consist of a photocell sensor and The AD5228 so the automatic brightness control can be achieved. The photocell’s resistance changes linearly and inversely with the light output. The lower photocell resistance and vice versa, the light output is brighter. Here is the schematic diagram of the circuit:
The AD5228 is used to set the voltage level which is is gained up by U2. The voltage level is used to drive N1 to a desirable brightness. The photocell is used as the variable feedback resistor and the R2 resistance changed by the change in the light output. So, U2 will drive N1 accordingly to regulate the output.
The PWM signal can be applied at the gate of N2 to switch the LED on and off without noticeable effect. So, the use of power would be more efficient. [Source: Analog Devices Application Note]