Energy-efficient Backlight Schematic Circuit Diagram

Efficient LCD Backlight Control: Switching Regulator vs. Microcontroller Regulation

In some LCD panels, backlight system consume considerable power, ranging from 20 mA to 100 mA. Typically, a series resistor determines the current flow, introducing additional power losses. A more efficient approach involves using a switching regulator IC, albeit slightly more complex. Alternatively, when an LCD panel is driven by a microcontroller, it can serve as a regulation source through software. Fortunately, precision in regulation isn’t overly critical in this context.

Key Components in the Regulation Circuit: P-Channel MOSFET and Microcontroller Integration

Central to our circuit is T1, a p-channel MOSFET, controlled by an inverted (active low) pulse-width modulated signal from the microcontroller. Components D1, L1, and C1 complete the standard step-down switching regulator setup. In the circuit diagram, the LCD backlight is symbolized by two LEDs; their current flow is measured by a shunt resistor, filtered, and then appropriately amplified. The amplified signal is fed into the microcontroller’s A/D converter using an operational amplifier. R1 plays a crucial role, ensuring the transistor completely turns off during microcontroller resets when all ports revert to inputs.

Versatile Compatibility: Universal Microcontroller Usage

This circuit is adaptable for use with any microcontroller capable of generating an inverted PWM signal within the frequency range of 10 kHz to 100 kHz. A demonstration program and a corresponding code module have been developed for the Atmel ATmega32, employing GNU C. Interested users can access the source code for download on the Elektor website at http://www.elektor.com.

Program Operation and Signal Generation: ATmega32 Integration

The program functions by generating a PWM signal at 31.25 kHz (when the processor clock is 8 MHz) on the OC2 output (PD2) of the ATmega32 microcontroller. With 256 available steps, the pulse width can be adjusted accordingly. Assuming an operational amplifier gain of 25.5, a current of 100 mA passing through the LEDs results in a voltage of 2.55 V at the A/D converter input. The ATmega32’s internal reference voltage is nominally 2.56 V, aligning with a ten-bit conversion result of 03FFh for a 100 mA LED current. Monitoring the top eight bits of this value allows the circuit to adjust the PWM signal’s pulse width based on deviations from the desired value, forming an integral controller.

Streamlining the Circuit: Simplified Version Implementation

While this solution may not match the simplicity of a series resistor, simplifications can be made by eliminating the regulation feedback loop. By removing the operational amplifier and surrounding circuitry and having the software output a fixed PWM signal, the circuit loses its ability to compensate for component variations and temperature fluctuations. However, practical applications often find such compensation unnecessary. The software also supports this simplified version of the circuit, offering a streamlined alternative.