PWM driven LED backlight for LCDs

There are several methods available to drive the backlight; one of the most popular is Pulse Width Modulation (PWM). The majority of LCD displays contain a LED (Light Emitting Diode) backlight.

Pulse Width Modulation is a popular way of dimming the LED backlight; it involves turning the LED ON/OFF at a specific rate that the eye perceives the LED being ON continuously but at a reduced brightness. The amount of the dimming is controlled by the duty cycle*. Typically the LED is turned ON/OFF between 60 to 240 times a second (Hz). Any slower than 50 Hz and the eye perceives flicker. Turning ON/OFF faster (higher frequency) is less efficient because of switching losses in the driver and can cause electrical noise (EMI).

*Note: Duty Cycle is a ratio of the amount of time something is ON versus the amount of time it is OFF.

Hardware Considerations for pulse width modulation

When designing the system (or PC board) special considerations need to be made for PWM dimming.

A driver is usually required for back-light type LEDs because of the current level. It cannot be driven directly from a digital output such as a micro-controller.

For many applications a simple logic level FET (Field-Effect Transistor) type transistor is usually used for a driver. The FET needs to be switched through a resistor on the gate to limit gate current when switching. If current limiting is needed, then a resistor is required. Be sure to check the LCD display data sheet for appropriate back-light driving voltages and currents.

A switching type LED driver may be used to drive the LED back-light for higher currents and higher efficiency. These drivers are more complex and usually use a specialized IC for the switching function. Many of these ICs have a PWM input specifically for dimming applications.

If using a micro controller, consideration should be made for connecting to an output pin that supports PWM (timer/counter) output if using hardware function for PWM.

PWM – Firmware/Software Considerations

Two common approaches to generating the PWM signal in firmware/software are controlling the port pin directly or using hardware to generate the PWM signal.

Controlling the port pins directly can be done through a timing function, such as a hardware interrupt or an operating system timing facility (ie: callback). The advantage of this is the PWM signal (to the driver) which can be connected to any output on the processor/computer. The disadvantage is that this uses more processing time (CPU overhead); although, in practice, it is minimal.

A more common approach on micro-controllers is to use a built-in hardware function. Most micro-controllers provide hardware timing facilities (such as a timer/counter module) that can be configured to output the PWM signals as needed with no CPU overhead. Once configured, the PWM (dimming level) is changed by writing new values to the appropriate hardware register.

This article on pulse width modulation / PWM was provided by: Gunn Systems, Inc. (www.gunnsys.com)

Gunn Systems, Inc. provides custom design services and product development. Our custom development capability can meet any customer’s software or hardware needs. We are experienced in the latest engineering systems design technologies. E-Mail: contact@gunnsys.com