Backlight Control Using Pulse-Width Modulation (PWM)
Pulse-Width Modulation (PWM) is a highly efficient method for driving LED backlights, offering several advantages over constant DC voltage or current approaches. The primary benefit is improved efficiency by delivering high-current pulses over short durations.
For example, the nominal LED driving current for this display is 120 mA, resulting in a typical brightness of 50 cd/m². Instead of applying a constant 120 mA, the same average current can be achieved by pulsing the LEDs at 600 mA (5× current) for 1/5 of the time. (See Fig. 1.) The average brightness measured over time remains the same, but the perceived brightness appears greater due to the eye’s response characteristics.
The human eye exhibits persistence of vision, meaning it retains an impression of light for a short duration after the source is removed. This is the principle that allows us to perceive motion on screens refreshing at 24–30 Hz as continuous. Similarly, when the LED is pulsed at high intensity for a short duration, the eye perceives a brightness level closer to the peak pulse brightness rather than the average DC brightness.
Advantages of PWM Backlight Driving
- Maximum Perceived Brightness:
To achieve the highest possible perceived brightness, the LEDs can be driven using a 1:4 on/off ratio with a 5× overdrive current. To prevent visible flicker, the PWM frequency should be kept above 100 Hz but below 1,000 Hz. - Power Savings:
PWM can reduce average power consumption while maintaining perceived brightness. For instance, a 50% or greater reduction in average power is achievable without noticeable brightness loss—ideal for battery-powered devices. - Smooth Brightness Control:
PWM provides a wide, linear range of brightness control by adjusting the duty cycle (on/off ratio). Unlike analog dimming through reduced DC current, which can cause uneven light output and visible LED elements at low current, PWM maintains uniform backlight appearance.
To implement this, set the peak current to the display’s specified typical current, and vary the duty cycle from nearly 100% (full brightness) down to near 0% (dim).
(See Fig. 2.)
This approach enables efficient, flexible, and visually consistent LED backlight control—critical for high-performance display applications.