PWM dimming enhances color purity in high-end LED video displays

Michael Day, Texas Instruments

Video Imaging DesignLine (10/31/2005 12:14 AM ET)

In Part 1 of this two-part series, “Creating Uniform LED Luminescence in RGB Displays”, the discussion focused on dot correction. This design technique is used for managing individual pixel brightness in large form-factor displays by adjusting the analog current supplied through individual LEDs in an array. While dot correction offers an excellent solution to compensate for lumen output variation between pixels, this adjustment of analog brightness is only the first-step in developing a high-quality LED display. This article addresses pulse width modulation (PWM) dimming, or grayscaling, the LEDs while maintaining color purity to create a superior picture quality.

The rapid migration from simple monochromatic alphanumeric displays to sophisticated full-motion video displays has been amazing. The demand for this technology has evolved from the transportation and education sectors that typically require simple text and information displays, to the ever-expanding advertising industry. Some examples of message delivery markets now include: convenience stores, gas stations, stadiums, and a myriad of other industrial venues that now require high-quality video displays. These displays demand sophisticated LED drivers capable of providing multiple brightness levels. The number of colors in the display is proportional to the number of brightness levels available for each of the red, green, and blue LEDs that make-up a single pixel in the overall display. Competition between display manufacturers is driving designers towards high-end LED drivers with integrated PWM functionality capable of delivering thousands of brightness levels. These brightness levels result in enhanced color shading and improved video quality.

Older alphanumeric displays seen in score boards, road signs, and simple alphanumeric displays only utilize two brightness levels: 100 percent and 0 percent, or “on” and “off”. Full-color video requires many brightness levels between 100 percent and 0 percent. Two methods are available to control LED brightness: analog dimming and PWM dimming. With analog dimming, LED brightness is controlled by changing the LED current. For example, if an LED is at full brightness with 20 mA of forward current, then 25 percent brightness is achieved by driving the LED with 5 mA of forward-current. While this simple dimming scheme works well for lower-end displays, the drawback with analog dimming is that an LED’s color shifts with changes in forward current. Figure 1 shows a “true” green LED’s color variation with changes in forward current. This LED’s full-brightness is specified at 20mA. Analog dimming to 25 percent brightness shifts the color spectrum from 525 nm to 531 nm. This color shift becomes unacceptable in displays requiring a true-color representation.

Figure 1 - Color Shift Caused by Analog Dimming

PWM dimming provides reduced brightness levels while reproducing an accurate color. Since PWM dimming maintains the full-forward current through the LED, it eliminates the color “shift” associated with analog dimming. Changes in LED brightness are achieved by modulating the total amount of time that forward current flows. In other words, the LED is rapidly switched “on” and “off”. If this switching frequency is greater than 100Hz, the human eye does not see the LED turn “on” and “off”. It averages the LED’s “on” and “off” times and perceives a reduced brightness that is proportional to the LED’s “on” time duty-cycle. A 25 percent brightness level is achieved by turning the LED on at full-current for 25 percent of the time. Pulsing the current provides precise brightness control while preserving the color purity. Figure 2 shows a comparison between analog and PWM dimming for a 20mA LED being dimmed to 25 percent brightness.

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Figure 2 - Analog Dimming vs. PWM Dimming

PWM dimming generates a discrete number of brightness levels for each LED, commonly referred to as grayscale steps. The total number of discrete steps available during any one period determines the LED’s brightness resolution. High-quality displays require hundreds to thousands of brightness steps to accurately reproduce the full-color spectrum necessary for full-motion video. Semiconductor manufacturers like Texas Instruments now offer LED drivers with 12-bits of pulse width modulation to meet this need. 12-bits of resolution provides 212 = 4096 shades for each LED. Each pixel in a color display is composed of three LEDs: red, green and blue. By simultaneously pulsing and mixing the red, green and blue LEDs, the RGB cluster is capable of a color pallet with up to 68.7 billion colors.

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Figure 3 - 3-Bit PWM Dimming Generates Orange Pixel

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