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Margery Conner Technical Editor Margery Conner's PowerSource streams the latest developments in electronic power design and related technologies. Follow Margery on Twitter at: http://twitter.com/margeryc.



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Friday, June 22, 2007

There’s more to backlighting displays than just turning on a white LED

Jun 22 2007 11:43AM | Permalink |Comments (5) |


Based on what seems like an unending stream of power control circuits and ICs for white LED-based display backlights, I had assumed that if you want to light up a hand-held display, you would automatically turn to a white LED. It seems I was naïve. Mark Davidson, marketing director, power management division for National Semiconductor was pointing out to a group of electronics editors and analysts this week that ”white” LEDs vary in their whiteness, and that their whiteness can vary over time. If you lock yourself into a single white LED for illumination, you give up the ability to compensate for LED aging, as well as variability in display color.  He suggests considering three discrete (RGB) LEDs along with control circuitry to allow you to maintain the correct degree of whiteness in the backlighting, as well as the on-times for the three LEDs. This degree of on-time and control can result in the use of less power than just turning on a white LED and leaving it on.


Related entries in: Components, Hardware, Interconnect | Power Sources/Controllers | 

Reader Comments



at 6/22/2007 6:47:49 PM, Paul Rako said:
My understanding is that white LEDs are really blue LEDs that have a white phosphor coating. This is why they have a bluish tinge. I can see the phosphors wearing out, especially if the LED was exposed to sunshine such as in an outdoor sign. RGB LEDs can balance the color but those colors have their own aging profiles. My question is what do you use to measure the RGB LED cluster to insure the light is still white? That is a non-trivial task. If you expect the user to balance the colors look at anyone who plays with the tint control of their TV, they invariable saturate some color. All I know is that when I was doing the Analog Seminar for National, an engineer in England walked up to be on a break and said that if National published one more white LED driver circuit he was going to firebomb the headquarters. I could sympathize—there was a period a few years ago when it seemed like every circuit and article was about LED backlights. There is a huge market for these in consumer electronics but I bet a lot of EDN readers are not involved in those designs. Maybe they can chip in their two cents on this issue.



at 6/25/2007 7:04:23 AM, Alvaius said:
Measuring the output of an RGB light source in a display application is pretty trivial. Companies such as Avago make specialized color sensors designed to do exactly that. You still need to place them properly (easy on a display) and calibrate the system, but once done, it is easy to keep the color balance extremely accurate.

I expect if you are designing cell phones, PDAs, industrial terminals, and the like, then you are likely very interested in white LED drivers. While these may represent absolutely massive quantities for the IC vendors, it likely represents a small amount of design engineers. However, many products do incorporate displays, they are increasingly becoming color, and they are likely LED backlight based....so maybe more engineers are interested. High powered LEDS for lighting also seems to be a growing field.



at 6/29/2007 7:36:19 AM, Paul Tamura said:
Unless you are displaying photographs, the change in color balance with time is probably not an issue. This is especially true if you already accept the initial blue cast.



at 7/3/2007 10:45:20 AM, Rennysoncemann said:
It's true that a mix of red, green and blue looks like white light, but it's not truly white light and I'd think there would be some cases where an item illuminated by rgb light would not look the same as it would under true white light, even though the rgb light itself is indistinguishable from the white - any comments?



at 7/4/2007 9:46:26 AM, Fred Grunert said:
I’m sure exactly color control isn’t a trivial task. The best solution for color control will be a color sensor with filter characteristics like human eye. Most color sensors have only rgb-filters and different from human eye color sensitivity. MAZeT offers under the brand JENCOLOR Color sensors like human eye . If you will have a stable color control (regarding temperature, long time behaviour and adapted on human eye), you has to use color sensors with features like JENCOLOR filters.

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