Strange backlight driver signal ? - Page 1
Forum section: electronics / projects

author: autobot
author_rank: Regular Contributor
timestamp: December 04, 2015, 11:04:31 pm
content: I'm looking at this review for a phone and they show the backlight signal:http://i0.wp.com/news.smartphone.bg/wp-content/uploads/2015/10/PWM-GSmart-Classic.jpgThey say it doesn't use PWM.But it looks weird - could it be high frequency non-binary PWM (the noisy blocks) , turning on and off ? or what is it ?Also, how do i look up more info about this kind of waveform ?http://laptopmedia.com/smartphone-review/gsmart-classic-review-a-pleasant-addition-to-the-budget-class/#display-and-sound

author: Berni
author_rank: Super Contributor
timestamp: December 05, 2015, 09:34:55 am
content: Well its clearly not PWM. The noise on there might be just some random noise from switching converters, CPU going in and out of idle. LCD refresh coming along, touch screen scanning causing interference etc.The point is that the screen doesn't look odd if you move it quickly in front of your eyes that PWM can cause.

author: autobot
author_rank: Regular Contributor
timestamp: December 05, 2015, 11:06:43 pm
content: Thanks Berni. Those are good possibilities.Looking a bit more into it, another explanation would be this:http://electronicdesign.com/content/content/62687/62687-fig3.jpg- image b, from >> The point is that the screen doesn't look odd if you move it quickly in front of your eyes that PWM can cause.Since a led turn-on/off time is say 300msec, it should act as a pretty good filter and strongly reject all that noise , right ?from:http://electronicdesign.com/boards/dynamic-backlighting-boosts-contrast-ratio-trims-power-small-lcds

author: Yansi
author_rank: Super Contributor
timestamp: December 05, 2015, 11:13:14 pm
content: QuoteDo you have a proof of that? I smell pretty bullshit.

author: autobot
author_rank: Regular Contributor
timestamp: December 06, 2015, 01:14:10 am
content: Ah you're right. Led response time is fast , closer to 20ns:http://www.ledsmagazine.com/articles/print/volume-4/issue-8/features/driving-led-lamps-some-simple-design-guidelines.html

author: dom0
author_rank: Super Contributor
timestamp: December 06, 2015, 09:15:05 am
content: It mainly depends on the LED itself. Signal LEDs can be very fast, but when you look at white LEDs using phosphorescence then the phosphor will have a afterglow of at least a few µs, but that's of course when you look at the light output - the LED itself switched much faster than that.

author: Berni
author_rank: Super Contributor
timestamp: December 06, 2015, 07:51:26 pm
content: Well the LED turnoff time is not really much of a factor here.But if you do want to go in to the details it might be the regulation loop of the switching converter that's producing the power to the LEDs. It might be operating in such a oscillating fashion where it switches for some time and then stops for a bit as a form of regulating the current. But since there is a output capacitor it keeps the LEDs lit trough that period.In any case this little bit of noise on top shouldn't be visible since it looks like it averages out to about the same light level and your eye certainly is not sensitive enough to pick up that tiny noise at such a high speed.

author: Yansi
author_rank: Super Contributor
timestamp: December 07, 2015, 01:31:03 am
content: Quote from: dom0 on December 06, 2015, 09:15:05 amMy friend actually tried to measure the phosphor time constant. Measured none. So again, have a proof of that time or another "I think it is" instead of measurement?He really couldn't see any light decay from the phosphor...I think there is NO led in the world with such properties as light intensity decay so long it could help smooth out anything like PWM from backlight driver.

author: DanielS
author_rank: Frequent Contributor
timestamp: December 07, 2015, 03:07:02 am
content: Quote from: Yansi on December 07, 2015, 01:31:03 amHow did he try to measure it? With a stopwatch? High speed camera? PV cell? Photodiode? Does whatever sensor he used have a linear and uniform response to all the wavelengths emitted by the LED?There is no such thing as a white LED. The most efficient "white" LEDs are blue LEDs with phosphors to convert part of that blue light to the rest of the spectrum to create light that looks white in overall appearance. While the blue part of that spectrum will have nearly instantaneous response and usually accounts for a large chunk of total output, the rest of the output generated by phosphor will have a fade time like all phosphors do, however fast that fade might be. Just because he couldn't measure it with whatever tools and components he had does not mean it isn't there.