Why do most PWM LED displays and backlights operate at a relatively low frequency?

Question

All around me are a bunch of electronics: digital clocks, my laptop, a refrigerator, a dimmable flashlight, and more. What they all have is perceptible flickering of their displays due to PWM, especially when I make quick eye movements (i.e. normal everyday vision).

I've played around with PWM and LEDs before; flickering becomes comfortably imperceptible at 1000Hz or so, which is trivial for a microcontroller, even perhaps inconsiderable.

I realize some devices may be governed by mains frequency, but as far as I know, a lot of my electronics use filtered DC power.

Why can't every LED display be designed so that no flickering occurs?
I can think of a few reasons for our current situation:

• We have a bunch of lazy engineers
• Cost reasons -- maybe they're using some absolute worst microcontroller to save a few pennies
• Efficiency -- I know PWM is more efficient than constant current, and I guess the higher the frequency the closer it is to constant current (can I assume that?), but I'd be surprised if there was a major difference between 100Hz and 1000Hz.
• I am literally the only person bothered by this.

Thoughts, anyone? I do hope I'm not the only one.

Answer 1

It's not really PWM, rather it's multiplexing of displays. I won't go over the advantages of multiplexing in detail here, but it's not power efficiency, rather it's a reduction in cost and complexity of drive components. It's possible to use a few cheap parts to drive a 4-digit LED display (32 segments) with only 12 port pins (on a single sided PCB if necessary).

Most of this kind of product will be using an 8-bit processor rather than some. 32 bit thing, and usually at a relatively low clock frequency such as 4 or 8 MHz. They will not be equipped with a hardware display controller generally, so an ISR will do the work. If there are other things that are high priority then the display digit brightness might be visibly modulated in brightness due to jitter in multiplexing - some level of that would be deemed to be acceptable if not entirely imperceptible. Same thing with flicker in the display. Even so the micro might be spending more than 20% of its bandwidth just controlling the display. Faster clock would mean more power consumption of the micro, more EMI and more cost. For an 8 digit display muxed at 200Hz a new digit must be handled every 600usec or so, +/- 30 usec (that would be a pretty high quality display for an application without vibration). If there is a lot of vibration maybe 5x faster.

Although a designer could propose using, say, a small FPGA to totally eliminate all timing constraints and a 6-layer board to deal with the EMI, that would likely be their final act at a consumer product company. The attitude is that a 5 cent reduction in cost would be sufficient to hire another engineer.

Digital LED mains powered clocks are a special case, and some use a clever biplexing scheme powering the display from an unregulated centre-tapped transformer secondary, so the mux frequency is tied to the mains frequency.

Answer 2

It's lazy engineers, period. I find it particularly annoying with regard to car taillights at night. When I sweep my eyes across the traffic, the "strobe" effect drives me nuts.

I have designed multiplexed displays, and I can tell you that it doesn't cost any more to multiplex/PWM a display at 1000 Hz than it does at 50/60 Hz.

Answer 3

At a guess (as I've never designed a PWM'd display):

• Cost: faster processors/IC's cost more
• Cost: higher frequency circuits (boards, components) cost more
• Cost: eliminating radio interference costs money, higher frequency = more likely to act like a transmitter and have unwanted side effects
• Cost: developing high frequency circuits that behave properly & don't generate RFI requires more effort
• Cost: Very few people notice or care that your alarm clock display flickers less, certainly won't pay more for that alarm clock.

Also cost. ;)

Answer 4

1. We have a bunch of lazy microcontrollers :)
2. Cost (anything said John U is true)
3. I haven't found any visual difference with 100 Hz and higher frequencies (except there's stroboscopic effects involved: vibration, rotation etc.)
4. In many cases embedded application uses repetitive timer with 50, 64, 100, 1000 Hz or so, used as main base for controlling time intervals. Often LCD refresh is also bound to this timer.
5. Sometimes device is clocked by 32768 Hz crystal oscillator (and it is enough).
6. Switching at audio frequencies can cause undesired continuous tone at speakers or some electronic parts such as inductors.
7. Sometimes LEDs are driven by switching voltage convertors and brightness is controlled, by turning it on and off. As startup requires some time it cannot be switched too often.

There's great difference driving led variable continuous current or PWM. Changing instant current cause not only brightness, but also can change emission spectrum (especially with white LEDs) and efficiency factor. Also controlling brightness by PWM duty cycle is strictly linear.