Re: PWMing of White LED LCD back light
- From: Sylvain Munaut <com.246tNt@tnt>
- Date: Sat, 23 Jul 2005 15:42:36 +0200
>>Hello,
>>
>>I have a CPU with programmable PWM output and a STN LCD with witheled
>>back lighting. All the whiteleds are in // so it's powered by classic
>>5V and draw about 150mA (the current limiting resistors are included in
>>the panel).
Ok, first thank you all for you answers.
See some comment below about Mochuelo answers.
> First, you don't need the NPN. If your MCU is fed also with 5 V, the
> PWM output can directly command the PMOS. You don't even need a
> resistor. You might want to include it, in series with the gate of the
> PMOS, if you were concerned about radiation (EMI). The PMOS must have
> a gate threshold voltage lower than 5 V (as it was also in your
> circuit). Just take into account that the LEDs will be on while your
> MCU output is 0. No problem at all. However, if you still want to
> complement it, most MCUs with PWM allow you to invert the output
> inside the MCU itself.
No, the MCU is a Au1100 (32bit MIPS) and has 3.3V IO and when in sleep
or early boot phase it's output might be Tristated or low and the
backlight must be off.
>>What I'd like to do is to control the intensity of the backlighting
>>by modulating the input. It doesn't need to be linear, nor precise,
>>just no artefact visible to the naked eyes. The PWM frequency output
>>can be from about 1Mhz to about 5kHz and from 0 to 100% duty in at
>>least 64 steps.
>
>
> Right. Ignoring interaction with the scan rate of the LCD, any pulse
> repetition frequency (PRF) of the PWM signal higher than about 1 kHz
> is ok. I have been designing circuits for lighting products based on
> high-power LEDs (1.2 W/LED), and using 3.9 kHz as the PRF, and no
> problem. The frequency is high enough not to be noticed by our eyes
> (even if you move your sight with respect to the source of light), and
> low enough to be able to use very small MOSFETs (with very low total
> gate charge). However, I would encourage you to take advantage of more
> duty ratio steps. I was using 256. At the low-light side of the range,
> the (linear) steps appear to be larger to our sight.
In fact I said 64 as a worst-case value. It depends on the divider.
Basically I have a 6Mhz clock that I can divide up to 4096 by setting a
divider. Then I can choose until which step it's off or on. So by
setting the frequency at 1.5 kHz, I would have up to 4096 steps to
choose from.
>>I know that for a simple led, I can just directly use that to PWM and it
>>works fine. But for the LCD backlight I'd like to know if inducing such
>>a discontinuous current might have on the life span on the back light
>>(for a simple led I don't care but for a LCD back light, that matters ;)
>
>
> No, no, don't worry. If you guarantee that there is no 't' for which
> v(t) or i(t) is going to exceed the ratings, the LED does not care
> about the shape of the excitation. Most LEDs in this world are being
> excited with rectangular waves.
Great, good to know, even easier then ;p
Sylvain
.
- Follow-Ups:
- Re: PWMing of White LED LCD back light
- From: Mochuelo
- Re: PWMing of White LED LCD back light
- References:
- PWMing of White LED LCD back light
- From: Sylvain Munaut
- Re: PWMing of White LED LCD back light
- From: Mochuelo
- PWMing of White LED LCD back light
- Prev by Date: Re: Have Fun With Gay Macon
- Next by Date: Re: Relocating - need advice
- Previous by thread: Re: PWMing of White LED LCD back light
- Next by thread: Re: PWMing of White LED LCD back light
- Index(es):
Relevant Pages
|
