Using a DAC instead of PWM to control a LED backlight

Hi,
Previously i was able to lit up the backlight of a Pixel Qi panel display using a PWM signal,
please check here the datasheet, pag 13,

So i now i got a MPC4725 DAC and tried to use an analog signal instead, but the results are not good.
setting dac.setVoltage(x, false) from 0 to around 940 the backlight is completely off, and from 940 to around 1040 it flickers a lot unsteadily, gradually increasing the brightness, and after 1040 its completely on.
The backlight has a cable for PWM control and another for VCC, i was going to try to send the signal to the VCC cable, but then i thought that that may burn the dac as too much current would flow through. I'd appreciate if anyone could lend a hand,
thanks

Stick to the PWM method. A DAC circuit delivers an analog voltage but has no capacity to supply current. PWM is the easiest and most effective method. Find other ways to play with the DAC.

Railroader:
Stick to the PWM method. A DAC circuit delivers an analog voltage but has no capacity to supply current. PWM is the easiest and most effective method. Find other ways to play with the DAC.

Hi, Thanks for reply. I understand. The fact is that i'd prefer to use a steady voltage instead of PWM because i think the flickering effect of PWM is bad for my eyes. I think i may try with a step down converter connected to the vcc pin of the backlight.

What is the frequency of the PWM output?
That is possible to change if needed. The normal freq is some 490 or 980 Hz depending on controller and output pin I was reading here not long ago.
TV uses a much lower freq…

LEDs can not be controlled by voltage, but you can try to control a constant current source by the DAC.

DrDiettrich:
LEDs can not be controlled by voltage, but you can try to control a constant current source by the DAC.

Hi , but how much current can pass through the mpc4725? I believe it's like 25ma. Won't I destroy it if I connect it to the VCC of the backlight?

amadeok:
Hi , but how much current can pass through the mpc4725? I believe it's like 25ma. Won't I destroy it if I connect it to the VCC of the backlight?

That is why the suggestion was to use a voltage controlled constant current source between the DAC and backlight.

Hi,
I'd like to control a 12 or 5v LED backlight using analog variable voltage, not PWM. I couldn't find any mosfet that would work at this low range. A DAC would not supply enough current.
Could anyone advice an approach? thanks

amadeok:
I'd like to control a 12 or 5v LED backlight using analog variable voltage, not PWM.

Why? What do you think that will give you that PWM won't.

Steve

duplicate post

If something is more or less impossible it doesn’t help to keep pushing.
Stretch for bringing down the moon but do it on Your own. Don’t bother skilled people to use time for writing nop-answers.

Duplicate topics merged

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Basic electronics point. LEDs are current controlled devices. There Vf is very stable and they can be used as zener diodes in many applications but each will have a different Vf that is why they cannot be paralleled. A light-emitting diode (LED) is a semiconductor light source that emits light when current flows through it. Electrons in the semiconductor recombine with electron holes, releasing energy in the form of photons. The voltage is only needed to overcome the Vf so current can flow. You can use a DAC if you want and it will work but you will have a very narrow range of counts like maybe 30 out of 255 that might work. This makes it very difficult to control and each LED will behave different.