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How bad it is to power LCD backlight from MCU IO?
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3dgeo:


Quote from: SiliconWizard on Today at 06:18:09 pm
And maybe I'm just being overly cautious and dispersion on Vf (or at least the impact of If on actual brightness) is not that bad.

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No, I'm really glad You bring this up! This was my goal – to find out things I don't know, I'm extreemly thankful for bringing things like this up, You people are awesome! Thank You!

I have 5V rail, I just wanted to see if I can reduce BOM. Well, I have free 10K resistor in a resistor array, and I already have 1.5K and 22R resistor in the BOM, so extra cost would be for extra transistor and 3 PnP parts.

Anyway, despite LCD brightness power from IO circuit should work and shouldn't give me any other issues?

I guess I'll do what I planned to do – I'll make modular PCB prototype where I can jump jumpers and test all powering options...
tooki:

--- Quote from: madires on March 12, 2020, 04:01:10 pm ---Yes, I meant backlight PWM. And don't skimp on bypass caps. ;)

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Just FYI, PWMing the backlight can cause some weird flickering effects on some LCDs, if the backlight and the LCD refresh rate happen to interact in unfortunate ways. I saw it when experimenting with a 1602 character LCD.
3dgeo:

--- Quote from: tooki on March 12, 2020, 08:43:16 pm ---Just FYI, PWMing the backlight can cause some weird flickering effects on some LCDs, if the backlight and the LCD refresh rate happen to interact in unfortunate ways. I saw it when experimenting with a 1602 character LCD.

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Yes, I see how this can happen, but it's easily fixable by changing PMW frequency. I'll test it, but the sceen is small and not that bright anyway, so dimming BL is not that nescesary feature, I'm OK with it being ON or OFF only. I might drop this feature entirely and just connect it straight to 5V and be done with it. But I want to explore all options first...
Siwastaja:
Also note the IO pin equivalent DC resistance is uncontrolled and varies somewhere between maybe 10 and 30 ohms depending on unit, temperature, etc. This will be in series with any external resistance.

OTOH, it can work out to be somewhat self-regulating: if the particular LED Vf is lower than you expect (a low Vf unit, or hot LED temperature), it draws more current, causing more heating in the MCU IO MOSFET, heating it up, causing the IO pin resistance to go up, lowering the current. Such a negative Rds(on) feedback loop can compensate for the positive feedback loop of the LED Vf per temperature.

Of course, do the IO pin current draw analysis for the worst case Vf of the LED.

Note that for better current regulation at Vled=5V and larger resistor value (as shown by SiliconWizard), you won't necessarily need external transistor: STM32 MCUs have IO pins which are 5V tolerant and have the open-drain output mode. I tend to use that when directly powering low-current LEDs with Vf too close to 3.3V.

If you decide to not use the IO pin to power the display directly, the transistor route isn't that much better; it's extra parts, and it's still kinda sucking at current regulation. Consider using a constant-current linear LED driver with enable pin instead: these are cheap and come in tiny packages, and the part count will be lower because no resistors are needed at all. They tend to come with some fixed current settings like 10mA and 20mA.
3dgeo:
You mean something like NSI45015WT1G ?

If so they cost 6cent in quantity and -+20%, that means 12 to 18mA – doesn't look very accurate as well...
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