I'm working on an STM32-based project to which I'm planning to connect an LCD (in my case, it should be 800x480, specifics irrelevant for the questions). I do have disco development boards with LCDs (F746 and F469), but I have never integrated custom LCD with custom MCU outside the dev boards. Did play with the dev boards for a while to get acquainted with them.
I've googled some questions of mine, of course, but I wanto to crystalize my knowledge before it solidifies in KiCad and will have a chance to explode in my face.
If you want some specific example, here's the LCD I'm planning to work with, so if you want to refer to some specifics, you can do it with this: LCD Datasheet PDF English
As for questions:
- LCDs in general have separate backlight cathode (-) pin. From what I understand, it can be connected to ground, but doesn't have to be. It could also be connected to some inverting charge pump so that the anode voltage doesn't have to be that high. But ground also works. What matters is the voltage difference between anode and cathode, since it's just a circuit with a few LEDs in there (I remember about current limiting resistor). The only thing to watch out for, as I see it, is the ground current path in case of PWM-control (make sure it doesn't induce noise into other stuff). Please, confirm or debunk these statements of mine (and why if they're wrong).
- Backlight can be controlled by PWM or by constant current source. First, how does it affect LED lifetime? (for example, the LCD I provided has 50,000h LED life ~ almost 6 years constant ON). I do realize it's very unlikely that a device with very specific purpose like mine will ever exhaust that time. But since I'm asking all the stuff about LCDs here, why not throw in all questions that interest me. So, LED-lifetime: PWM vs Constant Current? PWM is ON for less time, but there is turn on and off cycles and stuff. So if anyone knows, I would be greatful.
- Other reasons for constant current or PWM? I mean, nothing stops me from setting up a Timer to MCU pin to PWM backlight chip's enable, but I guess I could also control a constant current source, although it seems to me it became de-facto standard to use PWM (I fix laptops' logic boards and also occasionally see tablets and phones, even had one TV to fix, so I meet real circuits made by proper engineers unlike myself, and I use schematics a lot). Also, constant current circuit may seem more difficult and expensive to control for my inexperienced eyes comparing to a single boost converter with enable pin straight from the MCU. Same story here as in question 1 here, please, confirm or debunk my view of the things.
Before I got any replies, I think I'm considering using PWM, and since my LCD wants ~30V, I will need a boost converter for it. Then there are questions about PWM frequency and stuff, but I guess this is where I'll have to take into account all info from LCD datasheet (no mention of backlight control other than diode parallel-sequencial structure) and specific backlight IC