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Current/voltage limiting of a large array of LEDs ... without LED driver ICs?

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mariush:

--- Quote from: paulca on February 05, 2020, 03:40:27 pm ---
--- Quote from: I wanted a rude username on February 05, 2020, 10:10:10 am ---Those don't seem to have current limiting, do they?

--- End quote ---

The 595D does have have current limiting.  You set it by providing a resistor.  The datasheet has look up tables for the value of R to a given current.

--- End quote ---
If you're referring to the 3 cents one I linked to above, I don't see any way to set a register in the datasheet.

I am aware of such 8 channel led drivers that do allow you to set the current by using a resistor, only they're a bit more expensive

Here's led drivers with resistor for as low as 20 cents : mile long tme.eu led drivers link

ex cheapest is 16 channel 5..45mA per channel serial (like a shift register): https://www.tme.eu/ro/en/details/sct2024cssg/led-drivers/starchips-technology/
It's pin compatible with other chips like
https://www.digikey.com/product-detail/en/stmicroelectronics/STP16CPC26PTR/497-11923-1-ND/2757659
https://www.digikey.com/product-detail/en/texas-instruments/TLC59284DBQR/296-39245-1-ND/5143238
etc

8 channel models are about the same price, but gets you cheaper alternatives on digikey and other bigger stores, if you can't wait a week or whatever it takes for parts to come
ex. https://www.tme.eu/ro/en/details/sct2168cssg/led-drivers/starchips-technology/


I wanted a rude username:
Thanks for the advice, everyone!

The best option seems to be a buck converter plus PWM. This won't be as efficient as magically powering each row/column at the exact voltage it needs for the desired brightness, but is practical.

LED drivers with current control don't seem particularly efficient, especially at higher input voltages, because they regulate linearly. 60% efficiency at 5 V is normal. Some also have significant quiescent current (9 mA for the IS31FL3236A) and talking I2C would be less efficient than just bumping a shift register.

So, I thank you all for adjusting my expectations.  ;D

paulca:

--- Quote from: mariush on February 05, 2020, 04:28:15 pm ---If you're referring to the 3 cents one I linked to above, I don't see any way to set a register in the datasheet.

--- End quote ---

I was thinking of the TI TLC chips sorry.

paulca:

--- Quote from: I wanted a rude username on February 05, 2020, 09:47:58 pm ---The best option seems to be a buck converter plus PWM. This won't be as efficient as magically powering each row/column at the exact voltage it needs for the desired brightness, but is practical.

--- End quote ---

Correct me if I'm wrong, but if you pulse PWM through a buck convertor into an LED it will still fry the LED as for that instant the LED will be seen as a short on the buck converter so it will get the full current of the output cap discharging through it PWM or not the cap will discharge nearly instantly..  So you still need current limiting though each LED in parallel. 

Both Boost converters are not that highly efficient either mind.  Maybe 90-95% in their optimal point and as low as 60% outside of it.

I wanted a rude username:
Sorry, I wasn't clear: use a buck converter to power a 74HC595 shift register, then PWM the register via its output enable pin. The register will source to the LEDs and the microcontroller will sink.

As fcb noted, the impedence of the microcontroller's (and in this case also the 74HC595's) drivers limits current. In practice you can drive LEDs without resistors even at 5 V ... as long as you PWM them at a high enough frequency and low enough duty cycle. I might still end up doing that if, as you point out, the buck converter can't be made to run efficiently.

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