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

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Zero999:
I don't see how this is going to be that efficient.

What colour are the LEDs? If they're blue, white, violet or any phosphor converted chemistry, then a supply voltage of 3V, is close to the normal forward voltage and won't guarantee constant brightness, over the temperature range and life time of the LEDs.

The 74HC595 has a fairly high output impedance with a supply voltage of 3V and it will probably have a positive temperature coefficient, like all CMOS devices do, so the output voltage will drop more, at higher temperatures.

You say the supply voltage can vary between 3V and 5V, so what voltage are you going to set the output of the buck converter to? If you use a controller which can run at 100% output duty cycle then you could set it to 3V and accept there will be a small voltage drop, when the supply voltage falls to just over 3V.

I wanted a rude username:

--- Quote from: Zero999 on February 06, 2020, 11:36:09 pm ---what voltage are you going to set the output of the buck converter to? If you use a controller which can run at 100% output duty cycle then you could set it to 3V and accept there will be a small voltage drop, when the supply voltage falls to just over 3V.

--- End quote ---

That's the idea. Set the voltage just above the point at which the white LEDs show significant variance in brightness. Test power consumption against a more aggressively PWMed unregulated solution at the same perceptual brightness level and see which is more efficient at various input voltages.

Basically the buck converter method seems like the best option, and is easily tested and easily bodged out if it doesn't deliver benefits, so pursuing it is low risk.

I wanted a rude username:

--- Quote from: I wanted a rude username on February 05, 2020, 07:27:44 am ---PWM would work and is easy, but LEDs are more efficient when run at 100% duty cycle at a lower voltage.

--- End quote ---


--- Quote from: blueskull on February 07, 2020, 01:58:26 pm ---Bump voltage up to 5V wastes some power

--- End quote ---

The entire point of this quixotic quest is to minimise power consumption. A good buck converter like the TPS62120 isn't super cheap (50ยข), but is about 90% efficient in the target voltage and current range so it's definitely worth investigating. If it doesn't work, the plan is indeed to run everything straight off the battery and aggressively PWM the LEDs.

I wanted a rude username:
You're concerned that it won't run properly once the battery discharges to near 3 V, right? I'm OK with that ... not sure what the power supply will be yet, but if it's a Li-ion it won't get that low. Thanks for the reminder!

By the way, empirically I've found that the true green LEDs run pretty well down to about 2.5 V.

I wanted a rude username:
Even better: here's a graph of such an analysis I did on a 1 W LED a few months ago, but in 50 mV increments. Power increase is clearly exponential with voltage. Note the discontinuity above 3.2 V ... the LED's heatsink was small and it was overheating.

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