Drive an LED direct from a PIC IO pin (ie no current limiting resistor)

[HankMcSpank]

Still mulling my options here.

Here's the conundrum....I want to be able to run upto 12 leds off either AA - or more likely AAA - batteries. It's be cool if I could use rechargeables.

Therein lies a problem....3 x 1.2V is 3.6V ....an LDO would be no good here (they normally have a drop out voltage of about 0.8V.....meaning with 3 batteries the max I could expect to get regulated is about 2.7V (& that's with fresh batteries!)

I could up the battery quota to 4 batteries, but I'm really pushed for space...so I'd like to stick with 3 x 1.2V batteries (or 2 x 1.5V batteries)

Ok, blue LEDS ....yep, they normally have a forward voltage quoted around the 3.4V mark....having measured the batch I have, the current through 'em at that particular voltage is about 20mA. I actually want to 'under volt' them....the blue LEDs I tested only pull about 10-12mA at around 3.1V....but they're still plenty bright.


So here's what I was thinking....

use a 'LF' PIC variant, supplied by battery but the PIC gets it's supply via low quiescent current 2.5V regulator.

Drop the unregulated battery supply via a two resistor divider, so that the max that can be seen at the PIC ADC pin from the resistor the junction feeds is 2.5V.

So for example.....3 x normal AA batteries (fresh @1.6V initially), would mean 4.8V unregulated.....so eg use two resistors R1 2.3K, R2 =2.5K.

Ok, now with fresh batteries, there'll be 2.5V at the junction....feed this into a PIC, ADC it, now use a LUT....

If the PIC's ADC reading is 255, then the duty cycle (to the leds) should be 168 (66%)...therefore the effective voltage the LEDS 'receive' is 3.1V (4.8V x 67%)

Ok, now the batteries drop after usage, let's say they now amount to 3.8V in total ...this would read as an ADC value at the pic pin of 201, the duty cycle to the LEDs is 211 83%,so what I'm proposing is a simple lookup

if ADC value = 255 the duty_cycle = 168
if ADC-value = 201 then duty cycle = 211

& all values in between.

Or is there an LED driver IC (or an inductorless buck converter ....as inductors don't work well in guitars) to save me the bother? (to recap, I don't want to lose the 'spare/excess' voltage as wasteful heat (ie via a linear regulator)....but rather use PWM to 'manage/massage' the effective voltage that the LEDs ' receive'

(yes I know leds are current devices, but there's a good correlation between voltage applied across them & current they draw...also, again, I'm going to be underpowering them)