Re: 300 mA from a microcontroller pin

On Nov 29, 1:07 pm, Tomás Ó hÉilidhe <t...@xxxxxxxxxxx> wrote:
Arlet Ottens:

What are you driving that you need 3-state 300 mA outputs ? Depending on
the application, there may be alternative solutions.

A bi-colour LED that has only two pins (they're in parallel facing in
different directions). If the microcontroller pin is high, then it'll be
red. If low, it'll be green. If high impedance, it'll be off.

The maximum current rating for the LED package is 30 mA... except I'm
flashing a display and will only have them lit one sixteenth of the time,
so I'm gonna put a burst of 300 mA through them. (I've seen experiments
where people flashed an LED putting an entire amp through it, so I don't
think 300 mA will be a problem for a duty cycle of one sixteenth).

--
Tomás Ó hÉilidhe

There you go. Arlet said there might be alternate solutions and there
is one. You don't need the tri-state control. You can drive the LED
forward with one pin high and the other low, backward by switching the
two and off by bringing them both high or both low. Can't be much
easier than that! You will still need two pins from your controller,
but you don't need a complex driver circuit. Just a pair of FETs, 1 P
channel and 1 N channel on each leg of the LED. If you are driving
from a high enough voltage that the N channel FET will be fully off
(or using a low threshold part), you don't have to worry too much
about shoot through. To eliminate it, you can put your current
limiting resistors in between the drains of the two FETs and connect
the load between the drains of the N channel FETs. That's pretty
simple, four FETs and two resistors... about $0.40 per LED. You can
even get a pair of FETs in an SC-70 package, only 2 mm square. I'm
not sure SC-70 FETs are rated for 300 ma though. You may have to go
to the SOT-23 package at 3 mm square...
.