[sdiy] thanks for replies - Blue LED

[John Luciani]

On 10/22/06, Scott Gravenhorst <music.maker at gte.net> wrote:
> "John Luciani" <jluciani at gmail.com> wrote:
> >On 10/22/06, Bert Schiettecatte <bert at percussa.com> wrote:
> >
> >>
> >> I did some experiments with 2 and 3 cell AAA packs and it doesn't look
> >> like a pump circuit with a hex inverter and some 10uF caps can keep
> >> delivering the necessary current/voltage for the LED (around 30mA)...
> >
> >A problem with using the output of a hex inverter to source and sink current
> >is the output impedance is too high. It may be hard to find an inverter with
> >a low enough impedance that also meets your low cost requirement.
> >
> >An inverter made with a complementary P and N channel FETs would work
> >but again may not meet your price target.
>
> Use more than one hex inverter in parallel ?

The problem with parallel inverters is that you either need to diode-or them
together or place a series resistor between each output since the inverters
may not switch at the same time. You could have one inverter high and one low
(for a short time). It may be a small enough time that it would not
matter I don't
know. I have not seen designs with logic outputs paralleled (except
for open-drain (or open-collector) devices).

If it was only more current that was required then parrallel
open-drain inverters
would work well. Since the requirement was a higher voltage to bias the blue led
and a 30mA current Bert was looking at the charge-pump.

(* jcl *)


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