On Wed, 2009-04-15 at 20:03 -0400, solarwind wrote:
> I just sampled some MAX233A chips (TTL <---> RS-232 drivers). These
> are the newer ones that require no external caps for the charge pumps.
> However, the data sheet says to use a 1 uF capacitor from VCC to VDD.
> Why 1 uF? I have none of these. Would a 0.1 uF cermic cap work just as
> well or is there some weird requirement that it has to be 1 uf?

Generally, if the datasheet says use something, just use it. There is
often a reason for these things (the reason of which may be very
surprising) and it's best to just follow the directions.

That said, and this is pure speculation: those chips have charge pumps.
Charge pumps can sink relatively large amounts for currents for very
short periods of time. Specing such a large bypass is probably also to
ensure the droop on Vcc isn't beyond and acceptable level.

So, with that said, it would probably be OK to use two caps, a 0.1uF for
"normal" bypass reasons (i.e. noise on Vcc) and say a 10uF to minimize
the droop. Note that this recommendation is based on PURE speculation as
to the reason for the spec.

TTYL

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