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?

Unless the datasheet decides to explain the reasoning behind it or you can
talk to a Maxxim engineer envolved in the design, don't expect to ever know.
What you need to know is that's the spec.

My guess is that the internal charge pump draws current in bursts and can't
guarantee the output voltage and current specs if the supply is allowed to
droop.  They are probably assuming a barely good enough impedence of the
power supply especially once it reaches the chip, and they found 100nF
insufficient to cover their butts, so specified 1uF.

> 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?

There is a requirement since the datasheet clearly says so.  100nF would
probably work most of the time if you have a good power supply, but I would
use the 1uF.  The days of 100nF bypass caps are long gone for "normal" ICs
anyway.  Yes I know lots of people still tell you to use 100nF caps, but
that's a sign they haven't kept up and should be a warning they probably
don't know other things they are talking about either.  Today's 0805
multilayer ceramic 1uF caps have lower inductance and higher resonant
frequency than the 100nF caps of a few years ago, even though those were
considered adequate at the time.  I can only recall using less than 1uF
decoupling cap in one circuit in the last 8 or so years where 1uF 0805
ceramic caps have been cheaply and widely avaialable.  That was a RF circuit
where the higher resonant frequency of a 100pF cap of a specific model was
important.  Each power lead to the chip had its own 10ohms in series follwed
by this specific 100pF to ground.  The common supply point before all this
still had the 1uF to ground though.

So the point is if you don't have 1uF 0805 ceramic caps you should get some.

If you have a larger than 1uF ceramic cap you can use that instead.
Anything ceramic of 1uF of higher should be good enough as the power supply
cap for a MAX232 chip.  Do not substitute electrolytic though.  These don't
have the frequency response to work in this situation.


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