MAX232 and 12V?
Christian Hoene
Hi,
in a small portable computer system I want to use a max232 serial
driver.
There is already a 12V supply on board. Could I use the 12V supply for
V+ instead of the max232's charge pump?
Thanks
Christian
Kevin J. McCann
How are you planning to drive the 12V? The point of the 232 is to take TTL
levels and output 232 levels and vice versa.
Kevin
Charlie Allen
Christian Hoene wrote:
>
> Hi,
>
> in a small portable computer system I want to use a max232 serial
> driver.
> There is already a 12V supply on board. Could I use the 12V supply for
> V+ instead of the max232's charge pump?
>
> Thanks
> Christian
You still need to generate a negative voltage for the RS-232 signal, so
you still need one charge pump.
The MAX209 will make a full PC serial port (3 drivers, 5 recievers) with
just 1 0.1uF capacitor for the +12V to -12V charge pump.
If you want just 2 drivers and 2 receivers, the MAX202 is pin equivalent
to the MAX232 and uses 4 small, cheap 0.1uF ceramic caps, so there isn't
much sense in bothering to use the +12V.
If you need operation from 3V, then look at MAX3232.
Charlie Allen
(Inventor or MAX232)
Gary Reichlinger
Christian Hoene <ho...@cs.tu-berlin.de> wrote:
>in a small portable computer system I want to use a max232 serial
>driver.
>There is already a 12V supply on board. Could I use the 12V supply for
>V+ instead of the max232's charge pump?
Instead of using the MAX212, you should use the MAX211. The MAX211 is
designed for just such situations where a 12v supply is available and
accordingly costs less, requires fewer external capacitors, etc. Also,
there are newer parts than either the MAX212 or MAX211. The MAX202
replaces the MAX212 and the MAX201 replaces the MAX211. The difference
is that smaller capacitors can be used with these other parts. This is
fairly significant because you can then use ceramics which have better
temperature and noise characteristics than eletrolytics.
Charlie Allen
Peter wrote:
>
>
> Do you know the Rds and the turn-on times of the Maxim MAX232A FETs?
>
> I have found, in EMC testing on several products, that the MAX232
> generated far more RFI than the rest of the product together.
>
> It turned out that unless the tracks to the capacitors were very short
> (on the 2 caps which go *only* the the chip) and very directly and
> carefully routed (on the other two caps where they share VCC/GND pins)
> one ends up generating a vast amount of RFI from noise injected into
> the GND/VCC rails.
>
> I measured peak currents of nearly 1 amp, with risetimes below 10ns.
> That is a lot of RF power.
>
> Maxim don't AFAIK warn about this in their data, and I wonder if other
> vendors' MAX232A is better.
>
>
I have occasionally heard hints of what you describe, but never at that
severity. Normally the bypass capacitor from Vcc to Gnd picks up the
switching currents and if that loop is small the radiated noise is
small.
On the newer products, such as MAX202 and MAX3232 we have added more
deadtime on the charge pumps ---- not because of EMI, but to lower the
no-load Iq. In some other cases, the charge pump noise will cause
problems with very sensitive data acquistion circuits powered from the
same +5V.
The A version of the MAX232 has much faster charge pump switching that
the old non-A MAX232, or the 0.1uF replacement MAX202 family. Simply
changing to the pin-equivalent MAX202 will knock down the EMI , and also
be kinder to any analog circuitry sharing the 5V supply.
If you truly want to knock the input ripple current down by a large
factor (for either EMI or "kindness to other 5V analog circuits"),
simply increase the ESR of the capacitor C1 either by using a high ESR
capacitor or putting 2 to 5 ohms in series with it.
Charlie Allen
Matthias Weingart
In article <349df123...@news.navix.net>, rei...@navix.net says...
One disadvantage: the delivery time of the +5V/+12V supply parts (like Max231,
MAX211 ...) is long, and there is no second source for these parts. I have
switched to the MAX232 in my designs because of this.
Matthias