At this point the noise does not appear to be negatively affecting the rest
of my system - do you think I will encounter long term damage to the PIC by
powering the chip this way?  The max continuous current draw of the IC is
only 1mA.  I agree that a transistor is a good idea but I already have the
boards cut so I am somewhat committed to my design if it is unlikely to
cause problems down the road.

Thanks,
Nick



----- Original Message -----
From: "Olin Lathrop" <olin_piclist@EMBEDINC.COM>
To: <PICLIST@MITVMA.MIT.EDU>
Sent: Friday, January 09, 2004 6:24 AM
Subject: Re: [PICLIST] [PIC:] RS232 Driver IC Supply Noise


> James Nick Sears wrote:
> > I am using a TI MAX3221 RS232 driver IC with my 16F877A based
> > datalogger circuit and am having trouble getting the 3221 supply
> > cleaned up.  It is powered from a PIC pin and when it is turned on
> > there is a nasty 1V ripple at about 20kHz on the driver's Vcc pin
> > that appears to coincide with the switching waveform on C1 and C2 of
> > the charge pump.
>
> It is not appropriate to power these devices from a PIC pin since they
draw
> short bursts of relatively large current.
>
> Use a PNP transistor.  Emitter to +5V, collector to driver IC power input,
> base to PIC pin via 2Kohm resistor.  PIC pin low now enables the driver
IC,
> high disables it.  I would also put a 10uF electrolytic accross the driver
> IC power pins in addition to the 100nF ceramic.
>
>
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