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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