On Tue, 14 Mar 2000, Geoffrey Wossum wrote:
> > Another suggestion: use a charge pump to generate the +/- 5v. You
> > probably have a Maxim MAX232 for your serial connection anyway, right?
> > It has *two* buffers - use one for serial, the other for the LCD.
> MAX232 gives +/- 10V outputs in my experience. Probably less on 3.3V
> supply, if it would even run on 3.3 for you. You could regulate the
> output down to 5V I suppose.
> Big problem is I don't think MAX232 (or anything else in the family) would
> let you draw much current off of it. They're rated for around 850mW of
> power dissipation, which is 170mA @ 5V. Don't know if that's enough for
> an LCD. Or if it's a good idea to be pulling current off of it.
A while ago, when I was into Basic Stamp stuff, I had tried powering a
mouse using a MAX232. They can't handle very much current. The MAX3241
I'm using is designed for 3.3v, has 3 transmitters, and 5 receivers.
It was designed for laptop computers, and Maxim guarantees that it can
drive a mouse. Mice generally use about 10ma, which is similar to what
an LCD needs. The voltages it produces are available on the charge pump's
capacitors, so it wouldn't require devoting 2 transmitters to it. It's a
good idea. I will have to test the stability of the voltages it produces.
The negative supply can vary, because it goes through the contrast PWM
anyway, but my LCD has been fairly picky about its 5v supply. That might
work, and if it does it would save about $6, and of course reduce
complexity. Also, turning off the LCD would be as simple as turning off
the serial port.
> Geoffrey Wossum
> Project AKO - http://rover1.uta.edu/~ako
> Internet Imperialists - http://inetimperial.sourceforge.net
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