Thanks Dave, This looks like the best solution for me. I happen to have a 232A laying around, and I'm sure I can scrounge up the diodes and caps. ;-) Michael Brown Instant Net Solutions www.KillerPCs.net "In the land of the blind, he who has one eye is king" ----- Original Message ----- From: "Dave Dilatush" To: Sent: Saturday, March 09, 2002 11:39 AM Subject: Re: [EE]: -15 volts from +9 > Michael Brown wrote... > > >I need an adjustable negative supply voltage in the -13 to -16 volt range > >for the contrast control of an LCD module. I would like to generate this > >supply from a 9 volt battery that is also used for the positive supply of > >the rest of the circuit. I realize that I could just use a couple more 9 > >volt batteries and a pot to do it, but I would like it to be a little more > >compact than that. Can someone suggest some kind of simple charge pump that > >will generate the voltage from the normal supply voltage, without using any > >transformers? I am quite weak when it comes to "funky" analog electronics. > > Maxim's LCD Bias Supply chip will do this job; it even has a built-in > 6-bit DAC to allow the microcontroller to adjust the output voltage. It > requires a 22 microhenry inductor, two small electrolytic caps, a pair > of Schottky diodes, and a few other R's and C's. > > If you'd rather roll your own charge pumper using more readily available > parts, the attached diagram shows an approach using a MAX232A RS-232 > interface chip that'll give you about -18V unloaded. You can feed a > trimpot from that, to adjust the LCD contrast voltage. If you use a > MAX232 instead of a MAX232A, make all the capacitors ten times bigger to > compensate for the slower switching frequency. This design operates off > the +5V supply, which I assume you're regulating; and that eliminates > having to deal with a battery voltage that could be anywhere from 9V > (new) down to about 6V (discharged). > > The MAX232A is a very useful chip. The classical way of looking at it > is as advertised: it's an RS-232 interface chip with a built-in charge > pump for generating the interface voltages. An alternate way of looking > at it is a general-purpose charge pump chip that can generate +/- 8-10V > from a +5V supply, and which also happens to have some built-in RS-232 > I/O, just in case you need it. > > They're real handy for generating +/- voltage for "funky" analog stuff. > > Dave D. > -- http://www.piclist.com hint: The list server can filter out subtopics (like ads or off topics) for you. See http://www.piclist.com/#topics