Lpc2000

Dear Sean,

we had to deal with this issue on our unversal LCD controller boards - see 
at www.demmel.com - and we solved it as follows: We took the (very cheap) 
switcher IC TPS61041, which can produce up to + or -28 volts and made the 
output voltage variable by feeding a PWM controlled voltage of 0...5 (can 
be 0...3.3 volt as well) volt via an 100k resistor to the FB input. You 
only need the TPS61041 which only costs about 1.5 US$ (single pieces) and a 
ready to use Murata 10uH LQH32CN100K33L which costs less than 50 cent 
(price is also for single pieces). You only need a few passive components 
and a shottky diode like MBR0540T1 (about 40 cent) then and everything is 
working fine. You get all parts at www.farnell.com, TI has good reference 
schematics, you only have take care to use X7R capacitors. The TPS61041 can 
be supplied with voltages between 1.8 and 6 volts.

The good thing is, that you can control the voltage with the resolution of 
your PWM and you may save the setting directly in the uP's flash memory.

Hope that helps
Herbert

At 21:26 13.01.2006, you wrote:
>Sorry for the OT post, but I figure that since this is something common to
>embedded projects you guys might have an elegant solution.
>
>I'm looking to create a contrast voltage (typ -13V 10mA) and a LCD Drive
>voltage (min -23V 10mA), onboard I have +5V, +3.3V and Vin which is
>typically +12V.  Ideally the contrast should be digitally controllable.
>
>The big part of the problem: it needs to be relatively cheap.
>
>I've been having a hard time trying to figure out the best way to do
>this.  The best way that I can think of so far is going to be via voltage
>multipliers.  I.e. take 3.3, invert and quadruple to get -13.2V for the
>contrast, and then double again to get -26.4V for the LCD Drive.  I don't
>know however if I can handle 10mA this way after octupling the voltage.
>
>I'm trying to stay away from switched regulators, as they can get expensive.
>
>Thanks for any suggestions that you may have.
>
>-- Sean

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