Im working on a project that uses a PIC16F877 to controll a 240x64 LCD, and
I need to come up with a way
to generate an ajustable 0v to -18v negative output from 1.5 to 2.0ma for
the bias/contrast for the LCD.

I've been toying with a MAX749 which is meant specificly for this but I am
trying to avoid using an inductor, due to lack of ability to get one
cheaply and because of the noise generation (this will be for an in-car
audio  application).

I have also looked into various charge-pump inductorless types like the
MAX681 (and other voltage doubler-inverter chips) but the most they can
give me from +5v in is -10V, which is not enough. Tests have shown that I
will require atleast -16v for any viewability of the display.

The device will be used in a vehicle, so I can get at somewhat(?) regulated
12V power, so i can possibly get -12 or
-24 and resist it down to -18 but I am unsure about the clean-ness of the
12V supply from the car. Note that I will be taking it from an output from
under the dash which I beleave _is_ regulated at least to some degree, I'm
not sure how they regulate the power but there has to be some regulation or
the instrament lights/etc would brighten as the engine RPM's increse.
Anyway if anyone has an idea of a good way using as few parts as possible
that wont generate large amounts of RF noise please share the information.
Rember I require only up to 2ma and I have access to +5v regulated and +12
regulated power sources. (the +5 is regulated from the +12 of the car).

Thanks,
Tim Thompson


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