Lpc2000

Sean,

Herbert's solution looks good.
I did the voltage multiplier stuff (a capacitor/diode network) connected
to a PWM and that does generate the proper voltage for me (around - 4.5V)
but its voltage can not be controlled with pulse width or frequency
change. Voltage does change but there is no way to compute output voltage
using frequency/pulse width.

Rob

> 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
>