Schottky Diodes (BAT85s or similar) in place of the 1N4148s will get you
closer but you may need to go to a doubler or tripler arrangement to
overcome the diode losses.
I can't remember the circuit exactly but it is just an extension of a
common charge pump configuration. If you get stuck I should be able to find
it.

Richard P



Hi,
  I'm currently designing a portable hand-held terminal around a PIC16F628
(the cheapest variant of 18-pin PIC in my Box O' PICs). The problem is,
this thing has to run off a Lithium battery - that's 3V, 1AH (if I can get
one that big). I've got the power consumption of the chip itself down to
30uA (37kHz internal oscillator), but I still haven't found a way to
generate the -3V contrast voltage for the LCD display. I've built up a
charge-pump using two 1N4148s and some capacitors (a 0.1uF and a 0.22uF)
but
I can only get -1.8V out of it.
  Does anyone know how I might be able to get -3V out of a chargepump? I
don't want to add a chargepump controller IC due to the increase in current
consumption. It would be handy if the chargepump connected to RB3 (PWM Out)
as this would enable me to vary the PWM rate to adjust the contrast. The
other advantage to using the PWM controller would be that I could turn off
the PWM output at the same time as the LCD power, thereby reducing the
power consumption.
  Any ideas?

Thanks.
--
Phil.
philpem@dsl.pipex.com
http://www.philpem.dsl.pipex.com/

--
http://www.piclist.com hint: To leave the PICList
mailto:piclist-unsubscribe-request@mitvma.mit.edu

--
http://www.piclist.com hint: To leave the PICList
mailto:piclist-unsubscribe-request@mitvma.mit.edu