FAQ: a simple way to generate a negative voltage for an LCD bias
pedward
Posts: 1,642
This issue was raised in a post on Dangerous Prototypes today, I thought I would just throw a little ditty out here.
In the Maxim datasheet for the MAX232 series of transceivers, it says you can steal power from the charge pump of the chip. I have a few MAX233 chips laying around which have internal capacitors of which I have successfully used the negative supply output of these chips to provide LCD bias voltage.
Increasing the charge pump capacitor values to 1uf gives a stiffer supply without any negative effects.
The chips are rated to supply 10v and -10v at 10ma, but you can harvest up to 20ma from them, albeit at reduced voltages.
Even moderately sized LCDs like a 5.7 inch will be satisfied by these.
You can also use a transistor to modulate the bias with a PWM control -- I did this on a design from 10 years ago.
In the Maxim datasheet for the MAX232 series of transceivers, it says you can steal power from the charge pump of the chip. I have a few MAX233 chips laying around which have internal capacitors of which I have successfully used the negative supply output of these chips to provide LCD bias voltage.
Increasing the charge pump capacitor values to 1uf gives a stiffer supply without any negative effects.
The chips are rated to supply 10v and -10v at 10ma, but you can harvest up to 20ma from them, albeit at reduced voltages.
Even moderately sized LCDs like a 5.7 inch will be satisfied by these.
You can also use a transistor to modulate the bias with a PWM control -- I did this on a design from 10 years ago.
Comments
I was starting to think about how to get +/- 10 V for driving 3D glasses, and this looks like a cool way...