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Possible to PWM with 3.3V for a CFAG14432D?

nonarKitten

New member
Okay, I'm using an MCu that's 3.3V only (MCF52259) and would like to PWM drive the backlight and contrast of an CFAG14432D. I do have 5V available for power (for the backlight, for example), and would like to avoid the use of level shifters, etc, to keep complexity down.

1.) With the PWM, how many channels will I need; for example, are two enough (one for K and A, and the other tied to V0?). The scematic for the LCD controller also has GRN and BLU, which I'd assume I don't need, and VEE, which I can't correlate with anything on the CFAG14432D.

2.) I read elsewhere that it's possible to drive these at 3.3V, but you need to bring the VSS down to about 4.5 to 4.8VDC. Would a voltage divider work for this (see attachment)? It's only sourcing 2.2mA, so a linear voltage regulator seems overkill.
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The scematic for the LCD controller also has GRN and BLU, which I'd assume I don't need, and VEE, which I can't correlate with anything on the CFAG14432D.
You haven't mentioned which exact model number you are referring to, but I don't see any mention of GRN, BLU, or VEE. Can you supply a link to the document you are talking about?

To drive the display signals with 3.3 volt logic levels, it may be a better idea to use pull-up resistors on the display inputs, instead of reducing the Vdd voltage. I suggest 10K ohms for each pullup. Reducing Vdd without regulating it seems like it could cause problems (you want a rock steady voltage and a low power source impedance), you could use a LM317L adjustable regulator, set to 4.5 volts, if you don't like the idea of pull-up resistors.

As for PWM, you will need 2 channels.

For the backlight you will need a transistor to carry the B/L current (typically a N_channel FET is used), and a current limiting resistor to keep from frying the backlight.

Controlling contrast via PWM is trickier, because the voltage for best contrast is commonly near to 0 volts (ground), and in some cases you may need to go to a slightly negative voltage. You must also use resistance values that are small enough to allow a few mA to flow from the Vo pin to the negative supply (either gnd or -v), since the Vo circuit wants a current sink, not just a voltage. You should filter the voltage to avoid interference (beat patterns) due to the PWM frequency. See this thread for more info: https://forum.crystalfontz.com/showthread.php?t=6287
 
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