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LCD Contrast Bias Problems

I'm using an Okaya RC2004ARS-DSC-A (RC2004D, the baby 20x4) display in a 3.3V system. I originally planned on using a negative voltage charge pump driven by a PWM pin on a PIC (a circuit I've used before successfully.) When I couldn't get that to work I tried a ICL7660 neg chg pump. No luck on anything. Everything logic-wise appears to be operating normally, it init's fine, etc. When I put 5V on the system I can actually get the contrast up enough to start to see the characters.

There is an Okaya appnote that speaks of internal and external contrast adjustment on their modules. However, none of the jumpers they describe appear on the RC2004. So I'm assuming it doesn't have that capability (which could be causing the problem if somehow the internal contrast selection was jumpered on.)

When I probe the Vo pin, I get an impedance of about 3K ohms. When I test the ICL7660 without it connected to the V0 pin I get a good V of about -3.2V from a 3.3V supply. However, when I connect the ICL7660 output to the Vo pin all I get is -0.7V when scoped. The 4V bias obviously isn't enough when I need at least 5+ volts to start to see the characters when tested with a 5V Vdd. BTW, one would expect that when using the ICL7660 that would produce a bias of about 6.5V and the screen would be over contrasted and be dark. No such luck, it's as if there's no bias at all.

Any ideas or suggestions would be hugely appreciated,

John

Reply to
jecottrell65
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OK.

I've narrowed it down the Microchip MCP41010 SPI Digipot. If I replace the 41010 with a regular old pot everything works normally. I remembered there was a 1mA limit on the wiper of that part so I added a 4.7K current limiter. But that doesn't change anything.

Any ideas?

Thanks,

John

Reply to
jecottrell65

The Digipot needs all inputs to be between the power rails. Your negative voltage is being clamped by an internal diode to Gnd.

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Regards
Miguel Giménez
Reply to
Miguel Gimenez

OK. That makes sense. Thanks for the info. I think I see where that is mentioned in the datasheet.

Does anyone know of a digipot that isn't limited in this way?

Thanks,

John

Reply to
jecottrell65

Sounds like you are creating a negative voltage with the 7660, then adjusting it with the digipot to feed the LCD. Do it the other way. Create an adjustable positive voltage, either from a PWM or otherwise, then feed that to the 7660 to invert it, and feed the output direct to the LCD.

--
Regards,

Adrian Jansen           adrianjansen at internode dot on dot net
Design Engineer         J & K Micro Systems
Microcomputer solutions for industrial control
Note reply address is invalid, convert address above to machine form.
Reply to
Adrian Jansen

OK. That makes sense also. I had thought of maybe using the 7660 (as a voltage doubler) to supply the LCD and then use the digipot normally as a contrast adjust between Vss and Vdd, of course Vdd for the LCD now would be 2 x supply voltage to the 7660.

Thanks again,

John

Reply to
jecottrell65

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