On-chip LCD Driver pins

[@rt]

Hi, I have a Radio Shack/ Realistic/ Sangean radio with a dim LCD.
LCD is currently best viewed from a shelf above head height.

It has uPD75308 microcontroller with built in LCD driver, and I have identified these pins, and matched the to the radio schematic.
It it as I think, where LCD Bias is an AC output which is divided and fed back into the controller for column voltages?

I’m thinking I should be able to lower the value of R417 from 33K, to supply a higher voltage to the entire display.
Is this correct? Cheers!

[Miti]

You can try that, or you can use higher external bias.

Edit: I should have read your entire post. No, LCD bias is not AC, it is DC.

[@rt]

You can try that, or you can use higher external bias.

Edit: I should have read your entire post. No, LCD bias is not AC, it is DC.

Thanks for the reply but it took so long to get any reply at all, and I went ahead.
I found a Microchip document on it, and it appears this resistor divider arrangement is common.
The variable resistor is typically added to the top, and it changes the voltage of every tap.

Only issue is the 3 inputs are limited in the data sheet to the chip’s supply rail.
When I opened the device, the 33K resistor never existed. It was a 0 ohm link, which was already feeding the top of the divider with Vdd.
So I took the risk and applied a much higher voltage, about 5.5V in place of 3.7V, which fixes the display, but runs the microcontroller out of spec.
Any thought on this?

[Miti]

Only issue is the 3 inputs are limited in the data sheet to the chip’s supply rail.
When I opened the device, the 33K resistor never existed. It was a 0 ohm link, which was already feeding the top of the divider with Vdd.
So I took the risk and applied a much higher voltage, about 5.5V in place of 3.7V, which fixes the display, but runs the microcontroller out of spec.
Any thought on this?

That is never a good idea.

Edit: The operating voltage range is 2.7 to 6V and the absolute maximum Vdd is 7V, so I think the uC is safe. I'm not sure what other ICs are on the same supply line and are pushed over their limits. Are you sure that the supply voltage should be 3.7V? It doesn't seem like a standard value to me. Maybe it was supposed to be 5V but it lost few volts over time.

[@rt]

Only issue is the 3 inputs are limited in the data sheet to the chip’s supply rail.
When I opened the device, the 33K resistor never existed. It was a 0 ohm link, which was already feeding the top of the divider with Vdd.
So I took the risk and applied a much higher voltage, about 5.5V in place of 3.7V, which fixes the display, but runs the microcontroller out of spec.
Any thought on this?

That is never a good idea.

Edit: The operating voltage range is 2.7 to 6V and the absolute maximum Vdd is 7V, so I think the uC is safe. I'm not sure what other ICs are on the same supply line and are pushed over their limits. Are you sure that the supply voltage should be 3.7V? It doesn't seem like a standard value to me. Maybe it was supposed to be 5V but it lost few volts over time.

The schematic says it’s a 3V reg, but the data sheet of the reg quotes 3.7V.

I didn’t run the chip at a higher voltage, I fed the 5.5V into the top of the resistor ladder for the LCD voltage.
Still out of spec because the data sheet for the chip I posted above quoted Vdd (3.7V in this case) is the maximum voltage that should be input to the LCD supply pins VLCD0 - VLCD2.

[Miti]

Did you measure the supply voltage after the mod? It may have gone up through the clamp diode of the VLC0 pin, if it has one.