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Using module with internal dc converter in circuit that already has one

spectrum24

New member
I have a unit with an old LCD module that I would like to replace with a CFAG160128E-YYH-TZ. Understanding that I have to change to 5V for LED backlight, the other question is the existing unit
generates the -16V external to the module - verified by unplugging the LCD and checking the pins going to it. The unit also has software control of contrast using this external source. The CF modules has an internal converter that OUTPUTS -16V on that same pin. That seems like an issue. Is there an easy solution?

thanks
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You could ignore the CFAG160's Vee output (pin 5), and feed the external software controlled -16V to the contrast input (pin 4).

One thing to consider is whether the CFAG160's Toshiba T6963C controller is compatile with the software in the host device.
 

spectrum24

New member
You could ignore the CFAG160's Vee output (pin 5), and feed the external software controlled -16V to the contrast input (pin 4).

One thing to consider is whether the CFAG160's Toshiba T6963C controller is compatile with the software in the host device.
Yes, same controller. What powers the LCD itself then - the internal converter ? If so, it wont care that the contrast voltage is coming from a different source? I would think that the internal and external sources wouldnt be the exact same voltage..
 
The LCD is powered by the input to the contrast pin. The internal -16V converter does not power anything until you feed it (through a contrast adjust circuit) to pin 4. It doesn't matter if the external -16V and the internal -16V are not exactly the same; only one of them should be used. The contrast voltage is referenced to the Vdd (+5V) source, and pulls current to the negative supply from pin 4. The actual voltage at the contrast pin (4) will not necessarily be -16V for best contrast; perhaps -12 ~ -13 volts (approx) is optimum, thats why you need the contrast adjustment. What you're adjusting is the current being pulled; the voltage at the pin 4 is a side effect which depends on the internal impedance of the LCD.

BTW, the terminology in the datasheet is not consistant. Vdd and Vcc both refer to the +5V. Vadj and V0 both refer to the contrast input voltage (pin 4).
 

spectrum24

New member
The LCD is powered by the input to the contrast pin. The internal -16V converter does not power anything until you feed it (through a contrast adjust circuit) to pin 4. It doesn't matter if the external -16V and the internal -16V are not exactly the same; only one of them should be used. The contrast voltage is referenced to the Vdd (+5V) source, and pulls current to the negative supply from pin 4. The actual voltage at the contrast pin (4) will not necessarily be -16V for best contrast; perhaps -12 ~ -13 volts (approx) is optimum, thats why you need the contrast adjustment. What you're adjusting is the current being pulled; the voltage at the pin 4 is a side effect which depends on the internal impedance of the LCD.

BTW, the terminology in the datasheet is not consistant. Vdd and Vcc both refer to the +5V. Vadj and V0 both refer to the contrast input voltage (pin 4).

Very useful information, thank you! I have not worked with these kinds of displays before.
 

spectrum24

New member
The LCD is powered by the input to the contrast pin. The internal -16V converter does not power anything until you feed it (through a contrast adjust circuit) to pin 4. It doesn't matter if the external -16V and the internal -16V are not exactly the same; only one of them should be used. The contrast voltage is referenced to the Vdd (+5V) source, and pulls current to the negative supply from pin 4. The actual voltage at the contrast pin (4) will not necessarily be -16V for best contrast; perhaps -12 ~ -13 volts (approx) is optimum, thats why you need the contrast adjustment. What you're adjusting is the current being pulled; the voltage at the pin 4 is a side effect which depends on the internal impedance of the LCD.

BTW, the terminology in the datasheet is not consistant. Vdd and Vcc both refer to the +5V. Vadj and V0 both refer to the contrast input voltage (pin 4).
On the existing one, there is -16 (ish) being fed into the Vee Pin, and then a varying voltage into pin 4. Why would anything be fed into the Vee pin on the old one, if it really only is powered via pin 4?
 
... Why would anything be fed into the Vee pin on the old one, if it really only is powered via pin 4?
I don't know.
Are you sure there is -16V being fed into Vee (rather than being taken from Vee to create the pin 4 feed)?
Can you disconnect pin 5 and still get an image on the LCD?
With pin 5 disconnected, is the display actually outputting -16 on pin 5? Not all displays have the Vee generator circuit populated on board.
Hard to diagnose by remote control :D.
 

spectrum24

New member
I don't know.
Are you sure there is -16V being fed into Vee (rather than being taken from Vee to create the pin 4 feed)?
Can you disconnect pin 5 and still get an image on the LCD?
With pin 5 disconnected, is the display actually outputting -16 on pin 5? Not all displays have the Vee generator circuit populated on board.
Hard to diagnose by remote control :D.
Got that part working - now new issue, I posted on it. thanks..
 
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