Hi,
The datasheet should tell you. --> read the datasheet.
Also read about valid VCC range.
Most displays need a contrast voltage (VEE) that is related to +supply (VCC) instead of ground (GND)
In this case you need to keep (VCC - VEE) constant.
You may build an analog circuit to keep it constant.
With low VCC it may be (datasheet) that VEE needs to be negative.
Then a capacitive voltage inverter circuit should work.
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I usually use stable VCC.
And I usually use the microcontroller PWM to adjust VEE voltage. Thus I'm able to compensate for diplay production tolerance and temperature (contrast voltage depends on temperature).
Klaus