Summary of Getting character LCDs to work at 3 volts
This article explains how to operate standard HD44780-compatible character LCD modules at 3 volts. It identifies two main issues: the need for a negative voltage on the VLCD pin to achieve the required driving voltage, and a slower RC oscillator frequency that may require resistor replacement. The solution involves using a charge pump to generate the necessary negative voltage from a 2.7V to 3.3V supply.
Parts used in Character LCD 3V Operation:
- HD44780 compatible controller
- Character LCD module
- Charge pump circuit
- Rosc resistor
Generic character LCD modules contain an industry standard HD44780 compatible controller*, which can operate down at 3 volts. But the modules are usually specified to work only at 5 volts, unless you choose a specific one designed for 3V operation.
There are two reasons why the modules won’t work at 3 volts. First, the required LCD driving voltage is between 3.8V and 4.4V. The driving voltage is the voltage measured between VDD (pin#2 on pictures) and VLCD (pin#3 on pictures). There is no problem to adjust the VLCD voltage between 0.6V and 1.2V at 5 volts to get VDD-LVCD between 3.8V and 4.4V
But at 2.7V, 3V or 3.3V the voltage on VLCD (pin#3) should be negative to get the required LCD driving voltage (min. 3.8V) between VDD and VLCD.
Second reason is that the RC oscillator of the controller will get slower at 3 volts, so the Rosc resistor on the LCD module may need to be replaced if the program working with the LCD goes too fast for the LCD to execute the commands on the reduced frequency.
To solve the first problem, the circuit needs a negative voltage to set the required voltage on the VLCD (pin#3) pin. This is easily accomplished with a small charge pump which converts the supply voltage of 2.7-3.3 volts to -2.7 to -3.3 volts.
For more detail: Getting character LCDs to work at 3 volts
- Why do standard LCD modules not work at 3 volts?
They require a specific driving voltage between 3.8V and 4.4V and an RC oscillator that slows down at lower voltages. - What is the required LCD driving voltage?
The driving voltage must be between 3.8V and 4.4V measured between VDD and VLCD. - How can the first problem of low voltage operation be solved?
Use a small charge pump to convert the 2.7-3.3 volt supply to a negative voltage. - What happens to the RC oscillator at 3 volts?
The oscillator gets slower, which may cause the program to run too fast for the LCD to execute commands. - Can the Rosc resistor be replaced?
Yes, the Rosc resistor may need to be replaced if the program goes too fast for the reduced frequency. - What voltage range does the charge pump produce?
The charge pump converts the supply voltage to a negative range of -2.7 to -3.3 volts. - Which pins are relevant for measuring the driving voltage?
Voltage is measured between VDD (pin#2) and VLCD (pin#3).
