Using character LCD module at 3 volts

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Using character LCD module at 3 volts

Posted Date: 2024-02-03

Character LCD modules are suitable as display devices connected to small microcontroller systems. It's often used in electronic crafts because it's easy to use and relatively cheap, starting at around $10.

LCD module compared with seven-segment LED display screen, LCD module can display more characters than LED display screen. Most LCD modules consume less than 1 mA, making them suitable for battery-operated devices.

But there is a problem that they cannot be used at low supply voltages, such as 3V and 3.3V, since most of them are designed for 5V systems. However, they can be used at low supply voltages due to certain considerations.

picture. 1

Most character LCD modules use HD44780 (Hitachi) LCD controller, and there are many HD44780 compatible chips. This document assumes that the LCD module uses the HD44780 or compatible product.

The bias voltage between Vdd and Vop is divided by a resistor network on the module, as shown in Figure 1. The voltage is provided to the LCD controller and expansion driver as drive voltage levels. A bias voltage Vdd-Vop of approximately 4.0 to 4.5 volts is required, which also depends on temperature, number of rows, and LCD type (TN or STN). The supply voltage is divided by a trimmer resistor and provided to Vop as a bias power supply.

When lowering the supply voltage, even if Vop is connected to Vss, the display will brighten and disappear at about 3.5 volts. This means that the supply voltage of 3 volts is not enough to drive the LCD panel.

negative supply voltage

Figure II

When the supply voltage Vdd is 3 volts, the bias voltage applied to Vop becomes lower than Vss because a bias voltage of 4 volts is required between Vdd and Vop. The HD44780U also allows negative bias voltages.

The circuit diagram for the 3-volt system power supply is shown in Figure 2. It requires a charge pump circuit to generate negative supply voltage. Contrast can be adjusted via series resistor value or switching frequency. When using the microcontroller's pulse output instead, the contrast can be controlled in software.

When the power supply voltage decreases, the oscillation frequency of the built-in clock oscillator decreases. When an extension of the command execution time is unacceptable, Rosc should be changed to a value suitable for the supply voltage. Rosc is the 91k ohm resistor on the LCD module (may be a different value).

When the supply voltage changes, the interface timing is specified by different specifications for supply voltages from 2.7 to 5.5 volts and can therefore be considered in fast microcontrollers. For details, please refer to the datasheet of the LCD control chip on the module.

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