Temperature Compensation for LCDs
The optimal contrast setting for liquid crystal displays (LCDs) changes with ambient temperature. Within the standard operating temperature range of 0°C to +50°C, this variation is generally acceptable for most applications. However, many LCD modules offer an extended temperature range of -20°C to +70°C, over which contrast changes can become significant and unacceptable without compensation.
At lower temperatures, the LCD fluid requires a higher driving voltage to maintain consistent optical contrast. This behavior is illustrated in Figure 1. One method of compensation is to allow manual adjustment of contrast by the user. While simple, this approach is often impractical or undesirable.
Figure 1 Explanation:
The solid line in Figure 1 represents a temperature-compensated contrast voltage as produced by the circuit shown in Figure 2. The dashed line indicates how the LCD’s required operating voltage naturally varies with temperature.
To maintain good display contrast, the voltage at VL (contrast input) must be adjusted according to ambient temperature. For example, if a display performs optimally with -3V at VL at 25°C, it may require -2.7V at 50°C. This voltage can be estimated by adding the “relative voltage” shown in the graph to the known contrast voltage at 25°C.
While a microprocessor could monitor temperature and adjust VL accordingly, this method is typically too complex and costly. A more practical and widely used solution is the automatic compensation circuit shown in Figure 2.
Figure 2 – Automatic Temperature Compensation Circuit
This circuit utilises a negative temperature coefficient (NTC) thermistor to sense ambient temperature. For best performance, the thermistor should be mounted as close to the LCD module as possible. A PNP transistor, configured as an emitter follower, provides the required drive current to the LCD’s VL input.
The supply voltage VEE will vary depending on the LCD’s specifications. Note that both VL and VEE are referenced to the LCD’s VDD. For example, an extended temperature range character LCD may require approximately -7.8V at VL (relative to VDD) at 25°C, equating to around -2.8V with respect to ground. VEE should be about 25% higher than the required VL to ensure proper operation.
During development, VEE should be adjustable to fine-tune contrast. In production units, it can be fixed or remain adjustable depending on design requirements.
This compensation circuit is suitable for character LCDs and graphic LCDs up to 320 × 240 resolution. Larger modules typically do not support extended temperature operation.