4.4 Contrast Control

As mentioned in Section 4.2, when using the charge pump the V_LCD of the display is software controlled. This allows the user to easily adjust contrast in software. Changing V_LCD adjusts all of the other LCD voltages as well regardless of internal or external biasing, because all of the other voltages are simply divided down from V_LCD.

When using R03EXT bit to source V5 voltage externally, contrast can also be adjusted in hardware by changing the optional resistor Rx on the R03 pin in Figure 6 – this changes the voltage at the low end of the resistor divider. When using an external bias resistor ladder for generating the bias voltages, the sizing of R can also have an impact on contrast. If contrast is not even across all segments, the size of the resistors in the bias ladder may need to be reduced at the tradeoff of additional current consumption (see Section 4.3 on biasing).

The different biasing modes and the particular LCD being used can also have an impact on contrast ratio. As you can see in Table 2, the contrast ratio can be represented as V_RMS,ON/V_RMS,OFF, or the RMS voltage from the waveforms for a segment that is on, divided by the RMS voltage from the waveforms for a segment that is off. The higher the contrast ratio, the greater the difference in appearance of an on segment versus an off segment. Table 2 shows that there is better or worse contrast depending on the bias configuration and muxing of the display – this is because these settings affect the characteristics of the waveforms that are output. As shown in the table, higher mux rates tend to have more lower contrast ratios, which means that their performance is more sensitive to any tradeoffs that affect contrast – this means that a more sensitive LCD glass with a better threshold, or other factors to provide better contrast (such as higher V_LCD, smaller bias resistors, or the techniques from Section 4.10), might be needed for the desired LCD performance.

A typical approach to determine the V_LCD for good contrast, is to use the threshold voltage when the contrast is 10% and use this with the V_RMS,OFF/V_LCD ratio from the user's guide table to calculate a recommended V_LCD using this equation: V_LCD = V_th,10%/(V_RMS,OFF/V_LCD). The V_th,10% is a characteristic of the fluid used in the LCD display, so it varies with the display. The display information provided by the manufacturer typically lists a visual threshold voltage for 10%.

Some configurations trade off a reduced contrast ratio for a reduction of the full-scale LCD voltage V_LCD used. For example, on some modules 1/3 bias may give a better contrast, but the 1/3 bias mode may require a higher V_LCD to be used as well. See the LCD module-specific section in the family user's guide for more information pertaining to the particular LCD module and contrast ratio – the user's guides have tables like the one below with information specific to that module's muxing and bias options (Table 2 shows information for LCD_B).