LCD_A Controller Operation
19-7LCD_A Controller
The internal bias generator supports 1/2 bias LCDs when LCD2B = 1, and 1/3
bias LCDs when LCD2B = 0 in 2-mux, 3-mux, and 4-mux modes. In static
mode the internal divider is disabled.
Some devices share the LCDCAP, R33, and R23 functions. In this case, the
charge pump cannot be used together with an external resistor divider with 1/3
biasing. When R03 is not available externally. The V5 is always AVSS.
LCD Contrast Control
The peak voltage of the output waveforms together with the selected mode
and biasing determine the contrast and the contrast ratio of the LCD. The LCD
contrast can be controlled in software by adjusting the LCD voltage generated
by the integrated charge pump using the VLCDx settings.
The contrast ratio depends on the used LCD display and the selected biasing
scheme. Table 191 shows the biasing configurations that apply to the
different modes together with the RMS voltages for the segments turned on
(VRMS,ON) and turned off (VRMS,OFF) as functions of VLCD. It also shows the
resulting contrast ratios between the on and off states.
Table 191.LCD Voltage and Biasing Characteristics
Mode Bias
Config. LCDMx LCD2B COM
Lines Voltage
Levels VRMS,OFF
/VLCD
VRMS,ON/
VLCD
Contrast
Ratio
VRMS,ON/
VRMS,OFF
static static 00 X 1 V1, V5 0 1 1/0
2mux 1/2 01 1 2 V1, V3, V5 0.354 0.791 2.236
2mux 1/3 01 0 2 V1, V2, V4, V5 0.333 0.745 2.236
3mux 1/2 10 1 3 V1, V3, V5 0.408 0.707 1.732
3mux 1/3 10 0 3 V1, V2, V4, V5 0.333 0.638 1.915
4mux 1/2 11 1 4 V1, V3, V5 0.433 0.661 1.528
4mux 1/3 11 0 4 V1, V2, V4, V5 0.333 0.577 1.732
A typical approach to determine the required VLCD is by equating VRMS,OFF
with a defined LCD threshold voltage, typically when the LCD exhibits
approximately 10% contrast (Vth,10%): VRMS,OFF = Vth,10%. Using the values
for VRMS,OFF/VLCD provided in the table results in VLCD =
Vth,10%/(VRMS,OFF/VLCD). In the static mode a suitable choice is VLCD greater
or equal than 3 times Vth,10%.
In 3-mux and 4-mux mode typically a 1/3 biasing is used but a 1/2 biasing
scheme is also possible. The 1/2 bias reduces the contrast ratio but the
advantage is a reduction of the required full-scale LCD voltage VLCD.