
2000 Dec 07
8
Philips Semiconductors
Product specification
67
×
101 Grey-scale/ECB colour dot matrix
LCD driver
PCF8820
7.11
Bias voltage generator
The bias voltage generator generates 4 buffered
intermediate LCD bias voltages. It contains 4 operational
amplifiers and an input reference voltage generator. It can
operate in two voltage ranges:
Normal mode (from 7.0 to 14.5 V)
Partial screen mode (from 4.5 to 14.5 V).
7.12
High voltage generator
The high voltage generator contains a voltage multiplier
which uses a charge pump circuit supplied by V
DD2
and
V
DD3
.
The multiplier is software programmable with a factor from
2 to 8. In the direct drive mode the output voltage
V
LCDOUT
= V
DD2
.
7.13
Temperature compensation
The viscosity of the liquid crystal depends on the
temperature; so to maintain optimum contrast at lower
temperatures V
LCD
needs usually to be increased. Fig.2
shows V
LCD
as a function of the temperature for a typical
high multiplex rate liquid crystal.
Linear temperature compensation is supported in the
PCF8820. The temperature coefficient for V
LCDOUT
can be
set to one of 8 values by setting bits TC
2
to TC
0
.
7.14
Temperature sensor
The PCF8820 has a built-in temperature sensor. The
sensor monitors the temperature and writes an 8-bit
number into the status register. The temperature sensor
and status register can both be accessed via the I
2
C-bus
interface controller.
The temperature sensor allows any temperature
compensation to be implemented; any programmable
parameter can be optimized as a function of the sensor
read-out temperature.
7.15
LCD driver waveforms
The LCD waveforms are shown in Figs 3, 4 and 5.
At frame inversion, the PCF8820 generates a dummy row
cycle, where no row is selected. This ensures equal
conditions for the first row after frame inversion as for the
other rows. Therefore the effective multiplex rate in all
modes is 1 : (multiplex rate + 1).
7.16
DDRAM to display mapping
DDRAM to display mapping is shown in Fig.6.
handbook, halfpage
VLCD
(V)
MGT123
Tamb (
°
C)
0
(1)
(2)
Fig.2
LCD supply voltage as a function of the
temperature.
(1) LCD characteristic.
(2) Linear temperature compensation.