PCF8820U datasheet(8/60 Pages) PHILIPS

8 / 60 page

2000 Dec 07

Philips Semiconductors

Product speciﬁcation

× 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 VDD2 and

VDD3.

The multiplier is software programmable with a factor from

2 to 8. In the direct drive mode the output voltage

VLCDOUT =VDD2.

7.13

Temperature compensation

The viscosity of the liquid crystal depends on the

temperature; so to maintain optimum contrast at lower

temperatures VLCD needs usually to be increased. Fig.2

shows VLCD 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 VLCDOUT can be

set to one of 8 values by setting bits TC2 to TC0.

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 I2C-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

MGT123

VLCD

(V)

Tamb (°C)

(1)

(2)

Fig.2

LCD supply voltage as a function of the

temperature.

(1) LCD characteristic.

(2) Linear temperature compensation.

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