PCF8812USLASH2 datasheet(17/36 Pages) PHILIPS

17 / 36 page

2000 Nov 22

Philips Semiconductors

Product speciﬁcation

× 102 pixels matrix LCD driver

PCF8812

10.9

Temperature control

Due to the temperature dependency of the liquid crystals

viscosity the LCD controlling voltage VLCD must be

increased with lower temperature to maintain optimum

contrast. There are 4 different temperature coefficients

available in the PCF8812 (see Fig.13). The coefficients

are selected by bits TC1 to TC0. Table 6 shows the typical

values of the different temperature coefficients. The

coefficients are proportional to the programmed VLCD.

10.10 Set VOP value

The operating voltage VLCD can be set by software. The

generated voltage is dependent on temperature,

programmed Temperature Coefficient (TC) and the

programmed voltage at reference temperature (Tcut).

(1)

The voltage at reference temperature [VLCD(T = Tcut)] can

be calculated as follows:

(2)

The parameters are explained in Table 6. The maximum

voltage that can be generated is dependent on the VDD2

voltage and the display load current. Two overlapping

VLCD ranges are selectable via the command ‘HV-gen

control’. For the LOW (PRS = 0) range a = a1 and for the

HIGH (PRS = 1) range a = a2 with steps equal to ‘b’ in both

ranges. It should be noted that the charge pump is turned

off if VOP 6 to 0 and the bit PRS are all set to zero

(see Fig.14).

For MUX 1 to 65 the optimum operating voltage of the

liquid can be calculated as follows;

(3)

where Vth is the threshold voltage of the liquid crystal

material used.

Table 6

Typical values for parameters for the

HV-generator programming

As the programming range for the internally generated

VLCD allows values above the maximum allowed VLCD

(9 V) the user has to ensure, while setting the VOP register

and selecting the Temperature Compensation (TC), that

under all conditions and including all tolerances that VLCD

remains below 9 V.

handbook, halfpage

MGS402

VLCD

Tcut

Fig.13 Temperature coefficients.

LCD T

()

() 1T T

cut

–

() TC

()

LCD T = T

cut

()

SYMBOL

VALUE

UNIT

2.94 (PRS = 0)

6.75 (PRS = 1)

0.03

Tcut

°C

LCD

165

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6.85

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