OM6211 데이터 시트보기 (PDF) - Philips Electronics

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OM6211

48×84 dot matrix LCD driver

Philips Electronics 

Philips Semiconductors

48 × 84 dot matrix LCD driver

Product specification

OM6211

9.11 Temperature compensation

Due to the temperature dependency of the liquid crystals viscosity, the LCD controlling voltage (VLCD) must be increased

at lower temperatures to maintain optimum contrast. Figure 4 shows VLCD as a function of temperature for a typical high

multiplex rate liquid.

In the OM6211 the temperature coefficient of VLCD can be selected from 4 values by setting bits TC1 and TC0,

see Tables 4 and 8.

handbook, full pagewidth

VLCD

MGT848

T

Fig.4 VLCD as a function of liquid crystal temperature (typical values).

9.12 VLCD generator

The binary number VOP representing the operating voltage

can be set by the serial interface command and can be

adjusted (calibrated) by 5 input pins according to the

following formula:

VOP = VPR + VOS

(1)

where:

• VPR is an 8-bit unsigned number set by the serial

interface command

• VOS is a 5-bit two’s complement number set by the

5 input pins VOS4 to VOS0, see Table 9

• VOP is an 8-bit unsigned number used internally for

generation of the LCD supply voltage VLCD.

To avoid numerical overflow the allowed values of VPR

should be limited to the range 32 to 225 (decimal).

The corresponding voltage at the reference temperature,

Tnom, can be calculated as follows:

VLCD(Tnom) = (a + VOP × b)

(2)

The generated voltage at VLCD is dependent on the

temperature, programmed Temperature Coefficient (TC)

and the programmed voltage at the reference temperature

(Tnom).

VLCD = (a + VOP × b) × [1 + TC × (T – Tnom)]

(3)

Tnom, a and b for each temperature coefficient are given in

Table 4. The maximum voltage that can be generated is

dependent on the voltage of VDD2 and the display load

current.

As the programming range for the internally generated

VLCD allows values above the maximum allowed VLCD, the

user has to ensure while setting the VPR register and

selecting the Temperature Compensation, that under all

conditions and including all tolerances the VLCD limit of

maximum 9 V will never be exceeded.

For a particular liquid crystal, the optimum value of VLCD

can be calculated for a given multiplex rate. For a Mux rate

of 1 : 48, the optimum operating voltage of the liquid

crystal can be calculated as follows;

VLCD

=

----------1-----+---------4---8------------

2 × 1 – ----1-4---8--

×

Vth

=

6.06 × Vth

(4)

where Vth is the threshold voltage of the liquid crystal used.

2002 Jan 17

11

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