PCF8534AH datasheet(9/41 Pages) NXP

9 / 41 page

PCF8534A_0

Product data sheet

Rev. 00.90 — 4 February 2008

9 of 41

NXP Semiconductors

PCF8534A

Universal LCD driver for low multiplex rates

Data transfers on the I2C-bus must be avoided for 1 ms following power-on to allow

completion of the reset action.

7.2 LCD bias generator

Fractional LCD biasing voltages are obtained from an internal voltage divider of the three

series resistors connected between VLCD and VSS. The center resistor can be switched

out of the circuit to provide a 1

⁄2 bias voltage level for the 1:2 multiplex configuration.

7.3 LCD voltage selector

The LCD voltage selector co-ordinates the multiplexing of the LCD in accordance with the

selected LCD drive configuration. The operation of the voltage selector is controlled by

MODE SET commands from the command decoder.

The biasing configurations that apply to the preferred modes of operation, together with

the biasing characteristics as functions of VOP and the resulting discrimination ratios (D),

are shown in Table 5.

A practical value for VOP is determined by equating VOFF(rms) with a defined LCD threshold

voltage (VTH), typically when the LCD exhibits approximately 10 % contrast. In the static

drive mode a suitable choice is VOP

> 3 VTH.

Multiplex drive ratios of 1:3 and 1:4 with 1

⁄2 bias are possible but the discrimination and

hence the contrast ratios are smaller e.g.:

The advantage of these modes is a reduction of the LCD full-scale voltage VOP as follows:

• 1:3 multiplex (1

⁄2 bias):

• 1:4 multiplex (1

⁄2 bias):

These compare with VOP =3

× VOFF(rms) when 1⁄3 bias is used. Note: VOP =VLCD.

1.732 for 1:3 multiplex or

----------

1.528

⎝⎠

⎛⎞

for 1:4 multiplex

OFF rms

()

2.449V

OFF rms

()

---------------------

2.309V

OFF rms

()

Table 5.

Preferred LCD drive modes: summary of characteristics

LCD drive

mode

Number of:

LCD bias

configuration

Backplanes

Levels

static

∞

1:2

⁄2

0.354

0.791

2.236

1:2

⁄3

0.333

0.745

2.236

1:3

⁄3

0.333

0.638

1.915

1:4

⁄3

0.333

0.577

1.732

OFF ms

()

-----------------------

ON ms

()

--------------------

ON ms

()

OFF ms

()

-----------------------

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