PCF8576DT datasheet(10/41 Pages) PHILIPS

10 / 41 page

2004 Dec 22

Philips Semiconductors

Product speciﬁcation

Universal LCD driver for low

multiplex rates

PCF8576D

6.1

Power-on reset

At power-on the PCF8576D resets to the following starting

conditions:

• All backplane outputs are set to VLCD

• All segment outputs are set to VLCD

• Drive mode ‘1 : 4 multiplex with 1⁄3 bias’ is selected

• Blinking is switched off

• Input and output bank selectors are reset (as defined in

Table 4)

• The I2C-bus interface is initialized

• The data pointer and the subaddress counter are

cleared

• Display is disabled.

Data transfers on the I2C-bus should be avoided for 1 ms

following power-on to allow completion of the reset action.

6.2

LCD bias generator

Fractional LCD biasing voltages are obtained from an

internal voltage divider comprising three resistors

connected in series between VLCD and VSS. The middle

resistor can be bypassed to provide a 1

⁄2 bias voltage level

for the 1 : 2 multiplex configuration. The LCD voltage can

be temperature compensated externally via the supply to

pin VLCD.

6.3

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 VLCD and the resulting

Discrimination ratios (D), are given in Table 2.

A practical value for VLCD 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

VLCD >3Vth.

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

⁄2 bias are

possible but the discrimination and hence the contrast

ratios are smaller.

6.3.1

LCD BIAS FORMULAE

Bias is calculated by the formula

For 1

⁄2 bias, a = 1; for 1⁄3 bias, a = 2.

The LCD on voltage (Von) is calculated by the formula

The LCD off voltage (Voff) is calculated by the formula

where Vop is the resultant voltage at the LCD segment; N

is the LCD drive mode: 1 = static, 2 = 1 : 2, 3 = 1 : 3,

4=1:4.

Discrimination is the ratio of Von to Voff, and is determined

by the formula

Using the above formula, the discrimination for an LCD

drive mode of 1 : 3 with 1

⁄2 bias is

= 1.732, and the

discrimination for an LCD drive mode of 1 : 4 with 1

⁄2 bias

= 1.528.

The advantage of these LCD drive modes is a reduction of

the LCD full-scale voltage VLCD as follows:

1 : 3 multiplex (1

⁄2 bias):

VLCD =

= 2.449 Voff(rms)

1 : 4 multiplex (1

⁄2 bias):

VLCD =

= 2.309 Voff(rms)

These compare with VLCD =3Voff(rms) when 1⁄3 bias is

used.

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off

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