PCF8576CUSLASH7 datasheet(9/44 Pages) PHILIPS

9 / 44 page

1998 Jul 30

Philips Semiconductors

Product speciﬁcation

Universal LCD driver for low multiplex

rates

PCF8576C

6.1

Power-on reset

At power-on the PCF8576C resets to a starting condition

as follows:

1. All backplane outputs are set to VDD.

2. All segment outputs are set to VDD.

3. The drive mode ‘1 : 4 multiplex with 1

⁄3bias’ is selected.

4. Blinking is switched off.

5. Input and output bank selectors are reset (as defined

in Table 5).

6. The I2C-bus interface is initialized.

7. The data pointer and the subaddress counter are

cleared.

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

The full-scale LCD voltage (Vop) is obtained from

VDD − VLCD. The LCD voltage may be temperature

compensated externally through the VLCD supply to pin 12.

Fractional LCD biasing voltages are obtained from an

internal voltage divider of the three series resistors

connected between VDD and VLCD. The centre resistor can

be switched out of the circuit to provide a 1

⁄2bias voltage

level for the 1 : 2 multiplex configuration.

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 Vop =VDD − VLCD and the

resulting discrimination ratios (D), are given in Table 2.

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 >3Vth approximately.

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

⁄2bias are

possible but the discrimination and hence the contrast

ratios are smaller (

= 1.732 for 1 : 3 multiplex or

= 1.528 for 1 : 4 multiplex).

The advantage of these modes is a reduction of the LCD

full-scale voltage Vop as follows:

• 1 : 3 multiplex (1⁄2bias):

Vop =

= 2.449 Voff(rms)

• 1 : 4 multiplex (1⁄2bias):

Vop =

= 2.309 Voff(rms)

These compare with Vop =3Voff(rms) when 1⁄3bias is used.

----------

off rms

〈〉

()

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

Table 2 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(rms)

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

on(rms)

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

on(rms)

off(rms)

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

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