PCF8545ATT datasheet(18/54 Pages) NXP

18 / 54 page

PCA8576F

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Product data sheet

Rev. 3 — 3 December 2014

18 of 54

NXP Semiconductors

PCA8576F

Automotive 40 × 4 LCD driver

The host microcontroller maintains the 2-line I2C-bus communication channel with the

PCA8576F. The internal oscillator is enabled by connecting pin OSC to pin VSS. The

appropriate biasing voltages for the multiplexed LCD waveforms are generated internally.

The only other connections required to complete the system are to the power supplies

(VDD, VSS, and VLCD) and the LCD panel chosen for the application.

7.6 LCD voltage

7.6.1 LCD bias generator

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

impedances connected between pins VLCD and VSS. The center impedance is bypassed

by switch if the 1⁄2 bias voltage level for the 1:2 multiplex drive mode configuration is

selected. The LCD voltage can be temperature compensated externally using the supply

to pin VLCD.

7.6.2 LCD voltage selector

The LCD voltage selector coordinates the multiplexing of the LCD in accordance with the

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

mode-set command 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 16.

Discrimination is a term which is defined as the ratio of the on and off RMS voltage across

a segment. It can be thought of as a measurement of contrast.

A practical value for VLCD is determined by equating Voff(RMS) with a defined LCD

threshold voltage (Vth(off)), typically when the LCD exhibits approximately 10 % contrast. In

the static drive mode a suitable choice is VLCD >3Vth(off).

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.

Bias is calculated by

, where the values for a are

a = 1 for 1⁄2 bias

a = 2 for 1⁄3 bias

The RMS on-state voltage (Von(RMS)) for the LCD is calculated with Equation 1:

(1)

Table 16.

Biasing characteristics

LCD drive

mode

Number of:

LCD bias

configuration

Backplanes Levels

static



1:2 multiplex

1⁄2

0.354

0.791

2.236

1:2 multiplex

1⁄3

0.333

0.745

2.236

1:3 multiplex

1⁄3

0.333

0.638

1.915

1:4 multiplex

1⁄3

0.333

0.577

1.732

off RMS



LCD

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

on RMS



LCD

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

on RMS



off RMS



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

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

on RMS





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

LCD

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