PCF8576DTSLASH2 datasheet(9/57 Pages) NXP

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PCF8576D

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Product data sheet

Rev. 14 — 10 June 2013

9 of 57

NXP Semiconductors

PCF8576D

Universal LCD driver for low multiplex rates

7.3 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 6.

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)

where the values for n are

n = 1 for static drive mode

n = 2 for 1:2 multiplex drive mode

n = 3 for 1:3 multiplex drive mode

n = 4 for 1:4 multiplex drive mode

The RMS off-state voltage (Voff(RMS)) for the LCD is calculated with Equation 2:

(2)

Discrimination is the ratio of Von(RMS) to Voff(RMS) and is determined from Equation 3:

Table 6.

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

off RMS



–





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

LCD

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