PCF8576DTSLASH2 datasheet(9/52 Pages) NXP

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PCF8576D_7

Product data sheet

Rev. 7 — 18 December 2008

9 of 52

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 (see Section 7.17) 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 5.

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.

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 the equation

(1)

where VLCD is the resultant voltage at the LCD segment and where the values for n are

n = 1 for static mode

n = 2 for 1:2 multiplex

n = 3 for 1:3 multiplex

n = 4 for 1:4 multiplex

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

(2)

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

Table 5.

Discrimination ratios

LCD drive

mode

Number of:

LCD bias

conﬁguration

Backplanes Levels

static

∞

1:2 multiplex 2

⁄

0.354

0.791

2.236

1:2 multiplex 2

⁄

0.333

0.745

2.236

1:3 multiplex 3

⁄

0.333

0.638

1.915

1:4 multiplex 4

⁄

0.333

0.577

1.732

off RMS

()

LCD

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

on RMS

()

LCD

------------------------- D

on RMS

()

off RMS

()

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

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

on RMS

()

---

–

()

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





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

LCD

off RMS

()

–

()

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

LCD

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