PCF8579 datasheet(8/41 Pages) NXP

8 / 41 page

PCF8579_5

Product data sheet

Rev. 05 — 11 May 2009

8 of 41

NXP Semiconductors

PCF8579

LCD column driver for dot matrix graphic displays

Remark: Do not transfer data on the I2C-bus for at least 1 ms after power-on to allow the

reset action to complete.

8.2 Multiplexed LCD bias generation

The bias levels required to produce maximum contrast depend on the multiplex rate and

the LCD threshold voltage (Vth). Vth is typically deﬁned as the RMS voltage at which the

LCD exhibits 10 % contrast. Table 4 shows the optimum voltage bias levels and Table 5

the discrimination ratios (D) for the different multiplex rates as functions of Voper.

(1)

The RMS on-state voltage (Von(RMS)) for the LCD is calculated with the equation

(2)

and the RMS off-state voltage (Voff(RMS)) with the equation

(3)

where the values for n are determined by the multiplex rate (1:n). Valid values for n are:

n = 8 for 1:8 multiplex

n = 16 for 1:16 multiplex

n = 24 for 1:24 multiplex

n = 32 for 1:32 multiplex

Table 4.

Optimum LCD voltages

Bias ratios

Multiplex rate

1:8

1:16

1:24

1:32

0.739

0.800

0.830

0.850

0.522

0.600

0.661

0.700

0.478

0.400

0.339

0.300

0.261

0.200

0.170

0.150

oper

LCD

–

on RMS

()

---

–

()

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

oper

off RMS

()

–

()

()2

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

oper

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

oper

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

oper

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

oper

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

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