PCF85321 datasheet(6/44 Pages) NXP

6 / 44 page

PCF8532_1

Product data sheet

Rev. 1 — 10 February 2009

6 of 44

NXP Semiconductors

PCF8532

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

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

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.

Preferred LCD drive modes: summary of characteristics

LCD drive mode Number of:

LCD bias

conﬁguration

Backplanes

Bias levels

static

∞

1:2 multiplex

⁄

0.354

0.791

2.236

1:2 multiplex

⁄

0.333

0.745

2.236

1:3 multiplex

⁄

0.333

0.638

1.915

1:4 multiplex

⁄

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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