PCF85176 datasheet(7/43 Pages) NXP

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PCF85176_1

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Product data sheet

Rev. 01 — 14 April 2010

7 of 43

NXP Semiconductors

PCF85176

Universal LCD driver for low multiplex rates

• Display is disabled

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

the reset action to complete.

7.2 LCD bias generator

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

series resistors connected between VLCD and VSS. The center resistor is bypassed by

switch if the 1

⁄

2 bias voltage level for the 1:2 multiplex configuration is selected.

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 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 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

Table 5.

Biasing characteristics

LCD drive

mode

Number of:

LCD bias

configuration

Backplanes 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

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