PCA8553 datasheet(16/52 Pages) NXP

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NXP Semiconductors

PCA8553

Automotive 40 × 4 LCD segment driver

8.2 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 Display_ctrl_1 command (see Table 6). 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 11.

Number of:

LCD drive

mode

Backplanes Levels

LCD bias

configuration

static

∞

1:2 multiplex 2

1⁄2

0.354

0.791

2.236

1:2 multiplex 2

1⁄3

0.333

0.745

2.236

1:3 multiplex 3

1⁄3

0.333

0.638

1.915

1:4 multiplex 4

1⁄3

0.333

0.577

1.732

Table 11. Biasing characteristics

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 with Equation 2

(2)

The values for abias are:

abias = 1 for 1⁄2 bias

abias = 2 for 1⁄3 bias

The RMS on-state voltage (Von(RMS)) for the LCD is calculated with Equation 3:

(3)

where the values for n are

nMUX = 1 for static drive mode

nMUX = 2 for 1:2 multiplex drive mode

nMUX = 3 for 1:3 multiplex drive mode

nMUX = 4 for 1:4 multiplex drive mode

The RMS off-state voltage (Voff(RMS)) for the LCD is calculated with Equation 4:

(4)

Discrimination is a term which is defined as the ratio of the on and off RMS voltages

(Von(RMS) to Voff(RMS)) across a segment. It can be thought of as a measurement of

contrast. Discrimination is determined from Equation 5:

PCA8553

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Product data sheet

Rev. 4 — 1 October 2021

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