PCF8576CUSLASH2 datasheet(10/56 Pages) NXP

10 / 56 page

PCF8576C_9

Product data sheet

Rev. 09 — 9 July 2009

10 of 56

NXP Semiconductors

PCF8576C

Universal LCD driver for low multiplex rates

• The I2C-bus interface is initialized

• The data pointer and the subaddress counter are cleared

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

The full-scale LCD voltage (Voper) is obtained from VDD − VLCD. The LCD voltage may be

temperature compensated externally through the VLCD supply to pin VLCD.

Fractional LCD biasing voltages are obtained from an internal voltage divider comprising

three series resistors connected between VDD and VLCD. The center resistor can be

switched out of the circuit to provide a 1

⁄

2 bias voltage level for the 1:2 multiplex

conﬁguration.

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 < 3 × Vth.

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

Table 5.

Preferred LCD drive modes: summary of characteristics

LCD drive

mode

Number of:

LCD bias

conﬁguration

Backplanes Bias 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

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