PCF8537 datasheet(28/82 Pages) NXP

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PCF8537

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Product data sheet

Rev. 1 — 31 May 2012

28 of 82

NXP Semiconductors

PCF8537

Industrial LCD driver for multiplex rates up to 1:8

8.4.4 Temperature measurement and temperature compensation of VLCD

8.4.4.1

Temperature readout

The PCF8537 has a built-in temperature sensor which provides an 8 bit digital value,

TD[7:0], of the ambient temperature. This value can be read through the interface (see

Figure 47 on page 56 and Figure 51 on page 59). The actual temperature is determined

from TD[7:0] using Equation 2:

(2)

The measurement needs about 5 ms to complete and is repeated periodically as soon as

bit TME is set logic 1 (see Table 9 on page 10). The time between measurements is linked

to the system clock and hence varies with changes in the chosen frame frequency, see

Table 25.

The temperature sensor can be thought of as analog to digital converter. Like all A/D

converters, jitter will exist on the LSB of the output value. This is also true of the

temperature sensor in the PCF8537. Jitter of the LSB of TD[7:0] may lead to contrast

(1) VPR[7:0] = 42h.

(2) VPR[7:0] = 85h.

(3) VPR[7:0] = C6h.

Tamb =30 C; VDD1 =VDD2 =5V.

Remark: For driving the charge pump safely the VLCD and IDD(LCD) values have to be kept below

the flat part of the respective graph.

b. Charge pump configuration: VLCD =3  VDD2

Fig 19. Charge pump driving capability with VDD2 =5.0 V

013aaa663

0.4

0.8

1.2

1.6

IDD(LCD) (mA)

VLCD

(V)

(3)

(2)

(1)

Table 25.

Temperature measurement update rate

Selected frame frequency

Temperature measurement update rate

60 Hz

3.3 s

200 Hz

1 s

300 Hz

0.67 s

T (°C)

0.9375

TD 7:0

 40

–



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