PCF8537 datasheet(29/82 Pages) NXP

29 / 82 page

PCF8537

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Product data sheet

Rev. 1 — 31 May 2012

29 of 82

NXP Semiconductors

PCF8537

Industrial LCD driver for multiplex rates up to 1:8

stepping of the display due to the VLCD voltage is periodically changing between two

different target voltages. For this reason, a filter has been implemented to ensure that LSB

jitter does not affect the display performance.

Like any other filtering, the digital temperature filter (see Figure 20) introduces a certain

delay in the measurement of temperature. This behavior is illustrated in Figure 21.

This delay may cause undesired effects at start-up when the environment temperature

may be different than the reset value of the PCF8537 which is 20

C. In this case, it takes

up to 30 s until the correct measured temperature value will be available. A control bit,

TFE (see Table 22 on page 16), is implemented to enable or disable the digital

temperature filter. This bit is set logic 0 by default, which means, that the filter is disabled

and the unfiltered environment temperature value is available to calculate the desired

VLCD.

Fig 20. Temperature measurement block with digital temperature filter

(1) Environment temperature, T1 (

C).

(2) Measured temperature, T2 (

C).

(3) Temperature deviation,

T=T2  T1.

Fig 21. Temperature measurement delay during ramping up-down of the environment

temperature

TEMPERATURE

MEASUREMENT

BLOCK

DIGITAL

TEMPERATURE

FILTER

TD[7:0]

unfiltered

TD[7:0]

filtered

enabled or disabled

by bit TFE

To the readout register

via I2C-bus and to

the VLCD compensation

block

013aaa642

t (s)

160

120

013aaa643

(°C)

(3)

(1)

(2)

(°C)(3)

-4

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