LM75A_4

© NXP B.V. 2007. All rights reserved.

Product data sheet

Rev. 04 — 10 July 2007

9 of 24

NXP Semiconductors

LM75A

Digital temperature sensor and thermal watchdog

When a set-point register is read, all 16 bits are provided to the bus and must be collected

by the controller to complete the bus operation. However, only the 9 most significant bits

should be used and the 7 LSB of the LSByte are equal to zero and should be ignored.

Table 13 shows examples of the limit data and value.

7.5 OS output and polarity

The OS output is an open-drain output and its state represents results of the device

watchdog operation as described in Section 7.1. In order to observe this output state, an

external pull-up resistor is needed. The resistor should be as large as possible, up to

200 k

Ω, to minimize the Temp reading error due to internal heating by the high OS sinking

current.

The OS output active state can be selected as HIGH or LOW by programming bit B2

(OS_POL) of register Conf: setting bit OS_POL to logic 1 selects OS active HIGH and

setting bit B2 to logic 0 sets OS active LOW. At power-up, bit OS_POL is equal to logic 0

and the OS active state is LOW.

7.6 OS comparator and interrupt modes

As described in Section 7.1, the device OS output responds to the result of the

comparison between register Temp data and the programmed limits, in registers Tos and

Thyst, in different ways depending on the selected OS mode: OS comparator or

OS interrupt. The OS mode is selected by programming bit B1 (OS_COMP_INT) of

register Conf: setting bit OS_COMP_INT to logic 1 selects the OS interrupt mode, and

setting to logic 0 selects the OS comparator mode. At power-up, bit OS_COMP_INT is

equal to logic 0 and the OS comparator is selected.

Table 11.

Tos register

MSByte

LSByte

7654321076543210

D8

D7

D6

D5

D4

D3

D2

D1

D0

XXXXXXX

Table 12.

Thyst register

MSByte

LSByte

7654321076543210

D8

D7

D6

D5

D4

D3

D2

D1

D0

XXXXXXX

Table 13.

Tos and Thyst limit data and value

11-bit binary

(2’s complement)

Hexadecimal value

Decimal value

Value

0 1111 1010

0FA

250

+125.0

°C

0 0011 0010

032

50

+25.0

°C

0 0000 0001

001

1

+0.5

°C

0 0000 0000

000

0

0.0

°C

1 1111 1111

1FF

−1

−0.5 °C

1 1100 1110

1CE

−50

−25.0 °C

1 1001 0010

192

−110

−55.0 °C