Datasheet

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 kΩ, 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
D8D7D6D5D4D3D2D1D0XXXXXXX
Table 12. Thyst register
MSByte LSByte
7654321076543210
D8D7D6D5D4D3D2D1D0XXXXXXX
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