Datasheet
LM75A_4 © NXP B.V. 2007. All rights reserved.
Product data sheet Rev. 04 — 10 July 2007 10 of 24
NXP Semiconductors
LM75A
Digital temperature sensor and thermal watchdog
The main difference between the two modes is that in OS comparator mode, the OS
output becomes active when Temp has exceeded T
os
and reset when Temp has dropped
below T
hyst
, reading a register or putting the device into shutdown mode does not change
the state of the OS output; while in OS interrupt mode, once it has been activated either
by exceeding T
os
or dropping below T
hyst
, the OS output will remain active indefinitely until
reading a register, then the OS output is reset.
Temperature limits T
os
and T
hyst
must be selected so that T
os
> T
hyst
. Otherwise, the OS
output state will be undefined.
7.7 OS fault queue
Fault queue is defined as the number of faults that must occur consecutively to activate
the OS output. It is provided to avoid false tripping due to noise. Because faults are
determined at the end of data conversions, fault queue is also defined as the number of
consecutive conversions returning a temperature trip. The value of fault queue is
selectable by programming the two bits B4 and B3 (OS_F_QUE[1:0]) in register Conf.
Notice that the programmed data and the fault queue value are not the same. Table 14
shows the one-to-one relationship between them. At power-up, fault queue data = 0 and
fault queue value = 1.
7.8 Shutdown mode
The device operation mode is selected by programming bit B0 (SHUTDOWN) of register
Conf. Setting bit SHUTDOWN to logic 1 will put the device into shutdown mode. Resetting
bit SHUTDOWN to logic 0 will return the device to normal mode.
In shutdown mode, the device draws a small current of approximately 3.5 µA and the
power dissipation is minimized; the temperature conversion stops, but the I
2
C-bus
interface remains active and register write/read operation can be performed. The OS
output remains unchanged.
7.9 Power-up default and power-on reset
The LM75A always powers-up in its default state with:
• Normal operation mode
• OS comparator mode
• T
os
= 80 °C
• T
hyst
= 75 °C
• OS output active state is LOW
• Pointer value is logic 0
Table 14. Fault queue table
Fault queue data Fault queue value
OS_F_QUE[1] OS_F_QUE[0] Decimal
001
012
104
116