ADT7482 Datasheet, PDF (17/20 Pages) Analog Devices – Dual Channel Temperature Sensor and Overtemperature Alarm

ADT7482
If the ADT7482 is in the default temperature range (0°C
to 127°C), then THERM hysteresis must be less than the
THERM limit.
Figure 20 shows how the THERM and ALERT outputs
operate. If desired, use the ALERT output as a SMBALERT
to signal to the host via the SMBus that the temperature has
risen. Use the THERM output to turn on a fan to cool the
system, if the temperature continues to increase. This
method ensures that there is a fail−safe mechanism to cool
the system, without the need for host intervention.
TEMPERATURE
100°C
90°C
80°C
70°C
60°C
THERM LIMIT
THERM LIMIT−HYSTERESIS
HIGH TEMP LIMIT
50°C
40°C
ALERT
THERM
1
2
RESET BY MASTER
4
3
Figure 20. Operation of the ALERT and THERM
Interrupts
1. If the measured temperature exceeds the high
temperature limit, the ALERT output asserts low.
2. If the temperature continues to increase and
exceeds the THERM limit, the THERM output
asserts low. This can be used to throttle the CPU
clock or switch on a fan.
3. The THERM output de−asserts (goes high) when
the temperature falls to THERM limit minus
hysteresis. In Figure 20, the default hysteresis
value of 10°C is shown.
4. The ALERT output de−asserts only when the
temperature has fallen below the high temperature
limit, and the master has read the device address
and cleared the status register.
Pin 8 on the ADT7482 can be configured as either an
ALERT output or as an additional THERM output.
THERM2 asserts low when the temperature exceeds the
programmed local and/or remote high temperature limits. It
is reset in the same manner as THERM, and it is not
maskable. The programmed hysteresis value applies to
THERM2 also.
Figure 21 shows how THERM and THERM2 might
operate together to implement two methods of cooling the
system. In this example, the THERM2 limits are set lower
than the THERM limits. The THERM2 output could be used
to turn on a fan. If the temperature continues to rise and
exceeds the THERM limits, the THERM output could
provide additional cooling by throttling the CPU.
TEMPERATURE
90°C
80°C
70°C
60°C
50°C
40°C
30°C
THERM2
1
THERM
2
3
THERM LIMIT
THERM2 LIMIT
4
Figure 21. Operation of the THERM and THERM2
Interrupts
1. When the THERM2 limit is exceeded, the
THERM2 signal asserts low.
2. If the temperature continues to increase and
exceeds the THERM limit, the THERM output
asserts low.
3. The THERM output de−asserts (goes high) when
the temperature falls to THERM limit minus
hysteresis. In Figure 21, there is no hysteresis
value shown.
4. As the system cools further, and the temperature
falls below the THERM2 limit, the THERM2
signal resets. Again, no hysteresis value is shown
for THERM2.
The temperature measurement could be either the local or
the remote temperature measurement.
Applications Information
Noise Filtering
For temperature sensors operating in noisy environments,
the previous practice was to place a capacitor across the D+
pin and the D− pins to help combat the effects of noise.
However, large capacitance’s affect the accuracy of the
temperature measurement, leading to a recommended
maximum capacitor value of 1000 pF. While this capacitor
reduces the noise, it does not eliminate it, making it difficult
to use the sensor in a very noisy environment.
The ADT7482 has a major advantage over other devices
for eliminating the effects of noise on the external sensor.
The series resistance cancellation feature allows a filter to be
constructed between the external temperature sensor and the
part. The effect of any filter resistance seen in series with the
remote sensor is automatically cancelled from the
temperature result.
The construction of a filter allows the ADT7482 and the
remote temperature sensor to operate in noisy environments.
Figure 22 shows a low−pass R−C−R filter, with the following
values:
R + 100 W and C + 1 nF
(eq. 1)
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