ADT7482
http://onsemi.com
17
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.
Figure 21. Operation of the THERM
and THERM2
Interrupts
THERM2
LIMIT
905C
805C
705C
605C
505C
405C
TEMPERATURE
1
2
3
4
THERM
305C
THERM
LIMIT
THERM2
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 capacitances affect the accuracy of the
temperature measurement, leading to a recommended
maximum capacitor value of 1,000 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
This filtering reduces both common-mode noise and
differential noise.
Figure 22. Filter Between Remote Sensor
and ADT7482
100 W
100 W
1 nF
D+
D
REMOTE
TEMPERATURE
SENSOR
Factors Affecting Diode Accuracy
Remote Sensing Diode
The ADT7482 is designed to work with substrate
transistors built into processors or with discrete transistors.
Substrate transistors are generally PNP types with the
collector connected to the substrate. Discrete types can be
either PNP or NPN transistors connected as a diode (base
shorted to collector). If an NPN transistor is used, the
collector and base are connected to D+ and the emitter to D.
If a PNP transistor is used, the collector and base are
connected to D and the emitter to D+.
To reduce the error due to variations in both substrate and
discrete transistors, a number of factors should be taken into
consideration:
?SPAN class="pst ADT7482ARMZ-REEL_2296055_3"> The ideality factor, n
f
, of the transistor is a measure of
the deviation of the thermal diode from ideal behavior.
The ADT7482 is trimmed for an n
f
value of 1.008. The
following equation can be used to calculate the error
introduced at a temperature T (癈), when using a
transistor whose n
f
does not equal 1.008. Consult the
processor data sheet for the n
f
values.
(eq. 1)
DT +
n
f
* 1.008
1.008
273.15 Kelvin ) T
To factor this in, write the DT value to the offset register.
It is then automatically added to or subtracted from the
temperature measurement by the ADT7482.
?SPAN class="pst ADT7482ARMZ-REEL_2296055_3"> Some CPU manufacturers specify the high and low
current levels of the substrate transistors. The high
current level of the ADT7482, I
HIGH
, is 220 mA and the
low level current, I
LOW
, is 13.5 mA. If the ADT7482
current levels do not match the current levels specified
by the CPU manufacturer, it may be necessary to
remove an offset. The CPU data sheet advises whether
this offset needs to be removed and how to calculate it.
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