ADT7481
http://onsemi.com
17
Figure 19. Operation of the ALERT and THERM
Interrupts
1
32
4
THERM LIMIT
HIGH TEMP LIMIT
THERM LIMIT−HYSTERESIS
RESET BY MASTER
ALERT
THERM
100°C
TEMPERATURE
90°C
80°C
70°C
60°C
50°C
40°C
• If the measured temperature exceeds the high
temperature limit, the ALERT
output will assert low.
• 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.
• The THERM output de−asserts (goes high) when the
temperature falls to THERM
limit minus hysteresis. In
Figure 19, the default hysteresis value of 10°C is
shown.
• 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 ADT7481 can be configured as either an
ALERT
output or as an additional THERM output.
THERM2
will assert 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 also applies to
THERM2
.
Figure 20 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 20. Operation of the THERM and THERM2
Interrupts
THERM2 LIMIT
THERM LIMIT
THERM2
1
3
2
4
TEMPERATURE
THERM
90°C
80°C
70°C
60°C
50°C
40°C
30°C
When the THERM2 limit is exceeded, the THERM2
signal asserts low.
• If the temperature continues to increase and exceeds the
THERM
limit, the THERM output asserts low.
• , there is no hysteresis value shown.
• 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,
previous practice was to place a capacitor across the D+ and
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.
Factors Affecting Diode Accuracy
Remote Sensing Diode
The ADT7481 is designed to work with substrate
transistors built into processors or with discrete transistors.
Substrate transistors will generally be PNP types with the
collector connected to the substrate. Discrete types can be
either a PNP or an NPN transistor 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:
• The ideality factor, n
f
, of the transistor is a measure of
the deviation of the thermal diode from ideal behavior.
The ADT7481 is trimmed for an n
f
value of 1.008. Use
the following equation to calculate the error introduced
at a temperature, T (°C), when using a transistor where
n
f
does not equal 1.008. Consult the processor data
sheet for the n
f
values.
(eq. 2)
DT +
ǒ
n
f
* 1.008
Ǔ
ń1.008
ǒ
273.15 Kelvin ) T
Ǔ
To factor this in, the user can write the DT value to the
offset register. It will then automatically be added to, or
subtracted from, the temperature measurement by the
ADT7481.
• Some CPU manufacturers specify the high and low
current levels of the substrate transistors. The high
current level of the ADT7481, I
HIGH
, is 233 mA. The
low level current, I
LOW
, is 14 mA. If the ADT7481
current levels do not match the current levels specified
by the CPU manufacturer, it may become necessary to
remove an offset. The CPU data sheet will advise
whether this offset needs to be removed and how to
