ADT7461A
http://onsemi.com
16
Figure 20. Operation of the THERM and THERM2
Interrupts
THERM2
1
3
2
4
THERM LIMIT
THERM2 LIMIT
TEMPERATURE
THERM
905C
805C
705C
605C
505C
405C
305C
• 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.
• The THERM output deasserts (goes high) when the
temperature falls to THERM
limit minus hysteresis. In
Figure 20, 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
.
Both the external and internal temperature measurements
cause THERM
and THERM2 to operate as described.
Application Information
Noise Filtering
For temperature sensors operating in noisy environments,
the industry standard 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. Although this
capacitor reduces the noise, it does not eliminate it, making
it difficult to use the sensor in a very noisy environment.
The ADT7461A has a major advantage over other devices
when it comes to 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 ADT7461A and the
remote temperature sensor to operate in noisy environments.
Figure 21 shows a low−pass R−C−R filter, where R = 100 W
and C = 1 nF. This filtering reduces both common−mode and
differential noise.
Figure 21. Filter Between Remote Sensor and
ADT7461A Factors Affecting Diode Accuracy
D+
1nF
100W
REMOTE
TEMPERATURE
SENSOR
D–
100W
Remote Sensing Diode
The ADT7461A 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 are either
PNP or NPN transistors connected as diodes (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, consider several factors:
• The ideality factor, nF, of the transistor is a measure of
the deviation of the thermal diode from ideal behavior.
The ADT7461A is trimmed for an nF value of 1.008.
The following equation may be used to calculate the
error introduced at a temperature, T (°C), when using a
transistor whose nF does not equal 1.008. Consult the
processor data sheet for the nF values.
DT = (nF − 1.008)/1.008 x (273.15 Kelvin + T)
To factor this in, the user writes the DT value to the offset
register. It is then automatically added to, or subtracted
from, the temperature measurement.
• Some CPU manufacturers specify the high and low
current levels of the substrate transistors. The high
current level of the ADT7461A, I
HIGH
, is 220 mA and
the low level current, I
LOW
, is 13.5 mA. If the
ADT7461A 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 should advise whether this offset needs to be
removed and how to calculate it. This offset is
programmed to the offset register. It is important to
note that if more than one offset must be considered,
the algebraic sum of these offsets must be programmed
to the offset register.
If a discrete transistor is used with the ADT7461A, the
best accuracy is obtained by choosing devices according to
the following criteria:
• Base−emitter voltage greater than 0.25 V at 6 mA, at the
highest operating temperature
• Base−emitter voltage less than 0.95 V at 100 mA, at the
lowest operating temperature
• Base resistance less than 100 W
• Small variation in h
FE
(50 to 150) that indicates tight
control of V
BE
characteristics
Transistors, such as the 2N3904, 2N3906, or equivalents
in SOT−23 packages are suitable devices to use.
Thermal Inertia and Self−Heating
Accuracy depends on the temperature of the remote
sensing diode and/or the internal temperature sensor being
at the same temperature as that being measured. Many
factors can affect this. Ideally, place the sensor in good
thermal contact with the part of the system being measured.
