C
AD 746±A
TEMPERATURE
90°C
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+.
80°C
70°C
60°C
50°C
40°C
30°C
THERM LIMIT
THERM2 LIMIT
1
4
THERM2
THERM
To reduce the error due to variations in both substrate and
discrete transistors, consider several factors:
3
2
THERM
THERM2
Interrupts
Figure 21. Operation of the
and
•
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.
THERM2
When the
asserts low.
THERM2
signal
•
•
•
limit is exceeded, the
If the temperature continues to increase and exceeds the
THERM THERM
limit, the
output asserts low.
output deasserts (goes high) when the
THERM
The
ΔT = (nF − 1.008)/1.008 × (273.15 Kelvin + T)
THERM
temperature falls to
limit minus hysteresis. In
To factor this in, the user writes the ΔT value to the offset
register. It is then automatically added to, or subtracted
from, the temperature measurement.
Figure 21, there is no hysteresis value shown.
•
As the system cools further, and the temperature falls
THERM2
THERM2
below the
limit, the
signal resets.
THERM2
•
Some CPU manufacturers specify the high and low current
levels of the substrate transistors. The high current level of
the ADT7461A, IHIGH, is 220 μA and the low level current,
Again, no hysteresis value is shown for
.
Both the external and internal temperature measurements cause
THERM THERM2
I
LOW, is 13.5 μA. If the ADT7461A current levels do not
and
to operate as described.
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.
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.
If a discrete transistor is used with the ADT7461A, the best
accuracy is obtained by choosing devices according to the
following criteria:
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.
•
Base-emitter voltage greater than 0.25 V at 6 μA, at the
highest operating temperature
•
Base-emitter voltage less than 0.95 V at 100 μA, at the
lowest operating temperature
•
•
Base resistance less than 100 Ω
Small variation in hFE (50 to 150) that indicates tight
control of VBE characteristics
The construction of a filter allows the ADT7461A and the remote
temperature sensor to operate in noisy environments. Figure 22
shows a low-pass R-C-R filter, where R = 100 Ω and C = 1 nF.
This filtering reduces both common-mode and differential noise.
Transistors, such as the 2N3904, 2N3906, or equivalents in
SOT-23 packages are suitable devices to use.
100
Ω
D+
REMOTE
TEMPERATURE
SENSOR
1nF
100
Ω
D–
Figure 22. Filter Between Remote Sensor and ADT7461A
Factors Affecting Diode Accuracy
Rev. A | Page 19 of 24
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