ADT7461
http://onsemi.com
16
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 deasserts (goes high) when the
temperature falls to THERM
limit minus hysteresis.
No hysteresis value is shown in Figure 21.
4. As the system continues to cool 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. While this capacitor
reduces the noise, it does not eliminate it, making it difficult
to use the sensor in a very noisy environment.
The ADT7461 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 ADT7461 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
ADT7461 Factors Affecting Diode Accuracy
100 W
100 W
1 nF
D+
D
REMOTE
TEMPERATURE
SENSOR
Remote Sensing Diode
The ADT7461 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 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, several factors should be taken into
consideration:
" The ideality factor, nF, of the transistor is a measure of
the deviation of the thermal diode from ideal behavior.
The ADT7461 is trimmed for an nF value of 1.008. The
following equation may be used to calculate the error
introduced at a temperature T (癈), 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 (273.15 Kelvin ) T) (eq. 1)
To factor this in, the user can write the DT value to the
offset register. It is then automatically added to or
subtracted from the temperature measurement by the
ADT7461.
" Some CPU manufacturers specify the high and low
current levels of the substrate transistors. The high
current level of the ADT7461, I
HIGH
, is 96 mA, and the
low level current, I
LOW
, is 6 mA. If the ADT7461
current levels do not match the current levels specified
by the CPU manufacturer, it may become necessary to
remove an offset. The CPUs data sheet advises whether
this offset needs to be removed and how to calculate it.
This offset may be 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 being used with the ADT7461,
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.
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