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參數資料
| 型號： | ADT7461AR-REEL |
| 廠商： | ANALOG DEVICES INC |
| 元件分類： | 模擬信號調理 |
| 英文描述： | 【1∑C Temperature Monitor with Series Resistance Cancellation |
| 中文描述： | SPECIALTY ANALOG CIRCUIT, PDSO8 |
| 封裝： | MS-012-AA, SOIC-8 |
| 文件頁數： | 18/24頁 |
| 文件大?。?/td> | 854K |
| 代理商： | ADT7461AR-REEL |
ADT7461
Figure 20
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
When the THERM2 limit is exceeded, the THERM2 signal
asserts low.
Figure 20
shows how THERM and THERM2 might operate
THERM2 LIMIT
THERM LIMIT
0
THERM2
1
3
2
4
TEMPERATURE
90
°
C
THERM
80
°
C
70
°
C
60
°
C
50
°
C
40
°
C
30
°
C
1.
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. In
, 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
external temperature measurement.
APPLICATION INFORMATION
Noise Filtering
For temperature sensors operating in noisy environments, previ-
ous 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 1000 pF. While this
capacitor will reduce the noise, it will not eliminate it, making it
difficult to use the sensor in a very noisy environment.
The ADT7461 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 ADT7461 and the remote
temperature sensor to operate in noisy environments. F
shows a low-pass R-C-R filter, with the following values: R =
100 and C = 1 nF. This filtering reduces both common-mode
noise and differential noise.
igure 21
Figure 21. Filter Between Remote Sensor and ADT7461
0
D+
1nF
100
REMOTE
TEMPERATURE
SENSOR
D–
100
FACTORS AFFECTING DIODE ACCURACY
Remote Sensing Diode
The ADT7461 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 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, 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
ADT7461 is trimmed for an
n
f
value of 1.008. The follow-
ing equation may be used to calculate the error introduced
at a temperature T (°C), when using a transistor whose
n
f
does not equal 1.008. Consult the processor data sheet for
the
n
f
values.
(
n
T
f
008
.
008
.
–
)
(
)
T
Kelvin
+
×
=
15
.
273
To factor this in, the user can write the T value to the
offset register. It will then be 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 μA and the low level current, I
LOW
,
is 6 μA. 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 will advise 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.
Rev. 0 | Page 18 of 24
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