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参数资料
| 型号： | NCT1008DMT3R2G |
| 厂商： | ON SEMICONDUCTOR |
| 元件分类： | 模拟信号调理 |
| 英文描述： | %2B-1℃ temperature monitor with series resistance cancellation |
| 中文描述： | SPECIALTY ANALOG CIRCUIT, DSO8 |
| 封装： | 2 X 2 MM, 0.50 MM PITCH, LEAD FREE, DFN-8 |
| 文件页数： | 8/20页 |
| 文件大小： | 251K |
| 代理商： | NCT1008DMT3R2G |
NCT1008
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 NCT1008 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 NCT1008 and the
remote temperature sensor to operate in noisy environments.
Figure 21 shows a lowpass RCR filter, where R = 100
W
and C = 1 nF. This filtering reduces both commonmode and
differential noise.
Figure 21. Filter Between Remote Sensor and
NCT1008 Factors Affecting Diode Accuracy
D+
1nF
100W
REMOTE
TEMPERATURE
SENSOR
D–
100W
Remote Sensing Diode
The NCT1008 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 (baseshorted
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 NCT1008 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 NCT1008, IHIGH, is 220 mA and the
low level current, ILOW, is 13.5 mA. If the NCT1008
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 NCT1008, the best
accuracy is obtained by choosing devices according to the
following criteria:
Baseemitter voltage greater than 0.25 V at 6 mA, at the
highest operating temperature
Baseemitter voltage less than 0.95 V at 100 mA, at the
lowest operating temperature
Base resistance less than 100 W
Small variation in hFE (50 to 150) that indicates tight
control of VBE characteristics
Transistors, such as the 2N3904, 2N3906, or equivalents
in SOT23 packages are suitable devices to use.
Thermal Inertia and SelfHeating
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.
If it is not, the thermal inertia caused by the sensor’s mass
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