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参数资料
| 型号： | NCT65DMR2G |
| 厂商： | ON Semiconductor |
| 文件页数： | 4/6页 |
| 文件大小： | 0K |
| 描述： | IC TEMP MON TRIP POINT 8-MSOP |
| 标准包装： | 3,000 |
| 感应温度： | 70°C & 85°C 跳闸点 |
| 输出类型： | 开路漏极 |
| 电源电压： | 2.8 V ~ 3.6 V |
| 精确度： | ±1°C，±1.5°C |
| 封装/外壳： | 8-TSSOP，8-MSOP（0.118"，3.00mm 宽） |
| 供应商设备封装： | 8-MSOP |
| 包装： | 带卷 (TR) |
NCT65
REMOTE
100 W
Theory of Operation
The NCT65 is a remote trip point temperature sensor for
use in a vide variety of applications from smart phones to
embedded systems. The remote temperature is measured by
the NCT65 and then compared with a fixed limit set by the
internal device reference. The limit for the THERM1 pin is
70 ? C and the limit for the THERM2 pin is 85 ? C. If either the
remote temperature exceeds the defined limits the open
drain THERM pins are asserted low. Each THERM pin self
clears when the temperature drops 5 ? C below the THERM
limit. This is to prevent THERM jitter, where the
temperature hovers around the THERM limit.
THERM2 = 85
Hyst = 5
THERM1 = 70
Hyst = 5
Time
THERM2
THERM1
Figure 2. Trippoints
Measurement Method
A simple method of measuring temperature is to exploit
the negative temperature coefficient of a diode, or the
base-emitter voltage of a transistor, operated at constant
current. Unfortunately, this technique requires calibration to
null out the effect of the absolute value of V BE , which varies
from device to device.
The technique used in the NCT65 is to measure the change
in V BE when the device is operated at three different
currents.
This is given by:
q
D V BE + (n f ) kT In(N) (eq. 1)
Where:
k is Boltzmann’s constant (1.38 ? 10 –23 ).
q is the charge on the electron (1.6 ? 10 –19 Coulombs).
T is the absolute temperature in Kelvins.
N is the ratio of the two currents.
n f is the ideality factor of the thermal diode.
The NCT65 is trimmed for an ideality factor of 1.008.
To prevent ground noise interfering with the
measurement, the more negative terminal of the sensor is not
referenced to ground but is biased above ground by an
internal diode at the D ? input. If the sensor is operating in a
noisy environment an optional filter can be added. Its value
should be no more than 1,000 pF. See the Layout
Considerations section for more information on C1.
To measure D V BE , the operating current through the
sensor is switched among three related currents.
N1 ? I and N2 ? I are different multiples of the current, I.
The currents through the temperature diode are switched
between I and N1 ? I, giving D V BE1 ; and then between I and
N2 ? I, giving D V BE2 . The temperature is then calculated
using the two D V BE measurements. This method also
cancels the effect of any series resistance on the temperature
measurement. The resulting waveform is passed through a
65 kHz low-pass filter to remove noise, and then to a
chopper-stabilized amplifier that performs the functions of
amplification and rectification of the waveform to produce
a dc voltage proportional to D V BE . This voltage is input into
two comparators with a reference voltage. If the voltage
exceeds the reference voltage then the THERM output
asserts low.
Applications 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 NCT65 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 NCT65 and the
remote temperature sensor to operate in noisy environments.
The figure below 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.
100 W
D+
TEMPERATURE 1 nF
SENSOR
D ?
Figure 3. Filter between Remote Sensor and Factors
Affecting Diode Accuracy
http://onsemi.com
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