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ADT7481ARMZ-1R7 数据表(PDF) 7 Page - ON Semiconductor |
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ADT7481ARMZ-1R7 数据表(HTML) 7 Page - ON Semiconductor |
7 / 20 page ADT7481 http://onsemi.com 7 Theory of Operation The ADT7481 is a local and dual remote temperature sensor and over/under temperature alarm. When the ADT7481 is operating normally, the on−board ADC operates in a free−running mode. The analog input multiplexer alternately selects either the on−chip temperature sensor to measure its local temperature, or either of the remote temperature sensors. The ADC digitizes these signals and the results are stored in the local, Remote 1, and Remote 2 temperature value registers. The local and remote measurement results are compared with the corresponding high, low, and THERM temperature limits, stored in on−chip registers. Out−of−limit comparisons generate flags that are stored in the status register. A result that exceeds the high temperature limit, the low temperature limit, or remote diode open circuit will cause the ALERT output to assert low. Exceeding THERM temperature limits causes the THERM output to assert low. The ALERT output can be reprogrammed as a second THERM output. The limit registers can be programmed, and the device controlled and configured via the serial SMBus. The contents of any register can also be read back via the SMBus. Control and configuration functions consist of switching the device between normal operation and standby mode, selecting the temperature measurement scale, masking or enabling the ALERT output, switching Pin 8 between ALERT and THERM2, and selecting the conversion rate. Temperature Measurement Method A simple method of measuring temperature is to exploit the negative temperature coefficient of a diode, measuring the base−emitter voltage (VBE) of a transistor operated at constant current. This technique requires calibration to null the effect of the absolute value of VBE, which varies from device to device. The technique used in the ADT7481 measures the change in VBE when the device is operated at two different currents. Figure 14 shows the input signal conditioning used to measure the output of a remote temperature sensor. This figure shows the remote sensor as a substrate transistor, but it could equally be a discrete transistor. If a discrete transistor is used, the collector is not grounded and is linked to the base. 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. C1 may optionally be added as a noise filter with a recommended maximum value of 1,000 pF. To measure DVBE, the operating current through the sensor is switched among two related currents. The currents through the temperature diode are switched between I, and N x I, giving DVBE. The temperature can then be calculated using the DVBE measurement. The resulting DVBE waveforms pass through a 65 kHz low−pass filter to remove noise and then to a chopper−stabilized amplifier. This amplifies and rectifies the waveform to produce a dc voltage proportional to DVBE. The ADC digitizes this voltage producing a temperature measurement. To reduce the effects of noise, digital filtering is performed by averaging the results of 16 measurement cycles for low conversion rates. At rates of 16, 32, and 64 conversions/second, no digital averaging takes place. Signal conditioning and measurement of the local temperature sensor is performed in the same manner. Figure 14. Input Signal Conditioning VDD IBIAS N ×I I BIAS DIODE D+ D– C1 NOTE: REMOTE SENSING TRANSISTOR fC = 65kHz VOUT+ VOUT– TO ADC CAPACITOR C1 IS OPTIONAL. IT IS ONLY NECESSARY IN NOISY ENVIRONMENTS. C1 = 1000pF MAX. Temperature Measurement Results The results of the local and remote temperature measurements are stored in the local and remote temperature value registers and are compared with limits programmed into the local and remote high and low limit registers. The local temperature measurement is an 8−bit measurement with 1 °C resolution. The remote temperature measurements are 10−bit measurements, with the 8 MSBs stored in one register and the 2 LSBs stored in another register. Table 1 is a list of the temperature measurement registers. |
类似零件编号 - ADT7481ARMZ-1R7 |
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类似说明 - ADT7481ARMZ-1R7 |
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