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ADT7486AARMZ-RL 数据表(PDF) 9 Page - ON Semiconductor |
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ADT7486AARMZ-RL 数据表(HTML) 9 Page - ON Semiconductor |
9 / 14 page ADT7484A/ADT7486A http://onsemi.com 9 Temperature Measurement The ADT7484A/ADT7486A each have two dedicated temperature measurement channels: one for measuring the temperature of an on-chip band gap temperature sensor, and one for measuring the temperature of a remote diode, usually located in the CPU or GPU. The ADT7484A monitors one local and one remote temperature channel, whereas the ADT7486A monitors one local and two remote temperature channels. Monitoring of each of the channels is done in a round-robin sequence. The monitoring sequence is in the order shown in Table 11. Table 6. Temperature Monitoring Sequence Channel Number Measurement Conversion Time (ms) 0 Local Temperature 12 1 Remote Temperature 1 38 2 Remote Temperature 2 (ADT7486A only) 38 Temperature Measurement Method A simple method for measuring temperature is to exploit the negative temperature coefficient of a diode by measuring the base-emitter voltage (VBE) of a transistor operated at constant current. Unfortunately, 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 ADT7484A/ADT7486A measures the change in VBE when the device is operated at three different currents. Figure 16 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, which is provided for temperature monitoring on some microprocessors, but it could also be a discrete transistor. If a discrete transistor is used, the collector is not grounded and should be linked to the base. To prevent ground noise from 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 D1− input. If the sensor is operating in an extremely noisy environment, C1 can be added as a noise filter. Its value should not exceed 1000 pF. To measure DVBE, the operating current through the sensor is switched between three related currents. Figure 16 shows N1 x I and N2 x I as different multiples of the current I. The currents through the temperature diode are switched between I and N1 x I, giving DVBE1, and then between I and N2 x I, giving DVBE2. The temperature can then be calculated using the two DVBE measurements. This method can also cancel the effect of series resistance on the temperature measurement. The resulting DVBE waveforms are passed through a 65 kHz low-pass filter to remove noise and then through a chopper-stabilized amplifier to amplify and rectify the waveform, producing a dc voltage proportional to DVBE. The ADC digitizes this voltage, and a temperature measurement is produced. To reduce the effects of noise, digital filtering is performed by averaging the results of 16 measurement cycles for low conversion rates. Signal conditioning and measurement of the internal temperature sensor is performed in the same manner. Figure 14. Signal Conditioning for Remote Diode Temperature Sensors C1* D+ BIAS DIODE *CAPACITOR C1 IS OPTIONAL. IT SHOULD ONLY BE USED IN NOISY ENVIRONMENTS. VDD TO ADC VOUT+ VOUT– REMOTE SENSING TRANSISTOR D– I N1 x I N2 x I IBIAS LOW−PASS FILTER fC = 65kHz Reading Temperature Measurements The temperature measurement command codes are detailed in Table 12. The temperature data returned is two bytes in little endian format, that is, LSB before MSB. All temperatures can be read together by using Command Code 0x00 with a read length of 0x04. The command codes and returned data are described in Table 12. Table 7. Temperature Channel Command Codes Temp Channel Command Code Returned Data Internal 0x00 LSB, MSB External 1 0x01 LSB, MSB External 2 0x02 LSB, MSB All Temps 0x00 Internal LSB, Internal MSB; External 1 LSB, External 1 MSB; External 2 LSB, External 2 MSB |
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