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ADM1032ARZ bảng dữ liệu(PDF) 7 Page - ON Semiconductor |
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ADM1032ARZ bảng dữ liệu(HTML) 7 Page - ON Semiconductor |
7 / 18 page ADM1032 http://onsemi.com 7 Functional Description The ADM1032 is a local and remote temperature sensor and overtemperature alarm. When the ADM1032 is operating normally, the on-board A/D converter operates in a free running mode. The analog input multiplexer alternately selects either the on-chip temperature sensor to measure its local temperature or the remote temperature sensor. These signals are digitized by the ADC, and the results are stored in the local and remote temperature value registers. The measurement results are compared with local and remote, high, low, and THERM temperature limits stored in nine on-chip registers. Out-of-limit comparisons generate flags that are stored in the status register, and one or more out-of-limit results cause the ALERT output to pull low. Exceeding THERM temperature limits causes the THERM output to assert low. 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 • Masking or Enabling the ALERT Output • Selecting the Conversion Rate 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 VBE, which varies from device to device. The technique used in the ADM1032 is to measure the change in VBE when the device is operated at two different currents. This is given by: (eq. 1) DVBE + nf KT q In(N) 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 nf is the ideality factor of the thermal diode. The ADM1032 is trimmed for an ideality factor of 1.008. Figure 12 shows the input signal conditioning used to measure the output of an external temperature sensor. Figure 12 shows the external sensor as a substrate transistor, provided for temperature monitoring on some microprocessors, but it could equally well 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 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, C1 can optionally be added as a noise filter. Its value should be no more than 1000 pF. See the Layout Considerations section for more information on C1. To measure DVBE, the sensor is switched between the operating currents of I and N × I. 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 DVBE. This voltage is measured by the ADC to give a temperature output in twos complement format. To further reduce the effects of noise, digital filtering is performed by averaging the results of 16 measurement cycles. Signal conditioning and measurement of the internal temperature sensor is performed in a similar manner. Figure 12. Input Signal Conditioning LOW-PASS FILTER fC = 65 kHz REMOTE SENSING TRANSISTOR BIAS DIODE D+ D− VDD IBIAS IN × I VOUT+ VOUT− To ADC C1* * CAPACITOR C1 IS OPTIONAL AND IT SHOULD ONLY BE USED IN VERY NOISY ENVIRONMENTS. C1 = 1000 pF Max. |
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