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ADM1021AARQZ-R Folha de dados(PDF) 7 Page - ON Semiconductor |
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ADM1021AARQZ-R Folha de dados(HTML) 7 Page - ON Semiconductor |
7 / 15 page ADM1021A http://onsemi.com 7 Functional Description The ADM1021A contains a two−channel A−to−D converter with special input−signal conditioning to enable operation with remote and on−chip diode temperature sensors. When the ADM1021A is operating normally, the A−to−D converter operates in 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 stored in the local and remote temperature value registers as 8−bit, twos complement words. The measurement results are compared with local and remote, high and low temperature limits, stored in four on−chip registers. Out−of−limit comparisons generate flags that are stored in the status register, and one or more out−of−limit results will cause the ALERT output to pull low. The limit registers can be programmed and the device controlled and configured via the serial System Management Bus (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. On initial powerup, the remote and local temperature values default to –128 °C. Since the device normally powers up converting, a measurement of local and remote temperature is made, and these values are then stored before a comparison with the stored limits is made. However, if the part is powered up in standby mode (STBY pin pulled low), no new values are written to the register before a comparison is made. As a result, both RLOW and LLOW are tripped in the status register, thus generating an ALERT output. This can be cleared in one of two ways. 1. Change both the local and remote lower limits to –128 °C and read the status register (which in turn clears the ALERT output). 2. Take the part out of standby and read the status register (which in turn clears the ALERT output). This works only if the measured values are within the limit values. 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 the effect of the absolute value of VBE, which varies from device to device. Figure 13. Input Signal Conditioning C1* D+ I N y 1 D– REMOTE SENSING TRANSISTOR IBIAS VDD VOUT+ TO ADC VOUT– BIAS DIODE LOW−PASS FILTER fC = 65kHz * CAPACITOR C1 IS OPTIONAL. IT IS ONLY NECESSARY IN NOISY ENVIRONMENTS. C1 = 2.2nF TYP, 3nF MAX. The technique used in the ADM1021A is to measure the change in VBE when the device is operated at two different currents. This is given by: (eq. 1) DV BE + KT q 1n (N) where: K is Boltzmann’s constant. 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. Figure 13 shows the input signal conditioning used to measure the output of an external temperature sensor. This figure shows the external sensor as a substrate transistor provided for temperature monitoring on some microprocessors, but it could be a discrete transistor. If a discrete transistor is used, the collector will not be 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, one can optionally be added as a noise filter. Its value is typically 2200 pF, but it should be no more than 3000 pF. See the Layout Considerations section for more information. |
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