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ADM1023 データシート(PDF) 6 Page - Analog Devices |
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ADM1023 データシート(HTML) 6 Page - Analog Devices |
6 / 12 page ADM1023 –6– REV. A 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 may 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 will work only if the measured values are within the limit values. MEASUREMENT METHOD A simple method of measuring temperature is to exploit the nega- tive temperature coefficient of a diode, or the base-emitter voltage of a transistor, operated at constant current. Thus, the temperature may be obtained from a direct measurement of VBE where, V nKT q I I BE C S =× ln () (1) 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 ADM1023 is to measure the change in VBE when the device is operated at two different collector currents. This is given by: ∆V nKT q N BE =× ln ( ) (2) where: K is Boltzmann’s constant q is charge on the electron (1.6 × 10–19 Coulombs) T is absolute temperature in Kelvins N is ratio of the two collector currents n is the ideality factor of the thermal diode (TD) To measure ∆V BE, the sensor is switched between operating cur- rents of I and NI. The resulting waveform is passed through a low-pass filter to remove noise, then to a chopper-stabilized ampli- fier that performs the functions of amplification and rectification of the waveform to produce a dc voltage proportional to ∆V BE. This voltage is measured by the ADC, which gives a temperature output in binary 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 shows the input signal conditioning used to measure the output of an external temperature sensor. This figure shows the external sensor as a substrate PNP 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 will not be 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 inter- nal diode at the D– input. If the sensor is operating in a noisy environment, C1 may optionally be added as a noise filter. Its value is typically 2200 pF, but should be no more than 3000 pF. See the section on Layout Considerations for more information on C1. SOURCES OF ERRORS ON THERMAL TRANSISTOR MEASUREMENT METHOD EFFECT OF IDEALITY FACTOR (n) The effects of ideality factor (n) and beta (Beta) of the temperature measured by a thermal transistor are discussed below. For a ther- mal transistor implemented on a submicron process, such as the substrate PNP used on a Pentium III processor, the temperature errors due to the combined effect of the ideality factor and beta are shown to be less than 3 °C. Equation 2 is optimized for a sub- strate PNP transistor (used as a thermal diode) usually found on CPUs designed on submicron CMOS processes such as the Pentium III Processor. There is a thermal diode on board each of these processors. The n in the Equation 2 represents the ideality factor of this thermal diode. This ideality factor is a measure of the deviation of the thermal diode from ideal behavior. According to Pentium III Processor manufacturing specifica- tions, measured values of n at 100 °C are: nMIN = 1.0057 < nTYPICAL = 1.008 < nMAX = 1.0125 The ADM1023 takes this ideality factor into consideration when calculating temperature TTD of the thermal diode. The ADM1023 is optimized for nTYPICAL = 1.008; any deviation on n from this typical value causes a temperature error that is calculated below for the nMIN and nMAX of a Pentium III Processor at TTD = 100 °C, ∆T Kelvin C C MIN =× + ° = ° 1 0057 1 008 1 008 273 15 100 0 85 . – . . (. ) – . ∆T Kelvin C C MAX =× + ° = + ° 1 0125 1 008 1 008 273 15 100 1 67 . – . . (. ) . Thus, the temperature error due variation on n of the thermal diode for Pentium III Processor is about 2.5 °C. C1* D+ D– REMOTE SENSING TRANSISTOR IN I 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 TYPICAL, 3nF MAX. * Figure 12. Input Signal Conditioning |
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同様の説明 - ADM1023 |
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