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NE1617A Datasheet(PDF) 5 Page - NXP Semiconductors |
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NE1617A Datasheet(HTML) 5 Page - NXP Semiconductors |
5 / 27 page NE1617A_4 © NXP B.V. 2009. All rights reserved. Product data sheet Rev. 04 — 30 July 2009 5 of 27 NXP Semiconductors NE1617A Temperature monitor for microprocessor systems 7. Functional description The NE1617A contains an integrating A-to-D converter, an analog multiplexer, a status register, digital data registers, SMBus interface, associated control logic and a local temperature sensor or channel (refer to Figure 1 “Block diagram of NE1617A”). The remote diode-type sensor or channel should be connected to the D+ and D − pins properly. Temperature measurements or conversions are either automatically and periodically activated when the device is in free-running mode (both STBY pin = HIGH, and the configuration register bit6=LOW)or generated by one-shot command. The free-running period is selected by changing the programmable data of the conversion rate register, as described in Section 8.3.4. For each conversion, the multiplexer switches current sources through the remote and local temperature sensors over a period of time, about 60 ms, and the voltages across the diode-type sensors are sensed and converted into the temperature data by the A-to-D converter. The resulting temperature data is then stored in the temperature registers, in 8-bit two's complement word format and automatically compared with the limits which have been programmed in the temperature limit registers. Results of the comparison are reflected accordingly by the flags stored in the status register, an out-of-limit condition will set the ALERT output pin to its LOW state. Because both channels are automatically measured for each conversion, the results are updated for both channels at the end of every successful conversion. 7.1 Remote diode selection The method of the temperature measurement is based on the change of the diode VBE at two different operating current levels given by: (1) where: ∆V BE = change in base emitter voltage drop at two current levels K = Boltzman’s constant T = absolute temperature in ° Kelvin q = charge on the electron LN = natural logarithm N = ratio of the two currents The NE1617A forces two well-controlled current sources of about 10 µA and 100 µA and measures the remote diode VBE. The sensed voltage between two pins D+ and D− is limited between 0.25 V and 0.95 V. The external diode must be selected to meet this voltage range at these two current levels. The diode-connected PNP transistor provided on the microprocessor is typically used, or the discrete diode-connected transistor 2N3904 or 2N3906 is recommended as an alternative. Even though the NE1617A integrating A-to-D converter has a good noise performance, using the average of 10 measurement cycles, high frequency noise filtering between D+ and D − should be considered. An external capacitor of 2200 pF typical (but not higher ∆V BE KT q -------- LN N () × = |
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