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NVT210 Datasheet(PDF) 7 Page - ON Semiconductor |
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NVT210 Datasheet(HTML) 7 Page - ON Semiconductor |
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7 / 20 page  NVT210 http://onsemi.com 7 Theory of Operation The NVT210 is a local and remote temperature sensor and over/under temperature alarm, with the added ability to automatically cancel the effect of 1.5 k W (typical) of resistance in series with the temperature monitoring diode. When the NVT210 is operating normally, the on-board ADC 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. The ADC digitizes these signals and the results are stored in the local and remote temperature value registers. The local and remote measurement results are compared with the corresponding high, low, and THERM temperature limits, stored in eight on-chip registers. Out-of-limit comparisons generate flags that are stored in the status register. A result that exceeds the high temperature limit or the low temperature limit causes the ALERT output to assert. The ALERT output also asserts if an external diode fault is detected. Exceeding the THERM temperature limits causes the THERM output to assert low. The ALERT output can be reprogrammed as a second THERM output. The limit registers are programmed and the device controlled and configured via the serial SMBus/I2C. The contents of any register are also read back via the SMBus/I2C. Control and configuration functions consist of switching the device between normal operation and standby mode, selecting the temperature measurement range, masking or enabling the ALERT output, switching Pin 6 between ALERT and THERM2, and selecting the conversion rate. Series Resistance Cancellation Parasitic resistance to the D+ and D− inputs to the NVT210, seen in series with the remote diode, is caused by a variety of factors, including PCB track resistance and track length. This series resistance appears as a temperature offset in the remote sensor’s temperature measurement. This error typically causes a 0.5C offset per ohm of parasitic resistance in series with the remote diode. The NVT210 automatically cancels the effect of this series resistance on the temperature reading, giving a more accurate result, without the need for user characterization of this resistance. The NVT210 is designed to automatically cancel typically up to 1.5 k W of resistance. By using an advanced temperature measurement method, this process is transparent to the user. This feature permits resistances to be added to the sensor path to produce a filter, allowing the part to be used in noisy environments. See the section on Noise Filtering for more details. Temperature Measurement Method A simple method of measuring temperature is to exploit the negative temperature coefficient of a diode, measuring the base emitter voltage (VBE) of a transistor operated at constant current. However, 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 NVT210 measures the change in VBE when the device operates at three different currents. Previous devices used only two operating currents, but it is the use of a third current that allows automatic cancellation of resistances in series with the external temperature sensor. Figure 14 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, but it can equally be a discrete transistor. If a discrete transistor is used, the collector is not grounded but is 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. C1 may be added as a noise filter (a recommended maximum value of 1,000 pF). However, a better option in noisy environments is to add a filter, as described in the Noise Filtering section. See the Layout Considerations section for more information on C1. To measure DVBE, the operating current through the sensor is switched among three related currents. As shown in Figure 14, N1 I and N2 I are different multiples of the current, I. The currents through the temperature diode are switched between I and N1 I, giving DVBE1; and then between I and N2 I, giving DVBE2. The temperature is then calculated using the two DVBE measurements. This method also cancels the effect of any series resistance on the temperature measurement. The resulting DVBE waveforms are passed through a 65 kHz low-pass filter to remove noise and then to a chopper-stabilized amplifier. This amplifies and rectifies the waveform to produce a dc voltage proportional to DVBE. The ADC digitizes this voltage producing a temperature measurement. To reduce the effects of noise, digital filtering is performed by averaging the results of 16 measurement cycles for low conversion rates. At rates of 16-, 32-, and 64-conversions/second, no digital averaging occurs. Signal conditioning and measurement of the internal temperature sensor are performed in the same manner. |
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