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TMP12 Fiches technique(PDF) 9 Page - Analog Devices |
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TMP12 Fiches technique(HTML) 9 Page - Analog Devices |
9 / 14 page TMP12 REV. 0 –9– Measuring the TMP12 Internal Temperature As previously mentioned, the TMP12’s VPTAT generator repre- sents the chip temperature with a slope of 5 mV/K. In some cases, selecting the setpoints is made easier if the TMP12’s internal VPTAT voltage (and therefore the chip temperature) is known. For example, the case temperature of a high power microprocessor can be monitored with a thermistor, thermocouple, or other mea- surement method. The case temperature can then be correlated with the TMP12’s temperature to select the setpoints. The TMP12’s VPTAT voltage is not available externally, so indi- rect methods must be used. Since the VPTAT voltage is applied to the internal comparators, measuring the voltage at which the digital output changes state will reflect the VPTAT voltage. A simple method of measuring the TMP12 VPTAT is shown in Figure 20. To measure VPTAT, adjust potentiometer R1 until the LED turns ON. The voltage at Pin 2 of the TMP12 will then match the TMP12’s internal VPTAT. VPTAT TMP12 VREF SET HIGH GND V+ HEATER 1 2 3 4 5 6 7 8 200K 200K +5V NC R1 +5V R1 330 +5V LED OVER UNDER SET LOW Figure 20. Measuring VPTAT with a Potentiometer The method described in Figure 20 can be automated by replac- ing the discrete resistors with a digital potentiometer. The improved circuit, shown in Figure 21, permits the VPTAT volt- age to be monitored with a microprocessor or other digital controller. The AD8402-100 provides two 100 k Ω potentiom- eters which are adjusted to 8-bit resolution via a 3-wire se- rial interface. The controller simply sweeps the wiper of potentiometer 1 from the A1 terminal to the B1 terminal (digital value = 0), while monitoring the comparator output at Pin 7 of the TMP12. When Pin 7 goes low, the voltage at Pin 2 equals the VPTAT voltage. This Circuit sweeps Pin 2's voltage from maximum to minimum, so that the TMP12's setpoint hystersis will not affect the reading. The circuit of Figure 21 provides approximately 1 °C of resolution. The two potentiometers divide VREF by two, and the 8-bit potentiometer further divides VREF by 256, so the resolution is: Resolution = = 4.9 mV VREF 2 2N 2.5 V 2 28 = where VREF is the voltage reference output (Pin 1 of the TMP12) and N is the resolution of the AD8402. Since the VPTAT has a slope of 5 mV/K, the AD8402 provides 1 °C of resolution. The adjustment range of this circuit extends from VREF/2 (i.e. 1.25 V, or 23 °C) to VREF 1 LSB (i.e. 2.5 V 4.9 mV, or 226 °C). The VPTAT is therefore: VPTAT = 1.25 V + (Digital Count 4.9 mV) where Digital Count is the value sent to the AD8402 which caused the setpoint 1 output to go LOW. A third way to measure the VPTAT voltage is to close a feedback loop around one of the TMP12’s comparators. This causes the comparator to oscillate, and in turn forces the voltage at the comparator input to equal the VPTAT voltage. Figure 22 is a typical circuit for this measurement. An OP193 operational amplifier, operating as an integrator, provides additional loop-gain to ensure that the TMP12 comparator will oscillate. TEMPERATURE SENSOR & VOLTAGE REFERENCE 1 2 3 4 HYSTERESIS GENERATOR WINDOW COMPARATOR VPTAT VREF 7 8 5 6 TMP12 SERIAL DATA INTERFACE 15 6 7 8 9 10 11 B2 2 A2 3 W2 4 W1 12 A1 13 B1 14 100 DGND AGND SDI CLK CS AD8402–100 µC INTERFACE V DD RS SHDN OVER +5V +5V NC NC Figure 21. Measuring VPTAT with a Digital Potentiometer |
Numéro de pièce similaire - TMP12 |
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Description similaire - TMP12 |
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