Current-based temperature measurement devices and methods
A Proportional-To-Absolute-Temperature (PTAT) current source is used for high-resolution temperature measurement. The PTAT current source is coupled to a capacitor for a fixed amount of time so as to charge the capacitor to a voltage which is proportional to the current applied to the capacitor, and thus proportional to the temperature. The voltage on the capacitor is measured, and a temperature is calculated or determined from the measured voltage.
1. A temperature measurement system, comprising:
a proportional to absolute temperature (PTAT) current source;
a capacitor selectively coupled to the PTAT current source by a controllable switch, wherein the switch is controllable to decouple to capacitor from the PTAT current source such that a voltage across the capacitor remains constant;
a controller coupled with a timer and configured to control the switch to couple the capacitor to the PTAT current source for a charging time determined by the timer, wherein the capacitor is charged to a voltage during the charging time;
an analog-to-digital converter (ADC) having an input coupled to the capacitor, and configured for generating a digital representation of the voltage across the capacitor after the capacitor has been charged for the charging time determined by the timer and decoupled from the PTAT current source; and
an electronic circuit configured to calculate a temperature based on the digital representation of the voltage on the capacitor.
2. The temperature measurement system of claim 1 , further comprising a current mirror to increase the current available from the PTAT current source.
3. The temperature measurement system of claim 1 , further comprising a current mirror to adjust a temperature coefficient of the PTAT current source.
4. The temperature measurement system of claim 1 , wherein the temperature measurement system has a temperature measurement resolution selected from any one of the group consisting of less than 1° C., less than 0.5° C., less than 0.2° C., less than 0.1° C., and less than 0.05° C.
5. The temperature measurement system of claim 1 , wherein the charging time is selected from any one of the group consisting of less than 1 second, less than 100 milliseconds, less than 10 milliseconds, less than 1 millisecond, less than 100 μs, less than 10 μs, less than 1 μs, and less than 100 ns.
6. The temperature measurement system of claim 1 , wherein the charging time is about 625 ns.
7. The temperature measurement system of claim 1 , further comprising a shorting switch to discharge the capacitor before charging during the charging time.
8. The temperature measurement system of claim 7 , wherein the controller controls the shorting switch.
9. The temperature measurement system of claim 1 , further comprising a zero change to absolute temperature (ZTAT) current sink configured for adjusting a capacitor charging current to a desired value.
10. The temperature measurement system of claim 9 , wherein PTAT and ZTAT currents are created by band gap circuits.
11. The temperature measurement system of claim 9 , further comprising a current mirror to increase the current handling of the ZTAT current sink.
12. The temperature measurement system of claim 1 , wherein the capacitor, controllable switch, controller, timer, ADC, and electronic circuit are within a microcontroller.
13. The temperature measurement system of claim 12 , wherein the microcontroller comprises:
a microprocessor; and
a memory coupled to the microprocessor and storing a program for determining the temperature from the digital representation of the voltage on the capacitor.
14. The temperature measurement system of claim 12 , wherein the microcontroller comprises the ADC and the controllable switch.
15. A method for measuring temperature, said method comprising the steps of:
providing a current from a proportional to absolute temperature (PTAT) current source;
charging a capacitor with the current from the PTAT current source for a charging time determined by a timer;
disconnecting the PTAT current source after the charging time determined by the timer has been passed such that a voltage across the capacitor is held constant for subsequent measuring;
measuring the voltage across the capacitor after the charging time determined by the timer has been passed; and
converting the measured voltage to a temperature.
16. The method according to claim 15 , wherein the step of converting the measured voltage to a temperature comprises the steps of:
determining a reference voltage on the capacitor, wherein the reference voltage is equal to the current from the PTAT current source at a certain temperature multiplied by the charging time divided by a capacitance of the capacitor;
determining a current/temperature slope of the PTAT current source;
converting the current/temperature slope to a voltage/temperature slope over the charging time; and
calculating the temperature from the reference voltage and the voltage/temperature slope.
17. The method according to claim 15 , further comprising the step of adjusting the capacitor charging current to a desired value with a zero change to absolute temperature (ZTAT) current sink.
18. The method according to claim 15 , further comprising the step of increasing the current from the PTAT current source with a mirror current source.
19. The method according to claim 15 , further comprising the step of increasing a temperature coefficient of the PTAT current source with a mirror current source.
20. An apparatus for measuring temperature, comprising:
a microcontroller comprising
a microprocessor with memory,
an analog-to-digital converter (ADC) having an output coupled to the microprocessor,
a capacitor coupled to an input of the ADC,
a first switch having a first terminal coupled to a first node and a second terminal coupled with the capacitor,
a second switch coupled between the capacitor and a common voltage (ground),
a switch controller coupled to the microprocessor and the first and second switches, and
a timer coupled to the microprocessor; and
a proportional to absolute temperature (PTAT) current source coupled between a supply voltage and the first node;
wherein the first switch closes for a certain time determined by the timer and then opens;
the ADC measures a voltage across the capacitor after the certain time determined by the timer has expired and the first switch decouples the capacitor from the PTAT current source such that a voltage across the capacitor is held constant; and
the microprocessor converts the measured voltage to a temperature.
21. The apparatus according to claim 20 , wherein the second switch closes and discharges the capacitor before the first switch closes.
22. The apparatus according to claim 20 , further comprising a zero change to absolute temperature (ZTAT) current sink configured for adjusting a charging current provided by the PTAT current source to the capacitor.