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Digital Temperature Sensing in a Variable Supply Environment

Digital Temperature Sensing in a Variable Supply Environment. EE241 Term Project Matthew Spencer Steven Callender Spring 2009. Accurately Measures BJT Current Big, Power-Hungry, Analog Circuit. Classic Temperature Measurement is Analog.

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Digital Temperature Sensing in a Variable Supply Environment

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  1. Digital Temperature Sensing in a Variable Supply Environment EE241 Term Project Matthew Spencer Steven Callender Spring 2009

  2. Accurately Measures BJT Current • Big, Power-Hungry, Analog Circuit Classic Temperature Measurement is Analog [1] M. Pertijs, K. A. Makinwa, J. H. Huijsing. A CMOS Smart Temperature Sensor With a 3σ Inaccuracy of +/-0.1°C From -55°C to 125°C. IEEE JSSC, December 2005

  3. Measure Delay Pretty Well • Ignore Supply and Process Digital Replacements Ignore Real Effects [2] Poki Chen et al. An Accurate CMOS Time-to-Digital-Converter-Based Smart Temperature Sensor. JSSC 2005 [3] K. Woo, D. Ham et al. Dual-DLL-Based CMOS All-Digital Temperature Sensor for Microprocessor Thermal Monitoring. ISSCC 2009.

  4. Motivation • Current sensors: What makes them good? • Comparing digital sensors and Simulation • Digitally Assisted Supply Correction • Supply Insensitive Biasing Outline

  5. Vdd to delay constants mismatched • Optimal Resolution, not supply rejection A Worrisome Graph from a Paper …

  6. “Linear With Supply” Costs a Lot of Area

  7. Use Digital Algorithm to Help Measurement • Use Several f(C) to Guess Vdd and T Can the We Correct for Supply with Math?

  8. Good Vdd Guess, but Bad T Guess • Exponentially sensitive to Vdd Digital Correction Can’t Handle Vdd Errors

  9. Strong coupling between Vdd and temperature in delay variations • Very hard to isolate each effect for a given delay variation (How much is due to Vdd and how much is due to temperature?) • Solution: Supply insensitive biasing • Idea: “remove” any Vdd shifts from the point of view of the delay cells Accounting for Vdd shifts: A Better Approach

  10. Once delay variations are decoupled from supply shifts, problem becomes trivial • Supply insensitive biasing is the most viable approach, but there are still issues: • Device Sizes • Topology: New approaches may need to be explored Accounting for Vdd shifts: A Better Approach

  11. Literature ignores real environments • Correcting for supply hasn’t worked yet • Delay is a BAD temperature metric • … But finding something else is hard Conclusion

  12. Thank You • Questions?

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