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Digital Temperature Sensing in a Variable Supply Environment
EE241 Term ProjectMatthew SpencerSteven Callender
Spring 2009
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Classic Temperature Measurement is Analog
• Accurately Measures BJT Current • Big, Power-Hungry, Analog Circuit
[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
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Digital Replacements Ignore Real Effects
[2] Poki Chen et al. An Accurate CMOS Time-to-Digital-Converter-Based Smart Temperature Sensor. JSSC 2005
• Measure Delay Pretty Well• Ignore Supply and Process
[3] K. Woo, D. Ham et al. Dual-DLL-Based CMOS All-Digital Temperature Sensor for Microprocessor Thermal Monitoring. ISSCC 2009.
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Outline
• Motivation
• Current sensors: What makes them
good?
• Comparing digital sensors and
Simulation
• Digitally Assisted Supply Correction
• Supply Insensitive Biasing
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A Worrisome Graph from a Paper …
• Vdd to delay constants mismatched• Optimal Resolution, not supply rejection
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“Linear With Supply” Costs a Lot of Area
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Can the We Correct for Supply with Math?
• Use Digital Algorithm to Help Measurement• Use Several f(C) to Guess Vdd and T
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Digital Correction Can’t Handle Vdd Errors
• Good Vdd Guess, but Bad T Guess• Exponentially sensitive to Vdd
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Accounting for Vdd shifts: A Better Approach
• 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
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Accounting for Vdd shifts: A Better Approach
• 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
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Conclusion
• Literature ignores real environments
• Correcting for supply hasn’t worked yet
• Delay is a BAD temperature metric
• … But finding something else is hard
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Thank You
Questions?