Using Power Delivery Path Resistance For Precision CPU Current Monitoring
by Viktor Vogman, Power Conversion Consulting, Olympia, Wash., How2Power Today, Feb 15 2019
Focus:
To monitor CPU current (and power), typical CPU voltage regulator (VR) designs use the VR
inductor’s winding equivalent series resistance (ESR) and/or VR switching transistor’s on-
resistance as current sensors. When processor current is constant or varies slowly, the
inductor current (or MOSFET current magnitude) signal practically replicates the CPU current
waveform and such sensing techniques provide reasonable accuracy. But in very dynamic modes
with high current slew rates, the effect of LC-filtering results in latencies and level errors.
This article describes a current monitoring technique that addresses this issue by performing
nonintrusive, real-time current monitoring of the actual CPU current. The article begins by
discussing the threat posed by power viruses, and why this requires power monitoring at the
point of load. Then the current monitoring concept is explained: CPU current can be derived as
a sum of the currents flowing through the power plane and cavity cap array. Models of the power
delivery network are presented, sources of error are discussed, details of implementation are
explained, and experimental results are presented to verify the technique.
What you’ll learn:
- How to perform nonintrusive, real-time current monitoring of CPU current
- How to overcome errors in CPU current monitoring due to effects of LC filtering
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