What is the "worst-case power corner" used for power signoff?
From PDVerse Low-Power Physical Design Mentor Guide, part of the pdVerse Mentor Guide
Short Answer
For leakage, the worst case is fast process, maximum voltage and maximum junction temperature, because every one of those raises off-current. For dynamic and peak power, the worst case is maximum voltage with the activity of the busiest real mode. Average battery power is usually reported separately at typical conditions with realistic use-case activity.
Technical Explanation
- Leakage corner: FF process, highest supply in the spec, hottest junction (for example 125 °C), measured in the idle or standby mode.
- Peak dynamic corner: highest voltage and frequency, with activity from the heaviest mode such as a full CPU plus GPU load.
- Activity matters as much as PVT: a default toggle rate can under- or over-state dynamic power badly.
- Average power for battery life uses typical conditions and a weighted mix of modes, not the worst case.
- Peak power feeds IR-drop and power-grid signoff; leakage feeds standby budget and thermal limits.
- Write the corner and mode next to every power number, so a reviewer can tell a standby figure from a peak one.
- Mixing them up breaks things: grid sized for average current droops under peak load.
Common Mistake
The Trap: Using one corner with default activity for every power question.
- The standby budget is checked too cold, peak current is underestimated, and the grid and package are sized wrong.
- Keep a small table with one row per power question: process, voltage, temperature, mode and activity source.
Follow-up Question & Model Response
"Where does the activity for a power corner come from?"
Candidate Model Response: From simulation of a real workload, written as SAIF toggle counts or a VCD waveform, and read into the power tool. For peak power you pick the window with the most switching, often from an emulator run. Without it the tool falls back to vectorless default toggle rates, which are only good for early estimates. Garbage activity in gives garbage power out.
Practical Example
Design Scenario: (illustrative) A tablet SoC reports three numbers: standby leakage 45 mW at FF 0.99 V 125 °C with only the always-on domain powered, peak 6.5 W at FF 0.99 V 105 °C in a game workload, and average 1.8 W at TT 0.9 V 65 °C over a daily-use mix. The grid is sized from the 6.5 W case, the battery from the 1.8 W case. Check the currents: 6.5 W at 0.99 V is about 6.6 A, over three times the 2 A average at 0.9 V, so a grid sized from the average would droop.
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