During sign-off dynamic IR drop analysis, a localized region of standard cells shows a transient 15% VDD droop due to simultaneous switching. The physical designer resolves this by inserting decoupling capacitors (decaps) into the empty standard cell sites of that specific region. How do these decaps physically mitigate the dynamic IR drop?
The power grid has inherent inductance and resistance, which prevents the main power supply from instantly reacting to sudden current spikes. Decoupling capacitors store charge locally. When logic gates in the region switch simultaneously, the decaps act as local batteries, instantly supplying the necessary high-frequency current and preventing the localized VDD voltage from dropping.
