Magic Blue Smoke

 

Power Gating and Decoupling Cap cell Insertion

Hearing many concerns on topics related to power gated design and its relation to decoupling capacitor to reduce IR-Drop issues. Thought of writing a paragraph on this topic.

Power gated designs requires addition of decoupling capacitor  to power network to resolve dynamic IR-drop issues.  Decoupling cap insertion becomes more challenging in the power-gated designs compared to normal designs. Besides the issues of high leakage power in the de-caps , de-caps added to the virtual power network has significant impact on the wakeup latency and in-rush current.  Larger the decoupling capacitance, Larger the in-rush charge current and longer time to wakeup. On the other hand, de-caps, inserted in the permanent power network, does not affect the wakeup latency and in-rush current. Therefore, optimization of de-cap insertion in the power-gating design should not only minimize total capacitance of the de-caps but we should also consider adding some capacitance to the virtual power network while meeting the dynamic IR-drop target.