Instantaneous Voltage Drop (IVD) is a part of the landscape in Deep Sub-Micron (DSM) design. IVD is a droop in rail voltage prompted by large amounts of simultaneous switching in a circuit. Problems with IVD are especially common to high speed memories, which have potentially thousands of cells switching at a time.
A design with enough rail-to-rail capacitance becomes resistant to the effects of IVD, as the capacitance acts as a charge reserve supplying local current sinks briefly for the time of the event.
As such, it has become common in the past decade to add dcaps to the areas of an IC that otherwise have no cells, or add dcaps directly to problem areas such as high speed memories.
However, DCAPs normally come with a quite serious down-side. They are leaky devices. Too many DCAPs in a design can push leakage beyond the thermal runaway point, or at least beyond the customer's expectations.
For these reasons, DCAP insertion is a subject that requires more than casual attention of the layout designer.-- EOF-TRUEVUE --