An Improved Active Decoupling Capacitor for “Hot-Spot” Supply Noise Reduction in ASIC Designs

On-chip decoupling capacitors (decaps) are widely used to reduce power supply noise by placing them at the appropriate locations on the chip between blocks. While passive decaps can provide a certain degree of protection against IR drop, if a problem is found after the physical design is completed, it is difficult to implement a quick fix to the problem. In this paper, we investigate the use of an active decap as a drop-in replacement for passive decaps to provide noise reduction for these so-called ldquohot-spotrdquo IR drop problems found late in the design process. A modified active decap design is proposed for ASIC applications operating up to 1 GHz. Our improvement uses latch-based comparators as the sensing circuit, which provides a better power/delay tradeoff than previous designs and incorporates hysteresis to minimize unnecessary switching. It is implemented in a 1 V-core 90 nm CMOS process with a total area of 0.085 mm
²
and static power of 2.8 mW. Measurements from a number of test chips show that using an active decap can provide between 10%-20% noise reduction in the 200 MHz-1 GHz frequency range over its passive counterpart. Sizing and placement analyses are also carried out using circuit simulation. The active decap is most effective when placed in close proximity to the hot-spot, as compared to the passive decap which is less sensitive to the exact location. Overall, if sized and placed properly, active decaps can provide an additional 20% reduction in supply noise over passive decaps.