Best usage of free-space capacitors in ASIC regulators

Abstract. In this work we examine how to improve the performance of voltage regulators in application specific integrated circuits (ASICs) by placing capacitors into free layout space. The problem arising after layout, when there are areas not covered by functional elements, is where to connect the free-space capacitors (FSCs), as they can be connected to the input or the output net of a voltage regulator. Therefore we designed a testbench for mathematical calculations and one for simulations to identify the influence of a capacitance connected at these certain positions. We mainly focused on PSR analysis while not losing sight of transient effects. The results of calculation and simulation illustrate that the best solution is to split the capacitance half by half to both possible nets if no output capacitance was installed during design. Otherwise a ratio of one to one for input capacitance to output capacitance has to be set up for best performance.