Introduction to Non-Invasive Current Estimation (NICE)

It is notoriously difficult to measure instantaneous supply current to a device such as an ASIC, FPGA, or CPU without also affecting the instantaneous supply voltage and compromising the operation of the device [21]. For decades designers have relied on rough estimates of dynamic load currents that stimulate a designed Power Delivery Network (PDN). The consequences of inaccurate load-current characterization can range from excessive PDN cost and lengthened development schedules to poor performance or functional failure. This paper will introduce and describe a method to precisely determine timedomain current waveforms from a pair of measured timedomain voltage waveforms. This NonInvasive Current Estimation (NICE) method is based on established twoport network theory along with component and board modeling techniques that have been validated through measurements on demonstrative circuits. This paper will show that the NICE method works for any transient event that can be captured on a digital oscilloscope. Limitations of the method and underlying measurements are noted where appropriate. The method is applied to a simple PDN with an arbitrary load, and the NICE-derived current waveform is verified against an independent measurement by sense resistor. With careful component and board modeling, it is possible to calculate current waveforms with a root mean square error of less than five percent compared to the reference measurement. Current transients that were previously difficult or impossible to characterize by any means can now be calculated and displayed within seconds of an oscilloscope-trigger event by using NICE. ASIC and FPGA manufacturers can now compute the startup current for their device and publish the actual waveform, or provide a piecewiselinear SPICE model (PWL source) to facilitate design and testing of the regulator and PDN required to support their device.