By Benjamin Dannan and Steven Sandler, Picotest.com
Control loop stability is critical to the performance of all systems, as it influences all closed loop parameters, as well as system noise. Unfortunately, in many instances (particularly in the cases of voltage references, fixed voltage LDOs, and integrated POLs), a Bode plot assessment is not feasible because there is no feedback loop access to the part. In other cases, the feedback loop is difficult to access because the hardware is integrated or would require cutting a printed circuit board (PCB) trace. In yet other cases, the devices either contain multiple control loops (with only one of them being accessible) or the order of the control loop is higher than 2nd order, in which case the Bode plot is a poor predictor of relative stability. A further complication is that in many portable electronics such as cell phones and tablets, the circuitry is very small and densely populated, leaving little in the way of access to the control loop elements. In these cases, the non-invasive stability margin (NISM) assessment, which is derived from easily accessible output impedance measurements, is the only way to verify stability.
NISM computes stability from output impedance [1]. The 2-port shunt-through impedance measurement is the gold standard for measuring a VRM’s output impedance in the microOhm and milliOhm region [2]; however, it is not always possible to make these measurements without direct SMA connections to the PCB or Device Under Test (DUT). Therefore, when a designer makes these types of measurements with a Vector Network Analyzer (VNA), the method of connecting the DUT requires attention to detail to ensure inductance and various error sources are minimized to allow an accurate measurement. To get the most out of your VNA, you need to use the right probes and accessories to ensure your measurement is successful. With a browser probe like the P2102A, you can quickly characterize multiple VRMs to ensure stability or even check if your model is accurate during your initial power delivery network (PDN) design.
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