Feature: Analogue design


LED driver Bode plots using LTspice


By Keith Szolusha, Applications Director, and Brandon Nghe, Applications Engineer, Analog Devices


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roper control of loop phase and gain measurements should be made by factory experts possessing (expensive) equipment


and commensurate experience. For those without access to these, there is an alternative. Closed-loop gain and phase plots are


well-known tools used to determine the stability of the control loop in switching regulators. Gain and phase measurements when done properly require access to, and familiarity with, fancy network analyzers. Te measurements involve breaking the control loop, injecting noise and measuring the resulting gain and phase over a frequency sweep. Tis method of measuring the control loop is rarely applied to LED drivers. LED driver control loop phase and


gain measurements require a different approach (Figure 1) – a deviation from the typical resistive divider path to GND voltage regulator injection and measurement point. In both cases,


26 May 2022 www.electronicsworld.co.uk


Looking at the overall design cycle picture, even a five-minute simulation is orders of magnitude faster than designing, assembling and testing PCBs


benchtop measurements are the best way to guarantee stability, but not every design team has at its fingertips the required equipment and access to an experienced factory apps engineers. One alternative is to build the LED


driver and see how it responds to transients. Transient response observation requires an application board and more common benchtop equipment. Te results of transient analysis lack the Bode plots’ frequency-based gain and phase numbers, which can be used to guarantee stability, but they can act as a telltale for general control loop stability and speed. Large signal transients can be used


to check absolute deviation and system response time. Te shape of the transient


disturbance indicates the phase or gain margin and thus can be used to understand general loop stability. For instance, a critically-damped response might indicate 45-60° phase margin. Or, a large spike during the transient can indicate the need for more COUT


or a faster loop. A


long settling time can indicate the need to increase the bandwidth (and crossover frequency) of the loop. Tese relatively easy system checks enable on-the-fly characterisation of a switching regulator’s control loop, but gain and phase Bode plots are required for deeper analysis. LTspice simulation can be used to


generate both switching regulator output transients and Bode plots before circuits are fabricated. Tis can provide a rough


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