#LED MACSPICE HOW TO#
#LED MACSPICE DRIVER#

LT3950 DC2788A demonstration circuit LED driver LTspice model with control loop noise injection and measurement points. Large frequency sweep, stopping at fSW/2 (half of the switching frequency ofįigure 2. The entire control loop Bode plot, this measurement must be repeated over a The gain (dB) and the phase (°) at the injection frequency (f). Measurement points A, B, and C are used to calculate

Sine wave of given frequency (f) injected directly into the feedback path on the Figure 2 shows an LED driver ( LT3950) with an ideal LTspice simulation can be used to create a similar injection and measurement in To make the measurement, the control loop is broken and a sinusoidal perturbation pushes into a high impedance path, while the resulting control loop gain and phase is measured, enabling the designer to quantify the stability of the loop. Switching regulator control loop Bode plot measurement with a network analyzer for (a) voltage regulator and (b) LED driver. Figure 1 shows the setupįor making these measurements using a network analyzer.įigure 1. The crossover frequency relates to general loop speed. System, and that –10 dB gain margin is required for guaranteed loop stability. It is generally accepted that 45° to 60° of phase margin is required for a stable Standard switching regulator control loop Bode plots yield three critical measurements, which can be used to determine stability and speed:

#LED MACSPICE HOW TO#

How to generate LED driver current-sense feedback loop Bode plots in LTspice,Īnd in the lab. One not previously demonstrated in LTspice. Injection point or Middlebrook’s original recommendation in the feedback path, Resistance LED loads, should have a different injection point than either today’s The actual signal injection location laid out in Middlebrook’s method is not commonly used these daysīut has been adjusted over the years, resulting in the commonly used injectionįurthermore, LED drivers, with high-side sense resistors and complicated AC “ LTspice: Basic Steps in Generating a Bode Plot of SMPS”). The process of using LTspiceīased on Middlebrook’s original recommendation in 1975 is well documented (see Help to get a rough idea of control loop stability-a starting point for compensation component choices and output capacitor sizing. Transients and Bode plots before circuits are assembled or fabricated. LTspice ® simulation can be used to generate both switching regulator output Gain and phase Bode plots are required for deeper analysis. These relatively easy system checksĮnable on-the-fly characterization of a switching regulator’s control loop, but A long settling time can indicate the need to speed up the bandwidth Or, a large spike during the transient can indicate the need for more COUT or af aster loop. Instance, a critically damped response might indicate 45° to 60° phase margin. Or gain margin and thus can be used to understand general loop stability. The shape of the transient disturbance indicates the phase Large signal transients can be used to check absolute deviation and system Gain and phase numbers-which can be used to guarantee stability-but they canĪct as a telltale for general control loop stability and speed. The results of transient analysis lack the Bode plot’s frequency-based Response observation requires the application board and more common benchtopĮquipment. One option is to build the LED driver and see how it responds to transients. Phase and gain measurements are the best way to guarantee stability, but notĮvery engineer has at their fingertips the required equipment and access to anĮxperienced factory apps team. Regulator injection and measurement point. (see Figure 1)-a deviation from the typical resistive-divider-path-to-GND voltage LED driver control loop phase and gain measurements require a different approach Measuring the control loop is rarely applied to LED drivers. The resulting gain and phase over a frequency sweep (see Figure 1). Measurements involve breaking the control loop, injecting noise, and measuring Gain and phase measurements, whenĭone properly, require access to, and familiarity with, fancy network analyzers. Of the control loop in switching regulators. How to Use LTspice to Produce Bode Plots for LED DriversĬlosed-loop gain and phase plots are well-worn tools used to determine the stability

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