Feature: Analogue design


Figure 3: The measurement of points A, B and C in Figure 2 determine the gain and phase of the control loop at the injection frequency. Different injection frequencies yield different gain and phase. Figures 3a and 3b show the gain and phase of 10kHz ± 10mV AC injection. Figures 3c and 3d show the gain and phase of 40kHz ± 10mV AC injection. A sweep of frequencies and the measurements of gain and phase between B-C and A-C form the closed-loop Bode plot


between B-C and A-C form the entire closed-loop Bode plot. As mentioned earlier, this is typically accomplished on the bench using a fancy (and expensive) network analyzer. Such a sweep is also possible in LTspice, as shown in Figure 4. These results are confirmed by comparing them to those using a network analyzer; see Figure 8.


Creating a full Bode plot in LTspice To create a full Bode plot, a graphical sweep of gain and phase in LTspice for the control loop, follow these steps:


Step 1: Create the AC injection source In LTspice, insert the ±10mV AC injection voltage source and injection resistor and label nodes A, B and C, as shown in Figure 2. The voltage source value sets the 10mV peak and sweeps the frequency. The user can play with peak sine values between 1mV and 20mV. But, keep in mind that the


28 May 2022 www.electronicsworld.co.uk


Figure 4: Bode plot measurements with the LT3950 in LTspice showing gain (solid line) and phase (dashed line)


sense voltage for many LED drivers is 250mV and 100mV. Higher injection noise can create LED current regulation errors.


Step 2: Do the math Insert the .measure statements on the schematic as .sp (SPICE) directives. Tese directives perform Fourier transforms and


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