LM26480-Q1: AC Analysis

Jenny,

I apologize, but after opening the TINA-TI model for BUCK1 of the the LM26480 I see that the test bench is set up for a transient load step analysis. There is no way of knowing if this model would behave correctly if you attempted AC analysis to determine frequency response (Gain & Phase Margin).

However, there is a relatively simple test that can be run using the transient model to verify stability:

Apply an instantaneous load step and determine the amount of "ringing" at the output. If the buck regulator overshoots the desired output voltage after dropping due to the load being applied, or if the waveform oscillates as the voltage is regulated in reaction to the load step, then the circuit is unstable (not critically damped). If the voltage simply undershoots and recovers without any "ringing", then it is a stable circuit. The same rule applies when the load is removed, but in reverse: naturally, the regulator will overshoot and then settle to the correct voltage again.

Please refer to the highlighted regions in the below simulation:

Using the rule of thumb in the below image (source):

I would say that there is significant phase margin in the LM26480 test bench that was setup due to the lack of overshoot and ringing caused by the load step. You could verify this by forcing the circuit into unstable scenarios (using a very small inductor, for example) to see ringing and overshoot. However, the guidelines for output inductance and capacitance in PMICs typically ensures that the circuit will be stable with the internal compensation.

See the attached TINA simulation (.tsc file) I ran to modify the test bench and produce this result.

snvmb54_LM26480-BUCK1_mod.TSC