LT3791 Buck mode question

Question

Hello 
 Customer has and issue about LT3791EFE-1 and that is when the input voltage > output voltage, the buck-boost converter should be worked at buck mode, and the M1 & M2 should be work as buck converter, and M4 should be 100% on and M3 should be OFF, however when they measured the waveform at boost side of inductor, it is not a flat waveform like Vout, it is also a switching waveform, that violate customer understanding about H bridge buck-boost converter. 
 
 Why we see switching waveform, on boost side of inductor? Is it normal? Thank you.

Patrick

MarkusHP · Accepted Answer

Hello, 
 are you sure the Vin is large enough to operate in buck mode? if Vin is only slightly higher than Vout the Dutycycle might have a value where the part operates in BuckBoost Region: 
 First measure is to increase Vin and check if something changes at the switch nodes of the converter 
 It would be helpful to show measured waveforms for both Switch nodes and give more details on the real circuit.)

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LT3791 Buck mode question