LT3791 Buck mode 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

Parents
  • +1
    •  Analog Employees 
    on Jan 14, 2020 11:29 AM

    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.

Reply
  • +1
    •  Analog Employees 
    on Jan 14, 2020 11:29 AM

    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.

Children