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LM5176-Q1: Regarding Hiccup Mode Current Limiting Feature Operation.

Soundhar B
Prodigy 30 points
Part Number: LM5176-Q1

Tool/software:

Hello All, We have used this IC for the Buck Boost operation from 6V to 29V. Desired output = 24V, Desired Maximum Load current = 8A. It was giving 23.9V Properly even when we change the input voltage in the specified range. We are supplying 24V to this IC from a standard half brick which can can give upto 16A. When we load this LM5176-Q1 buck boost (with 24V input and 24V output), It was working properly till 8A and it trips after that. This operation is well known according to the IC hiccup mode current limit feature. But after 8A, this buck boost stage made input side also to trips. We have checked this. After 8A, the standard power brick itself is tripping and shutting down. When we load power brick separately, we were able to get till 15.5A. In Buck-boost stage, there was nothing much current ripples even not voltage ripples. Didn't get this behaviour properly. Attached schematic.

  • 0
    TI__Genius 12945 points

    Hi Soundhar,

    Thanks for reaching out.

    Can you please send scopeshots of the behavior you see?

    I would like to see Vin, Vout and both switchnodes.

    Also, what half bridge are you using?

    Best regards

    Moritz

  • 0 in reply to Moritz
    Prodigy 30 points

    Hello Moritz, Sorry for the delay and inconvenience. We found the logic which causes the input to turn off. This buck boost output is going to a microcontroller supply, when this buck boost trips when overloaded, this also turns off the microcontroller supply. In our logic, microcontroller enables the input supply to the Buck Boost initially. Thats how we designed it. When buck boost trips, microcontrollers loses the supply and turns off the input as well. This we have got it. 

    Now, we designed this buck boost for 8A, but it trips when load current reaches 6.1A itself. We probed both the sides of switch nodes and attached the waveforms. It seems there are switch node noise present at the waveform.We suspect current waveform will also have the spikes, that might cause the trip even before the set limit. MOSFETs we are using is (SQJA86EP-T1_GE3). Suggest us to how to mitigate and reduce them in an efficient way, which might improve the design and meets the requirements. Here in all pictures Yellow is input 24V, Purple is Ouput of Buck Boost (24V), Cyan is Switchnode 1 and green is Switch node2. Thanks.

  • +1 in reply to Soundhar B
    TI__Genius 12945 points

    Hi Soundhar,

    You can reduce these spikes by adding RC snubbers to both switchnodes (From SW to GND).

    Best regards

    Moritz

  • 0 in reply to Moritz
    Prodigy 30 points

    I will add it. thanks Moritz