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LM5116: LM5116 oscillation when borderline CCM/DCM

Jason Cuadra35
Intellectual 260 points
Part Number: LM5116

Hello,

I have a buck converter using the LM5116:

18-51 V input
12V / 3A output

I get an output oscillation that occurs at a narrow band of load current, at light load. The current at which it happens varies according to input voltage. It happens right when the converter is in borderline CCM/DCM.  I have the part in diode emulation mode.

The load step response at heavy load (e.g. 50% - 100% load) is only slightly underdamped.
The load step response at light load, where it stays in DCM, also looks fine. 

The waveforms below show choke current, and output ripple voltage (AC coupled). Load is 0.4A and input is 25 V.

Can you give me a list of things to check?

Thanks.



  • 0
    TI__Mastermind 25590 points
    Jason,

    Please share the schematic.

    Please also share this same scope setup with FB on CH3 and COMP on CH4.

    Thanks,
    -Sam
  • 0 in reply to Samuel Jaffe
    Intellectual 260 points

    Please see below.

    From top to bottom:

    - output voltage ripple
    - choke current
    - FB pin
    - COMP pin

    It looks voltage-modey in that the choke current lags the COMP pin by 90*.  More specifically, once the choke triangular current "touches" zero (DCM), there is a lag of about 10 switching cycles from when the COMP pin begins rising, and the current begins rising up.  At the peak of the current waveform (CCM), there is only about a 3 switching cycle lag from when the COMP pin begins dropping, and the current begins dropping.

    I did a step response test at heavy load (CCM), and the lag is only 3 switching cycles, so it looks much more like current mode.





    Schematic:

  • 0 in reply to Jason Cuadra35
    TI__Mastermind 25590 points
    I've contacted the product expert to help. We will get back to you in the coming days (holidays may delay us).

    -Sam
  • 0 in reply to Samuel Jaffe
    TI__Guru 73742 points

    Hi Jason,

    The lead network across the upper feedback resistor is typically not connected for a current-mode controller. Please forward a completed LM5116 quickstart file for your circuit (this Excel file is available by download from the product folder). 

    The DEMB resistor is normally 0 Ohms to enable diode emulation mode. Does the oscillation appear if diode emulation mode is disabled?

    Regards,

    Tim