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LM5122:Output current does not reach the desired specification

Lee Jong Chul
Prodigy 40 points
Part Number: LM5122

Tool/software:

Hello.

I am currently verifying the converter with 12V 88A specifications.

I want to drive it in a 4-phase interleaved manner, but the output current is at 1/3 level, so I am conducting the test with one phase.

The current of one phase should be 22A, but it is currently only outputting about 9A.

When the load is increased to a current of more than 9A, the output fluctuates. I think this is hiccup mode.

I tried modifying the values ​​of Loop Compensation Components such as Ccomp, Rcomp, CHF, etc. to improve the output current, and also modified the Rslope value, but the output current did not change.

When the RES pin is connected to AGND, the hiccup mode does not occur, so the output current is high, but I think the hiccup mode is necessary.

I would like to ask in which direction I should proceed with the test in this situation.

  • 0
    TI__Mastermind 29400 points

    Hi Lee,

    Thanks for using the e2e forum.
    The hiccup feature is not necessary for running the device, but it can be a useful tool to limit damage within the board in case of an output short for example.

    As you confirmed that hiccup mode is triggering, the next step should be to find out why overcurrent was triggered in the first place.
    You can double check the expected current flows within the design with our quickstart calculator:
    https://www.ti.com/tool/download/LM5122-BOOST-CALC
    If the current sense resistor looks okay and should not trigger OCP during normal operation, you can consider a filter at the CSP-CSN pins to reduce possible noise within the sensing system.
    Please also check the switch node voltage during normal operation to see if there is oscillation or strong overshoots.

    If you have any additional questions regarding the device, please let me know.

    Best regards,
    Niklas

  • 0 in reply to Niklas Schwarz
    Prodigy 40 points 
    Hi Niklas,
I'm asking because I don't understand. 
    Does installing a filter on the CSP-CSN pin mean adding a new component?
Although it is operating normally, there is noise in the output voltage, which is expected from the switching frequency. 
    It is a 4-phase interlibrary circuit and the switching frequency is LMC555CMX and CD4017BM96, but noise seems to occur here. 
    Are there any expected problems and solutions? And can this problem occur if the output capacitor value is insufficient?
  • 0 in reply to Lee Jong Chul
    TI__Mastermind 29400 points

    Hi Lee,

    Please have a look at our EVM reference board for LM5122:
    https://www.ti.com/tool/LM5122EVM-2PH

    Here you can see that between sense resistor and CSP-CSN pins there are two 100 Ohm resistor and a small capacitor to reduce noise at the pins.
    This is what I meant by "add a filter".

    In general, a switching regulator will always be more noise than an LDO for example. A small ripple at the output is to be expected and the switching noise will also affect the EMI behavior.
    However, it is possible to reduce the output ripple to a minimum through design of the boost circuit. Increasing output capacitance is just one way of reducing output voltage ripple.

    The question would be if the noise is your system is within your design goal specs, or if it is abnormally high and need to be compensated.

    Best regards,
    Niklas

  • 0 in reply to Niklas Schwarz
    Prodigy 40 points

    Hi Niklas,
    I have checked the answer you gave. However, there was no significant change in the oscilloscope even after modifying the CSP-CSN filter. I modified the capacitors on both ends from 100pF to 3.3uF, but there was no difference. I think the first thing to do is to remove the noise as you advised. The current output voltage waveform is as shown in the figure.

    There was no change even after modifying the snubber circuit.
    Best regards,
    Lee

  • 0 in reply to Lee Jong Chul
    TI__Mastermind 29400 points

    Hi Lee,

    You according to the drawing, you see noise peaks on the output voltage every time a switching event happens?
    Here the output capacitors are normally responsible to filter such noise.
    We recommend to use low ESR ceramic capacitors placed closely behind the high side switch. The caps are better at filtering switching noise than high ESR bulk capacitors.

    Best regards,
    Niklas