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LM5116: Higher peaks in inductor current

Martin Seeger
Expert 5411 points
Part Number: LM5116

Dear Team,

A customer uses the LM5116 in a sync. buck converter for following specs:

  • Vin: from 8.4 V to 62.4 V
  • Vout: 5 V (5% tolerance)
  • Iout: 2.65 A max
  • Compensation circuitry for a 3kHz cutoff frequency.

(please find more information in the comment section)

While measuring the inductor current, the customer noticed higher peaks every ~3.2ms / ~300Hz

Could you please advise which part of the circuitry could cause those peaks?

Thanks and best regards
Martin

  • 0
    TI__Mastermind 39980 points

    Hello Martin,

     

    I can't quite make out how many cycles the inductor current is peaking.  But the first thing that needs to be rules out here is the load current, please verify nothing is loading the buck load side?.  The average inductor current is the load current, so if the inductor current is moving, this typically means something is happening load side?

     

    Regarding your schematic I noted that you have two ~5k resistors in series with the CS/CSG called Rg, you should reduce Rg to ~100ohm, for RDSon sensing 16mR FET, with a Temp Coefficient of ~1:1.5.  You have drastically reduced the gain of the internal current sense amplifier for the RDS on you are using and the amount of voltage presented to the PWM comparator will be drastically reduced, from a gain of 10 to a gain of less than 2.  Please reduce the series resistor you have installed.

    Please reduce C8021 from 560pF to 470pF, to increase the amount of Slope comp.

     

    I would cross over ~10 kHz, I would reduce Rcomp to 10k and Comp to 10nF.

     

    Hope this helps?

  • 0 in reply to David Baba
    TI__Expert 5411 points
    Hello David,
    thanks a lot for the answer and details.

    Please find below the answer:

    Regarding the current sense and capacitor ramp change, it has improve the behavior of the converter at high load. Therefore it has helped.

    Regarding the feedback resistor and capacitor, increasing the bandwidth to 10 kHz as suggested had led to instability for all possible input and load conditions. They have observed that the design present high parasitic inductances within the switching loop. The question is how these parasitics affect the control. We do assume this is the source of the instability problems?

    Best Regards
    Martin
  • 0 in reply to Martin Seeger
    TI__Mastermind 39980 points

    Hello Martin,

     

    If you have very high parasitic inductance from Cin to the power FETs, this will indeed cause instability.  Fast di/dt's will cause excessive voltage ringing causing instability.  But from the waveforms presented, I am not sure that this is the issue?  It’s possible, but have you ruled out the load causing the problem?

     

    Further, the layout may certainly affect your current limit levels, and increasing the series resistors in effect, is lowering the gain of the current sense amplifier, reducing Rsense will have a similar effect. 

  • 0
    TI__Expert 4365 points

    Hello Martin,

    I was looking back through older posts and wondering if this topic had been closed.

    Any idea what current probe brand and model is being used in this test?

    I notice that the input impedance is correctly selected as 50 ohm but the bandwidth is at 1GHz.

    Unlikely that it should be set much higher than 100MHz because of current probe performance limits.

    I also notice there is an oscilloscope pushed off the top of the screen. There is the opportunity for crosstalk between channels with digital oscilloscopes that use shared data converters on multiple channels. I think it would be wise to disable the unviewed channels and repeat this measurement. Also you should be able to speed up the horizontal sweep rate and trigger on the peaks, if they are actually present. If they are actually present you may get a better indicator of the issue if you can see it on the same time scale as the inductor current waveform.

  • 0 in reply to Alan Martin
    TI__Expert 5411 points
    Hi Alan,

    thanks for reaching out.
    Yes, the problem is solved. I forgot to mark it as solved.

    The customer was able to solve the problem by changing the resistors in charge of measuring the current through the bottom MOSFET as suggested by you. Changing them, the unexpected behavior of the inductor current was not observed again. Therefore the topic can be closed.

    Best Regards
    Martin