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LMZM33606EVM: Regulated output levels change with load current

Chad Klippert
Prodigy 70 points
Part Number: LMZM33606EVM

I've been testing this regulator (using the EVM with no modifications).  My intent was to simply get an idea of the EMI profile so I could implement filtering input circuitry.

I noted that when I leave the EVM in DCM, with the output set to 5V (and Fsw set to 500kHz) I get different regulated output levels with different loads.

Under each load value (simple resistive loads), the EVM is powered up with the load present and the current and output voltage measured. I had the following results (once again, no changes to the jumper settings between these tests, and no circuit modifications applied to the EVM)

Load = 0.99A; Vout = 5.085V

Load = 1.9A; Vout = 5.118V

Load = 2.76A; Vout = 5.154V

All voltage measurements were made with the same DMM so they will be precise, regardless of the accuracy.

I measured the output voltage both with and without the current measurement DMM in circuit to ensure that the ammeter wasn't impacting the measurements.

The datasheet for the part indicates that it should have a 0.1% load regulation over the entire load range, and I've covered less than half of the range.  

I read through the datasheet to see if I've missed an application setting to maintain the regulation to spec, but can't find that anything is amiss, plus this is the EVM with no changes.

Any recommendations to ensure that the part remains in spec?

  • 0
    TI__Guru 50120 points

    Hi Chad,

    Where are you placing your DMM measurements across? It is recommended to connect the DMM across the output voltage monitor test point designated as VOUT S+ and VOUT S- .

    On my end, I configured the LMZM33606EVM and measured the output voltage with a DMM on the output voltage monitor test points and the results are as follows:

    Configuration: VIN = 12V ; VOUT = 5V; Fsw = 500kHz; Mode = DCM 

    Results:

    1. IOUT = 1A; VOUT = 5.0502V
    2. IOUT = 2A; VOUT= 5.0492V
    3. IOUT = 3A; VOUT = 5.0479V
    4. IOUT = 4A; VOUT= 5.04649V
    5. IOUT = 5A; VOUT = 5.04463V 

    Regards,

    Jimmy 

  • 0 in reply to Jimmy Hua
    Prodigy 70 points

    I've used S+ and S- as well.  I did find it unusual that the voltage was increasing with load.  Your measurements are much more what I would expect.

    I'm going to run it again and report back. It was a few weeks ago that I did this.  I will skip the current measurement.

  • 0 in reply to Chad Klippert
    TI__Guru 50120 points
    Hi Chad,

    For reference the measurements I listed was done with a stable bench power supply and the load was a Kikusui ELOAD that I manually ramped for the different loading conditions.

    Regards,
    Jimmy
  • 0 in reply to Chad Klippert
    Prodigy 70 points
    Turns out I was measuring at TB2. I've re-run it and can confirm no issues now. Thanks