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LMZ13608 input peaks

Lukas Heeden
Prodigy 30 points
Other Parts Discussed in Thread: LMZ13608

We have problems with a LMZ13608 module. Some devices function as expected, and some consume 12% more power. They do so under different load conditions. We have also seen with a thermal camera that these modules are warmer. When we replace a module the current consumptions is normal again.

When we observe the input voltage we see some strange very high amplitude pulses on the devices that does not work properly. The device that draw less current does not show the spikes. The input capacitors are two PANASONIC 50SVPF68M devices with a low ESR (20mOhm), the output capacitors are four PANASONIC 6SVPE220MW devices with a low ESR (15mOhm) to.


 
The input voltage is 17 – 28V, the output voltage is 3,5V and is used to power some LED displays.
Attached two screenshots of my scope. The probe was connected directly to the pins of the module. I made two different measurements, with different loads (first less load than the second).

Meas 1 - ok:

Meas 1 problem:

Meas 2 ok:

Meas 2 problem:

Probe connection, direct to module:

  • 0
    TI__Expert 3840 points
    Hi Lukas,

    Let me move your post to the correct E2E Forum (Power Modules). From your picture, the positioning of the input capacitor bank is relatively very far away. You should expect considerable input noise on this VIN pin. From the same picture I see that your measurements are from VIN to AGND and you see the noise as expected. Please share with us your PCB layout and see if there's a way to move your input capacitors closer. Try soldering a low ESR capacitor (ceramic) across VIN to AGND, just like you did for your probe testing.

    Best Regards,
    -JP
  • 0 in reply to Juan Paulo Fung
    Prodigy 30 points

    Hi Juan Paulo Fung,

    Below an image of the PCB. The trace from the capacitors to the module is 13,7mm long. Both legs to the capacitors are 5mm. The PCB has four layers. Both outer layers are the signal layers, and filled with GND for the cooling. The inner layers are a GND and a power layer (output from the module).

    I tried to add some ceramic, but without result at this moment. The peaks are reduced, but still the power is 12% higher as it is for modules without the peaks. I only had 1uF capacitors of a sufficient voltage rating available and tried 10 in parallel. I ordered some 10uF, but not tried them yet.

    The problem appear only on a few devices and the 12% less efficiency is a bit strange to. Could you imagine a failure mode with this efficiency loss as a result (same input and output voltage, different output currents all seems to give a 12% loss compared to a good module).

    Thanks, Best Regards,

    Lukas

  • 0 in reply to Lukas Heeden
    TI__Guru 60145 points

    It could also be bad soldering on the EP.  Or maybe the internal bypass cap has a problem?