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LMZ31520: Switcher output is not working(no output) and PH pin is not 500KHz

Part Number: LMZ31520

The PCB Board having LMZ31520 IC was kept for burnin of 192 hours.The IC was functioning properly till the end, but suddenly failed(non output). Please find the attached waveforms captured at the PH Pin of the LMZ31520 IC. Graph1 is for the IC having ouput voltage issue, and Graph2 corresponds to the working IC. As per the observation, Oscillator frequency in the working IC is 500KHz which is in accordance with the IC datasheet whereas in the IC with ouptut issue the internal oscillator frequency is varying. Also with changing the resistor between V5V and FREQ_SEL pin in the failed IC,waveform at PH pin not changing(i.e. Switching frequecny). Kindly help me out with issue      

Note : Verified the input at PVIN pins, V5V Pin output,and the resistor values between VADJ and AGND,SS_SEL to PWRGD resistor value and all were found Ok.

Graph 1: IC having output issue (Captured @ PH Pin output)

Graph2 : Working IC(Captured @ PH Pin Output)

  • ,

    When you say the unit failed ("non output"), can you please provide more detail - for example, is the output going into a repeated startup (hiccup)? I am trying to understand the context of the waveform you provided - is it part of a repeating cycle that can be seen on a longer timescale? What does the output look like? Perhaps you can take another oscilloscope capture with 40 or 100us/div, or even 1ms/div so the longer timescale behavior can be observed.

    Have you compared the pin impedances, for example. Vout to GND on working versus non-working unit?

    Was the burn in test at some elevated temperature?

    Can you share the schematic?

    Regards,
    Kris

  • Kris,

    Thanks for the reply. The output is not going into a repeated startup(constantly showing zero/no output).

    As per your suggestion, VOUT to GND impedance is showing open(Kohm to Mohm range). Also when we tried to measure the impedance between PH pin and VOUT,its showing open(i.e. internal to the chip inductor is showing open). Does that mean that the inductor internal to the IC has gone bad? If so,what could be the cause? 

    The burn in test was at normal room temperature(20-25deg).

      

    The output is going to Virtex 6 FPGA Core Voltage thorough bypass capacitors.

  • ,

    Just to confirm - you see the PH waveform you shared in your first post, but you do not see anything at all at VOUT, just 0V? Is the PH waveform consistently switching, or intermittent?

    I have not encountered such a case where the inductor connections became open in a power module, and it it would be even more surprising that it would happen with an encapsulated/overmolded module such as the LMZ31520. Do you see any physical differences in the module between the normal working part and this device with no output?

    Can you please annotate where and how you are measuring VOUT to PH on your schematic (or snippet of your schematic)? If you do not want to share your schematic on this forum, please email to bsr-mvhc-support@ti.com with LMZ31520 in the email subject and it will get to me.

  • Kris,

    The switching is happening with varying frequencies,repetivity of the waveform was found in between.

    No, there is'nt any physical difference between the working and failed part.

  • Hi ,

    If PH continues to have pulses, it would make me think there is some feedback/control action taking place.

    Can you please provide the zoomed out x-axis PH and VOUT waveforms I mentioned earlier?

    Measured in the exact same points on a working device/board with the same multimeter, you measure short between PH and VOUT, right?

    Thanks for sharing your schematic. I do not see anything obvious that would point me to any specific potential problems.