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LMZ31506: No voltage output from VOUT port

Masahito Takei
Intellectual 300 points
Part Number: LMZ31506


The board was working fine, but at some point the LMZ31506RUQR's VOUT output (+3.3V) went to 0V.


・When I measured the resistance value between VOUT and GND after it became 0V, it was 1.9Ω.
・The resistance value between VOUT and GND of the board in normal operation was 311Ω.
・+3.3V is input to INH/UVLO.
・+12V is input to VIN.
・When I probed the PH port, it was fixed at 0V.
・The overcurrent protection circuit is not operating, and there is no periodic voltage fluctuation of VOUT.

What could be the cause of the problem with the LMZ31506RUQR?

Please let me know if there is anything I should check with LMZ31506RUQR.

I'll attach a schematic excerpt.

  • 0
    TI__Expert 5838 points

    Hi Masahito,

     

    It is possible that the module may be damaged. One thing I noticed from your schematic is that you have Cout = 100uF x 8 which is a lot of capacitance. Per our application design on the datasheet, we do somewhere closer to Cout ~ 200uF for 3.3Vout. It is possible that this is triggering the device to not perform properly and therefore shutdown. If possible, try reducing the output capacitance as the first step. Also, make sure to check Vin, EN pins are within stable range. It would also help to check the PWRGD pin output to get more information on the device status. I'm assuming the part is working at room temperature, but if not it is possible that the parasitic properties of some components change with extreme temperatures. Also try disconnecting the INH/UVLO and see if it powers up, I couldn’t see much on that pin, however per our datasheet, we recommend connecting it to an open drain/collector device or a suitable logic gate.

    Regards,

    Eileen

  • 0 in reply to Eileen Hernandez Hernandez
    Intellectual 300 points

    Hi Eileen,

    Thank you for your answer.

    Is it possible to damage the modReplyule by mounting a lof of capacitors?
    The COUT capacitor was selected to be the same as the value output from WEBENCH due to the DC bias characteristics and effective capacitance value.

    Regards,

    Masahito

  • 0 in reply to Masahito Takei
    TI__Expert 5838 points

    Hi Masahito,

     

    I have ran Webench for your conditions and it recommends Cout = 261uF (https://webench.ti.com/appinfo/webench/scripts/SDP.cgi?ID=EC012BCD302DA105). When running simulations with Cout = 100uF x 8 = 800uF as per your schematic (https://webench.ti.com/appinfo/webench/scripts/SDP.cgi?ID=204DB355845F8808), the startup waveform gives an error which is usually a sign of an issue due to the components selected. I have no issues running startup with the recommended Cout=261uF.

    My guess of what could be happening is that the large amount of output capacitance is disrupting startup. When the device is attempting to start, the caps are getting charged and drawing current. If the there's too much output capacitance and/or if the SS time is too short, the device may feel like the current demanded is too high and therefore hit current limit and enter hiccup mode. In this case, you could try to increasing SS time and/or reduce Cout.

     

    Regards,

    Eileen

  • 0 in reply to Eileen Hernandez Hernandez
    Intellectual 300 points

    Hi Eileen

    Thank you for your reply.


    If hiccup mode occurs constantly, will it damage the module?
    We are investigating what caused the module state to change.

    Also, the data sheet states that the upper limit of the capacitance of the ceramic capacitor is 1500uF.
    Is this value not applicable when +3.3V is output?

    https://www.ti.com/jp/lit/gpn/lmz31506    page.4

    Regards,

    Masahito

  • 0 in reply to Masahito Takei
    TI__Expert 5838 points

    Hi Masahito,

    The hiccup mode is a self recoverable protection and shouldn't damage the module. The table on page 4 is based on Vout=1.8V. 

    Regards,

    Eileen

  • 0 in reply to Eileen Hernandez Hernandez
    Intellectual 300 points

    Hi Eileen,

    Thank you for your reply.
    Is there a way to determine the upper limit of the capacitance of the capacitor for +3.3V?
    Please let me know if there is a table or calculation formula that contains the information necessary to determine the upper limit.

    Regards,

    Masahito

  • 0 in reply to Masahito Takei
    TI__Expert 5838 points

    Hi Masahito,

    From our Webench simulations, the max ceramic Cout for 3.3Vout is about 700uF however not ideal. I ran some simulations for transient response for different Cout = 800uF(red), 700uF(dark blue), and 200uF(light grey) for reference of the tradeoff of going with a capacitance higher than recommended. As observed, a more oscillatory response is observed with higher capacitance.  

    Regards,

    Eileen