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LMZM23600: Tchnical inquiry DC/DC Converter LMZM23600V3SILR

Part Number: LMZM23600

We chose PFM-mode to improve "efficiency" of this device at light load.
During the evaluation, an abnormal unit higher than the average current consumption was found.
While looking for the difference from the normal unit, I found a difference in the attached waveform.
Since the output voltage is regulated to 3.3V, I think that the control system circuit is operating normally but that the inductor performance is not working.

I think the inductor is in failure mode, short circuit.
What is the cause of the failure ?

The cause is considered as follows:
・Input overvoltage
・Output overcurrent
・External heat stress

I think that the cause of the failure to be external heat stress.
I want to identify whether the steps that occurred were due to operations or manufacturing processes.

Please tell me the suspicious cause.

Please Best regards.

LMZM23600V3SILR_20200330.pdf

  • Hi,

    Can you comment on the following points below:

    1. Can you reconfirm that your application condition is as follows: VIN = 6V; VOUT = 3.3V; IOUT = near zero / low; Tambient = 25C (room temperature).
    2. What peak temperature was the device reflowed at? Was the device hand soldered or did it go through a controlled reflow process?
    3. The two waveforms show the part is switching and regulating, can you measure the input currents for Figure 1 (Normal) and Figure 2 (Abnormal)? I suspect the input current for the abnormal board is much higher than the normal board. 
    4. Can you provide the date and lot code for these parts? This information is usually detailed in the shipping label on the package.
    5. How many failures has this been observed on? I'm guessing this failure is seen on the same site on different boards?

    Regards,

    Jimmy 

  • Hi,

    Can you please provide feedback on the questions above? If your application is 6Vin to 3.3Vout, I doubt it will be an input over-voltage stress on the pin, unless there was another power supply that could surge the input pin up above the recommended absolute input maximum.

    Regards,

    Jimmy 

  • Hi, Jummy-san,

    Thank you, and please wait a little.
    JES Nagase,
  • Hi, Jummy-san,

    I answer below:
    ans1:
      ans1: yes
    ans2:
      This package is made by EMS company in reflow process.
      Currently, I do not know the peak temperature at soldering.
      I will contact EMS company later.
    ans3:
       #added Figure.3 of abnormal2 (refer to the attachment)
     
       Consumption (VIN=6V)       Normal ----- 3.5mA
                                                   Abnormal -----  7mA
                                                   Abnormal2 ----- 42mA
    ans4:
      I do not have lot code information.
    ans5:
      I have adopted this device for the first time, and encountered two failures.
      Therefore, we did not have any information on such failures.
    Best regards.
    JES Nagase.

  • Please provide both schematic and PCB layout for review. 

    Below are my replies to the inquiries listed in the pdf.

    Q1. Is Figure 2 normal or faulty? 

    • Figure 2 is not the typical switching waveform for a device in PFM condition. In PFM condition, the switch node should have some ringing similar to Figure 1 because of the parasitic in the package and inductor.

    Q2.  Where is the broken part?

    • The inductor is suspected to be the root cause of the issue. 

    Q3. Can you predict the cause of the failure

    • Figure 2 shows a damped recover which would indicate that the inductor has more resistance (either DCR or ACR) than expected. This can be a result of a faulty inductor. The inductor can be fault for a multitude of reasons: like over-heating the device during reflow, over-voltage the inductor outside spec, etc.

    Summary:

    The application condition has initially been ruled out as an issue since running a 6V input/ 3.3Voutput will not overvoltage the power module. Again please provide schematic and pcb layout for review to solidify this assertion.

    From the switching waveform, it looks like the output voltage is regulated but Figure 2 and Figure 3 show switching waveform not typical of PFM condition. This indicates that the inductor may be the root cause of this failure. The result of a faulty inductor would be increased supply current consumption, overheating/over-temperature, and or no output voltage. It is important to understand how the device is soldered as peak temperature over the recommended reflow temperature and uncontrolled reflow process may result in damaged inductors. Knowing the date code/ lot code of the device will help the internal quality support team track the device and internal components' history. 

    Regards,

    Jimmy 

  • Hi, Jummy-san,
    I answer below:

    ans0:
    Refer to the attachment, schematic and PCB layout.

    ans1:
    Figure.2 is abnormal, we have judged so.

    ans2:
    I think so, too.

    ans3:
    I think so, too.
    I think the cause of the breakdown is as follows:
      Overcurrent; Open mode failure
      Thermal stress; Short mode failure
    ps:
    We do not know the details of the internal circuit structure.
    So I inferred as follows.
    Since the output voltage is kept at 3.3V;
      Voltage detection feedback circuit is normal.
      This device output circuit is not an open mode failure.
    No ringing;
      The inductor do not charge energy.
    As a result, is the inductor a short mode failure ?
    Best regards.
    JES Nagase.
  • Just wanted to let you know that I have received your updated pdf document and am in the process of reviewing it.

    From the information and discussion we have had up until now, I am inclined to believe the inductor is the root cause due to switching waveform characteristic that is observed in your measurement. I am not sure what you mean by short mode failure, but I think the inductor may be compromised. This means that either the inductance, DCR or ACR of the inductor is higher than the expected typical (typical: 10uH +/- 20% ; DCR= 430mOhm; ACR usually not speced in inductor datasheet). The inductor is indeed charging energy (albeit much lower than expected) since you get a stable output, however the input current is significantly larger than typical resulting in a much lower efficiency reading. 

    The next train of thought now is how did the inductor get compromised. Again it is crucial to know what the peak temperature / reflow process the customer use to ensure that the inductor on this part did not experience any thermal stress over the recommended temperature. Also knowing the date code/ lot code based on the shipping label from the tape-and-reel package will help TI trace the inductor batch history back to the inductor supplier. 

    As it stands right now, I don't think the application of 6Vin/3.3Vout at such a low load current is an issue. I will revisit this post after reviewing the pcb layout and schematic sometime this week. Perhaps this failure may be a SMT process with reflow peak temperature profiles outside the datasheet specs. 

    Regards,

    Jimmy 

  • Hi, Jummy-san,

    I measured the DC resistance of figure.1 and figure.3.
      figure.1 : 0.45 ohm
      figure.3 : 0.39 ohm
    Please review each technical issue.
    We appreciate your support.
    Best regards,
  • Can you provide feedback/comments on the questions below?

    Does the digital tester also allow measurement of inductance and and Q factor? If it does, can you provide measurement for a good and bad circuit as well?

    I've attached a pdf going over the schematic and PCB layout review. Japan Elevator Service Holdings LMZM23600 Review (Schematic_PCB Layout).pdf

    The schematic looks correct. I would recommend increasing the output capacitance to 33uF to match the datasheet for 3.3V output. Also if you can change one of the input capacitor to a higher valued capacitor this may help with further input noise reduction since you are cascading different capacitor values. However this should not be causing the failure mode you are seeing. 

    One thing I was not sure of is the resistor highlighted in yellow. What is this resistor and why is it pulling feedback to ground? 

    Again I just want to reiterate that I think the rootcause is the inductor based on the symptoms that you've provided from the switching waveform and input current measurement. During SMT process if the inductor has not been in a controlled reflow profile with proper peak temperature, the inductor can experience thermal stress over the datasheet specs which will compromise the component. If you can provide the reflow profile used to solder these power modules onto your board, this will help clear my concern here. Also having a date code/ lot code will help my team track the history of these inductors back to the inductor vendor. 

    Also attached is the nano module SMT guidelines

    Nano Module SMT Profile.pptx

    Regards,

    Jimmy

  • Hi, Jummy-san,
    Q1. What is this resistor, highlighted in yellow ?
    ans1:
            This circuit board is designed for 'Adjustable output' type.
            Actually, the feedback resistor (exclud 0 ohm) is not mounted because '3.3V output' type is used.
            Refer to the attatchement, updated circuit.
    Q2. Measurement of inductance and Q factor.
    ans2:
            I can not measure on my digital tester.
    Best regards,
    JES Nagase,
  • Thank you for the update. This updated schematic makes more sense to me now. As it stands, there is nothing wrong with the schematic, pcb layout, or application use of 6Vin/3.3Vout at near zero loading. 

    Please ensure that you are following proper nano module SMT reflow profile detailed in powerpoint above. Has the EMS company provided any feedback on the peak temperature they used? I would press on this information to confirm they are not damaging the inductor during reflow.

    Regards,

    Jimmy  

     

  • Hi Jimmy-san,

    I appreciate your follow-up and advice.

    I will go in the right direction.
    Thanks!
    JES Nagase