• State Resolved
  • Locked Locked
  • Replies 5 replies
  • Subscribers 64 subscribers
  • Views 1294 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

LM3150 Problem with unstable 200 KHZ output.

James Karpy
Prodigy 30 points
Other Parts Discussed in Thread: LM3150

1157.online tool lm3150-design.pdf7711.LM3150_5X_QS_MOS_Selector-A02.xlsI am getting a 200KHZ pulse at the output caps of the LM3150. I used the provided TI tools to calculate all values. The online tool originally configured my frequency at 118KHZ which is below the minimum frequency for this part. I have bumped it up to 200KHZ but that does not solve the problem.5706.Power supply-BOM-B01.xls0317.PWR-DIST-FAB-B01.zip

If you need more info, I will be checking back when ever possible.

  • 0
    TI__Genius 15810 points

    Hello,

    It would be helpful to see the SW node waveform. It looks like there is some instability which is causing a narrow and wide pulses. A ripple of switching frequency at Vout is seen if the output cap has some ESL. The equivalent series inductance will cause small voltage spikes at the output voltage. I'd suggest that you put in a couple of small value ceramics rated for the right voltage. This will help reduce the size of the ripple.

    I am also wondering why Webench calculated 1.6mF of output capacitance. I will ask a colleague to see if some BOM value can be modified for better performance.

    Regards,
    Akshay

  • 0
    TI__Genius 10115 points

    Hi James,

    I took a look at your layout and though there are a few things I would change (such as adding a few more vias near the low side mosfet and thickening the gate and boot strap traces) nothing stands out as an obvious layout issue.

    When I look at your schematic and bill of materials.  My first concern is the low resistance on the low side Mosfet and the low dcr of your output capacitors.  I think that you might have swapped M1 and M2 in your bill of materials.  I think the lowside mosfet should be the Si7884 but in your BOM M2 is the BSC027N04 from Infineon.  Try replacing M2 with the Si7884 and see if this improves the operation of the part.

    If this is not enough, I would also look at adjusting the output capacitors.

    There are two things to try with the capacitors 1) adding a small 1uF ceramic in parallel with C27 to help remove high frequency spikes that might be coupling to feedback.  2) Removing one of the 820uF capacitors to increase the dcr and supply more in phase ripple to the 3150.

    Regards,

    Marc

  • 0 in reply to Marc Davis-Marsh
    Prodigy 30 points

    Thanks Marc, I will look at all these suggestions.  A friend of mine also suggested a problem with the fets.

    Thanks!

  • 0 in reply to Akshay Mehta
    Prodigy 30 points

    Hi Akshay, Sorry I missed your input yesterday. I will try this also. I will provide the switch node readings as soon as I get my scope back from the shop.

    I am curios, Why does the TI website state that the minimum frequency for the LM3150 is 200KHz? I Could not find that limitation in the data sheet and both Web bench and the Excel file allow it to be set lower than 200KHz. In fact Web bench sets the frequency lower than 200 automatically.

    Thanks for your help guys,

    James

  • 0 in reply to James Karpy
    TI__Genius 15810 points

    Hello,

    I didn't that 200 kHz was the minimum either and also didn't find that mention in the datasheet. We might have to ask someone to remove that.

    Regards,
    Akshay