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LMR14006 with PMEG6010 diode problem

GEOFF JACKSON
Expert 1535 points
Other Parts Discussed in Thread: LMR14006

I used WEBENCH to design a 3.3 volt regulator on the LMR14006 and originally set the load value @ 200mA. I used all of the recommended components including the PMEG6010 output diode and the 12TPC10M 10uF capacitor. The only difference was my inductor was 33uH instead of 27uH

With the load close to zero, the regulator gave the correct 3.3V output but as soon as it was loaded, with only 15mA, the output volts dropped to 2.9V. I tried swapping a few components and found that the regulator worked fine when I used an SD103AW diode instead of the PMEG6010.

Having got the regulator working I measured the normal load current and found it to be around 60mA, so I put this value into WEBENCH and recalculated. The recommended diode is still the PMEG6010 device but the inductor value goes up to 82uH, so my 33uH inductor value is now low.

Does any of this explain why the regulator is OK with the SD103AW but not with the PMEG6010 ?

Thanks

Geoff

  • 0
    TI__Genius 15810 points
    Hello,
    Please send us your design layout and schematic and your design conditions.

    The inductance is inversely proportional to the ripple current, and the ripple current is a certain percentage of the load current. Therefore when your load decreases, the ripple decreases consequently and thus increases the inductance. Practically you cannot have varying inductance for varying loads. Therefore it is better to design the inductor for a 30% ripple current at a nominal load. The nominal load is something that the device will experience during normal operating conditions. If your nominal load is 200 mA then you would calculate your inductance for that.

    Between the two diodes, the SD103AW is rated for 40 V and 350 mA and the PMEG6010 is rated for 1 A at 60 V. I am not sure why they make the design behave differently. Have you noticed how the ICs are performing thermally? Are they heating up a lot when you load the device?

    Regards,
    Akshay
  • 0 in reply to Akshay Mehta
    Expert 1535 points

    Hello Akshay,

    Thanks for you reply and apologies for not being able to get back to you earlier.

    Nothing is getting hot when I load the regulator. The supply volts are 12 and as I said in my original post, load current is only around 60mA.

    I'm attaching the schematic and layout as suggested; I hope this helps to indicate what the problem may be.

    Regards,

    Geoff

  • 0 in reply to GEOFF JACKSON
    TI__Mastermind 23845 points

    Hi Geoff,

    I think the C2 (BOOT Capacitor) should be connected between pin 1 and pin 6 instead of pin 1 and VOUT. 

    Try this connection and hopefully that helps.

    Thanks

    -Arief

  • 0 in reply to Arief Hernadi
    Expert 1535 points
    Hi Arief,

    What an idiot I am !

    Thanks for spotting that. It's so obvious when you see it that a boot capacitor isn't going to be charging without a switching waveform on one end of it.

    Thanks again,

    Geoff
  • 0 in reply to GEOFF JACKSON
    TI__Mastermind 23845 points
    No worries, it happens all the time.
    Hope that fixes your issue

    Thanks
    -Arief