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LM3668: Resistance between VIN and GND while unpowered

Rob O'Brien
Prodigy 100 points
Part Number: LM3668

I have a prototype device which uses a LM3668 buck-boost converter to regulate 5V down to 3.3V. Recently, the device failed and there is now an electrical short between the net tied to VIN pins (V_UNREG) and the net tied to the GND pins (GND) on the LM3668 while the device is unpowered. What is the expected resistance when measuring between the two nets shown in the schematic below when power is not applied to the DC-DC converter? I am seeing around 2 Ohms and am unsure if the problem is a failed LM3668 or another device on the V_UNREG net.

  • 0
    TI__Guru* 75865 points
    If the part is not powered, the impedance between PVIN and GND of the IC alone should be > Mega Ohm. Check if any damage to C800 or C803, or other circuit tied to the V_UNREG rail? A quick way may be remove the LM3668 and re-measure impedance between the two nodes, and/or replace the IC to see if the problem is resolved.
  • 0 in reply to Youhao Xi
    Prodigy 100 points
    I removed the LM3668 and the impedance between the two nodes increased drastically. A new LM3668 was installed and the circuitry is functioning normally now. I think my external power supply caused the issue so I changed it and will watch for this issue again. Thanks for the help!
  • 0 in reply to Rob O'Brien
    TI__Guru* 75865 points
    Thank you Rob for the updates. Good luck in your project!
  • 0
    Guru 117855 points

    Greetings,

    I was surprised by the "overlap" between poster Rob's, "LM3668 issue" and a near identical report - from one of my firm's clients.     We were able to resolve that client's issue - and our diagnosis & "fix" may prove of value "here" - as well...

    The Abs. Max. spec'ed for "PVIN" is 6V0 (6.0V) - and that's (unlikely) to be exceeded when V_UnReg is provided by a, "Single, well charged, Li-Ion cell."    Such is very much, "Not the case" - when "Cheap/Dirty" AC driven supplies are employed - for example - during  "test/verification."

    In our client's case - after we had installed one of our uber-compact, "Voltage Logger/Monitors" - the presence of a "Turn-On" voltage - peaking at nearly 7V2 - was (often) noted.     And - in time - "did in" the (unfortunate) LM3668.      Now the client DID scope their AC Supply's output - but NOT during its (critical) "Turn On."     Armed w/the,  "Notification provided automatically by that "Logger/Monitor" (which measured multiple circuit nodes) - the issue was quickly identified...

    As good as this vendor's APP circuit is - would not (our suggestion - to our client) of  ADDING a 0.01µF, quality, ceramic cap - as close as possible to PVIN - offer improved, "Transient, Over-Voltage Protection?"    (we found (both) the 10 & 1µF caps (C803, C800) to be, "too slow in responding" to  transient over-voltages - not so w/our smaller value cap!)      

    While our client's supply WAS the cause - and "KILLED" several of these neat devices (LM3668) the "tiny addition" of that  faster acting cap - solved this client's problem.    (and has lasted beyond 120 days, now.)

    Even when such a "Transient Generating Supply" is not deployed - the "extra insurance" provided by this addition - appears to make (very) "good sense!"

  • 0 in reply to cb1_mobile
    Prodigy 100 points

    I noticed that the abs. max on the PVIN pin is 6 Volts and that concerned me because the designer specified the external power as 5 Volts from a power supply OR a USB host, which may very well be dirty and have high transient voltages. In my case, I was powering the board with a benchtop SMU but I inherited the board and don't know how it was powered in the past. So perhaps the damage was inflicted before my possession. In either case, I did notice that the output of the SMU exhibited a negative pulse when it switched measurement ranges, which occurs quite often as additional circuitry powers up on the board and the power consumption changes. This also may have damaged my LM3668. I'm in contact with the SMU manufacturer regarding that finding because their initial response regarding my finding was "that shouldn't happen".

    Your suggestion of adding the additional 0.01uF cap is a good one, especially when our typical input voltage is so close to the abs. max of the LM3668 PVIN. Thanks!!

  • 0 in reply to Rob O'Brien
    TI__Guru* 75865 points
    Thank you cb1_mobile for sharing.