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LP3921: IQ(STANDBY) slowly increase above max spec on some boards

Ulf Wilhelmson
Prodigy 30 points
Part Number: LP3921

Within a prototype batch of 50 PCBs some of the LP3921 show a strange behaviour with a slowly increasing IQ(STANDBY)  to a high level above Max spec.

The boards are powered with a 4.2V  LiPO cell and have a few other components that always draw current i (2-3µA) in shutdown mode.

The total current drawn from the battery by a normal board is ca 4-5µA in room temperature. It makes sense as the expected load in the design.

Some of the boards, ca 10% of them, deviate and draw more.

The load is slowly increasing and stabilises after a few minutes.

The graph below is from a board that eventually reach ca 11µA.

(Time scale Seconds)

The worst board reach 16µA, where 13-14µA is drawn from the  LP3921.

It was investigated further.

When cutting the power-line to the BATT input the remaining load decreased to 2.6µA.

Reconnecting the BATT input again gave 16µA

  • 0
    TI__Genius 10885 points

    Hello Ulf,

    If I understand correctly, the 2.6uA current was drawn from the LiPo while still connected to VIN1, VIN2, VDD, and other components on the board, but just disconnected from the VBATT pin.  Then, connecting only the VBATT pin caused the current to reach 16uA, is this accurate?

    Does the behavior follow the IC if it is placed on a different board?

    Best Regards,

    Rick S.

  • 0 in reply to Richard Solarski
    Prodigy 30 points

    I was not clear enough in the description, Sorry.

    What I called VBATT is in reality the group of all four power inputs, VBATT, VIN1, VIN2 and VDD.  They, with proper decoupling, are connected together in a small local net connected via a 0R resistor to the main battery voltage, The VBAT-net.

    With the 0R in place => 16µA, without, => 2.6µA.  I managed to desolder one of the 10µF capacitors too, but that did not change the current behaviour. 

    The PCB densely populated, without thermal reliefs and ground flooded on all layers. There are also some heavier components on the opposite side just below the LP3921.

    I have not dared to try desoldering other components in fear of messing things up and destroying the board.

  • 0 in reply to Ulf Wilhelmson
    TI__Genius 10885 points
    Hi Ulf,

    This additional current will be hard to resolve without removing components to locate the source. If you can isolate the inputs, it may be possible to identify a potential leakage path.

    If the trend follows the individual ICs, they may have to be investigated for damage.

    Best Regards,
    Rick S.
  • 0 in reply to Richard Solarski
    Prodigy 30 points

    The only way to insulate the individual inputs would be to take the IC off the PCB completely as pins 4 and 7 are connected to pin 17 and 21 via a path under the IC.

    Currently there are only three possible routes for the increased current.
    1. Though the IC via pin 4, 7, 17, 21
    2. Via one of the remaining decoupling capacitors
    3. Stray current via remains on the PCV from the soldering process.

    I will remove the remaining decoupling capacitors, to eliminate point #2 above.

    How can the ICs be investigated from the damage?

    We had a first production run of 300+ boards at a different site. The problem is present on some of those boards too.

    The test-rig measures the total hibernation current. Please see the attached pdf file

    We had to set the max level to 20µA, not the from pecs calculated level around 6µA

    Hibernation current 300+.pdf

  • 0 in reply to Ulf Wilhelmson
    TI__Genius 10885 points

    Hi Ulf,

    The remaining points #1, #3, and the damage investigation, would all necessitate removing the IC from the board.

    An ABA swap can help with all three, by using one good board and one bad board, and swapping the PMICs. This will show if the behavior follows the IC or is instead dependent on some other external conditions (such as remains on the board).

    Once the behavior is confirmed to follow a bad IC, it can be submitted for a failure analysis, where it can be inspected for damage.

    Best Regards,

    Rick S.