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ADM2487E Inrush current

Is the inrush current range on the ADM2487E documented anywhere?   We have some product that are not working properly.  On powerup, the unit draws too much current that our 3.3V LDO goes into a foldback mode and just draws a lot of current.  I tested two parts on a bench supply, one powered up on 3.3V w/ a 50mA current limit setting, and the other (problem part) required 3.3V with a 750mA current limit setting.  We are using the coilcraft DA2303-AL transformer.  Any thoughts?



corrected p/n
[edited by: sel82 at 4:29 PM (GMT -5) on 16 Jan 2022]
  • Hello, 

    Please confirm the part number, ADM2847E is not a recognized ADI part. Did you mean the ADM2487E? 

    Regards,

    Neil

  • I'm sorry, yes.  ADM2487E

  • The inrush current of the ADM2487E is related to the capacitive load on the secondary side of the transformer. What value capacitor is placed between the secondary windings of the Coilcraft transformer and the LDO? If this is greater than 2.2uF, I would recommend replacing this with a 2.2uF capacitor. 

    This will reduce the inrush current to below 300mA, and should avoid triggering the overcurrent protection on your primary side regulator. 2.2uF is still sufficient for most LDO's to have good dynamic load regulation. 

    Regards,

    Neil 

  • Hi Neil.  Thanks for the quick reply.  We are using larger capacitors, largely following the application circuit recommendations. 

    This is the resulting waveform on power up.  Takes about 10s of current limit.

    This is a close up of the first attempt to power up:

    And a close up of the success after 10s:

    Our secondary side LDO 'doesn't need input capacitance for stability' but recommends .1 to 1u. I changed our 10u ceramic to a 1u ceramic and shaved off a couple of seconds:

    Removing the 4.7u tantalum from the isolated/regulated 3.3v side gave me another second:

    Removing the secondary LDO entirely (only 1u ceramic on the rectified output, no LDO or ADM2487E) and it still takes 5s to power up:

    I would have thought it had no issue with that, so i'm wondering what else could be happening.  Also the output capacitance tolerances wouldn't explain why some ADM2487E parts turn on right away w/ a 50mA current limit and others take 10s on a 300mA limit or require a 750mA limit to turn on immediately.  It seems like it should behave more consistently but we don't have failures on every unit.  We have noticed if we hack a 10 ohm resistor in series w/ the transformer windings we can get it to turn on more immediately, but hoping for more of a component change to get our failed production lots back into spec without a board change.

    Thanks for your help!

    Scott

  • Hi Scott, 

    Thanks for the detail, will help get to the bottom of this.

    While we review this internally, can I ask are the plots above taken with the ADM2487E VCC supplied from a benchtop power supply, or from the on board LDO?  

    Regards,

    Neil 

  • These specifically are from the on board LDO (diodes inc part AP7313-33SRG-7).  When i discuss the 50mA vs 750mA variation, thats with a bench supply providing the 3.3V.  I can take more captures if you need.  

    Thanks,

    Scott

  • Hi Neil.  I overnighted an EV kit for this part.  Just hooked it up out of the box with no mods... 3.3V bench supply set at 700mA current limit, it took about 3s to start up.  With a 500mA current limit, it never seemed to, or at least i gave up on waiting.  That seems really excessive to me.  Any thoughts?

    Thanks,

    Scott