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Original question:

LMR33620: Sequence between Vin and EN.

LMR33620: Unexpected big current when starting with empty load

Manu Chang
Intellectual 880 points
Part Number: LMR33620

Hi,

My customer is doing evaluation with LMR33620 with two similar schematics and PCBs.

For the one w/o EMI filter on input 33620-62850, the current will directly go beyond the current limit of the digital power supply (set up as 1A) when starting up with empty load.

However, for the 33620-87702 with EMI filter, the current seems normal and start successfully with empty load. 

The schematics are as attached, does an EMI filter a must for the LMR33620? What might be the reason cause the unexpected high current during start up with empty load?

33620-62850.pdf33620-87702.pdf

BR,

Manu

  • 0
    TI__Expert 5170 points

    Hi Manu,

    The input EMI filter should not have an impact on the input current. 

    Looking at the schematic, Cff as 1uF is too large. From Datasheet Equation9 Cff=3.3V*22uF/(120*100k*0.55)=11pF can be a starting point. Can you remove the Cff and see if the issue remains?

    Also can you make the symmetric input capacitors on the two VIN pins? i.e. 10uF+220nF for each VIN pin. This is not related to the issue but can be improved from the schematic point of view.

    Is it possible to cut off the connection of the EMI filter and give the supply directly to the VIN of LMR33620 of pdf-87702 and see if large Iin can be observed?

    Regards,

    Hongjia

  • 0 in reply to Hongjia-Wu
    TI__Intellectual 880 points

    Hi Hongjia,

    Thanks for reply, I will let customer try to change the Cff and see the effects.

    Just curious, are there any specific reason to maintain symmetric on the input capacitors? Could you pls kindly explain this?

    Thanks a lot!

  • 0 in reply to Manu Chang
    TI__Expert 5170 points

    Hi Manu,

    If there is symmetry in the input capacitors’ layout, the opposing magnetic fields generated by the input power loops are within the symmetric loops, thereby minimizing emissions to nearby systems.

    Regards,

    Hongjia