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LM3481: OCP

Allan Fan
Expert 6910 points
Part Number: LM3481
Other Parts Discussed in Thread: PMP10352, , PMP

Tool/software:

Hi expert,

My customer used LM3481 and refer to the PMP10352 circuit for PoE 15W output application. 

There is no problem to load 15W at output of LM3481 without main board. However, it seems LM3481 will trip OCP when main board is connected and operation.

Also, we checked Vin and UVLO. Vin seems stable but don't know why UVLO would drop and retry. 

Customer had tried lower Rsns = 10mohm but would still trip OCP. 

Could you please have a look and clarify why the current drawing at the input of LM3481 would impact the OCP of LM3481 as well? Or is there anything might goes wrong? 

Ch1: V(ISEN) Ch2: 48Vout

Ch1: 12Vin, Ch2: UVLO

Regards,

Allan

  • 0
    TI__Mastermind 26710 points

    Hi Allan,

    Thanks for using the e2e forum.

    I am confused by their measurement on VIN & UVLO. In the PMP design, UVLO is connected to a voltage divider from the VIN voltage.
    There is no internal mechanic that pulls down UVLO during shutdown, so the UVLO voltage should be proportional to the VIN voltage at all times.
    In their schematic, do they have any other connections on the UVLO pin besides the voltage divider?

    Even if OCP is triggered, the device should keep operation with limited duty cycle. There could be other protections that can cause the device to shutdown, like overtemperature or undervoltage lockout.
    Based on the information we have right now, it is difficult to determine the root cause for this behavior.

    Can customer please do the following checks:
    - Replace IC with a new one to check if device got damaged
    - Double check the schematic for any differences from PMP design (they can also send us the schematic for review)
    - Measure additional signals: VCC pin (is this voltage stable as well?), SW node (is there strong oscillation/overshoots?)

    Thanks and best regards,
    Niklas