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LM25066: current limitation

christopher2
Intellectual 515 points
Part Number: LM25066

 Hello I have LM25066EVK. The shunt was changed to 10mOhm ( stock 0,5mOhm) to gest 2.5A current limit at Vref 25mV. Configuration of the board is as follow: CL to GND, CB to GND, RT to GND.

Vin 12V from DC power supply limited to 5A and I perform hard short circuit on the output. So few question to the experts.

What I see is that during limitation period (about 8-9ms) current is limited to about 0,5A not to 2.5A.

1. Why is it so, why not 2.5A?
I enclose screen.

2.The  first peak is about 3.5 Amps - why there is such peak about 400us long?. Is my shunt 7mOhm or Vref 35mV?

Pink is current 0.5A/div Yellow is the gate voltage refered to the Vout(Drain).
Power limit is never exceed 3.5A x12V gives about 42W (LM25066EVK spec 80W)

  • 0
    TI__Genius 16305 points
    Hi

    CB = CL = GND, sets Circuit breaker operation with CB threshold 1.8 times the CL, current limit value.
    The peak current seen is due to circuit breaker threshold, now this is 1.8 x 2.5 = 4.5A. This should be the current through the FET.
    However during the short the input capacitor would provide some current which is not probed by current probe (most cases difficult to probe).
    That is why a value lower than 4.5A is seen, here it is 3.5A.

    After tripping, device puts the FET into power limit mode where the power dissipated by the FET is limited (against the current through the FET).
    Vds is huge 12V and hence current limit value seen during power limit will be much lower than the programmed value.

    What is the programmed power limit here?

    Regards,
    Kari.
  • 0 in reply to Karikalan Selvaraj
    Intellectual 515 points

    As it stays in EVK manual 80W. I have not changed RPWR (8k).

    I measured almost  the real output current - the output Caps I have de-soldered ( 2x330u, only 100nF left). This to be sure that I measure the real output current not sum from the FET and Caps.

    I think input C has nothing to do with 3.5Amps peak if devices limit FET current. How much time does it need to enter current limitation?.
    My Power supply is capable to provide 5Amps so if the input C has not enough energy the power supply should be able to provide the rest.

    However the most sensable explanation is with the power limit, i must verify that.

    ----

    Yes the RPWR which defines power limit is defined by the Rsense which was changed, so that the reason why current was limited to 0.5A within those 9ms.

    But how about this 3.5Amp in the peak instead of 2.5. How is the length of this spike defined? Is that perhaps related with FET gate capacitance.