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LM74700-Q1: Threshold for reverse current blocking

Shunsuke Nagata
Genius 13100 points
Part Number: LM74700-Q1
Other Parts Discussed in Thread: CSD18540Q5B

Hi,

I would like to confirm V(ak) threshold for reverse current blocking. My customer will use LM74700-Q1 and CSD18540Q5B. The Ron of CSD18540 is 2.2m ohms. V(ak_rev) is -11mA in the datasheet. this mean is 5A (=11m / 2.2m ) may flow in the silicon. Could you please let me know if this condition has no affect for silicon such as damage, incorrect work , etc?

Regards,
Nagata.

  • 0
    TI__Genius 16305 points
    Hi Shunsuke Nagata,

    LM74700-Q1 controllers regulates the MOSFET Vds to 20mV. It keeps adjusting the gate voltage based on the load current to meet 20mV.
    For higher load currents, Gate voltage will be higher and low currents (light load), it will be low voltage close to gate threshold voltage of MOSFET.

    This linear regulation mechanism ensures zero DC reverse current. But it allows small transient reverse current before the LM74700-Q1 can react within 1us. The small transient reverse current is based on the RdsON of MOSFET and reverse current threshold (Vak_rev) -11mV. For CSD18540Q5B with 2.2mohm RdsON, A peak reverse current of -11mV/2.2m = 5A will be seen only for 1us. Note that this 5A peak reverse current will be seen only when there is a hard short at the input side. Any other slow slewing input will results in zero reverse current due to the linear regulation mechanism, because due to the slow slew input, LM74700-Q1 will have time to react and keep increasing the RdsON, eventually leading to zero reverse current.

    Please refer to the waveform on the first page of datasheet "Reverse current blocking during input short". This shows reverse current blocking behaviour of LM74700-Q1 when a sudden input short is seen at the input side of LM74700-Q1. When the input is short there is a potential of large reverse current to flow from output to input and LM74700-Q1 reacts quickly within 1us to turnoff MOSFET. MOSFET used for this test is DMT6007LFG with a 6mohm RdsON and -11mV/6mohm = -1.8A of reverse current peak is seen for <1us.

    Regards,
    Kari.
  • 0 in reply to Karikalan Selvaraj
    TI__Genius 16305 points
    Hi Shunsuke,

    Let me know if you have more questions.
    If your question is answered, click "this resolved my issue" button at the end.

    Regards,
    Kari.