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LM74700-Q1: ORing circuit questions

Sherry Liang
Genius 10255 points
Part Number: LM74700-Q1
Other Parts Discussed in Thread: LM74800-Q1, INA302, TPS2663

Dear team,

My customer has the ORing circuit requirement, two power supplies and only one load. Can 74700 meet below requirements?

1. These two power supplies can't interface with each other;

2. When any of one power supply has failures, such as short circuit, open circuit and overvoltage, etc., the other one power supply shouldn't be affected by these failures, and it can work normally.

Thanks & Best Regards,

Sherry

  • 0
    TI__Guru* 95911 points

    Hi Sherry,

    What is the operating voltage and currents in each path ?

    Best Regards, Rakesh

  • 0 in reply to Rakesh
    TI__Genius 10255 points

    Hi Rakesh,

    Both are 12V.

    Thanks & Best Regards,

    Sherry

  • 0 in reply to Sherry Liang
    TI__Guru* 95911 points

    current levels ?

  • 0 in reply to Rakesh
    TI__Genius 10255 points

    We don't the detailed spec currently.

  • 0 in reply to Sherry Liang
    TI__Guru* 95911 points

    Please check and let us know.

    Thanks & Best Regards, Rakesh

  • 0 in reply to Rakesh
    TI__Genius 10255 points

    Hi Rakesh,

    Thanks for your reply!

    When input is 12V, the input current is 3~4A; When input voltage drops down to 6V, the input current is around 6~7A.

    My question is,

    1. When Vout-Vin>11mV, 74700 will turn off the Gate. When Vout-Vin<11mV, mosfet in turn-on mode. Considering the worst case, VIN1=12V, VIN2>12V, 12<Vout<12.11V. Due to Vout-Vin1<11mV, there will be reverse current flowing the mosfet 1, if the Rdson of mosfet 1 is 2mohm, then there will be around 5A reverse current flowing the mosfet 1, right? This current will impact the selection of the mosfet. Have you seen this phenomenon before in other customers?

    2. My customer will do negative pulse test, and the -100V negative pulse will be added at the VIN1/VIN2, our 74700 will be on/off repeatedly.  This on/off repeatedly operation won't damage our device, right?

    Thanks & Best Regards,

    Sherry

  • 0 in reply to Sherry Liang
    TI__Guru* 95911 points

    Hi Sherry,

    LM74700 does not provide short circuit and overvoltage protection.

    If Q100 device is needed, LM74800-Q1 as per Figure 15 in https://www.ti.com/lit/an/slvaes2/slvaes2.pdf can be used. But we need to add INA302 for short circuit protection in the path.

    If Q100 device is not a must, Please consider TPS2663 as per Figure 53 in the datasheet.

    Best Regards, Rakesh

  • 0 in reply to Rakesh
    TI__Genius 10255 points

    Hi Rakesh,

    Thanks for your reply!

    Before I recommended dual mosefets+LM74800-q1 solution to them, but they think it is too expensive for them. In my understanding, they can use TVS to achieve overvoltage function with our 74700. For the short protection, I don't know why there is this fault. Short to power? But VIN is already connected to the battery power. Short to ground? At this time, mosfet will turn off, so I think no need to consider the short failure. Have yo seen other short failure mode?

    Could you please tell me why you recommend 7480 to me? 74700 can't cover the current level? I point out two questions yesterday, but I think using 7480 can't solve my questions. Could you please help give the answer for my questions?

    Best Regards,

    Sherry

  • 0 in reply to Sherry Liang
    TI__Guru* 95911 points

    Hi Sherry,

    LM74700 does not provide output short circuit protection. The path from battery to the output still exists through the body diode of the MOSFET. 

    For your other two questions.

    1A) Yes, the magnitude of the reverse current depends on the Rdson of FET. This also plays role in FET selection in addition to the point discussed in Section 10.1.1.2.2 MOSFET Selection of the datasheet

    2. My customer will do negative pulse test, and the -100V negative pulse will be added at the VIN1/VIN2, our 74700 will be on/off repeatedly.  This on/off repeatedly operation won't damage our device, right?

    2A) It won't damage as the -ve pulse will be clamp by the input TVS diode

    Best Regards, Rakesh

  • 0 in reply to Rakesh
    TI__Genius 10255 points

    Hi Rakesh,

    Thanks for your help!

    Thanks & Best Regards,

    Sherry