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LM2733: Short Circuit Protection

Sean Rauchfuss
Intellectual 865 points
Part Number: LM2733
Other Parts Discussed in Thread: TPS61376

Posting for a customer:

I’m designing in a boost converter to one of our products and I’m curious as to how LM2733Y handles a short of its output to GND. Right now I have an inline resettable/PTC fuse (F2 in the picture below) to handle any overcurrent condition. I’m wondering if it is even needed at all.

For reference, here’s the circuit I’m working with:

 

 This boost converter takes in 4.75 – 6.8V and outputs 24.25V. I’ve sized L2 as per the calculations in the datasheet to safely supply 130mA’s without violating the 1A switch current limit. My typical max current during normal operation is 100mA.

D5 is a low Vf diode that blocks external supplies connected in error by a user from backpowering my boost converter. In other words, user-facing output is coming out from the cathode of D5.

 My questions are:

  • Does LM2733Y have short-circuit protection? The datasheet says it has “cycle-by-cycle current limiting” but I’m not sure if that handles an “output-connected-to-ground” fault condition.
  • If LM2733Y doesn’t have short-circuit protection or I decide to go with an inline PTC fuse anyway, is there anything I should be concerned about during that fault condition?
  • 0
    TI__Expert 7475 points

    Hi Sean,

    Thanks for reaching out. For your two questions:

    Sean Rauchfuss said:
    Does LM2733Y have short-circuit protection? The datasheet says it has “cycle-by-cycle current limiting” but I’m not sure if that handles an “output-connected-to-ground” fault condition.

    LM2733Y does not have short-circuit protection, and it does not support short-circuit condition. In the case of short circuit, it will cause damage to inductor L2 and diode D19.

    "Cycle-by-cycle current limit" means that the chip will sense current of low-side FET, and limit the low-side FET current below the current limit we set, but it can't promise output current.

    Sean Rauchfuss said:
    If LM2733Y doesn’t have short-circuit protection or I decide to go with an inline PTC fuse anyway, is there anything I should be concerned about during that fault condition?

    I am not sure what the PTC fuse spec is, especially its response time. I think the response time of this PTC fuse should be as short as possible, otherwise inductor L2 and diode D19 would damage because of the large current.

    What's more, I think TPS61376 may be an alternative for customer's condition, TPS61376 can provide Vout up to 25V and have short-circuit protection, it will release in end of Aug this year. If customer is interested in this device, please provide more information about customer's application so that we can check weather it is suitable for customer. Thanks.

    Best Regards,

    Nathan Ding