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LM338: Short Circuit Protection

Feryanlie Feryanlie
Prodigy 90 points
Part Number: LM338

Hi TI Support,

When I read the datasheet, it says that it has output short-circuit protection. But, on the same page also, it says, "Because the regulator is floating and receives only the input-to-output differential voltage, supplies of several hundred volts can be regulated as long as the maximum input to output differential is not exceeded; do not short-circuit output to ground." I assumed that since the IC has current limit of 5A and differential voltage between the input and output is 30V, when it tries to limit the current from short circuit, it cannot do it so it will blow up. But, if the differential voltage is less than 30V, it will succeed to limit the current.

Therefore, based on this assumption, I tested it, but it didn't work. It blew up instead, with the IC broken up to 2 pieces. That made me scratching my head, where is the short-circuit protection? Can you kindly explain how does the short circuit protection work? What would be the best protection aside from using fuse at the output?

  • 0
    TI__Mastermind 37065 points

    Hi Feryanlie,

    The linear regulator will current limit the output as shown in figure 1-3 in the datasheet.
    But you are right, the input voltage differential must not exceed 30V.
    If there is a sustained short or heavy current on the output, then the device will exceed this rating if larger than 30V is on the input.
    This is what causes the IC to break. 

    Can you tell me what your Vin, Vout and Iout needs are?

    Thanks,

    - Stephen

  • 0 in reply to Stephen Ziel
    Prodigy 90 points

    Hi Stephen,

    Thank you for your response. The Vin is 18V max, 15.5V min. The output is 14.3V with continuous load of 5A. I just need to have the IC to limit the current to 5A.

  • 0 in reply to Feryanlie Feryanlie
    TI__Mastermind 37065 points

    Hi Feryanlie,

    If your maximum Vin is 18V then we are in luck.
    Even with a short circuit on the output, you will never exceed the maximum differential voltage from Vin to Vout.

    I'm not certain how tight your 5A requirement is, so please take a look at the current limit specifications in the Electrical Characteristics table.
    For slow transients or steady state conditions, the typical current limit across temperature is 8A but the current limit not engage before 5A.

    Thanks,

    - Stephen

  • 0 in reply to Stephen Ziel
    Prodigy 90 points

    Hi Stephen,

    Thanks for confirming that 18V at Vin shouldn't break the chip. However, the actual test result was the IC blew up after 2-3 seconds of short circuit. So, to make sure that it was the IC and not the circuit, can you kindly confirm that there's nothing wrong with my schematic if the J7 connector is shorted?

    Also, I suspect that my supplier gave me a counterfeit IC which did not have any short circuit protection. Although, I did test that the thermal protection worked well.

  • 0 in reply to Feryanlie Feryanlie
    TI__Mastermind 37065 points

    Hi Feryanlie,

    It is possible that you have a faulty IC.
    There is nothing in your design that tells me the IC should fail, especially if thermal protection was working.

    Please make sure you have the minimum load current of 5mA for stability.

    Thanks,

    - Stephen

  • 0 in reply to Stephen Ziel
    Prodigy 90 points

    Hi Stephen,

    Thanks for your help. I think so too. I am suspecting either I bought many faulty ICs, since the price was much cheaper (about a half of regular price), or it was a counterfeit.