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TPS7A49: Regarding current limit feature during LDO output short circuit

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Part Number: TPS7A49

Hi there,

We are using TPS7A49 LDO to convert 15 V DC to 5 V DC (Current Requirement: 20 mA). Our 15V DC source can supply maximum current of 200 mA. Please let us know that if the 5V output of LDO is short-circuited, then would the LDO start taking maximum current from the 15 V source? As mentioned in your datasheet:

" The fixed internal current limit of the TPS7A49 family helps protect the regulator during fault conditions. The maximum amount of current the device can source is the current limit (309 mA, typical), and is largely independent of output voltage. For reliable operation, the device does not operate in current limit for extended periods of time."

Does the fault condition which is mentioned in above line also includes the LDO output short circuit fault? If so, then please explain that if the LDO output short circuit remains for a long period of time, then for reliable operation, will the LDO go into shutdown mode until the output short-circuiting is removed?

Thanks and Regards,

Surya Mohan

  • Hi Surya,

    The limitation will probably not be the current limit but thermal shutdown.  The junction to ambient thermal resistance is 63.4 C/W or 47.7 C/W, depending on which package you are using.  At 10V across the LDO and 300mA current limit, you are dissipating 3W across the device.  3W * 63.4 C/W = 190.2 C rise, and 3W * 47.7 C/W = 143.1 C rise.  If you are operating at room temperature (25C) then both of these would put you above the thermal shutdown temperature of 150C.  So the device after 10's of miliseconds would enter thermal shutdown, turn off, then after a small period of time of cooling down it would turn back on into current limit (and then enter thermal shutdown again).  This process would repeat until the short was removed on the output.

    Your 15V supply can only provide 200mA.  If the input supply was not capable of sourcing 300mA then the input supply would limit the current to the LDO.  If you are using the DGN package (63.4 C/W thermal resistance) then even at ambient temperature you will probably enter thermal shutdown as described above. 

    If you are using the DRB package, you may not enter thermal shutdown but the LDO current limit would remain until the short was removed.

    Thanks,

    - Stephen

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