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LM117: ADJ pin ratings

Steve Widener
Genius 11010 points
Part Number: LM117


We have  situation where we'd like to connect (short) the ADJ pin to the VIN pin momentarily (say 500msec). The circuit would be similar to the typical application diagram on page 1 of the datasheet. 

Would this damage the device?

What are the ABS max ratings for the ADJ pin?

  • 0
    Prodigy 90 points

    While not a direct answer to your question, I'll note that in the functional block diagram in section 8.2 that the ADJ pin is connected to VOUT by a diode and series 50 ohm resistor.

    Another piece of information is in the last paragraph of 8.4.2 which effectively states that the ADJ pin can survive a discharge of a 10uF cap at 23.75V, if the output is shorted to ground.   23.75V is derived from the conditions stated in that paragraph: 25V output - 1.25V (see section 8.1).  If the condition might exceed this, it is stated that a diode must be added  between the ADJ and VOUT pins.

    Since, the internal design of the LM117 effectively places a diode in series with 50 ohms between ADJ and VOUT, could you design your circuit such that you simply add a diode and 50 ohm resistor in parallel with the LM117 with a switch to VIN, rather than shorting the ADJ pin to VIN?  The end result should be equivalent, but you'll be able to size the discrete components appropriately to handle the current.

  • +1
    TI__Mastermind 37155 points

    Hi Steve,

    The previous response by 4537667 is correct.  You can view this problem as the energy flowing into the ADJ pin.  You will want to stay within the guidelines of section 8.4.2 for the maximum energy placed into the ADJ pin or add the appropriate protection around the device (diodes).

    Thanks,

    - Stephen