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LM350A: overload recovery confirm

Allen Lu
Intellectual 2490 points
Part Number: LM350A
Other Parts Discussed in Thread: LM1085

Hi team,

Currently, my customer using LM350AT/NOPB, the problem of abnormally high short-circuit temperature rise occurs.

 Please help to guide the analysis:
Application conditions: 18-20V input voltage, 15V/1A output, the output current is always around 2A during short circuit;
Abnormal phenomenon: The temperature of the device rises by 80°C during normal operation. After short circuit, the temperature rises from 80°C to 120°C within 20 seconds;
Requirement: Can the constant current value of this IC be set to within 1.5A when short-circuited?

 beside, we found the LM1085 has Overload Recovery function. but the curve is not good to understand,

 for examples, 20V/15V=1.33, the short circuit current will limit to 1A?

 but when the output of 15V decrease to ≈ 0， the 20V/0V will become higher ,and final limit to <500mA?

  • 0
    TI__Mastermind 18466 points

    Hello Allen, 

    When the output experiences an overload condition, the LDO will limit the current to protect the device. If the Junction of the device is at 25C and the difference between IN and OUT is 5V, then the current will be limited to 5.5A. 

    Best,

    Juliette

  • 0 in reply to Juliette
    TI__Intellectual 2490 points

    Hi Juliette,

     tks for the comments here .  for further check , we found the Vin abs is for Maximum Input to Output Voltage Differential

     what if 15V/1A output, the differential is 29V, does it means the Vin can cover to 15+29V?

    better to share with us what`s Vin abs here for clear understanding.

     

  • 0 in reply to Allen Lu
    TI__Mastermind 18466 points

    Hey Allen, 

    Because this part doesn't have a GND pin, and instead has an ADJ pin, we cannot give an abs MAX for VIN. 

    The Maximum Voltage differential would be limited by the thermals/power dissipation across the die. 

    If VIN -VOUT = 29V, and your load is 1A, your power dissipation is 29W. 

    With a Theta-JA of 50C/W

    Your Junction rise would be, approximately:

    TJ ~ TA + 1450C

    This is considerably above what the part is rated for and will not work. 

    You typically want to keep the power dissipation across a device less than 2W, depending on the Theta-JA

    Best,

    Juliette

  • 0 in reply to Juliette
    TI__Intellectual 2490 points

    Hi Juliette，

     tks here.

     regarding to LM1085IT-ADJ/NOPB  with below formula, can you help to check why 50C/W and how comes with 1450C?

    With a Theta-JA of 50C/W

    Your Junction rise would be, approximately:

    TJ ~ TA + 1450C

  • 0 in reply to Allen Lu
    TI__Mastermind 18466 points

    I am calculating for TJMAX.