This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

LP38691-ADJ: Current limitation problem

Part Number: LP38691-ADJ


I have a serious problem with limitation current of the device.

On an actual PCB we use LP39681-adj LDO regulators, and I have problem with them...

Current limiting doesn't work anyway,  if I short output or if I connect an empty 20µF capacitor,

I have a current (peak) of 1.3A (!!) instead of the 850/350mA limitation given in the documentation....

input power supply do not accept such current and all the other alimentations of the board are shut down !!

On simulation  I have correct currents but in real life I don't have them....

Why do I have such current on output ?


  • Hi Thierry,

    Do you have any scope shots of Vout, Vin, and Iout that you can share? Please note that current limit is not instantaneous as it will take time for the current limit circuitry to detect the over-current and engage the current limit.

    Very Respectfully,
  • Hello,

    Witch kind of scope would be usefull ?

    On my PCB I "only" short Vout to GND directly through an Ampere Meter, such as this may occurs in real life....

    I have seen that the ENABLE pin may disable the current foldback protection system, but there is no pin ENABLE

    ont the ADJ version of the device.

    Device is powered by a stepdown regulator [15v@5.5v] to have about 5.05v on output.

    I have 5 of these regulators on each board that power external devices...

    all of them have the same comportment, and since main power @5v5 is made by a 2A limited stepdown

    regulator, power collapse anytime there is a problem !!!!


  • Hi Thierry,

    Thanks for the additional information. It sounds like the route cause is inrush current. Unfortunately the LDO you selected does not have either an EN pin or a NR/SS pin; therefore, we do not have many options to sequence the LDOs so that they do not have the inrush current at the same time or reduce the inrush current through the LDO.

    Increasing Cin and decreasing Cout will reduce the inrush current through the LDO; however, increasing Cin will increase inrush current through your supply. Are you able to increase the current limit for the upstream supply?

    You could add a feedforward capacitor to help reduce inrush current as well; however, this alone will not reduce the inrush enough for your application.

    Very Respectfully,
  • Hi Ryan,

    This is not really the inrush current that cause me a problem, this is the regulator limitation (500mA base,

    350mA or 850mA that depend on delta Vin-Vout) that doesn't work...

    I've never seen such behavior.... but the inrush current may sometimes cause a problem, really !!!

    Thanks to the input voltage @5.5v the power dissipation is lower than with higher input voltages, but in case 

    of extexnal device failure, this wil give 7.5W in a little WSON case, instead of nominal 0.2w (or maybe 5w in

    bad condtions that may appear with limitations...) and this can destroy the board !! ( perhaps solder will melt before and component 

    fall in the case, making a "fuse protection" !!! ;-) )

    I've no way to change current limit on main power supply, we use L7985 (ST) that has an internal 2.6A limitation...

    (-> previously regulator chosen was L7986 with 3.7A limitation ) and logic supply @3v3 is also disturbed, making the

    bord to be reseted....

    If I knew this behavior, I had put  polyswitchs and increase a little the voltage to protect main power supply on

    each of the 5 regulators ....

    Is there similar regulators with same case and functionalities (LDO / adj ) that could replace this one ?

    Thanks for support,


  • Hi Thierry,

    I am not aware of a simple pin to pin device swap that would fix your peak current. The peak current occurs because current limit is not instantaneous. Perhaps the easiest way to reduce the peak current would be to place a series resistor before the LDO.

    Very Respectfully,