LP38691-ADJ: Current limitation problem

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,
Ryan

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 !!!!

Thierry

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,
Ryan

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,

Thierry

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,
Ryan