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LP38691-ADJ: Current limitation not working

Thierry Leveque
Prodigy 70 points
Part Number: LP38691-ADJ
Other Parts Discussed in Thread: LP38691,

Hello, 

I've see your answer to previous similar question, but I was too busy to answer you..

Neitheless I found differents working of the same component on the same board !!!

On the board we made, the same regulator is use five times, with the same

common input power supply ( Stepdown converter) four of them have no working

current limitation on a shortcut of the output, the 5th will operate correctly !!

I use it on separates prototype board and it also operate correctly... 

The only thing that may change is PCB track lenght which are very short to the output ..

they are less than 14mm, the other that have no problem has copper track lenght of 

76mm (some with and some without via )...

Another board I made has a total lenght of 55mm with 2 0R resistor and some via

on the power path.....

Does this may help to have a correct working of the component ?

I may send you a partial view of each routing where the LP38691 is used if needed..

thanks for support.

Thierry

  • 0
    TI__Guru 55940 points
    Hi Thierry,

    The layout should not impact how current limit behaves for the LDO; however, minimizing the inductance/resistance of the traces will help with other aspects of the system level performance. How are you determining if the current limit is operating correctly? If possible, could you provide a scope shot with Vin, Vout, and Iout?

    Very Respectfully,
    Ryan
  • 0 in reply to Ryan Eslinger
    Prodigy 70 points

    Hi Ryan

    the regulator is used to power external devices on USB female connector @ 5.04v. When I connect a production device with its VCC shorted (failed production we have our own CMS production machine and fuser), the main step-down running @500Khz has a 2A limitation and will collapse with a sound as its protection is active, it is an  audible sound....

    As other tests, I short manually the outputs with a 50cm wire four of them will always collapse on all our production devices, the 5th will not ! This can occurs on bad use of our device, as it will be used in some schools  !! 

    Vin is theoretically 5.4v to prevent some heat dissipation, I've tried to increase it to 8v with no more results...

    Measured current with an multimeter will give a current of more than 1.6A, the other 400mA that may be use with the rest of the board I suppose (protection of the step-down regulator is @ 2A ) !!

    As I know, in the data-sheet, the maximum current limitation given for this device is dependent of Vin-Vout and should not exceed 800mA ... One works correctly, not the others !!!

    What sort of scope do you need   and synchronization on which signal ?

    I can give you schematics and routing of the devices if needed.... 

    Thierry

  • 0 in reply to Thierry Leveque
    TI__Guru 55940 points
    Hi Thierry,

    Since you are attempting to start the LDO up into a short, it would be good if the scope shot has Vin (DC), Ven (DC) and Iout (DC). If you set the scope to trigger off of EN rising, this would capture the inrush current and current limit. Vout would not be required in this case as it is tied to GND. You should see the current go from zero (the LDO is disabled) to inrush current which may exceed the current limit to the current limit.

    The schematic and routing around the LDOs would be helpful. You may attach them to your next post or send them in a private message:

    e2e.ti.com/.../3032.08-adding-friends-sending-private-messages

    Very Respectfully,
    Ryan
  • 0 in reply to Ryan Eslinger
    Prodigy 70 points

    Hi Ryan, 

    Please note that the LP38691-ADJ has no enable pin, but I can sync on fall of Vout, please let me some time to make the captures, at this time, I can give you schematics and routing...

    Schematic PDF file is joined below and gerber.

    I think yo have a Gerber viewer, for such rapid views or analyze, I use GerberLogix from EasyLogix.de or other tools like Gcprevue

    Thierry