• State Resolved
  • Locked Locked
  • Replies 3 replies
  • Subscribers 63 subscribers
  • Views 403 views
  • Users 0 members are here
Support feedback
Options
Options
Related

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.

LM22680MRE-ADJ/NOPB - Startup problems

Tobias Diendorfer
Prodigy 130 points

Dear Support.

We ware using the "LM22680MRE-ADJ/NOPB" in an new design to create a +5V Power Rail from 12-24VDC input. the Input mostly comming from other switching power supplys where we have No influence in type, electrical characteristics ....

So first, here is the schematic: 1884.schematic.pdf

My problem is the startup of the LM22680MRE-ADJ/NOPB, it happens that ~25ms the input voltage is on the output pin!!!!! So this in not very good for the componetns on the board!!!

We have tried the following:

  • Connect the SS (Pin 2) with an 39nF cap to GND
  • Connect the SYNC (Pin 3) with an 61,9k resistor to GND
  • disconnect the load
  • remove two of the three output caps (each 100µF), now there is only one 100µF cap at the output rail
  • remove the input choke (L801)
nothing solved the problem!
Our next step was to replace the input switching power supply with an other linear power supply, and it solved the problem!
but this is no solution, because in the wild there are many different power supplys for our dwvice!
The result you can see in the attached photos of our oscilloscope.
Correct startup with linear power supply, also workbench simulation shows a simelar graph:
CH1 = VOUT
CH2 = VIN
Fault startup with switched power supply!! +5V power rail is ~25ms at VIN!!!!
CH1 = VOUT
CH2 = VIN
What can i do to work around my problem?
Please help me.
Thank you
Tobias Diendorfer

  • 0
    TI__Guru 73235 points

    This is very odd problem.  Please take scope plots of the input and output voltage zoomed in of the edge for both

    the good and bad conditions.  Also, what are you doing with the EN pin?  You may try to hold it low until the input supply

    is fully on.

    Frank D

     

  • 0 in reply to Frank De Stasi
    Prodigy 130 points

    Dear Frank D.

    Thank you for your reply.

    Here are a few more shoots, zommed in @ 250ns. The VOUT is at the VIN level!!! Sometimes for very long durations  (see my first screenshots).

    CH1 = VOUT
    CH2 = VIN

    I have played around with the EN pin. In our circuit (see pdf) we have left it floating as descibed in the datasheet!

    If ill tie the EN to low, power up main supply and let EN pin floating the startup is correct, see next screenshot:

    CH1 = VOUT
    CH2 = VIN

    But this is no solution because the LM is our main supply, so we have no PowerGood signal. The device could also be powered from +24VDC, which is the dead for the chips onboard if VIN = VOUT for a short period of time.

    As described in the datasheet the max voltage for EN pin is 6V, is there an circuit example how to delay the startup of the LM without the need of too much external components?

    Thank you

    Tobias

  • 0 in reply to Tobias Diendorfer
    Prodigy 130 points

    Solved!!!!

    Sorry for wasting your time ;-)

    It was a measure problem -> GND connection to our oscilloscope.

    Thread could be closed.

    best regards

    tobias