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The LMZ21701 in my circuit doesnt give the correct output voltage

Fidi Lubi
Prodigy 100 points
Other Parts Discussed in Thread: LMZ21701

Hi, 

In my circuit i use two LMZ21701, one to regulate 12V to 3.3V, other to regulate 12V to 5V.
The problem i have is on the regulator "12V to 3.3V", i get as output voltage 2.14V.
On my second regulator "12V to 5V" i get 3.28V as output voltage.

On both regulator circuit, the voltage at the FB pin is 0.8V. 
Why dont i get the correct output voltages?

  • 0
    TI__Mastermind 23815 points
    Hi,

    Do you have the schematic for your circuit?
    Also you can try to put a load lets say 0.2A - 0.5A and see if the output voltage change. This will tell you if the part regulating or not

    Thanks
    -Arief
  • 0 in reply to Arief Hernadi
    Prodigy 100 points

    Thanks for the hint, i will try it and let you know about the result.

    I have also uploaded the shematic.

  • 0 in reply to Fidi Lubi
    TI__Mastermind 23815 points
    Hi Fidel,

    Schematic looks ok. Where do you tie the VOS pin on the board? That is the sense output voltage pin.
    Do you have layout that you can share?

    Thanks
    -Arief
  • 0 in reply to Arief Hernadi
    Prodigy 100 points

    VOS pin is between resistor R21 and capacitor C34, for the other regulator between R26 and C42. 

    This are the lay out pictures

  • 0 in reply to Fidi Lubi
    TI__Mastermind 23815 points
    Hi,

    DId you say that you measure 0.8V on the FB pin? If that is true, then can you double check the feedback resistor value (R11, R12) and make sure that they are correct?

    The layout looks ok

    Thanks
    -Arief
  • 0 in reply to Arief Hernadi
    Prodigy 100 points
    Thanks a lot for all your tips, my problem seems to be solved.
    My PCB was a verry dirty, i hadjust soldered component on it . I have cleant the PCB with pcb cleaner, and i dont know how, now is the regulator working good.

    I now get 2.9 V for the regulator that must convert to 3.3v. It seems a lot better, but the voltage is quitte low(in my opinion), one of the feedback resistor (the one with value 1.21M ohm) hasnt 1% toleration(as in datasheet). Is it possible that this gives a little issue?

    For the regulator that must convert 12v-->5V, i get 4.8V. For the feedback resistors, i use both 1% tolerance resistors, may be that's the reason why here i get a good regulation and on the other regulator not so good.
  • 0 in reply to Fidi Lubi
    TI__Mastermind 23815 points
    Hi,

    Maybe you can scale the feedback resistor by a value of 10. So the 1.21Mohm become 121kohm and the bottom feedback resistor has to be also scaled down by 10 and see if that improves it.
    As long you have 0.8V on the feedback pin, that is the one we regulate at.

    Thanks
    -Arief
  • 0 in reply to Arief Hernadi
    Prodigy 100 points

    Hi?

    Again, thanks for all your tips. I have found the source problem of all my problems, it was'nt because of cleaning my pcb that solved my problem, what really solved my problem is letting the circuit ON for a couple of minits. I discovered it by building a second circuit(PCB).  I had the same problem, the voltage of both regulator were low, i had let the circuit on for 5 min because i must do something,when i then measured the voltage it gave me a higher voltage. 30 min later, it gave me the correct voltage. The regulator of 12v to 5V, in 30 min had 4.8V,what's is a great. The regulator of 12V tot 3.3v took longer to be at 3.3V, i had let the circuit on for 4 hours.  Now when i start the circuit, the circuit is now at the correct voltage.

    I think my problem had to do with softstart capacitor, is it possible? It's the first time i've seen this. But i'm happy now,that the regulator does what it must do. It would be interresting if someone explained me this phenomen, is it normal?

    Thanks ;). 

  • 0 in reply to Fidi Lubi
    TI__Mastermind 23815 points
    Hi,

    The soft start should not do this. It should startup right away. It is interesting though that you have the voltage settled after 5 minutes.
    Maybe some soldering issue? Try press down on unit and see if the voltage comes up faster. Also if you have a picture of the unit, i can probably visualize what's happening there.

    Thanks
  • 0 in reply to Arief Hernadi
    Prodigy 100 points

    The starting problem,once it had reached the correct voltage, 5V and 3.3V(the first time) by waiting 5min, all next time a started it, it was direct on the right voltage(so its works now normaly). Indeed it is strange, may be a soldering issue. Thanks a lot for your help and patience.

    Greetings. 

  • 0 in reply to Fidi Lubi
    TI__Mastermind 23815 points

    Yes, it could be might as well a soldering issue, contact

    I am glad it works normally now

    Thanks

    -Arief