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Help me!! Big Problem with TPS7A49xx

Alessio Olivieri
Prodigy 50 points
Other Parts Discussed in Thread: TPS7A49, TPS7A30

Hello,

I have a problem that I can not find solution. I used the LDO's TPS7A4901 and the TPS7A3001 regulators to make a complementary power supply. The schematic and the PCB are the same for both LDO. Are the typical datasheet schematic :

 

The negative part with TPS7A3001  work successfully with zero problem, but the positive part  with TPS7A4901   work for maximum 5/10 minutes whit an unstable output range (12 to 14 Volt) and after  it destroys!! I have solderet five PCB's and i tried to change the ratio resitances (R1= 470K R2=47K), but all give roughly the same problem!!!

Could you help me to solve the problem? thing is that it changes from a negative to a positive LDO? What changes from a negative to a positive LDO?

thank you

 

Alessio

 

  • 0
    TI__Guru 55940 points

    Hi Alessio,

    Do you have any scope shots that you could share?  It would be very helpful to see Vin, Vout, and Iout if possible.

    Also, could you share a schematic showing how you have the LDOs interacting?

    Very Respectfully,

    Ryan

  • 0 in reply to Ryan Eslinger
    Prodigy 50 points

    HI Ryan,

    i don't have in this moment a scope measures, but the test value are:

    VIN= 32Vdc  VOUT=14Vdc. In my five test i have i used a  constat load with a 470 ohm resistor (at 14V IOUT= 30mA)

    This is my simplified dual power supply used for TPS7A49 and TPS7A30 test

    thank you.

  • 0 in reply to Alessio Olivieri
    Intellectual 460 points

    Hi Alessio,

    which kind of capacitor do you use for input decoupling ?

    Are they ceramic / low ESR ?

    Best regards

    Stefano

  • 0 in reply to Stefano Bisi
    Prodigy 50 points

    Ciao Stefano,

    i use Aluminium Polymer cap with very low esr.

    bye

  • 0 in reply to Alessio Olivieri
    Intellectual 460 points

    Alessio good morning,

    I take a look to some datasheet because I don't know this kind of capacitor.

    It seems to me that they are not suitable to replace a Ceramic capacitor; please check Impedance-Frequency characteristics of your capacitor.

    "LOW ESR and Z" have different meanings for the different kind of caps. Is better to check numbers on datasheets.

    Another concern is about voltage of your cap; they are 50 V rated ? Please remind that capacitance become low when bias voltage increase.

    Stefano

  • 0 in reply to Stefano Bisi
    Prodigy 50 points

    Thank you Stefano...

    The Caps that use are CONDUCTIVE POLYMER ALUMINUM SOLID ELECTROLYTIC CAPACITORS 10Uf 50V with ESR=50mohm. The datasheet of TPS7A49 says only that:" to ensure stability, maximum ESR must be less than 200mohm" . So I guess it's all right!!

    Anyhow I remember you that the negative regulator TPS7A30 with same PCB, same components, same conditions, works perfectly!!


    bye

    Alessio


  • 0 in reply to Alessio Olivieri
    TI__Guru 55940 points

    Hi Alessio,

    Thank you for the added information.  It is difficult to debug your board with only the information provided so far.  I know you mentioned that you do not currently have scope shots, but it would help a lot in the debugging process if you would be able to take a few scope shots of the failed and working conditions.

    Very Respectfully,

    Ryan

  • 0 in reply to Ryan Eslinger
    Prodigy 50 points

    Sure I understand!!

    it is my responsibility to provide it as soon as possible!!

    thank you

    Alessio

  • 0 in reply to Alessio Olivieri
    TI__Mastermind 22610 points

    Possibly a problem with start-up, especially if negative Vout starts first and pulls positive Vout pin below ground.

    Fix for bipolar regulators was adding diodes across each output to prevent reverse bias.