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LM25085: LM25085 for CCCV application problem

Part Number: LM25085

Hi Sir,

I find application note- AN-2157 introduce CCCV application.

http://www.ti.com/lit/an/snva486a/snva486a.pdf 

my application is input 34.2~40Vdc and output is  33.6V/1.75A, how to design this circuit? i think need use LM318 to replace LM321 and change gain to 34.3, how about other parameters in external circuit?

thanks~

Best Regards,

Driscoll

  • Hello Driscoll,

    I quickly derived this for you.  

    See attached the subject of the formula is Rsense for the values selected in the app note, Where:

    R1 = 1k (input resistor to +ve pin of OP amp)

    R2 = 24k  (resistor from +ve pin of OP amp to Gnd)

    R3 = 1k (input resistor to -ve pin of OP amp)

    R4 = 24k (Rf resistor)

    Iout = 2A

    Vout= 5V

    Vref = 1.24V

    Result:

    Rsense = 23mR

    All other components to remain the same.

    Also Check that the Max Cathode voltage is not exceeded.

    Always make  R1 = R3 and R2 = R4.  You can adjust R1 and R3 to modify the "knee" of the constant current engagement point.

    Hope this helps?

    David.

    6523.CC Trip point Rsense calculation R1.pptx

  • In reply to David Baba:

    Hi David,

    thanks for your quick reply.
    but as I mention before, LM321 and LMV431A may not enough rating voltage. it should up to 40V for my application.
    Could you help describe more detail for this design?
    thanks~

    Best Regards,
    Driscoll
  • In reply to Driscoll Tsai:

    Hi David,

    For your formula use at my condition: input 34.2~40Vdc and output is 33.6V/1.75A
    the Rsense = -2.6 mR !
    it is negative value!

    I think R2=R4=34.3K , then Rsense = 5m R


    Best Regards,
    Driscoll

  • In reply to Driscoll Tsai:

    Hello Driscoll,

    My sincere apologies, I made a slight error in my calculation.  See attached.  This is now correct.

    The circuit works by regulating the voltage across Rsense to a point where the output of the  Op amp is = to the Vref = 1.24V.

    I think you will find that for your conditions stated above you will need to keep everything the same, only you will need to increase Rsnese from 20mR to ~30mR.

    Hope this helps?

    David.CC Trip point Rsense calculation R2.pptx

  • In reply to David Baba:

    Hi David,

    I draw LM25085 40V CCCV application circuit as attached file, pls help check this circuit.

    Due to LM318/TLV431 do not have enough rating voltage. So change INA168, but could not find suitable device to replace TLV431.

    Regards,

    Driscoll

    LM25085 40V CCCV application.pptx

  • In reply to Driscoll Tsai:

    Hello Driscoll,

    I think a better approach is to find an amplifier that has a higher common mode voltage range.  there are many available.

    As for the reference, I think a better approach is to place a zener diode on the Anode of the LMV431 to hold off the voltage below the max cathode voltage specified.  for a 40V input, suggest a 12V zener?

    also I have further simplified the equation assuming R1= R3 and R2 = R4.

    Rsense = (R1*Vref)/(Iout*R2)

    Now with the zener diode the equation will changes to 

    Rsense = (R1*(Vref+Vz))/(Iout*R2)

    Hope this helps?

    David.

  • In reply to David Baba:

    Hi David,

    Could you help check this circuit (as attached file) has any issue?

    thanks~

    BR,

    DriscollLM25085 40V CCCV application-0510.pptx

  • In reply to Driscoll Tsai:

    Hello Driscoll,

    The placement of the Zener diode is correct. 

    Regarding the amplifier, as mentioned I would use the same approach as the application note using an amplifier with a higher CM voltage range as previously mentioned.

    Hope this helps?

    David.

  • In reply to David Baba:

    Hi David,

    I had implement this circuit, but output is incorrect, only around 20V.
    if i modify this circuit to low output voltage, 5V, and it could work normally.
    if i change zener diode from 12V to 27V, output voltage could attached 33.6V but no CC function.
    do you has any idea?

    May I know how to calculate 100 ohm 5k ohm in this circuit?
    thanks~

    Regards,
    Driscoll

  • In reply to Driscoll Tsai:

    Hello Driscoll,

    Can you please share the schematic of what you implemented?

    Thanks.

    David.

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