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# LM25085: LM25085 for CCCV application problem

Part Number: LM25085

Hi Sir,

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

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,

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?

• 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,

• 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.