Part Number: LM25085
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?
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
Vref = 1.24V
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?
6523.CC Trip point Rsense calculation R1.pptx
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In reply to David Baba:
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
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.
David.CC Trip point Rsense calculation R2.pptx
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.
LM25085 40V CCCV application.pptx
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)
Could you help check this circuit (as attached file) has any issue?
DriscollLM25085 40V CCCV application-0510.pptx
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.
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
Can you please share the schematic of what you implemented?
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