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LM25085: LM25085 Current limiting

Farzaneh Faridi
Intellectual 585 points
Part Number: LM25085

Dear members,

we have used LM25085  to control a FAN by changing the duty cycle. I want that this circuit limits the current at 4A. Input voltage will be 0 to 12V according to duty cycle. 

Rsense= 0.030 ohm. Having 30mOhm I observed that with maximum duty cylce it limits the current at 2.3A. I reduced the Rsense to 22mOhm and 15mOhm but nothing changed.  I know that the FET also has an internal R that may affect the limiting. reducing the Rsense, I didnt get a correct result. 

I will appreciate any suggestion

Thanks

Farzaneh

  • 0
    TI__Expert 3766 points

    Hi Farzaneh,

    Can you send your schematic? 30mOhm should be the right value for a current limit of 4A. Are you running your system at 4A? If you are setting current sense across an Rsense resistor, it shouldn't be dependent on RDS_on of the FET. Finally, what are your system parameters? (VIN, VOUT, IOUT)

    Regards,

    Henry

  • 0 in reply to Henry Kou
    Intellectual 585 points

    Dear henry,

    Our system specification:

    Vin= 0 to 12v (according to duty cycle it changes) and based on the Vin , V out = 0 to 12V. and I out =4A. You can see the SCH below:

    there is an error on the SCH for R sense which is R10, on my design it's 30mOhm while you see 10R on the SCH. 

    Regards

    Farzaneh

  • 0 in reply to Farzaneh Faridi
    TI__Expert 3766 points

    LM(2)5085_Quick_Start_01262023.xls3005.Dynamic_Output_Change.pptx

    Hi Farzaneh,

    Thank you for the schematic. It seems like you are controlling the fan by raising the VOUT of the converter dynamically using a PWM waveform.

    For this solution I recommend designing to the highest power output that your converter will run, which from your description is 12V VOUT, 4A IOUT.

    Your current design due to limitations of VIN, minimum inductance, and Radj would cause a lower current limit.

    I recommend implementing the values from the excel quickstart calculator attached. This would include raising VIN to a constant 24V, raising your R_ADJ value to 7K Ohms, and adding some more COUT.

    Note this converter has limitations of minimum VIN = 4.5V where the device will hit shutdown, but with VIN > VOUT, you will not hit this point. 

    I also recommend reading an article attached noting the ways to achieve a dynamic VOUT. You should be able to use a PWM waveform to achieve a voltage swing from 1.25V to 12V.

    For a quick debug in your system I would increase R_ADJ, with VIN = 12V, VOUT = 5V, to see if your current will increase. Please proceed with caution with the high currents.

    Regards,

    Henry

  • 0 in reply to Henry Kou
    Intellectual 585 points

    Dear Henry,

    I changed the Radj to 4k64 just to test if the current limit would increase. Surprisingly I observed that it's not limiting anymore. I increased the electronic load little by little and it didn't limit the current. I supposed it should limit the current arround 3A or a bit more but it goes up to 4A. and I've seen 4.6A. I changed the Radj to 2K2 y Rsense 30mOhm and now it limits at 3A. I'm confused. 

    Farzaneh

  • 0 in reply to Farzaneh Faridi
    TI__Expert 3766 points

    Hi Farzaneh,

    Increasing your R_ADJ will increase your current limit. I would also use the WEBENCH tool for this device with your system parameters to test out your behavior of your circuit.

    Regards,

    Henry