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LM3481: Current limit and ripple

Part Number: LM3481

Hello

I have designed a Boost converter with LM3481 (for Battery charging application)as per Webench and attached here as a PDF document.

Specification:

Vin : 13 to 14.5vdc

Vout : 16.5v

Iout 4A

Fsw : 300Khz

Issues faced:

1. Ripple on the output

2. Current is not limited at 4A 

Please find various waveforms attached

1. Vout 

2. Vout vs Fet drive

3. FET Drive

4. FET Drive

5. Current Feedback

     LM3481_SCHEMATIC.pdf

  • hello ,

    Please help to reply here . the project is at very critical stage . 

    need help from TI experts .

    Thanks

    Mandar

    Arrow Electronics , India 

  • Hi Abhijit and Mandar,

    Thank you for using the LM3481.  For all these waveforms, what are exactly the operating conditions: Vin and Iout?  

    The waveforms show intermittent switching (Skipping pulses), implying that your circuit may hit the min ON time limit (571ns).  From 14.5Vin to 16.5Vout, you just need 15% duty, will the Min ON yields 17% duty, greater than needed.  To verify this, can you lower your frequency to 200kHz under the same Vin and Iout condition, and see if you resolve the intermittent switching?  

    Thanks,

    Youhao Xi,  Applications Engineering

  • Hi Youhao

    Thanks for the reply and it makes sense

    We will try it today.

  • Dear Youhao

    No it did not work.

    I took the Fsw down to 200 Khz and then to 70Khz but we have the same observation.

  • Hi Abhijit,

    Can you delete C102? It can cause the loop stability issue. 

    You should also reduce R122 and R123 by at least 20x.  High impedance at the FB pin can easily cause noise issue.

    Can you try these out?

    Thanks,

    Youhao 

  • I will check with reducing Feedback resistors.

    C102 anyways is not populated.

    One change I tried is increasing the Vop.

    I changed that to 18V and the problem is resolved.

    I had to change the inductor to 3.3uH and the shunts to a higher value of 25mE + 13mE in series.

    Now I get continuous drive waveform and current is limited.

    Vin was applied from 13.6v to 14.5v

    EDIT : It seems , the shunt resistor change has solved the issue.

    When I change the resistor to 13mE , it goes back to intermittent switching

    I tried reducing the feedback resistors by 20x, but there is no change with the issue.

  • Hi Abhijit,

    Thank you for the updating.  Its sounds strange about the 13mOhm resistor, unless you have a noise issue that hides the sensed current signal.  Anyway, glad that you get progress.  Let us know if you have further questions.

    Thanks,

    Youhao

  • Hi Youhao

    Thanks for getting back.

    Although the resistor change has resolved the problem for now , I have few questions and doubts!

    I believe that the device is operating at the least duty cycle to maintain the voltage and the proposed current. Whenever the battery SoC is very low , the battery demands a lot of current.

    We expect to limit the current to 4A , but there can be no further decrease in the duty and so the current is at 5A, (which is probably what the battery can accept)

    So do I have to live with this? What if there is a bigger battery and it demands more current? Does this mean that there will be no current limit?

    Please get back

  • Hi Youhao

    Changing the Resistor to 50mE limits the current to 3A.

    So it seems , the shunt resistor is the key here.

  • It seems you have layout noise immunity issues.  Hope changing the CS resistor resolves your problem.

    Let me close the thread here. If you have further  questions, you can always re-open it by adding a new post.

    Good luck in your project.


    Thanks,

    Youhao