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LM3478 limit current

Joaquim Martinez
Expert 1270 points
Other Parts Discussed in Thread: LM3478

3755.Config LM3478 13.5V@40mA.pdf

Hi all,

I have designed a step up based LM3478 with Webench. My requirements are: 

Vin min: 7.2V

Vin max: 7.2V

Vout = 13.5V

Iout max = 40mA

In normal mode the Iout will be about 13.5V@5mA.

I wish to limit the current output to 40mA. In the design report the Rsense is 680mOhm. If I understand well, when Iout reaches the limit values (40mA) the duty cycle will be 0% and Vout will drop. 

My problem is that the Vout drops to 10V for a 20mA ouput current only. 

I have tried to reduce the Rsense to 100mOhm, even 0mOhm.

Can anybody help me?

Attached is the Webench design report.

thanks.

  • 0
    TI__Genius 10115 points

    Hi Joaquim,

    Can you tell me a little bit more about your setup. 

    What type of load are you using to draw the current?

    Can you show some waveforms showing

    VIn

    Vout

    and the Switch Node

     

    Regards,

    Marc

     

  • 0
    TI__Expert 3950 points

    Hello Joaquim,


    could you please share the webench design here?

    i am the apps engineer for this part and i will be happy to help you.


    Thanks,

    GIuseppe


  • 0
    TI__Expert 3950 points

    Hello Joaquim,

    i just realized that you already attached the schematic.

    I took a look at the circuit and the following are my considerations:

    1. remove Rs1 since you don't need any additional ramp and furthermore the filter cap Csense should be placed directly on the Isense pin

    2. use a 900mohm for Rsense (CSR1206FKR900)

    3. change the compensation as Ccomp = 47nF, Ccomp2 = 22pF, Rcomp = 45.3kohm

    I think that the problem occurs because there is not enough phase margin anyway now it should be fine.

    If the problem is still there, please take a snapshot of the following and send it to me:

    1. waveforms at the switching point (drain of M1)

    2. input voltage waveforms

    3. output voltage ripple waveforms

    4. output voltage waveforms

    5 waveforms on the Isense 

    Regards,

    Giuseppe

  • 0 in reply to Giuseppe Pinto
    Expert 1270 points

    Hi Giuseppe,

    Thank you very much for your help.

    I have done your setup and now it works fine.

    Thank you and thank to Marc too.

    Joaquim.

  • 0 in reply to Joaquim Martinez
    Expert 1270 points

    Hi Giuseppe,

    I changed the phase margin and now I have a 0.2V-0.3V drop at 30mA. This is ok for the application. But when the 50mA are reached (limit current), I thought that the output voltage would drop immediately to 0V (more or less), but it drops progressively when current is increased. Some values of Voutput are: 11V@50mA, 10V@60mA, 9V@70mA, 8V@80mA. These values are aprox. I would desire a function like a switch when the current limit is reached. Is it possible?

     Thanks.

     PS: I can send you more measured values of Vout, Iout, Vsense, etc. I send you my schematic and webench design for more info.