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LM25149: Omitting the Sense Resistor

Elyza Agosta
Prodigy 10 points
Part Number: LM25149

I don't want to use the current sensing feature included in the LM25149 buck which brings up two questions:

1) Can I short ISNS+ to VOUT so that the overcurrent will never trip? Are there unintended consequences of doing that?

2) The equation 35 in the datasheet for the final calculation of the buck inductor includes the value of the sense resistor. This is for internal slope compensation. If I omit the sense resistor(Rs = 0) then this equation no longer works. Can I just use equation 33 and forger about equation 35?

Thank you!

  • 0
    TI__Mastermind 22301 points

    Hi elyza,

    I dont recommend not using the current sensing.

    The device is a current-mode control, shorting the current sense will make it a voltage-mode, and difficult to stabilize.

    It can work, it will just be hard to stabilize.

    If you dont want to have a resistor I at least recommend DCR current sensing from the inductor.

    I think you should have a current limit such that the MOSFETs don't damage from overcurrent.

    Hope this helps.

    -Orlando

  • 0 in reply to Orlando Murray
    Prodigy 10 points

    Orlando,

     Thank you for the response. I didn't realize it used the current sensing for control. I assumed it was just for overcurrent protection so and I was planning on just using a fuse. I was considering using the DCR current sensing, if I use this method how does that factor into the inductor equation? Do I still use equation 33? 

  • +1 in reply to Elyza Agosta
    TI__Mastermind 22301 points

    Elyza,

    It uses current sensing for both. 

    Note using a fuse will limit your average current output, but not your peak inductor current.

    SMPS work with ripple on inductor current, so there is always peak inductor current higher than average output current. 

    You need to make sure to limit the peak inductor current to lower than the inductor saturation current spec.

    Operating with saturated inductor is always avoided in SMPS.

    The inductor DCR will determine current limit, see figure 8.6 for more. 

    You will use the DCR in equation 38, double-check that meets your peak current requirement in equation 35.

    Hope this helps,

    https://e2e.ti.com/blogs_/b/powerhouse/posts/waveform-audit-inductor-core-saturation-saturation-current 

    -Orlando