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LM2731: Compensation and Feedback Resistor Question

Connor Connaughton
Expert 4460 points
Part Number: LM2731

Hi Team,

I have a couple questions regarding the LM2731.

  1. The datasheet specifies that a compensation capacitor is to be used to form a zero at 6 kHz in the feedback path of the boost converter circuit.

In my design, the resistor in parallel is 200 kΩ, and so when the equation is solved, [2 * pi  * 200,000 * 6,000]-1 equates to around 133 pF.

We have 100 pF and 150 pF on hand.  Can we adjust the zero with 150 pF, equating to a little less than 6 kHz?  I think it works out to around 5.3 kHz.

 2. Also, in the datasheet, to set the output voltage, it uses the typical voltage divider feedback.  It claims that the minimum value for R2 is 13.3 kΩ to establish 92 µA through the voltage divider.  I chose a value that is near this, but which allows us to use an R1 value we have in stock.  Our R2 is 15 kΩ.  Are either of these an issue?  What’s special about 92 µA?

 

I’m fairly certain these are a non-issue, this is a fairly low power situation and transients are not usually an abrupt step.

Thanks!

Connor

  • 0
    TI__Expert 7535 points

    Hi Connor,

    Thanks for reaching out.

    I think 5.3kHz zero and 15kohm feedback resistor are both ok, as long as the value is around the value we recommend and doesn't differ too much, it will not make a big difference. But it will be better if you can use a value which is closer to the value we recommend.

    As for your question about 92uA, there is a special circuit inside this chip which is very different from other devices, so the value of R2 and the current through it is more strict.

    Best Regards,

    Nathan Ding