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LM3409 - Coff range

Guru 20090 points


Could you please let me know the min/max range of Coff?

In the datasheet P.33 , there is the discription that "Assume a Coff value (from 470pF to 1nF)"
Can we use our of this 470pF to 1nF range ? (for example, 330pF or 1.5nF)

Best Regards,
Ryuji Asaka

  • HI Asaka-san,

    I do not see anything in the datasheet that says you can not use 330pF. However, every schematic I see with this device uses 470pF. I will confirm with the design team as to what the minimum capacitance value can reasonably used on this pin.

  • That is really just a recommended range for reasonable resistor values for the off timer. You can go lower and you can go higher. But keep in mind there are still practical limits.

    If you go too low the pin capacitance (probably 3 or 4 pF) itself will give you a noticeable error and you may have noise coupling issues. You may also get into resistor values of 1Mohm and above which is costly. I would probably not go below 220pF to be safe.

    If you go too high you will simply lower the resistor value, increase the cap charging current, and impact efficiency. There is an internal pull-down each cycle to discharge the cap as well. That means you could damage the pull-down or perhaps not discharge the cap completely before the next cycle resulting in oscillation. So I would not go higher than 2.2nF to be safe.