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About the capacity of the bypass capacitor of LT4321


The LT4321 data sheet P7 has the following description regarding bypass capacitors.

・ It should be 0.12uF or less according to the provisions of IEEE802.3.
・ Share 0.1uF or place 0.047uF near each IC.

Due to the delivery time, it was difficult to prepare a 0.047uF capacitor, so I placed a 0.1uF capacitor on the LT4321 side and a 0.01uF capacitor on the PD side.
The PoE connection was successful and I was able to apply a load of 52W.

If it is 0.12uF or less, is there any problem with the combination of 0.1uF and 0.01uF?

I would be grateful if you could reply.

Best Regards,

  • Hello,

    The input capacitance must be between 0.05uF and 0.12uF,  per the IEEE standard. I would prefer if this capacitance was split closer to 50/50 between the LT4321 and LT4295 equally. These capacitors bypass the local circuitry for each IC, including the logic and charge pump.

    Best Regards,