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I'm trying to connect an RJ-45 jack with integrated magnetics (Belfuse L829-1X1T-91 datasheet) to an Ethernet PHY chip (Micrel KSZ8041TL datasheet). The TX+/-, RX+/-, and the LED pins are all easy enough to figure out, but I'm not sure what to do with the center taps (TCT and RCT) for the transformers. Should they be connected to 3.3 V power? To the 1.8 V output of the PHY?

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Arrrgh, another one of those data sheets that don't tell you what you need... I guess we have to read betweeen the lines: Note 4 on p. 49 says that the transformer draws additional current on the 3.3 V rail, so I guess this is the way to go...

There may be differences among various types of PHYs, and I wasn't able to find better information for the exact type you mentioned, but it is very common to connect the center tap (CT) to the supply rail that is also powering the PHY itself (e.g. 3.3 V). A bypass capacitor from the CT to GND (0.1 uF, ceramic) should be used to provide a low-impedance source for transient currents.

Click here for a detailed application note for a similar PHY. It might be worth giving it a try...

See this app'note for some different types of PHYs with more options (cf. Fig. 2) and indeed a connection of the CT to the internal regulator.

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  • \$\begingroup\$ Some experimentation with a soldering iron and the circuit in question has suggested that your answer is correct. I'm not 100% sure yet, but it's definitely sending and receiving packets. Thanks! \$\endgroup\$ – pingswept Sep 6 '10 at 0:15
  • \$\begingroup\$ The links provided by you doesn't work now! \$\endgroup\$ – abhiarora Sep 21 '18 at 10:14
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In figure 3 of this appnote the connection to the magnetics is shown for the KSZ8041.

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0.1uF to ground? Just going from this circuit from this article on circuit cellar.

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    \$\begingroup\$ Could be another possibility, of course. Depends on the output stage (transformer driver) of the PHY. Considering the similarity to other parts by the manufacturer in the question, my guessing goes towards a connection to the supply rail (3V3). \$\endgroup\$ – zebonaut Sep 4 '10 at 13:36
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    \$\begingroup\$ @zebonaut - yeah, there are undoubtedly several different ways to build the output stage of the PHY. I think the simple cap to ground would probably only work if both sides of the TX differential have push-pull drivers. It definitely wouldn't work if the TX outputs were simple open collector type things, then you'd have to put the C.T. at +V. \$\endgroup\$ – JustJeff Sep 4 '10 at 14:44
  • \$\begingroup\$ The links provided by you doesn't work now! \$\endgroup\$ – abhiarora Sep 21 '18 at 10:15

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