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LAN8700 MII Ethernet Transceiver, datasheet download

January 13th, 2011 Leave a comment Go to comments

The LAN8700 MII is the Ethernet Transceiver form SMSC (Smart Mixed Signal Connectivity). The part is however, not recommended for new designs.  For new designs SMSC recommends the newer parts LAN8710 and LAN8720. The 8700 provides 8kv or ESD (Electro Static Discharge) suppression without needing any external component.

The IO voltage works on a wide range from 1.6V to 3.6V. The LAN8700 feature a low 39mA current for 10BASE-T operation and 79mA for 100BASE-TX mode operations.

LAN8700 has been used in a large number of new designs including the reference designs from freescale in its MX51 based designs. This gives users a confidence on the chip.

The datasheet of the LAN8700 can be downloaded from http://www.smsc.com/media/Downloads_Public/Data_Sheets/8700.pdf

The reference designs are also available directly from smsc at http://www.smsc.com/index.php?tid=149&pid=46&cid=&tab=5.There are two reference designs one for the MII and the other for RMII. It also comes with PCB design guidelines.  Here are some points to be considered for PCB designs when doing the layout

1. The magnetics should be at some distance, say, 1 inch or more away from the PHY. This ensures that the effects of the magnetics does not enter into the PHY.

2. The distance between the magnetics and the RJ45 connector should not more than typically 1 inch.

3. The TX+ and TX- lengthe should be matched typically withing 50 mils. Same thing for RX+ and RX-

4. The ground should not exist under the TX and RX pair and under the magnetics.

5. Maintain 100 Ohm differential impedance between the TX and RX lines between the traces between the PHY and magnetics.

Unfortunately, some of the design rules mentioned in the PCB design seem to be incorrect. This includes when they say – avoid 90 degree bend. Also it seems that they want warn to keep traces away from the differential pair, without specifying a distance. The truth is the differntial pair are pretty much immune to noise and any distance more than 15 mils should be good enough.  Designers will have to modify the rules a bit to suite them. Nonetheless a decent guide to start with.

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