Which Ethernet PHY to use for 100Base-FX (SFP) operation?

For new designs I primarily recommend the Texas Instruments DP83822. It comes in a 5x5mm QFN package and provides RMII, MII and even RGMII interfaces to the Microcontroller or other Ethernet MAC.

**Be sure to select the DP83822xF -**the F means Fiber!

Since at the time of writing this article, the DP83822has supply shortages, the following alternatives are available for 100Base-FX operation:

• DP83869HM is a Gigabit Ethernet transceiver that supports 100Base-FX but does not support MII or RMII (only the Gigabit interfaces RGMII & SGMII)
• DP83620 is a plain 10/100 PHY with RMII & fiber support
• BCM5221 (MII & RMII) & BCM5241 (MII) are plain old 10/100 PHYs but in typical broadcom fashion, they don’t even give you the datasheet to download on their website. But you can find it via Google
• BCM5248 is a 8-port PHY with fiber support
• ST802RT1 (LQFP48)
• Marvell’s 88E3015 & 88E3018 feature fiber support and have **good documentation on the fiber interface.**They are available in QFN packages but only support MII & RGMII - RMII is not supported !
• KS8721BL, KS8721SL & KSZ8721CL (LQFP/SSOP)
• KSZ8001L (LQFP/SSOP)
• KSZ8041FTL (LQFP/SSOP - you must buy the FTL variant!)
• (LQFP/SSOP)
• The VSC8211 is a Gigabit Ethernet PHY but supports 100Base-FX & RMII interface. It is available in a 117-pin BGA package and hence rather difficult to use.
• The LAN9355 3-port Ethernet switch features two 100Base-FX fiber interfaces and a RMII interface. It is more complex to use than a standalone PHY but can forward traffic without software interaction.
• The KSZ8893FQL 3-port Ethernet switch features one 100Base-FX fiber interface and a RMII interface. It is rather expensive
• The Cortina LXT971A is a simple 100Base-FX PHY from a rather unknown manufacturer. It only has MII, not RMII! Cortina appears to have been bought by Intel.
• LU3X34FTR is a 4-port 10/100 PHY with fiber support

Compared to the DP83822, within the context of 100Base-FX operation, there are few technical differences in whether you use the DP83822. In my experience, Ethernet PHYs are mostly difference with regards to their electrical immunity (ESD and so on) which is not really relevant in the fiber context unless someone directly touches the PCB, and the ability to compensate for a degrated Ethernet signal (which is not really relevant for fiber contexts). **The only real difference between the DP83822 and many other parts is that the DP83822 comes in a small VQFN package, which the Micrel/Microchip KSZ devices come in much larger SSOP or LQFP packages.**My recommendation is to select based on availability first, on size second.