Hello,
Getting back to you with our expert answer:
Double-checking STM32F746G-DISCO schematics, there is well a ferrite bead present in the schematics which is L2. This ferrite bead is located between 3V3 and the input pins of LAN8742A VDD1A and VDD2A. The schematics is not 100% exact with the datasheet reference schematics since the 4 x R90 to R93 resistors are not connected at the same potential than VDD1A and VDD2A but the impact is probably low.
Firstly, the aim of this ferrite bead is to act as a low-pass filter and improve voltage immunity for high frequencies on VDD1A and VDD2A. Please consider that Microchip also proposes a second reference design where the ferrite bead is no more present in the case of two power supplies.
Anyway, my recommendations are normally to follow the design of PHY datasheet in the case of a new design even if there are some contradictory examples, in Figure 3.21 and figure 3.22 of the datasheet, where the ferrite bead is not there... Also, mentioning the presence of a ferrite bead without indicating any values has a limited interest.
Please notice that the STM32F746G-DISCO is also an old design from 2015. For example, newer designs implementing Ethernet and LAN8742A have been changed and improved.
These designs use now a dedicated 25MHz oscillator to replace the signal connected to the XTAL1 input of LAN8742A. Also, the pull-up resistors 50R are now changed to 49R9 and they are now connected at the same voltage than VDD1A and VDD2A pins as shown in the reference design in the DS. I suggest using this new version of Ethernet peripheral for a new design (example NUCLEO-H563ZI)
Hope it does answer your question.
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