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Question:
When designing a custom baseboard for the SwitchCore Module:
- Can the polarity within an Ethernet pair be swapped, for example RX_N and RX_P or TX_N and TX_P, to simplify PCB routing?
- Can a Schottky diode be used at the SwitchCore power input instead of the P-channel MOSFET used on the reference baseboard?
Answer:
The polarity within the Ethernet differential pairs should not be swapped. TX+, TX−, RX+, and RX− should be connected according to the SwitchCore baseboard/reference design.
This is separate from Auto-MDIX. Auto-MDIX can handle swapping the transmit and receive pairs, but it does not mean the positive and negative signals within an individual differential pair can be reversed. If routing requires the traces to cross, a better approach is to use vias and perform the crossover on another PCB layer.
A Schottky diode can be used instead of the P-MOSFET at the power input, provided its forward-voltage drop and current capability are taken into account.
The SwitchCore draws approximately 1.5 W maximum. At a 5 V input, that corresponds to roughly:
1.5 W / 5 V = 0.3 A
With a Schottky forward-voltage drop of up to approximately 0.6 V, the diode would dissipate:
0.3 A × 0.6 V = 0.18 W
The voltage delivered to the SwitchCore would also be reduced by the diode’s forward drop. For that reason, using an input voltage of around 6–7 V is recommended when using a Schottky diode, giving additional margin for the voltage drop.