I've been working on a smart home control panel. It's basically a 10.1" fat-a$$ Android tablet having two ethernet ports, USB 2.0 ports, RS-485 interface, KNX interface, etc.
The first ethernet interface has a KSZ8081RNA Eth Phy which is connected to the processor through the RMII interface. The other ethernet interface has a LAN9512JZX Eth-to-USB bridge + USB hub combo and is connected to the processor through the USB upstream port.
Both ethernet ports work correctly. No problems in terms of connection and performance. But, when I unplug the ethernet cable from the 1st ethernet port, the KSZ8081RNA gets damaged which results in short-circuiting of the supply pins. I tried this 3 times on different boards (same components) and got the same result twice. The 3rd try gave me a different result: PMIC got damaged. I don't see any problems with any plug-unplugs to the other ethernet port.
I can't share the schematics of the full board here, but here's the schematic of the problematic ethernet interface:
Please note that the design of the ethernet interfaces is based on the respective eval boards: EVAL8081 and EVAL9512. One main difference: I'm using this mag-jack instead of an RJ-45 connector plus external magnetics.
Some extra technical info:
The system is driven by a Rockchip PX30 processor and Rockchip RK809 PMIC + codec combo. PMIC gets a clean 5VDC from a synchronous buck and generates all the necessary voltages (3.3V for sensors and peripherals, 1.2V for RAM, 1.8V for ADC, etc). Both ethernet controllers are supplied from the same 3.3V output of the PMIC. This 3.3V output is taken from one of the buck converters of the PMIC and is used only for ethernet interfaces.
Questions and Thoughts
Both eval boards use linear regulators. I'm using a buck regulator. Could this be the issue? The PMIC has LDOs but I'm using those for analog supply requirements of PLL, USB, PMU, etc.
I'm using this ferrite bead on the 3.3V supply line before applying it to the chips. Could this cause a back EMF? I know, this is a silly question. But I can't perform any test as there are nearly no remaining test boards.