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I have designed a custom pcb utilizing the ENC28J60 ethernet chip to be used with raspberry pi. The board has 5V in which powers both the ENC28J60 (through a AMS1117-3.3 LDO regulator) and the pi. Power and internet is received from a POE splitter which has RJ45 and microUSB 5V out.

This design works fine during initial testing, but when the modules are left to run 24/7, after a few weeks of operation the ENC28J60 chip gets damaged (still detected by rpi, but does not detect ethernet cable). However this only occurred on roughly half of them, the rest work fine.

On the broken boards I have diagnosed, that the 5->3.3v voltage regulator is now outputing almost 5v, basically acting as a pass through. So the ENC28J60 (rated for 3.3V) gets overvolted (5v) and breaks, that part seems clear, but I cannot figure out what could cause the voltage regulator to act/break in such a way.

The ENC has a draw of 250mA max according to datasheet meaning it is well within the AMS1117 3.3 rated 1A current. I have read about "floating ground pins" however running an additional ground to the regulator did not affect the almost 5V output. I do not think overheating is the issue, as in the enclosure, the cpu temp of the pi does not surpass 50C.

Is the regulator insufficient quality for this application of constant usage? Could the POE splitter (amazon link) have spikes in voltage that cause this? or is something else the problem here?

Schematic of power in: Schematic of power in

Schematic of ENC28J60 (green net ports go to rpi):

Schematic of ENC28J60

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    \$\begingroup\$ Some guesses: Lack of ESD and surge protection circuits at the power input? Lack of the LDO backfeed protection diode from Vout to Vin? \$\endgroup\$
    – 比尔盖子
    Sep 18 at 17:20
  • \$\begingroup\$ Check the datasheet for the LM1117 or whatever variant you are using. They recommend a protection diode for certain power removal conditions. \$\endgroup\$
    – Aaron
    Sep 18 at 20:43

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The actual current you can get from a regulator is usually dominated by power dissipation not the rating on the front page of the data sheet.

(5-3.3)*0.25=0.425W. Now this isn’t terrible and might be ok but if you’re in a small enclosure temps could be getting higher than you realize.

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  • \$\begingroup\$ The 1117 family of parts are thermally protected. So, they might shutdown, but that wouldn't cause them to pass 5V. \$\endgroup\$
    – Aaron
    Sep 19 at 13:46
  • \$\begingroup\$ Rad the fine print and you’ll find the thermal protections are not guaranteed (and often above the stated and max) If you actually operate for long periods of time at or near the thermal limit it can cause a failure such as this. \$\endgroup\$
    – asdf30
    Sep 19 at 14:07
  • \$\begingroup\$ Uhg I can’t edit the typos. But reference spx1117 for example and it’s explicit that the thermal shutoff is above the abs max. Many other 1117’s don’t characterize the thermal shutoff at all. The thermal limits are helpful but can’t be fully relied upon. I agree it might not be the most likely cause in this case. But it’s a possible one. \$\endgroup\$
    – asdf30
    Sep 19 at 14:45

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