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Recently I've been developing a board, which for me was the most complex I've ever built. Almost all components are SMT, which was new to me. And I'm running into some point of trouble. All relevant board files are available on GitHub. A schematic PDF is also available.

After some testing I could confirm my "charging section" (which is basically the part powered by the left USB-C port) looks to be working just fine. However, I couldn't get the board into it's "Programming mode". To explain: In programming 'mode' the board should show up as a regular USB storage device to a PC, so one could upload software to the board.

The board is essentially an "extended" Raspberry Pi Pico, following the guidelines RPi gives here

I'll say upfront that I'm not sure if my design is the most ideal or efficient way of doing things. I've made some choices to keep the costs relatively low, or decided on workarounds because I couldn't really find the proper way to implement what I wanted. (Which is why I have two USB-C ports, instead of one).

So the problem here is that the right USB-C port seems end up shorting somewhere, and I'm trying to identify where. At first I thought I might have put some IC's the wrong way around, but after double/triple checking with the respective datasheets and PCB design it seemed to be just fine.

When connecting a (decent) power supply to the USB-C port it immediately jumped into short-circuit protection mode, which is what led me to believe there's a short circuit.

I found out that removing U5 (which is one of two TI LM66200's removes the short-circuit state.

Upon closer inspection I was unable to find any shorted pins on either the footprint or the IC itself. Re-flowing the IC re-instated the short-circuit condition.

However, even with the short-circuit condition removed I still couldn't get power to the RP2040 (U3). The NCP1117LPST33T3G powering it is only receiving 0.2V whilst measuring between the USB-port and it's entry-point on U10 (the other LM66200) reads a proper 5.1V.

So as far as I can trouble-shoot there is something going wrong aroudn those two LM66200's, but I'm puzzled as to what. Double checking my design with TI's datasheet and application information didn't lead me to any insights.

For some functional explanation: The LM66200 is used as a power-mux, in order to favor a USB device connected to port J7 over the battery that could be connected simultaneously.

Could anyone help me shed some light on these two issues? I'm at this point not even sure if they're related, but any insights would be greatly appreciated.

Thanks!

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    \$\begingroup\$ Please add a schematic PDF to the Github, otherwise just looking at the design requires downloading the whole thing and opening it in KiCad. Most people won't bother. You should also include the schematic in the question itself. \$\endgroup\$ Mar 17, 2022 at 21:28
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    \$\begingroup\$ @Kubahasn'tforgottenMonica I've added a schematic PDF as you've requested. It's in the original post and in the Github repo as well. \$\endgroup\$
    – justSem
    Mar 17, 2022 at 21:46
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    \$\begingroup\$ FYI. Use the diode range on your multimeter to determine if you have a short circuit or a semiconductor junction. A real short measures close to 0 Ohms. A junction measures like a diode. This gives you a clue as to what might be the cause. \$\endgroup\$
    – Kartman
    Mar 17, 2022 at 23:31

2 Answers 2

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Zener diode Z1 is mounted upside down and shorts SIM_PWR to GND.

That may not be the only problem though.

For example, the USB-C connectors may never work, as the MicroUSB connector is just replaced with USB-C connector without making any changes to as which kind of device the USB-C connector identifies the device with resistors.

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  • \$\begingroup\$ Nice find on the Zener diode! I double checked it against the datasheet and you're right indeed. For the USB-connector. I was under the impression that I could just connect it as a "normal" micro USB connector if I were to use USB-C in USB2.0 mode. Do you have some resources I could read upon to see what I should change on these parts? \$\endgroup\$
    – justSem
    Mar 17, 2022 at 22:19
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    \$\begingroup\$ @justSem you need to connect at least the CC lines with resistors -- this should point you towards the right search terms. \$\endgroup\$ Mar 17, 2022 at 22:33
  • \$\begingroup\$ @DamienD That lead me to some interesting reading, thank you! \$\endgroup\$
    – justSem
    Mar 18, 2022 at 13:08
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LM66200's datasheet states "The ST pin is an open drain output that must be pulled up to an external voltage for proper operation.". It seems like you left the ST pins floating for both chips. Pull them up to the USB supply or the output, that might fix your problem.

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    \$\begingroup\$ How does pulling up an unused open-drain output will solve any problem? It's an output, and unused. \$\endgroup\$
    – Justme
    Mar 30, 2022 at 7:33
  • \$\begingroup\$ Not entirely sure but going with the datasheet's recommendations is often the safest option. \$\endgroup\$
    – potblitd
    Apr 1, 2022 at 0:20
  • \$\begingroup\$ The output could be left floating, I checked that to be sure. Eventually the problem turned out to be something completely stupid I overlooked in my design. Re-tracing some of the SIM868 Pins solved the short-circuit issues. \$\endgroup\$
    – justSem
    Apr 1, 2022 at 18:38

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