BOM for the FEZ Cobra II net board

I’ve created a custom board based on the cobra II board. It’s almost identical except I got rid of the expansion headers and exchanged the sd card reader for a serial flash chip from spansion.

My custom board has an issue with the power source. the 3rd pin of the MIC4680-50ym doesn’t give me 5v but only 1v and I can’t figure out what I am doing wrong.

So I was thinking if there is a BOM for the fez cobra II board? I need one to match mine.

Are you sure you are using the 5v version the chip, not the adjustable one?

Yes I’m sure I’m using the 5v version. I’ve ordered 998-MIC4680-5.0YM from mouser and the chip has MIC4680 50 on it.

Pin 3 won’t have a steady state voltage in it. This is a switching regulator so this output will be switching on and off so if you check with a meter on dc, you’ll see a lower voltage than expected.

What’s the output from the actual circuit, ie, the voltage on the other side of the coil? If this OK?

If you have a scope, is pin 3 switching on and off?

If there is no output on the other side of the coil from pin 3, can you show a schematic of the circuit you created?

Hi Dave, I checked the voltage on the MIC4680 of a regular Cobra II board from GHI and it gives a steady 5v.

The voltage on the custom board on the other side of the coil is 0.7v I don’t have scope (yet).

I’ll put my schematic and an image of the circuit later today.

[edit: added schematic and circuit]

Hi Patrick,

I am not 100% familiar with Eagle but I can see from the GHI design that the top ground plane is actually +3.3V.

I see that the 5V runs around the board but that 3.3V at the Gadgeteer sockets (pin 1) terminate nowhere so I assume these connect to the 3.3V top plane. Eagle works very different to Altium Designer that I use. I have not figured out how to show planes yet in Eagle (OK I found it. Ratsnest causes the planes to appear) :slight_smile:

On my Cobra 2 board, pin 3 of the MIC4680 is clocking at around 200Khz. The datasheet indicates that this drops to around 60Khz if there is a short on the output from the oil. This might explain why you are seeing 1V at this pin.

Can you check for any short circuits?

(Edit) If you want to send me a copy of the Eagle files, I can have a quick look over them for you. Assuming they are ok to send?

Hi Dave, thanks so far. I’ll send you the eagle files later today (when I get home from a meeting).

Hi Patrick,

The design looks OK and all the power rails look OK too.

Does the board power up if you connect to USB (preferrably via a current limited power supply and not actual USB port)? ie, does the 5V look OK when you do this? If 5V is OK, is 3.3V OK?

If both 5V and 3.3V are OK on the USB supply input, then the fault lies in the MIC area. Diode D2 will cause a failure of the power supply if the wrong way around. Double check all capacitor polarity too. The line on these SMD represents the pos terminal, unlike the electrolytic types where it marks the NEG terminal. I’ve seen some PCB houses put these the wrong way around too. :slight_smile:

If there is no 5V when powered on the USB side, then you have a short circuit on the 5V rail somewhere, but could also be in the 3.3V and isolating the input to IC2 will confirm this. There is not a lot connected to the 5V rail but IC4 or IC3 if the wrong way around could cause it to fail. These things are so small and seeing the align mark for pin 1 needs a good magnifying glass at my age now :slight_smile:

Let us know how you get on.

Hi Dave,

I’ll check your suggestions this evening. Good to hear the design looks ok. Justin also checked the power source part (he uses it in his design) and he didn’t find anything wrong in the design.

Maybe the PCB house made a mistake by turning a diode other way around.

Thanks for your time and input!! Really appreciate it!

I’ll get back on this…

Ok, I checked the board. The results:

The polarity of the capacitors are OK.
The polarity of the diodes are OK


I’ve checked the diodes, and they report different values then those on the original cobra II board. Some of them are higher and some of them are lower compared to the original Cobra II board. (what should the values be? I think around 0.600 v)

After that I connected the board with a usb power adapter and the board still doesn’t power up. When I measure the voltage at the first diode it says 0v.

So I think there are multiple errors: a short around the mini USB connector and maybe wrong kind of diodes.

@ Dave: are you willing to check my BOM, and confirm that I am using the right components?