Custom PCB with MDBT40

[Rosina Weiss]

Hi there,

we are a student project and are using the nRF51822 Chip with the MDBT40 module. We designed our own custom PCB and followed the instructions for the circuit as well as the antenna. We already manufactured the PCB. Unfortunately something is wrong with it. When we connect our PCB via USB to the power supply, our voltage regulator gets very hot and somewhere a short develops. Our PCB functions perfectly if we don't solder the MDBT40 module so therefore we assume that the problem lies with the module or the modules circuit and not the rest of our PCB. We also solderd some cables to all used pins of the MDBT40 and connected them to our normal UART, VCC, GND debug pins of the board and everything worked fine. We tried to solve this problem with mulitple PCBs so we don't think the problem accurs due to a soldering short.

Can someone help us? Becuase we have no idea anymore what could cause this problem.

Thanks anyone so much in advance!

[ketiljo]

There's nothing wrong with the schematic or layout as far as I can see. Check of you have the correct voltage on the correct pins before you mount the module. The module shouldn't need more than VDD on two pins plus ground to operate. Check for solder issues as well. 

[Rosina Weiss]

Thank you very much for your answer. So I checked the voltage before mounting the module. My VDD pins are 3.3V as well as the UART TX pin. All the other pins are low. Is that correct?

I just mounted a new one and only soldered the two VDD and five ground pins. Befor connecting it to the power supply, I checked with a multimeter for a short. Non was found. As I connected the power supply the same happend as with the last ones: it got hot and "died". Afterwards I checked again for shorts and there was one between VDD and GND. After I removed the module kind of forcefully I could also see that there was no shortage between the pins on the backside of the module.

[ketiljo]

Sorry for the late reply. 

Very strange! Can you measure the voltage on the 3.3 V regulator with an oscilloscope to see if there's any overshoot when you turn on the power? Cal you also replace the regulator with a power supply with current limiting capabilities, set to ~50 mA and see if the same thing happens? The module is probably broken because of overvoltage on some pin

[Rosina Weiss]

Hmm yes that would make sense. I will try your recommendation and then maybe implement some ESD protection.

Thank you very much for your help!

[ketiljo]

Is L1 a ferrite bead or an inductor?

ESD shouldn't be the problem here.