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nRF52840 Startup Surge

I've been doing some testing with the nRF52840 (I purchased the Raytac MDBT50Q-1M), and I'm seeing a pretty hard startup surge.  I've attached the results from my scope.  The power supply is 3.3V from my bench.  I thought maybe I had a faulty chip, but I am seeing the same behavior on the nRF52840 Feather from Adafruit (uses the same Raytac variant).

Having said that, I did see there's a new chip revision out, but I was just wondering if this is normal?  It's a pretty big drop.  I added a small capacitor between the supply and ground, and it definitely smoothed things out; however, the surge is still there.

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  • Thanks for the response.  So, I think the problem IS the DCDC converter.  Using an external LDO, I don't think I'll see the sudden drop in voltage (or at least the LDO will buffer it).  Normally, I don't think this is an issue on startup; however, I will be putting a FET at the source (LiFePO4) battery that will most definitely be affected by the voltage drop.

    LiFePO4 batteries run around 3.2V and peak at 3.6V (much different than a LiPo).  I think I'll have to run an external 3V LDO regulator and disable both LDO and DCDC on the chip.  Is there a register I need to set in code to do that or do I just follow the schematic?

    I'm surprised others haven't seen this issue.

Children
  • Do mind sharing the datasheet of your battery?

    "I've been doing some testing with the nRF52840 (I purchased the Raytac MDBT50Q-1M), and I'm seeing a pretty hard startup surge. "
    This surge is only seen when you first attach the load to the source right? In other words, is this surge present if you reset the chip via it's reset pin? 


    "however, I will be putting a FET at the source (LiFePO4) battery that will most definitely be affected by the voltage drop." 
    Our regulators are not causing this issue. My bet is that your battery cannot source the required current in order to charge the 4.7µF capacitor without dropping voltage. 
    I recommend that you supply your FET via another capacitor so that it does not lose its charge when you attach your load to your source. 

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