nRF52840 Startup Surge

[czuvich]

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.

[haakonsh]

I believe that's the supply capacitor getting charged. See C9 in Schematic QFAA and QFAB QFN48 with internal LDO setup. You should see a similar curve if you charge 4.7µF capacitor from your PSU. 


It looks to be a 2.5V drop, that is quite extreme. How much current can your power-supply supply?

[czuvich]

The power-supply is at 3A.  This is definitely going to cause issues with my board setup.  Is there a reason for such behavior and is it normal?

[haakonsh]

Then there's something wrong. Charging the supply cap should require ~80-150mA for a few µs. 

Do you mind measuring the current consumption during start-up?

[czuvich]

Unfortunately, it's too fast of a spike to really measure. The power supply source is being pulled down upon connecting power to the chip. The spike occurs whether the module is programmed or not.

[czuvich]

Something else I just noticed, there are supply configurations that are specific to voltages below 3.6V.  Right now, I have it configured for Reg0 DC/DC Enabled; however, should I have it configured for Reg0 DC/DC and LDO Disabled since I am running at 3.3V?  I am going to be running LiFePO4 batteries without a regulator to the board.

[haakonsh]

Place a weak resistor in series and scope the voltage across it with your scope. The curve should be the inverse of your supply voltage scope, but you will get a reading of a few mV that you can translate to current with the resistance of the series resistor.