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?

[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.

[haakonsh]

I suggest you use DCDC for both reg0 and reg1. The LDO's will burn about 40-50% of the energy you supply them, and they are worse the higher the voltage drop. 

If you plan on using reg0 to step down the voltage to 3.3V then you really should use reg1's DC-DC converter as well since the voltage drop from 3.3V to 1.8V is quite large. 


If you could guarantee that your supply voltage would not exceed 3.6V then you can use reg1 only, and skip reg0. But a fully charged lithium cell is usually way above 3.6V...

[czuvich]

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.

[haakonsh]

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.