Troubleshooting SC20100S issue... voltage low on 3.3v bus

I have my redesigned board reflowed here and I’m troubleshooting an issue… The most obvious symptom is the 3.3v power bus is sitting at 0.840 volts and the 500ma LDO regulator gets warm, so it’s obviously a power consumption issue. I’ve triple checked with a magnifier I don’t have any solder bridges on the SC20100S or other chips, so that isn’t the problem. I also re-checked I have the right pins connected to power and ground, both by referencing the GHI pinout PDF and the processor datasheet. I know this chipset takes more power than the older G80 chip, but I should still be within within spec. Here are my 3.3v power consumers that are of any significance:

SC20100S - typical while executing code = 226ma, 25*C max = 348ma (per datasheet @ 480mhz, all peripherals on)
RN4678 bluetooth module = max 25ma
WinBond flash = 25ma max (<1ma in standby)

My current consumption should be ~400ma worst case, but typically around 250ma… So I may be on the edge and should probably go to a 1a regulator, but I don’t think the lack of power output from the regulator here is the problem… thoughts?

Schematic: https://www.dropbox.com/s/c2qz96tygmcnzyy/SC20100S%20schematic.pdf?dl=0

How many volts into regulator?

Does the 3.3v drop immediately, or does it take a while? If it takes awhile, might indicate thermal shutdown.

12v into the regulator. The drop is instantaneous. I don’t think it’s a thermal shutdown as it doesn’t get super hot, only unusually/moderately warm… I had a short previously (other board) and it dropped to 0.0v and got super hot. I also can’t feel any hot spots other than the regulator (using ESD protection of course), although I have a thermal camera on order to get more “scientific” data to see if I can find another hotspot.

12v to 3.3v will generate a lot of heat even if the regulator supports it, the heat will be a problem.

It can actually be up to 28v input… The heat itself from the 3.3v regulator is actually insignificant compared to other components when it is powering equipment. There is plenty of heat dissipation on the PCB both in terms of heat sinks and airflow in the equipment environment… Or at least it was with the G80 :slight_smile:

I have 1a version of the regulator on the way to test with, we’ll see if that makes a difference also.

Try lowering the voltage to 5V and see what happens.

The internal heat of the regulator determines thermal shutdown.

1 Like

Mystery solved… I had contemplated moving to a higher capacity linear regulator previously and it turns out the newer part I ordered months ago and had in the box-o-components (and forgot about) has the same footprint but reverses the pin/pad.

One I had sitting in the box and forgot it changed:
https://www.digikey.com/htmldatasheets/production/2956966/0/0/1/ifx27001.html
vs
One I just ordered: https://www.mouser.com/datasheet/2/348/bdxxfc0wefj-e-1874088.pdf

So in a way…the new higher amperage one I just ordered WOULD have fixed the problem as the pinout was the same :slight_smile:. In the meantime however I put one of the correct 500ma regulators on and it works great. BTW total current draw for the whole board (including 5v components) is 220ma. I’m guestimating 200 of that is the 3.3v bus…

2 Likes