5.5V input on Li+ / VUSB pin (Photon 2 / Argon)

Hardware
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We've previously been powering the Argon via the 3.3V (pulling the EN pin to ground with a 10k resistor; VEN is about 0.200V) - and only recently found out about possible issues with powering the Argon this way when the Photon 2 failed to boot :stuck_out_tongue:

In any event, we have a 5V5 line that can be tied to either the Li+ or VUSB pin.

Concept is to keep the internal converter disabled, still provide power via the 3V3 - but providing the voltage to one of these two pins (Li+/VUSB) so that there is sufficient voltage at VIN to ensure the converter is disabled.
We can also put a small delay to make sure the 3V3 comes up after the 5V5, although that may be overkill - something I'll play around with to see how we can ensure stable operation.

Part of this (continuing to provide power from the 3V3) is that it is easier to argue against a significant EMI change if we don't have another SMPS making noise (for making a revision for the already-qualified Argon-supporting board; we'll have to do some delta tests for a Photon 2).

Advantage with Li+ pin is that we can still have a USB cable connected while in system for monitoring, since there is no diode between the USB connector and VUSB. From what I can see on the schematics and part #s, there should not be any particular issue - apart from potentially the P2pin12 (VBAT_MEASURE) - that said, it is supposed to have an upper bound of 5.0 as full register - but there is no electrical maximum rating on that pin (just the 0-5V range) - but the realtek datasheet I can see is not particularly great at calling out what the max electrical tolerances are for the specific pins. Going this way, it may just make sense to do some current limiting + voltage divider to drop to 5V for safety's sake)

As USB is specified to 5V5 in any event, I don't expect that there would be any issues providing it via the VUSB pin.

I can attest that both solutions are functioning (having done some minor soldering/rework on some in-house test units).

So the question is which less likely to encounter difficulties - or what we would be trading off?

2

Hello!

We strongly advise against using an external 3.3V supply for the Photon 2 due to several critical reasons:

  1. The 3v3 regulator (and reset circuit) is very sensitive on the photon 2 due to the current switched by the MCU (especially noticeable in scenarios like 5G WiFi operations).
  2. The built in DC2DC (VCC to 3v3) is under a shield, with much reduced EMI and components to tune out noise from impacting the RF section of the wifi/ble chip. Using an external 3v3 supply will require you to manage any FCC or other emissions based issues on their own, including shielding the power supply and implementing your own reset IC to manage brown outs.

I am going to spin this post into an internal ticket so we can further assist you with this. Thank you!

1 Like
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Hi Juffin - fair points; keeping in mind that we aren't actually using the photon 2's wifi/BLE capabilities.

I'll likely design in some 0R resistors for the various power options for our risk-reduction spin, just to make life easy(ier).

Thanks,

James