Providing high current on battery pin from USB gets it booting

Andy Green andy at
Mon Jun 30 22:51:47 CEST 2008

Hash: SHA1

Somebody in the thread at some point said:
| Andy Green wrote:
|> Anyway the main battery + pin shows no signs of any dents or load and
|> sits there at about 0.4V itself before during and after the fun on Vsys.
|> ~ I don't think this can be it.
| I added an 100uF bypass to VB_SYS and a 470uF capacitor where the
| battery ought to be. That monster didn't impress the system at all,
| and it happily continued to run.

Now my backup battery has charged up (to ~1.3V), the behaviour changed,
I couldn't hear the ticking sound from the mic any more but it is still
spinning with Vsys trouble.  However the Vsys broken part is much more
variable and can oscillate -- the attached pic shows a 250us outage
happening at low probability.

| I then just shorted the battery. That got me some "motorboat"
| effect. So the battery isn't completely ignored.

I also tried to hook VBUS (USB +ve after the host switch) to the Main
battery +, it starts beautifully.  But a LOT of current is pulled that
way, hooking it to the battery + via 5R did not help the symptom, a
chunk of wire did.  So the system tries to pull >2A I would guess from
Vsys starting the whole disaster.  Where is all that current going!

It's a strong indication USB power is disconnected from Vsys by the PCF
due to USB overcurrent as suspected, we fill in the hole from the
"battery" and we go on.

| I'm confused :-(

It all comes down to where is all the current going at startup.  We'll
get unconfused real quick when we work that out.  The regulators are
current limited during this phase and shouldn't be able to add up to
~10W that is suspected.  It seems the culprit is via Vsys path not via
battery directly.

- -Andy
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