Providing high current on battery pin from USB gets it booting

Joerg Reisenweber joerg at
Tue Jul 1 13:00:11 CEST 2008

Am Di  1. Juli 2008 schrieb Andy Green:
> Somebody in the thread at some point said:
> |> | To test it might be a simple setup to feed some 3V to IO_3V3 from
> |> external
> |> Gah I read this as I just finished doing that test on STBY_1V2, 1V8 and
> |> IO_3V3 rails from a second A5 that was powered first -- the first two
> |> make no difference, but we get a good start on the unit under test if I
> |> gave it "3V3 transfusion" from the other A5.  So good idea!
> |>
> |> This is clearly a big clue, but our PMU variant says that it current
> |> limits the Auto regulator to 400mA during its startup.
> |
> | had no look at the particular power-up sequence. So just an idea:
> precharge
> | via a diode from some low-power lower-voltage more-early-in-sequence
> | regulator?
> There is no sequence on our PMU variant, it all comes up on "activation
> phase 3" in a big lump, which doesn't help anything.  Except STBY_1V2
> which is on an LDO off 1V8 and doesn't come up until 1V8 is 1V6 and
> more.  But the transfusion test on that seems to show it isn't relevant.
> I didn't understand why we appear to have large alleged-inrush current
> at startup particularly on 3V3 rail.  I tried to remove the 47u last
> night to see about the cap charging idea and managed to trash some
> components around it :-( so I can't test this idea that the inrush is
> coming from ~60u or whatever on 3.3V.  But at the time of the problem
> the voltages are ramping slowly and do look constant-current (the ramp
> into C is linear).  The voltages are at around 1V but we pull estimated
> ~10W on the regulator input side, it suggests instantaneous inrush
> current up at ~10A since we are only at 1V :-/  But the current limit on
> the 3.3V rail set at 400mA means we should only suck down 400mW there.
> -Andy

Test on R1721 (0R). maybe make this 1R / 10R for test purposes

If we see current spike on this PMU-power-out, the current-limiter is crap, 
and we (almost) know where the root cause(TM) comes from. ;-)
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