No battery Vsys issue is USB overcurrent?

Werner Almesberger werner at
Mon Jun 30 14:53:19 CEST 2008

Andy Green wrote:
> I was up until 2am looking at the "charging problem"...

It's a tricky beast, isn't it ? ;-)

> I believe this
> is not a charging problem at all but precisely the old "I can't come up
> without a battery" problem and nothing more (or, unfortunately, less).

I think both have a lot in common. I see the charging problem if I
do the following:

- start the system with battery and USB power
- bring it to a state where it can run from USB power alone
- remove the battery
- enable the charger

What happens on VB_SYS then looks exactly like what happens when I try
to boot without a battery. (Looks like your traces.)

So I think solving the "can't boot without battery" problem will also
solve the charging issue.

By the way, since you're seeing the same VB_SYS breakdown now, does
this mean that your GTA02 now also refuses to do anything if there's
no battery ?

> Currently the hypothesis I am trying to work with is that we pull way
> too much current at power up, and the loss of Vsys is a reasonable
> response from the pcf50633.

That sounds plausible, yes. I estimated that the systems draws about
3W from the capacitor bypassing VB_SYS.

> So where does all the power go?  I didn't find out yet (or indeed prove
> it is true).

> I sniffed around the GSM stuff for a while, because the charger action
> can basically power it all since it is directly powered from "battery",

Ah, that's an interesting theory. The PMU's GPIOs are stable in
Standby, so you could control the GSM power switch. However, our
default should already do this right.

There's something we don't control with this switch, and that's the
RF amplifier. A radical way to test whether GSM is to blame would be
to remove B1701 and see what happens.

I haven't really looked at what we have on the primary side of the
PMU, but now that you mention it, that sounds like a good place to

Let's see, we have a bypass capacitor for 22uF on VB_GSM (for the RF
amp), 10uF on VB (right at the input), and another 22uF on VB (for USB
host power). There are a few smaller bypasses as well. That's a total
of 54uF.

When I added an external 47uF bypass to VB_SYS, I sometimes got the
system to start, sometimes not. Adding 4.7uF made it almost always
work. 47uF+4.7uF =~ 52uF. Could this be conincidence ?

To summarize, my system has:

Internal VB_SYS bypass (from rework):		  10uF
External VB_SYS bypass:				  52uF
Bypasses sitting on VB and VB_GSM:		- 54uF
VB_SYS bypass recommended by NXP:		- 22uF
						- 10uF

We also feed the vibrator directly from VB, but removing it didn't
change anything.

- Werner

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