[Gta04-owner] USB host vbus current draw

Andreas Kemnade andreas at kemnade.info
Wed Jun 18 17:15:15 CEST 2014


On Tue, 17 Jun 2014 21:44:14 +0200
"Dr. H. Nikolaus Schaller" <hns at goldelico.com> wrote:

> Am 17.06.2014 um 19:35 schrieb Paul Kocialkowski <contact at paulk.fr>:
> > A few days ago, it was reported to me that some USB flash drive was not
> > working with the GTA04 USB host. The required current from the device
> > was 300mA, while I set the limit to 200mA a few months back.
> > 
> > So that got me wondering what the actual limitation of the USB host port
> > really is. To find out, I used a few resistors with very small values
> > and measured what current and voltage the USB host port provides.
> > 
> > * Off-load, vbus was measured at 5.12V
> > * With a 50.3 Ohm resistor (theoretical current of ~100mA with no
> > voltage drop), I get 4.9V and 95-100mA current draw.
> > * With a 15.7 Ohm resistor (theoretical current of ~326mA with no
> > voltage drop), I get 4.4V and 280mA current draw.
> > * With any value below 15.7 Ohm, vbus collapses to 0V and no current is
> > drawn at all until the device is rebooted.
> > 
> > Looking at these figures, it seems like 100mA is still reasonably-well
> > handled while anything beyond 300mA is impossible.
> > So I think it makes sense to keep the limit at 200mA (4.4V seems
> > unreasonably low, so I think even 250mA would be too much).

One point here is that the logic behind the vbus out charge pump is
quite quick in reacting to overcurrent. So I think that having some
margin here is important.

> Nice experiments!
> The official data says:
> a) TPS65950 TRM:
> "6.4.3 USB Charge Pump
> When the device acts as an A-device, the USB charge pump provides 4.8 V/100 mA to the VBUS pin."
> b) TPS65950 data sheet 
> "Table 4-17:
> 							min		typ		max
> Short-circuit limitation current 	250 mA	350 mA	450 mA"
> I have not found a note about the current limit behaviour of the CP. I.e. if it is possible to restart it without a reboot by (re)setting some registers.
maybe you use
echo off > /sys/bus/platform/devices/twl4030_usb/vbus_out
then insert power supply for a short time and attach your mini-a-plug
with your a device again.

> So your results confirm what the data sheet says and I think your conclusions about the limit current are the best we can do.

If your usb device can live with battery voltage (because it just uses
the 5v to power a 3.3v regulator (maybe integrated into some chip)),
you can misuse the linear charging functionality. There is *no* reverse
current protection, so you can happily discharge your gta04 over usb
when linear charging is enabled. Probably current just goes through an
open mosfet (and then the same heat is produced there as it would be in
the event of real charging).
I have never tested that excessively but I have already drawn something
in the magnitude of 2A out of it as an accident for a few seconds. I
immediately shut down it after I have seen the astronomical values in 
And if you manage to kill your device that way, do not forget to add
yourself for the preorder. ;-)

Andreas Kemnade
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