[Gta04-owner] USB host vbus current draw

Dr. H. Nikolaus Schaller hns at goldelico.com
Tue Jun 17 21:44:14 CEST 2014

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).

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.

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


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