STUSB1700 not supplying more than 5V@500mA to a Samsung tablet

[Ivan_]

Hi everyone,

I'm currently using an STUSB1700 as a 5V 3A power source for one of the USB ports in my project. I'm able to draw up to 2.7A with Apple devices (iPhones and iPads) on that port. Using the STUSB GUI, I can connect to the STUSB1700 via I2C and read the messages. I've looked through the datasheet for the register in the NVM but can't seem to find any that would resolve this issue. If anyone has faced the same problem and found a solution, I would greatly appreciate hearing about it. I've attached a screenshot showing what the STUSB GUI dashboard looks like when I plug in the Samsung tablet (it is written that the device is a STUSB1600A but it is a STUSB1700). For the design I've followed the recommended schematic from the datasheet, for it to advertise 5V 3A capabilities when under negotiation (RP_HIGH/RP_DEF pulled up to 5V).

Thanks in advance!

Ivan.

[Didier HERROUIN]

Hello Ivan,

The GUI snapshot is perfectly normal, we can see that 5V/3A has been advertised to the SINK.

The STUSB1700 has no way to limit the current. It embeds a voltage monitoring that can detect a under-voltage and an over-voltage: if the drop voltage (mainly through the MOS FET) is too high at high currents, it could trigger a error and open the MOS.

If I well understand your complain, you would expect that the Apple devices draw 3A ? Only the SINK (so the Tablet) can limit the current. The USB-C standard specifies that the SOURCE must protect itself against an over-current. So if the SINK asks for more than 3A, the SOURCE power supply output is allowed to collapse (under-voltage will be triggered then). It this under-voltage occurs <3A, it could mean that your power supply is not able to deliver 3A.

But if you measure 2.7A without any error (the MOS staying closed), it means that the Apple devices draw only 2.7A and nothing can be done from your side.

Regards,

Didier

[Ivan_]

Hi Didier,

My concern is not about the current draw of Apple devices but of Samsung devices.

Maybe I didn't explain it clearly but Apple devices are able to draw as much current as they see fit but Samsung devices only draw 500mA.

I've got Samsung devices to draw more current but adding voltage dividers on D+ and D- line but that will affect USB communication. I was wondering if you knew why Samsung devices wouldn't draw more than 500mA even if they are connected to a source IC (STUSB1700) advertising 5V 3A on CC1 and CC2.

Thanks

Ivan.

[Didier HERROUIN]

The Samsung device is supposed to follow the Type-C standard, so it should be able to draw up to 3A like the Apple device (it may start with a lower current, then it increases progressively). I do not know why it finally draws 500mA only, the decision is taken by the Tablet itself.
You mention that adding resistors on D+/D- increases the current: it is a kind of of backward compatibility to the legacy standard (Dedicated Charging Port DCP, supplying 1.5A with a short between the D+ and D- lines). I guess that some USB-C chargers (pure charger, with no data) have also implemented this short-circuit.