VDD pin connection of STUSB1600

[MoniR]

Hi, 

I am trying to use STUSB1600 as a front end interface to my USB Type-C connector and interface it with STM32L4R9 series microcontroller. It is a dual role type application but with no power delivery. Unable to understand why VDD of STUSB1600 is connected between source of two PMOS as shown in the figure below (snippet taken from STUSB1600 datasheet).

Why not simply connect it to VBUS path directly. Was there any specific reason behind doing this?

Also, under power supplies section it is mentioned that if both VDD and VSYS are present then the precedence is given to VSYS provided it is greater than 3.1V. Does that mean if we are connecting both VDD (as VBUS) and VSYS (say of 3.3V) as shown in figure 1, then STUSB1600 will never take power from VDD but always from VSYS?

Can someone please help me figure this out

Thanks.

[FBL]

Hi @MoniR 

Two PMOS are used to take control of the direction or the orientation of the power supply for dual role application to switch modes either to sink Vbus or sources Vbus.

About your second question, according to datasheet, 

If VSYS is present and its voltage is above 3.1V, the STUSB1600 will use VSYS as its power source.

Update check @NBALL's comment 

[NBALL]

Hello

When STUSB1600 has no supply (i.e deadbattery mode), Vsys is assumed to be null and in other to start the system, stusb1600 needs to be supplied. As stusb1600 provides deadbattery termination to the source, it will get VBUS. Using the PMOS diode, VDD will be supplied: stusb1600 will start and vbus_en_snk will be grounded to turn on the sink power path, after tccdebounce.

Best regards

Nathalie

[MoniR]

Thanks for the reply.

Can you please also explain why VDD pin connection is always made specifically between back to back PMOS in the application schematic? And why not at the VBUS path directly as drawn in the below pasted figure?

[NBALL]

Hello MoniR

It depends on application. As I explained in my previous post, If we assume that Vsys is a battery voltage, battery is sometimes fully depleted.
In this condition (vsys=0), with your propose connection,  as VDD won't be supplied, there is no way to start (vbus_en_snk pin is Hz)
With our proposal, because of parasitic diode of PMOS, VDD will be polarized, STUSB1600 will start and will assert vbus_en_snk=0 in order to close the power path.

If Vsys is permanent supply, other connection can be done.

Best regards

Nathalie