STUSB4761 vs STUSB4700 PD Source for compatibility with STUSB4500 PD Sink

[kinesphere]

Hi, I'm working on a board that has a USB PD input and a USB PD output.

The board initialises via a generic PD 100W charger connected to the USB-C input, negotiating power for 20V / 5A via the STUSB4500. After this, I'm trying to source the voltage to a secondary board, connecting the STUSB4500 output directly to a PD Source chip, which will again connect to an STUSB4500 in this secondary board. I tried to do this with the STUSB4761 PD Source chip, but the chip is not negotiating or outputting any voltage. I realised the chip has a feedback pin and probably only works with AC/DC supplies. Is there anything I can do to make this work? In other words, this is the scheme I tried:

External power supply (100W) > USB-C CABLE > PCB#1: STUSB4500 + STUSB4761 > USB-C CABLE > PCB#2: STUSB4500 and so on... (this will repeat on a daisy-chain for at least 6 PCBs).

One more question, would it be better to have a STUSB4700 for the source in this situation? Can I link the STUSB4500 to an STUSB4700, and vice-versa for the next negotiation (at 20V / 5A), like in the scheme above but replacing the STUSB4761?

Any help will be very much appreciated!

Thank you,
Ian

[NBALL · Accepted solution]

Hello Ian,

As mentioned in our stusb4761, this device is dedicated to AC/DC applications.

About the application you are targeting, can you please confirm that you will implement step down?
USB always start at 5V. Thus, when 1st sink has negotiated 20V, it cannot be directly pass thru to a source port. It needs to supply 5V and will raise to 20V only after sink has made power nego.

stusb4700 or stusb4710 are not able to check that cable is 5A capable. Only stusb4761 has cable messaging.

To be compliant with USBPD standard, to output 100W with stusb4700 or stusb4710,  connector needs to be a plug instead of receptacle (so called captive cable configuration). On those 2 devices, 20V/5A PDO can be setup.

Best regards

Nathalie

[kinesphere]

Hi Nathalie,

Yes, my application has the regulator for stepping down. Thanks for clarifying all this. I think the way for my application will be to use a PD source from another manufacturer as a captive cable is not suitable.

Regards,
Ian