LDO U3 at STEVAL-USBPD27S

[DRH]

Hi together,

the VCONN voltage at STEVAL-USBPD27S is generated with a LDO. 

https://www.st.com/content/st_com/en/products/evaluation-tools/solution-evaluation-tools/psu-and-converter-solution-eval-boards/steval-usbpd27s.html#overview

USB power delivery specifies a cable supply power of max. 1W at VCONN that means that in worst case, the SOT23 LDO generates about 1.7W loss at 9V USB output voltage. More than that, VCONN is specified with 5V, not 3.3V.

How does this work? Is the duration of the VCONN at 1W limited? Is there an option to skip VCONN if output power stays at 3A, not 5A?

Thank you and Regards,

DRH

[Filippo Bonaccorso]

Hi DRH,

 thanks a lot for your question.

 Let me note two things by Type-C specs:

 - vVCONNValid (The voltage range over which VCONN is considered valid on Source side) is between 3V and 5.5V

 - Power for Sources in USB Suspend or without TX/RX Signals may provide 100mW as minimum while 1W is mandatory in case of TX/RX Signals (USB Data) or VPD support.

 Said this, as stated by Type-C specs, Source may disable VCONN. If not otherwise required, VCONN power may be removed after the source has read the cable’s eMarker and has determined the cable current capability.

 This is the case of the STEVAL-USBPD27S that, once identified the cable capability, removes VCONN supply. The identification is done at attach when the LDO is supplied at 5V.

 Note that PDOs offered are adjusted to cope with cable capability.

 I hope this helps, feel free to comment/ask

Filippo