Current limit not being enforced in PPS mode

[nsluhrs]

Hello,

I was doing some testing with my shiny new PD Micro and it doesn't seem to be respecting the current limiting. I can pull 380mA despite setting the current limit to 200mA. Just as a sanity check here's the init line I'm using. Everything else is the same as the pps example file. PD_UFP.init_PPS(PPS_V(9.2), PPS_A(0.2)); The load is 25 Ohm at dc and 66mH (not that inductance should matter in this case.

I honestly kind of doubt this is a problem with the PD Micro, I suspect it may be an issue with the USB PSU I'm using, but any debugging advice would be nice.

Best, Nik

[ryan-ma]

Hi Nik,

The current limit in PPS is quite coarse. The PD spec only requires the PD power supplies to provide accuracy of 150mA when souring more than 1A, or +/-5% when souring more than 3A.

USB_PD_R3_0 V2.0 20190829.pdf - page 246 - 7.1.4.4 Programmable Power Supply Current Limit

......
The PPS Operating Current regulation accuracy during Current Limit is
defined as iPpsCLNew. The minimum programmable Current Limit level is iPpsCLMin.
......

USB_PD_R3_0 V2.0 20190829.pdf - page 309 - 7.4.1 Source Electrical Parameters

......
iPpsCLMin (Minimum Current Limit setting)
- Minimum: 1A
iPpsCLNew (Current Limit accuracy)
- 1A ≤ Operating Current ≤ 3A : +/-150mA
- Operating current > 3A : +/- 5%
......