The reference design for this USB-C chip recommends capacitors to ground between 200 and 600pF on the CCx lines. I have never seen a capacitor to ground on a digital signaling line. Does anyone know what these might be for? Other similar chips also seem to include them.
Best Answer
The Power Delivery protocol uses slow, 300 kbps signaling rate, with edges limited to 300 ns. And the driver uses 50 Ohm impedance. With 400 pF load the time constant of this network is just 20 ns, so the cap doesn't have any effect on PD digital communication. Yet the CC pins are multi-functional, so smoothing the signal a bit doesn't hurt. In fact, the BMC PD receiver does require the 200-600 pF cap per PD specifications, see Table 5-19. Note that the old (faster) BFSK signaling mode is depreciated in PD 3.0 Specifications.