First of all, I'm not a USB expert. I tend to fall into the "if ain't broke, don't fix it" and if it was on a previous working design, stick with it.
So our previous designs with USB have series 22 ohm resistors on the DP and DM signals (no vbus). In the BQ24392 datasheet (http://www.ti.com/lit/ds/symlink/bq24392.pdf), they list 2.2 ohm resistors on the DP, DM and VBUS lines.
"VBUS, DM_CON, and DP_CON are recommended to have an external resistor of 2.2-Ω to provide extra ballasting to protect the chip and internal circuitry."
So my first thought was maybe this was a typo and they were meant to be 22. But its stated in multiple places and the datasheet went through several revisions over years, so that makes me trust the 2.2 ohm values as intended and not a careless typo. If it was a typo, I'd think someone at TI would get fired by now.
What is the reasoning for 2.2 ohm resistors on the BQ24392 and 22 ohm on other designs? Also, what would be a minimum size of the 2.2 ohm resistor on the VBUS? If it needs to support up to 1.5A charging current, thats a 5W resistor! So does "recommended" mean we can leave off the 2.2R on VBUS?
Thanks!
