Solution
A USB power distribution switch is used in the USB design of the PXIe-8135. It implements multiple protection features one of which being Short-Circuit Protection. When the output voltage drops below 1.5V, which is caused by an over-load or a short-circuit, the devices limit the output current down to a safe level. The short-circuit current limit is used to reduce the power dissipation during short-circuit conditions. If the junction temperature reaches over-temperature threshold, the device will enter the thermal shutdown (Over-Temperature Protection).
We did a test during V&V on the limits and short conditions using E-load to verify the protection circuitry. The limits are the following:
Two side-by-side USB2.0 ports shared about 1.7A typical current limit (e-fuse limit range between 1.5A to 2.0A).
Two side-by-side USB3.0 ports shared about 2.3A typical current limit (e-fuse limit range between 1.9A to 2.5A).
The limits are similar among all PXIe controllers.