When a load draws current through a long or thin wire, there is an IR drop that reduces the voltage delivered to the load. In the vehicle from the voltage regulator 5-V output to the V_{PD_IN} (input voltage of portable device), the total resistance of power switch r_DS(on) and cable resistance causes an IR drop at the PD input. So the charging current of most portable devices is less than their expected maximum charging current.

Figure 8-1 Voltage Drop

The TPS254900A-Q1 device detects the load current and applies a proportional sink current that can be used to adjust the output voltage of the upstream regulator to compensate for the IR drop in the charging path. The gain G_(CS) of the sink current proportional to load current is 82 µA/A.

Figure 8-2 Cable Compensation Equivalent Circuit