When a load draws current through a long or thin wire, there is an IR drop that reduces the voltage delivered to the load. Cable droop compensation linearly increases the voltage at the CSN/OUT pin of TPS2584x-Q1 as load current increases with the objective of maintaining V_{BUS_CON} (the bus voltage at the USB connector) at 5 V, regardless of load conditions. Most portable devices charge at maximum current when 5 V is present at the USB connector. Figure 10-19 provides an example of resistor drops encountered when designing an automotive USB system with a remote USB connector location.

Figure 10-19 Automotive USB Resistances

Figure 10-20 Voltage Drop

The TPS2584x-Q1 detects the load current and increases the voltage at the CSN/OUT pin to compensate the IR drop in the charging path according to the gain set by the R_SNS, R_SET, and R_IMON resistors as described in R_SNS, R_SET, R_ILIMIT, and R_IMON.

The amount of cable droop compensation required can be estimated by the following equation: ΔV_OUT = (R_SNS + R_{DSON_NFET} + R_WIRE) I_BUS× R_IMON is then chosen by R_IMON = (ΔV_OUT × R_SET × 2) / (I_BUS × R_SNS), Where ΔV_OUT is the desired cable droop compensation voltage at full load.

In most cases, the recommended voltage across R_SNS must be 50 mV. See the R_SNS, R_SET, R_ILIMIT, and R_IMON section. In type-C application, typical R_IMON resistors value are listed in Table 10-3 given the condition full load current = 3 A, R_SNS = 15 mΩ and R_SET = 300 Ω.