It is possible to put a positive load line on a power supply with a handful of inexpensive parts. In many cases, the added complexity of this circuit is small compared to remote sensing, and it is safer.
Figure 3 shows the measured performance of this circuit.
There are three curves: the uncorrected Vo with 0.25 Ohms of resistance of the cable, the power supply output with cable drop compensation, and the load voltage with cable drop compensation. The uncomp Vo curve shows that, without cable drop compensation, the output voltage would fall outside a 5 percent window. The comp PS Vo curve shows the output of the power supply with cable compensation ranges about 600 mV over a 2.5-amp load current range.
The no-load voltage regulation point is 4.92 volts compared to a desired 5.00 volts, for an error of about 1.6 percent. This is less than the potential worst-case error. The significant error terms are reference accuracy (0.7 percent), divider resistors R4 and R7 (1.6 percent with 1 percent resistors), and U4A offset voltage (0.3 percent). The comp cable Vo curve shows the voltage at the end of the cable connected to the load with cable compensation. The goal of 5 percent accuracy is easily achieved with this approach.
To summarize, it is possible to put a positive load line on a power supply with a handful of inexpensive parts. In many cases, the added complexity of this circuit is small compared to remote sensing. Furthermore, it is safer due to concerns over fault conditions with the remote sense connections. The impact on the loop compensation is minor as long as the current sense connection is on the downstream side of the output capacitor.
Please join us for the next Power Tip, where we will look at the implication of envelope tracking on supplies.
Check out TI Power Lab Notes for a designer's perspective on his power supply designs. For more information about this and other power solutions, visit: www.ti.com/power-ca.