Let's assume you design a power supply to power a piece of equipment a few feet away from the supply. We will stipulate that your design is well done — stable with various loads, a varying input voltage, and temperature. And it's accurate to within a fraction of a percent. You have several feet of wire (the loop length going out and coming back) and the wire gauge is suitably large. And let's assume the current draw is more than just milliamps. What problems do you encounter?
Probably you'll find that the voltage at the piece of equipment (henceforth called the load) is low (maybe even too low). Why? Because of the voltage or IR drop in the wiring. What to do? Well, you could just turn up the supply voltage a little bit to compensate for the IR drop. But if the load current changes, the voltage at the load will change. That's generally not acceptable — after all, that's why you built that high-performance, regulated supply with the excellent specs.
You could increase the wire gauge by a couple sizes. That would help, but not completely solve the problem. Plus, it'll cost more. Another approach that is sometimes used is a load-sensing configuration. That's where you have two extra wires running from the power supply to load. These monitor the voltage at the load and send that back as a differential signal to the power supply's voltage feedback monitoring circuitry. That gives the proper regulation right where you need it. Of course, you need two more wires, so that'll cost more, too.
Here's another approach that splits the difference (or perhaps blends the difference) with the above methods. Linear Technology Corp. (LTC) has a new device that is used as a high-side current monitor. The device (LT6110) measures the load current and produces a proportional signal that can be used to slightly tweak the output voltage of most any standard voltage regulator. It does this by pulling down the voltage feedback point of the regulator just a tiny amount which makes the VOUT rise slightly. This compensates for the IR drop in the wiring. Through selection of a resistor value at the IC, this voltage compensation can be tailored to your specific set of conditions (VOUT and load current).
The resistor labeled RIN provides the necessary calibration. Without voltage compensation, here's what you typically get when the load changes. This is showing a load change going from 1.6A to 1.0A and back.
The voltage regulator pretty quickly compensates for the change as all good regulators do. But at the load, there is a ΔV of 200mV. With the LT6110 in place and functioning on the same circuitry, the load voltage is stable.
Of course, the voltage at the output of the supply steps around, but that is just as expected. Note that there are some transients and ringing as the load current and voltage change. This can be reduced somewhat with proper compensation capacitor (C1) value selection.
Additional bypass capacitors are certainly present in typical designs. These also mitigate excessive voltage transients.
The part is available in an 8-lead plastic DFN package (2mm X 2mm) and an 8-lead plastic TSOT-23 package. It is fully spec'd from -40°C to +125°C.
Have you had problems with excessive IR drop in power connections between source and load? How did you deal with the problem?