The July 2010 issue of the Journal of Analog Innovation from Linear Technology included an article about how remote sensing can overcome voltage drops in power-supply connections. Engineers usually compensate for such drops--due to inherent wiring resistances--by using a sense connection at the point of load or using a point-of-load regulator. The former technique adds two extra connections and the latter adds an circuit for regulation at the load. Both take up PCB space and can add cost.

Linear Technology takes a different approach, called "virtual remote sense" (VRS) that measures the impedance of the power-supply connections and adjusts voltage output from the supply circuit accordingly. Circuits that apply this technique use a capacitor across the load and a Virtual Remote Sense chip (LT4180) at the power supply. The article includes many helpful schematic diagrams of power-supply circuits. You can download the complete article, "Virtual Remote Sensing Improves Load Regulation by Compensating for Wiring Drops Without Remote Sense Lines," at the Linear Technology Web site: http://cds.linear.com/docs/LT%20Magazine/LTJournal-V20N2-00-Cover-LT4180-Tom_Hack.pdf.

The LT4180 and its surrounding components also add cost, but you balance that against better voltage regulation. You can include the LT4180 in your power-supply circuit, where it provides feedback to the switching-regulator section.

Here's how Linear Technology's authors, Tom Hack and Robert Dobkin, explained the chip's operation...

The LT4180 VRS solves this [voltage-drop] problem by interrogating the line impedance and dynamically correcting for the voltage drops. It works by alternating the output current between 95-percent and 105-percent of the required output current. In other words, the LT4180 forces the supply to provide a DC current plus a current square wave with a peak-to-peak amplitude equal to 10% of the DC current. The decoupling capacitor at the load (which normally insures low impedance for load transients in non-VRS systems) takes on an additional role by filtering out voltage transients from the VRS square wave.

Because the capacitor is sized to produce an “AC short” at the square-wave frequency, the interrogating voltage square wave produced at the power supply is equal to V_SUPPLY(AC) = 0.1 × I_DC × R, measured in V_P-P. The voltage square wave measured at the power supply has a peak-to-peak amplitude equal to one tenth the DC wiring drop. This is not an estimate—it is a direct measurement of the voltage drop across the wiring over all load currents.

Minor signal processing creates a DC voltage from this AC signal, which is introduced into the power supply’s feedback loop to provide accurate load regulation.

The July issue of this journal also contains other articles that relate to power and analog circuits. It's worth taking a look. --Jon Titus