Correction voltage
Negative feedback
Linear regulator ICs offer simple, efficient, and effective solutions in a multitude of low- to moderate-power voltage regulation applications. They also provide opportunities for many simple application tricks that stretch their performance and push their suitability into tasks that need a bit more capability and versatility than the ICs can manage on their own. One example is supplying power to a load that’s separated from the regulator by interconnecting wiring with substantial resistance—for example, when the regulator is located some distance from the load.
The classic solution to poor load regulation due to interconnection resistance is, of course, the four-wire Kelvin connection, in which the regulator sense connections are run separately from the load current conductors. Clever off-chip circuits have long been available to implement Kelvin connections for IC regulators.1 But sometimes the added connections and wiring of the Kelvin topology is not convenient or cost effective. This idea describes an alternative that doesn’t use them.
The extra sense wires can be eliminated if the necessary VOUT correction voltage is developed from a separate current-sampling resistance: RS1 (Fig. 1). RS1 should have roughly the same resistance as the interconnection (wiring) resistances RS2 and RS3 (generally a few tens to hundreds of milliohms). In fact, it can be as simple as a meandering printed-circuit board trace. The op amp then sums a scaled version of the RS1 sampled voltage with the regulator output V1 to produce corrected feedback and net load power VOUT that is constant and independent of load current.
The necessary polarity of the compensatory feedback depends on the style of regulator being used. Regulators that maintain a constant voltage differential between output and adjust pins (for example, the LM317) require positive feedback. Regulators that use a ground-referred reference voltage (for example, the LM2941) need negative feedback (Fig. 2). But in both cases, the feedback scale factor is adjusted using the same two-step procedure:
• With no load applied (IL = 0), adjust RV Adj for the desired VOUT.
• Apply full load (IL = max) and adjust RS cancellation Adj to restore the target VOUT.
REFERENCE
1. Jim Williams, Application Note No. 2, “Performance Enhancement Techniques for Three Terminal Regulators,” Linear Technology, 1984, Fig. 13.