Philip Karantzalis, Linear Technology
Design Note 529
Introduction
A common problem in power distribution systems is loss of regulation due to the cable/wire voltage drop between the regulator and the load. Any increase in wire resistance, cable length or load current increases the voltage drop over the distribution wire, increasing the difference between actual voltage at the load and the voltage perceived by the regulator. One way to improve regulation over long cable runs is to measure voltage directly at the load via a 4-wire Kelvin connection between the regulator and the load.
Unfortunately, this solution requires routing additional wires to the load as well as a Kelvin resistor placed near the load, impractical when the load is inaccessible for modification. Another method minimizes the voltage drop by employing large diameter wire, lowering the resistance from the regulator to the load. This is electrically simple, but can be mechanically complicated. Increasing the size of cable conductors can significantly increase space requirements and cost.
An alternative to additional wiring is to compensate for the voltage drop at the regulator with the LT6110 cable/wire drop compensator without additional cabling/wiring between the regulator and load. This article shows how the LT6110 can improve regulation by compensating for a wide range of regulator-to-load voltage drops.
The LT6110 Cable/Wire Compensator
Figure 1 shows a 1-wire compensation block diagram. If the remote load circuit does not share the regulator’s ground, two wires are required, one to the load and one ground return wire. The LT6110 high side amplifier senses the load current by measuring the voltage, VSENSE, across the sense resistor, RSENSE, and outputs a current, IIOUT proportional to the load current, ILOAD. IIOUT is programmable with the RIN resistor from 10 μA to 1 mA. Cable/wire voltage drop, VDROP compensation is accomplished by sinking IIOUT through the RFA feedback resistor to increase the regulator’s output by an amount equal to VDROP. An LT6110 cable/wire voltage drop compensation design is simple: set the IIOUT • RFA product equal to the maximum cable/wire voltage drop.
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| Figure 1. | No Extra Wires Are Required to Compensate for Wire Voltage Drop to a Remote Load. |
The LT6110 includes an internal 20 mΩ RSENSE, suitable for load currents up to 3 A; an external RSENSE is required for ILOAD greater than 3 A. The external RSENSE can be a sense resistor, the DC resistance of an inductor or a PCB trace resistor. In addition to the IIOUT sink current, the LT6110 IMON pin provides a sourcing current, IMON, to compensate current-referenced linear regulators such as the LT3080.
Compensating Cable Voltage Drops for a Buck Regulator
Figure 2 shows a complete cable/wire voltage drop compensation system consisting of a 3.3 V, 5 A buck regulator and an LT6110, which regulates the voltage of a remote load connected through 20 feet of 18 AWG copper wire. The buck regulator’s 5 A output requires the use of an external RSENSE.
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| Figure 2. | Example of a High Current Remote Load Regulation: A 3.3 V, 5 A Buck Regulator with LT6110 Cable/Wire Voltage Drop Compensation. |
The maximum 5 A ILOAD through the 140 mΩ wire resistance and 25 mΩ RSENSE creates an 825 mV voltage drop. To regulate the load voltage, VLOAD, for 0 A ≤ ILOAD ≤ 5 A, IIOUT • RFA must equal 825 mV. There are two design options: select IIOUT and calculate the RFA resistor, or design the regulator’s feedback resistors for very low current and calculate the RIN resistor to set IIOUT. Typically IIOUT is set to 100 μA (the IIOUT error is ±1% from 30 μA to 300 μA). In the Figure 2 circuit the feedback path current is 6 μA (VFB/200 k), the RFA resistor is 10 k and the RIN resistor must be calculated to set IIOUT•RFA = 825 mV.
and
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for RFA = 10 k, RSENSE = 25 mΩ and RWIRE = 140 mΩ, RIN = 1.5 k.
Without cable/wire drop compensation the maximum change in load voltage, ΔVLOAD, is 700 mV (5 • 140 mΩ), or an error of 21.2% for a 3.3 V output. The LT6110 reduces ΔVLOAD to only 50 mV at 25 °C, or an error of 1.5%. This is an order of magnitude improvement in load regulation.
Precision Load Regulation
A modest improvement in load regulation with the LT6110 does not require accurate RWIRE estimation. The load regulation error is the product of two errors: error due to the wire/cable resistance and error due to the LT6110 compensation circuit. For example, using the Figure 2 circuit, even if the RSENSE and RWIRE calculation error is 25%, the LT6110 still reduces VLOAD error to 6.25%.
For precise load regulation, an accurate estimate of the resistance between the power source and load is required. If RWIRE, RSENSE and the resistance of the cable connectors and PCB traces in series with the wire is accurately estimated, then the LT6110 can compensate for a wide range of voltage drops to a high degree of precision.
Using the LT6110, an accurate RWIRE estimation and a precision RSENSE, the ΔVLOAD compensation error can be reduced to match the regulator’s voltage error over any length of wire.
Conclusion
The LT6110 cable/wire voltage drop compensator improves the voltage regulation of remote loads, where high current, long cable runs and resistance would otherwise significantly affect regulation. Accurate regulation can be achieved without adding sense wires, buying Kelvin resistors, using more copper or implementing point-of-load regulators – common drawbacks of other solutions. In contrast, compensator solutions require little space while minimizing design complexity and component costs.
