Control the Voltage of a Remote Load Over Any Length of Copper Wire – Design Note 529 Philip Karantzalis 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. 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 10µA to 1mA. 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. The LT6110 includes an internal 20mΩ RSENSE, suitable for load currents up to 3A; an external RSENSE is required for ILOAD greater than 3A. 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. An alternative to additional wiring is to compensate for the voltage drop at the regulator with the LT®6110 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. VIN IN OUT REGULATOR FB L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners. ILOAD VREG VFB I+IN RFA + – +IN V+ RSENSE 20mΩ RG IIOUT CLOAD VSENSE RIN RFB RWIRE IOUT IMON LT6110 RS VLOAD REMOTE LOAD –IN + – V– DN529 F01 Figure 1. No Extra Wires Are Required to Compensate for Wire Voltage Drop to a Remote Load 08/14/529 Compensating Cable Voltage Drops for a Buck Regulator Figure 2 shows a complete cable/wire voltage drop compensation system consisting of a 3.3V, 5A 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 5A output requires the use of an external RSENSE. 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%. The maximum 5A ILOAD through the 140mΩ wire resistance and 25mΩ RSENSE creates an 825mV voltage drop. To regulate the load voltage, VLOAD, for 0A ≤ ILOAD ≤ 5A, IIOUT • RFA must equal 825mV. 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 100µA (the IIOUT error is ±1% from 30µA to 300µA). In the Figure 2 circuit the feedback path current is 6µA (VFB/200k), the RFA resistor is 10k and the RIN resistor must be calculated to set IIOUT • RFA = 825mV. 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 ∆VLOAD compensation error can be reduced to match the regulator’s voltage error over any length of wire. IIOUT = VSENSE /RIN, IIOUT • RFA = VDROP and RIN = RFA • and 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. RSENSE RSENSE •R WIRE for RFA = 10k, RSENSE = 25mΩ and RWIRE = 140mΩ, RIN = 1.5k. Without cable/wire drop compensation the maximum change in load voltage, ∆VLOAD, is 700mV (5 • 140mΩ), or an error of 21.2% for a 3.3V output. The LT6110 reduces ∆VLOAD to only 50mV at 25°C, or an error of 1.5%. This is an order of magnitude improvement in load regulation. VIN 5V TO 40V 10µF VIN EN OUT BOOST SS SW LT3976 RT 0.01µF 100k FB SYNC GND VISHAY IHLP4040DZE 6.8µH 0.47µF PDS540 2Ω 100µF VREG 10k 470pF VFB 1.197V 180pF 340k 200k 1 8 +IN NC 2 7 EN V+ LT6110 3 6 IMON RS 4 GND –IN 5 1.5k 0.1µF VISHAY VSL2512R0250F RWIRE 140mΩ 20 FT, 18AWG VLOAD 3.3V 220µF LOAD 5A DN529 F02 Figure 2. Example of a High Current Remote Load Regulation: A 3.3V, 5A Buck Regulator with LT6110 Cable/Wire Voltage Drop Compensation Data Sheet Download www.linear.com/LT6110 Linear Technology Corporation For applications help, call (408) 432-1900, Ext. 3761 dn529f LT/AP 0814 111K • PRINTED IN THE USA 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com LINEAR TECHNOLOGY CORPORATION 2014