design ideas Compensate for Wire Drop to a Remote Load Philip Karantzalis A common problem in power distribution systems is degradation of regulation due to the 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 voltage at the load and the voltage programmed by the regulator. Remote sensing requires routing additional wires to the load. No extra wiring is required with the LT6110 cable/wire drop compensator. 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 sinks a current, IIOUT, proportional to the load current, ILOAD. IIOUT scale factor is programmable with the RIN resistor from 10µ A to 1m A. 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/ Figure 1. No extra wires are required to compensate for wire voltage drop to a remote load COMPENSATING CABLE VOLTAGE DROPS FOR A BUCK REGULATOR 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. VIN IN OUT REGULATOR FB 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, ILOAD VREG VFB 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 . I+IN RFA + – +IN V+ RSENSE 20mΩ RG IIOUT IOUT IMON LT6110 VLOAD CLOAD VSENSE RIN RFB RWIRE RS REMOTE LOAD –IN + – V– October 2014 : LT Journal of Analog Innovation | 29 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 are accurately estimated, the LT6110 can compensate for a wide range of voltage drops to a high degree of precision. VIN 5V TO 40V 10µF VIN OUT EN BOOST SS SW LT3976 100k PDS540 2Ω 100µF VREG 10k 470pF RT 0.01µF VISHAY IHLP4040DZE 6.8µH 0.47µF FB SYNC GND VFB 1.197V 180pF 340k 200k 8 1 +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 Figure 2. Example of a high current remote load regulation: a 3.3V, 5A buck regulator with LT6110 cable/wire voltage drop compensation or design the regulator’s feedback resistors for very low current and calculate the RIN resistor to set IIOUT. Typically IIOUT is set to 100µ A (the IIOUT error is ±1% from 30µ A to 300µ A). In the Figure 2 circuit the feedback path current is 6µ A (VFB /200k), the RFA resistor is 10k and the RIN resistor must be calculated to set IIOUT • RFA = 825mV. IIOUT = VSENSE/RIN IIOUT • RFA = VDROP and RIN = RFA • RSENSE RSENSE • R WIRE so 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. 30 | October 2014 : LT Journal of Analog Innovation PRECISION LOAD REGULATION CONCLUSION A modest improvement in load regulation with the LT6110 only requires a moderately 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 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. n 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 are accurately estimated, 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.