DN529 - Control the Voltage of a Remote Load Over Any Length of Copper Wire

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
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