Compensate for Wire Drop to a Remote Load

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.
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