Datasheet
Table Of Contents

LT6110
23
6110fa
For more information www.linear.com/LT6110
applicaTions inForMaTion
Copper Resistor Made from an R
F
Inductor
An inductor made of copper wire will have a small DC
resistance, DCR or R
COIL
, with a temperature coefficient
that matches that of the copper wire connecting the remote
load. Copper wire resistance has a positive temperature
coefficient of approximately +3900ppm/°C. If the current
sense resistor and the remote load are in the same operat
-
ing env
ironment and
subject to an increase in temperature,
the resistance increase in R
SENSE
will increase both V
SENSE
and the LT6110 compensation current to directly track and
cancel the increase in wire voltage drop to the load(refer
to the Temperature Errors section). Table 3 shows a list
of small air core inductors suitable for use as external
R
SENSE
resistors.
Table 3. Coilcraft Air Core Inductors for External R
SENSE
COILCRAFT PART
NUMBER
INDUCTANCE
(nH)*
DCR NOMINAL (mΩ)
(±6% TYPICAL)
I
RMS
(A)
0908SQ-27N 27 8.5 4.4
2222SQ-221 221 9.8 5
1010 US-141 146 3.1 14
*Inductance is not relevant for current sense.
PCB Copper Resistor
In a high load current application without a high preci-
sion load
regulation specification, the cost of an external
R
SENSE
resistor can be eliminated using the resistance of
a printed circuit board, PCB, trace as a sense resistor. The
resistance, R
PCB
, is a function of copper resistivity (ρ), PCB
copper thickness (T), trace width (W) and trace length (L),
R
PCB
= ρ (L/(T • W)). The typical manufacturing of PCB
fabrication limits the trace resistance tolerance to ±15%.
A simplified R
PCB
calculation sets the length equal to the
width (L/W = 1) and approximates 0.5mΩ and 0.25mΩ
per square trace area for 1oz and 2oz copper respectively.
The maximum current of a PCB trace depends on the
trace cross sectional area, trace width (W) times cop
-
per thickness
(
T) and the amount of heating of the trace
permitted. Figure 15 plots PCB trace current vs PCB trace
width for 1oz (T = 1.4mils) and 2oz (T = 2.8mils) copper
for less than 10˚C temperature
rise (this graph provides
a conservative maximum trace current estimate based on
the ANSI IPC2221 standard).
Example: Design a 2oz copper PCB trace sense resistor to
compensate for wire voltage drop for an I
LOAD(MAX)
of 10A.
A V
SENSE
of 60mV is large enough to minimize the com-
pensating IOUT current
error due to the input offset voltage
of the LT6110.
R
PCB
=
V
SENSE
I
LOAD(MAX)
=
60mV
10A
= 6m
Ω
Using Figure 15, the 2oz copper minimum trace width for
10A is 150mils. This sets the current handling capability
of the trace.
The resistance of the trace resistor is set by the length of
the trace. Each 150mil wide square of 2oz copper will have
a resistance of 0.25mΩ. A total resistance of 6mΩ will
require 24 squares (6mΩ/0.25mΩ/square). The length of
the PCB trace will then be 24sq × 150mils or 3.6 inches.
A serpentine layout can be used to reduce the footprint of
R
PCB
. Figure 16 shows a serpentine layout for a 6mΩ PCB
sense resistor and the V
SENSE
connections to the LT6110.
The corners of the serpentine resistor count as 3/4 of a
square. In Figure 16, R
PCB
consists of six 3.5 square rect-
angular traces (
two whole squares and two 3/4 squares).
The R
PCB
six rectangular traces equal 21 0.15in × 0.15in
squares. Using a 2oz copper trace the resistance of the
21 squares is 5.25mΩ at 25°C (21 • 0.25mΩ per square).
An additional very small trace resistance is due to the
0.015in × 0.15in trace that connects the rectangular
PCB TRACE WIDTH (MILS)
0
PCB TRACE CURRENT (A)
12
16
20
24
400 450
6110 F15
8
4
10
14
18
22
6
2
0
50 100 150
200 250
300 350
500
2oz COPPER
1oz COPPER
Figure 15. PCB Trace Current vs Trace Width
(<10°C Temperature Rise)