LINER LT6110 Cable/wire drop compensator Datasheet

LT6110
Cable/Wire Drop
Compensator
Features
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Description
Improve Voltage Regulation to a Remote Load by 10×
Ideal for Resistor-Adjustable Voltage Regulators
Gain Configurable with a Single Resistor
High Side Current Sensing:
Integrated 20mΩ Sense Resistor for Up to 3A
Ability to Use an External Sense Resistor
300µV Maximum Input Offset Voltage
Output Current Accuracy of 1% Maximum
30µA Maximum Supply Current
2V to 50V Supply Range
Fully Specified from –40°C to 125°C
Available in Low Profile (1mm) ThinSOT™ and
(2mm × 2mm) DFN Packages
Applications
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The LT®6110 is a precision high side current sense with a
current mode output, designed for controlling the output
voltage of an adjustable power supply or voltage regulator.
This can be used to compensate for drops in voltage at
a remote load due to resistance in a wire, trace or cable.
The LT6110 monitors load current via a series-connected
internal or external sense resistor. Two current mode outputs, one sinking and one sourcing, are provided that are
proportional to the load current. This allows the LT6110 to
adjust the output voltage of a wide variety of regulators.
Either output may be used to monitor the load current.
Low DC offset allows for the use of a small sense resistor, as well as precise control of small variations in wire
voltage drop.
L, LT, LTC, LTM, Linear Technology, the Linear logo and µModule are registered trademarks
and ThinSOT is a trademark of Linear Technology Corporation. All other trademarks are the
property of their respective owners.
Automotive and Industrial Power Distribution
USB Power
DC/DC Converters
Plug-In DC Adapters
Power over Ethernet
Typical Application
IN
OUT
REGULATOR
RWIRE
ILOAD
VREG
VLOAD
RIN
FB
+IN V+
RS
–IN
IOUT
LT6110
+ –
VLOAD UNCOMPENSATED
5V
2A
IMON
V–
REMOTE
LOAD
5V
GND
500mV/DIV
VIN
1A
VLOAD COMPENSATED
6110 TA01
ILOAD
200µs/DIV
6110fa
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1
LT6110
Absolute Maximum Ratings
(Note 1)
Total Supply Voltage (V+ to V–)..................................55V
+IN, –IN, IOUT, IMON to V– Voltage............................. V+
+IN, -IN, IOUT, IMON Current..................................10mA
IOUT to IMON Voltage.....................................36V, –0.6V
V+, +IN to IOUT Voltage..............................................36V
Differential Input Voltage............................................. V+
RSENSE Current (Note 2)
Continuous..............................................................3A
Transient (<0.1 Second)...........................................5A
Specified Temperature Range (Note 3)
LT6110I.................................................–40°C to 85°C
LT6110H.............................................. –40°C to 125°C
Junction Temperature ........................................... 150°C
Storage Temperature Range................... –65°C to 150°C
Lead Temperature (Soldering, 10 sec)
TS8.................................................................... 300°C
Pin Configuration
TOP VIEW
TOP VIEW
NC* 1
IOUT 2
IMON 3
V– 4
8 NC*
+IN 1
8 +IN
7 V+
6 RS
5 –IN
V+ 2
RS 3
–IN 4
TS8 PACKAGE
8-LEAD PLASTIC TSOT-23
TJMAX = 150°C, θJA = 195°C/W
*NC PIN NOT INTERNALLY CONNECTED
9
V–
7 IOUT
6 IMON
5 V–
DC PACKAGE
8-LEAD (2mm × 2mm) PLASTIC DFN
TJMAX = 150°C, θJA = 80.6°C/W
EXPOSED PAD (PIN 9) IS V–, MUST BE SOLDERED TO PCB
*NC PIN NOT INTERNALLY CONNECTED
Order Information
Lead Free Finish
TAPE AND REEL (MINI)
TAPE AND REEL
PART MARKING*
PACKAGE DESCRIPTION
SPECIFIED TEMPERATURE RANGE
LT6110ITS8#TRMPBF
LT6110ITS8#TRPBF
LTGCQ
8-Lead Plastic TSOT-23
–40°C to 85°C
LT6110HTS8#TRMPBF
LT6110HTS8#TRPBF
LTGCQ
8-Lead Plastic TSOT-23
–40°C to 125°C
LT6110IDC#TRMPBF
LT6110IDC#TRPBF
LGCP
8-Lead (2mm × 2mm) Plastic DFN
–40°C to 85°C
LT6110HDC#TRMPBF
LT6110HDC#TRPBF
LGCP
8-Lead (2mm × 2mm) Plastic DFN
–40°C to 125°C
TRM = 500 pieces. *Temperature grades are identified by a label on the shipping container.
Consult LTC Marketing for parts specified with wider operating temperature ranges. *The temperature grade is identified by a label on the shipping container.
Consult LTC Marketing for information on lead based finish parts.
For more information on lead free part marking, go to: http://www.linear.com/leadfree/
For more information on tape and reel specifications, go to: http://www.linear.com/tapeandreel/
2
6110fa
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LT6110
Electrical Characteristics
The l denotes the specifications which apply over the full specified
temperature range, otherwise specifications are at TA = 25°C. V+ = 5V, V– = VIMON = 0V, I+IN = 100µA, VIOUT – VIMON = 1.2V, unless
otherwise noted.
SYMBOL
PARAMETER
V+
Supply Range
CONDITIONS
VOS
Amplifier Input Offset Voltage
∆VOS/∆I+IN
Amplifier Input Offset Voltage Change
with I+IN
I+IN = 10µA to 1mA
0°C ≤ TA ≤ 85°C (Note 6)
∆VOS/∆VIOUT
Amplifier Input Offset Voltage Change
with IOUT Voltage
VIOUT = 0.4V to 5V
∆VOS/∆VIMON
Amplifier Input Offset Voltage Change
with IMON Voltage
VIMON = 0V to 1V
∆VOS/∆T
Amplifier Input Offset Voltage Drift
MIN
0°C ≤ TA ≤ 85°C (Note 5)
85°C ≤ TA ≤ 125°C (Note 5)
–40°C ≤ TA ≤ 0°C (Note 5)
2.0
IOS
Amplifier Input Offset Current
V+ = 5V
PSRR
Power Supply Rejection Ratio
V+ = 2.0V to 36V
V+ = 36V to 50V
IOUT Current Error (Note 4)
(Referred to I+IN)
I+IN = 10µA
0°C ≤ TA ≤ 85°C, (Note 6)
V
µV
µV
µV
µV
0.15
0.3
0.5
1.5
mV/mA
mV/mA
mV/mA
l
0.005
0.02
mV/V
l
0.3
1
mV/V
l
l
l
l
l
1
35
l
I+IN = 100µA
0°C ≤ TA ≤ 85°C, (Note 6)
I+IN = 1mA
0°C ≤ TA ≤ 85°C, (Note 6)
IMON Current Error (Note 4)
(Referred to I+IN)
I+IN = 10µA
0°C ≤ TA ≤ 85°C, (Note 6)
I+IN = 100µA
0°C ≤ TA ≤ 85°C, (Note 6)
I+IN = 1mA
0°C ≤ TA ≤ 85°C, (Note 6)
UNITS
300
400
500
550
l
Amplifer Input Bias Current (–IN)
MAX
100
V+ = 5V
IB
TYP
50
l
l
l
96
90
µV/°C
70
100
nA
nA
1
nA
110
100
dB
dB
0.6
1.6
2
2.5
%
%
%
0.5
1
1.5
2.3
%
%
%
0.75
2.5
3
4
%
%
%
1.5
3
3.5
5
%
%
%
1.5
3
3.5
5
%
%
%
1.7
l
l
4
5
6
%
%
%
l
l
l
l
l
l
l
l
l
l
∆IIOUT/VIOUT
IOUT Current Error Change with
IOUT Voltage (Note 4)
VIOUT = 0.4V to 3.5V
VIOUT = 0.4V to 5V
l
l
0.2
0.4
%/V
%/V
∆IIMON/VIMON
IMON Current Error Change with
IMON Voltage (Note 4)
VIMON = 0V to 3.1V, VIOUT = 5V
l
0.2
%/V
1
mA
16
30
50
µA
µA
30
50
100
µA
µA
0.02
0.0225
+IN Current Range
IS
Supply Current
l
V+ = 5V, I
+IN = 0µA
V+ = 50V, I+IN = 0µA, VIOUT = 25V
RSENSE
RSENSE Resistance
(Note 2)
BW
Signal Bandwidth (–3dB)
I+IN = 100µA, RIOUT = 1k
tr
Rise Time
0.01
l
l
0.0165
Ω
180
kHz
2
µs
6110fa
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3
LT6110
Electrical Characteristics
Note 1: Stresses beyond those listed under Absolute Maximum Ratings
may cause permanent damage to the device. Exposure to any Absolute
Maximum Rating condition for extended periods may affect device
reliability and lifetime. In addition to the Absolute Maximum Ratings, the
output current and supply current must be limited to insure that the power
dissipation in the LT6110 does not allow the die temperature to exceed
150°C. See the Applications Information section Power Dissipation for
further information.
Note 2: RSENSE resistance and maximum RSENSE currents are guaranteed
by characterization and process controls.
Note 3: The LT6110I is guaranteed to meet specified performance from
–40°C to 85°C. The LT6110H is guaranteed to meet specified performance
from –40°C to 125°C.
Note 4: Specified error is for the LT6110 output current mirror and does
not include errors due to VOS or resistor tolerances. Since most systems
will not have 100% correction, the total system error can be compensated
to less than the specified error with proper design. See the Applications
Information section for details.
Note 5: Measurement errors limit automatic testing accuracy. These
measurements are guaranteed by design correlation, characterization and
testing to wider limits.
Note 6: The 0°C ≤ TA ≤ 85°C temperature range is guaranteed by
characterization and correlation to testing at–40°C, 25°C and 85°C.
Typical Performance Characteristics
NUMBER OF UNITS
175 VIOUT = 1.2V
VIMON = 0V
I = 100µA
150 +IN
400
800 UNITS
125
100
75
50
25
0
–350 –250 –150 –50 50 150 250
INPUT OFFSET VOLTAGE (µV)
I+IN = 100µA
VIOUT = 0.4V
VIMON = 0V
300
TA = 125°C
200
TA = 85°C
100
TA = 0°C
0
TA = –40°C, –55°C
TA = 25°C
–100
–200
350
VOS vs Supply Voltage
0
5
10 15 20 25 30
SUPPLY VOLTAGE (V)
35
VOS Temperature Coefficient
NUMBER OF UNITS
VIOUT = 1.2V
10 VIMON = 0V
I+IN = 100µA
400
40 UNITS
–40°C TO 125°C
INPUT OFFSET VOLTAGE (µV)
V+ = 5V
8
6
4
2
0
0
1.0
2.0
–3.0 –2.0 –1.0
3.0
INPUT OFFSET VOLTAGE TEMPERATURE COEFFICIENT (µV/°C)
6110 G04
4
VOS vs Supply Voltage
I+IN = 100µA
VIOUT = 25V
VIMON = 0V
300
200
TA = 85°C
TA = 125°C
100
TA = 0°C
0
TA = –40°C, –55°C
–100
–200
TA = 25°C
40
45
SUPPLY VOLTAGE (V)
35
6110 G02
6110 G01
12
40
400
VOS vs IOUT Voltage
300
200
10
V+ = 36V
VIMON = 0V
I+IN = 100µA
TA = 25°C
TA = 125°C
TA = 85°C
100
0
–100
–200
0.1
TA = –55°C
TA = 0°C
TA = –40°C
VOS vs IOUT Voltage
V+ = 50V
VIMON = 25V
I+IN = 100µA
9
8
7
6
TA = 125°C
5
TA = 85°C
TA = 25°C
4
3
TA = –55°C
2
TA = 0°C
1
1
10
IOUT VOLTAGE (V)
40
6110 G05
50
6110 G03
INPUT OFFSET VOLTAGE (mV)
V+ = 5V
INPUT OFFSET VOLTAGE (µV)
VOS Distribution
INPUT OFFSET VOLTAGE (µV)
200
0
30
35
TA = –40°C
40
45
IOUT VOLTAGE (V)
50
6110 G06
6110fa
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LT6110
Typical Performance Characteristics
VOS vs VSENSE Voltage
10
V+ = 5V
VOS vs IMON Voltage
TA = 125°C
200
TA = 85°C
100
TA = 0°C
0
8
TA = –40°C, –55°C
–100
TA = 85°C
7
6
5
TA = 125°C
4
3
2
TA = 25°C
1
0.5
0
1.0
1.5 2.0 2.5
VSENSE (V)
3.0
3.5
0
0.1
4.0
3
800 UNITS
100
50
–0.8 –0.4
0
0.4
0.8
IOUT CURRENT ERROR (%)
TA = –55°C
1
0
TA = 25°C
–1
TA = –40°C
–3
1.2
0
–1
TA = 0°C
TA = 85°C, 125°C
TA = 25°C
–2
TA = 85°C, 125°C
–4
0.001
0.1
0.01
+IN CURRENT (mA)
1
2
0
5
10 15 20 25 30
SUPPLY VOLTAGE (V)
V+ = 5V
VIMON = 0V
2 I+IN = 100µA
35
6110 G13
2
VIOUT = 25V
I+IN = 100µA
2
TA = –40°C
TA = –55°C
1
0
TA = 0°C
TA = 85°C, 125°C
TA = 25°C
–1
–3
40
35
40
45
SUPPLY VOLTAGE (V)
0
TA = 25°C
–1
50
6110 G12
IOUT Current Error vs Output
Voltage
3
TA = –40°C
TA = –55°C
1
–3
1
–2
TA = 85°C, 125°C
TA = 0°C
–2
–3
0.1
0.01
OUTPUT CURRENT (mA)
3
3
TA = –55°C
TA = –40°C
TA = 0°C
6110 G09
IOUT CURRENT ERROR (%)
1
TA = –55°C
–200
0.001
IOUT Current Error vs Output
Voltage
IOUT CURRENT ERROR (%)
IOUT CURRENT ERROR (%)
200
6110 G11
IOUT Current Error vs +IN Current
TA = –40°C
TA = 25°C
IOUT Current Error vs Supply
Voltage
TA = 0°C
6110 G10
V+ = 5V
VOUT = 1.2V
TA = 85°C
0
–2
0
–1.2
TA = 125°C
400
VIOUT = 0.4V
I+IN = 100µA
2
150
2
600
IOUT Current Error vs Supply
Voltage
200
3
40
V+ = 5V
VOUT = 1.2V
6110 G08
IOUT CURRENT ERROR (%)
NUMBER OF UNITS
V+ = 5V
VIOUT = 1.2V
250 VIMON = 0V
I+IN = 100µA
1
10
IMON VOLTAGE (V)
6110 G07
IOUT Current Error Distribution
300
TA = –40°C, –55°C
VOS vs +IN Current
800
INPUT OFFSET VOLTAGE (µV)
TA = 25°C
IOUT CURRENT ERROR (%)
300
–200
1000
= V+ = 36V
VIOUT
9 I+IN = 100µA
INPUT OFFSET VOLTAGE (mV)
INPUT OFFSET VOLTAGE (µV)
400
V+ = 36V
VIMON = 0V
2 I+IN = 100µA
TA = –40°C
TA = –55°C
1
0
TA = 85°C, 125°C
TA = 25°C
–1
TA = 0°C
–2
0
0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0
OUTPUT VOLTAGE (V)
6110 G14
–3
0
10
20
30
OUTPUT VOLTAGE (V)
40
6110 G15
6110fa
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5
LT6110
Typical Performance Characteristics
IMON Current Error vs Supply
Voltage
7
800 UNITS
200
150
100
50
5
4
TA = –55°C
3
TA = 0°C
2
–1
TA = –40°C
TA = 25°C
1
TA = 85°C, 125°C
0
0
0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4
IMON CURRENT ERROR (%)
0
5
10 15 20 25 30
SUPPLY VOLTAGE (V)
35
I+IN = 100µA
I+IN = 10µA
0
–55 –35 –15
4
TA = –55°C
3
TA = –40°C
TA = 0°C
2
1
TA = 125°C
0
–1
5 25 45 65 85 105 125
TEMPERATURE (°C)
TA = 25°C
TA = 0°C
TA = –55°C
0
–1
TA = 125°C
TA = 25°C
TA = 85°C
0
0.1
0.01
+IN CURRENT (mA)
1
2
6110 G22
6
50
5
4
TA = –55°C
3
TA = –40°C
TA = 0°C
2
1
TA = 125°C
0
–1
0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0
OUTPUT VOLTAGE (V)
TA = 25°C
0
5
TA = 85°C
10 15 20 25 30
OUTPUT VOLTAGE (V)
35
40
TA = 85°C
TA = 125°C
30
TA = –40°C
20
0
TA = 25°C
TA = –55°C
0
5
10 15 20 25 30 35 40 45 50
SUPPLY VOLTAGE (V)
6110 G23
40
6110 G21
Supply Current vs +IN Current
10
I+IN = 0µA
10
–2
–3
0.001
40
45
SUPPLY VOLTAGE (V)
V+ = 5V
VOUT = 1.2V
50
TA = –40°C
1
35
V+ = 36V
6 VIMON = 0V
I+IN = 100µA
Supply Current vs Supply Voltage
SUPPLY CURRENT (µA)
IMON CURRENT ERROR (%)
2
TA = 85°C, 125°C
7
TA = 85°C
60
V+ = 5V
3
TA = 25°C
1
6110 G20
IMON Current Error vs +IN Current
4 VOUT = 1.2V
TA = 0°C
IMON Current Error vs Output
Voltage
5
6110 G19
5
TA = –40°C
2
6110 G18
IMON CURRENT ERROR (%)
I+IN = 1mA
TA = –55°C
3
–1
40
V+ = 5V
I+IN = 100µA
6
IMON CURRENT ERROR (%)
IOUT TO IMON VOLTAGE (V)
7
0.6
0.2
4
IMON Current Error vs Output
Voltage
V+ = 5V
VIMON = 0V
∆IOUT ERROR < 1%
0.4
5
6110 G17
Minimum IOUT to IMON Voltage
vs Temperature
0.8
VIOUT = 25V
I+IN = 100µA
6
0
6110 G16
1.0
7
VIOUT = 0.4V
I+IN = 100µA
6
IMON CURRENT ERROR (%)
NUMBER OF UNITS
V+ = 5V
VIOUT = 1.2V
250 VIMON = 0V
I+IN = 100µA
SUPPLY CURRENT (mA)
300
IMON Current Error vs Supply
Voltage
IMON CURRENT ERROR (%)
IMON Current Error Distribution
1.0
0.1 TA = 85°C
TA = 125°C
TA = –40°C, –55°C
0.01
0.001
TA = 25°C
0.1
0.01
+IN CURRENT (mA)
1
2
6110 G24
6110fa
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LT6110
Typical Performance Characteristics
Output Short-Circuit Current vs
Temperature
50
OUTPUT SHORT-CIRCUIT CURRENT (mA)
180
160
120
TA = 125°C
100
80
TA = –40°C
60
40
TA = 85°C
20
0
TA = –55°C
0
5
TA = 25°C
40
V+ = 36V
30
20
V+ = 5V
10
TA = 25°C
TA = 0°C
0
–200
0
0.5
1
1.5 2 2.5 3 3.5
INPUT VOLTAGE (V)
4
4.5
–5
I+IN = 1mA
I+IN = 100µA
–10
–15
–20
TA = –40°C T = –55°C
A
–100
I+IN = 10µA
0
TA = 85°C
100
PSRR vs Frequency
100
V+ = 5V
5 VIOUT = 1.2V
VIMON = 0V
VIOUT = 0.4V
VIMON = 0V
I+IN = 100µA
GAIN (dB)
INPUT OFFSET VOLTAGE (µV)
200
5
6110 G28
I+IN = 10µA, RIN = RIOUT = 10k
–25 I+IN = 100µA, RIN = RIOUT = 1k
I+IN = 1mA, RIN = RIOUT = 100Ω
–30
100
10
1k
10k
100k
FREQUENCY (Hz)
1M
V+ = 5V
I+IN = 100µA
RIN = RIOUT = 1k
90
80
70
60
50
40
30
20
10
0
10
1k
10k
FREQUENCY (Hz)
100k
0µA to 1mA
IOUT Current Step Response
VSENSE
VSENSE
VSENSE
50mV/DIV
50mV/DIV
50mV/DIV
VIOUT
VIOUT
VIOUT
20µs/DIV
V+ = 5V
R+IN = 1k
R–IN = 0Ω
RIOUT = 1k TO 1.2V
1M
6110 G30
0µA to 100µA
IOUT Current Step Response
6110 G31
100
6110 G29
0µA to 10µA
IOUT Current Step Response
20µs/DIV
V+ = 5V
R+IN = 10k
R–IN = 0Ω
RIOUT = 10k TO 1.2V
5 25 45 65 85 105 125
TEMPERATURE (°C)
6110 G27
Frequency Response
10
V+ = 5V
TA = 125°C
15
6110 G26
Minimum Input Voltage
300
20
5
–55 –35 –15
5 25 45 65 85 105 125
TEMPERATURE (°C)
6110 G25
400
25
10
0
–55 –35 –15
10 15 20 25 30 35 40 45 50
SUPPLY VOLTAGE (V)
RSENSE vs Temperature
30
POWER SUPPLY REJECTION RATIO (dB)
INPUT BIAS CURRENT (nA)
140
35
VIOUT = V+
VIMON = 0V
SHORT DURATION = 1ms
RSENSE (mΩ)
Input Bias Current vs Supply
Voltage
6110 G32
20µs/DIV
V+ = 5V
R+IN = 100Ω
R–IN = 0Ω
RIOUT = 100Ω TO 1.2V
6110 G33
6110fa
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7
LT6110
Typical Performance Characteristics
0µA to 30µA
IMON Current Step Response
0µA to 300µA
IMON Current Step Response
0µA to 3mA
IMON Current Step Response
VSENSE
VSENSE
VSENSE
50mV/DIV
50mV/DIV
50mV/DIV
VIOUT
VIOUT
VIOUT
6110 G34
R+IN = 10k
R–IN = 0Ω
RIMON = 3.4k TO GND
VSENSE = 5mV Step Response
VSENSE = 50mV Step Response
VSENSE
20mV/DIV
VIOUT
50mV/DIV
100µs/DIV
V+ = 5V
R+IN = 49.9Ω
R–IN = 0Ω
RIOUT = 1k TO 1.2V
6110 G36
VSENSE = 500mV Step Response
VIOUT
50mV/DIV
VIOUT
50mV/DIV
20µs/DIV
V+ = 5V
R+IN = 499Ω
R–IN = 0Ω
RIOUT = 1k TO 1.2V
20µs/DIV
R+IN = 100Ω
R–IN = 0Ω
RIMON = 34Ω TO GND
VSENSE
200mV/DIV
6110 G37
6110 G38
20µs/DIV
V+ = 5V
R+IN = 4.99k
R–IN = 0Ω
RIOUT = 1k TO 1.2V
6110 G39
VSENSE = 1V Step Response
Balanced Inputs
VSENSE
500mV/DIV
VSENSE
500mV/DIV
VIOUT
50mV/DIV
VIOUT
50mV/DIV
20µs/DIV
V+ = 5V
R+IN = 10k
R–IN = 0Ω
RIOUT = 1k TO 1.2V
V+ = 5V
VSENSE
20mV/DIV
VSENSE = 1V Step Response
Unbalanced Inputs
8
6110 G35
20µs/DIV
V+ = 5V
R+IN = 1k
R–IN = 0Ω
RIMON = 340Ω TO GND
20µs/DIV
V+ = 5V
6110 G40
20µs/DIV
V+ = 5V
R+IN = 10k
R–IN = 10k
RIOUT = 1k TO 1.2V
6110 G41
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LT6110
Pin Functions
(TSOT-23/DFN)
NC (Pin 1/Pin 8): Not Internally Connected.
IOUT (Pin 2/Pin 7): Sinking Current Output. IOUT will sink
a current that is equal to VSENSE/RIN. VSENSE is the voltage
developed across the sense resisor.
IMON (Pin 3/Pin 6): Sourcing Current Output. IMON will
source a current that is equal to 3 • VSENSE/RIN.
V– (Pin 4/Pin 5): Negative Power Supply. Normally connected to ground.
–IN (Pin 5/Pin 4): Negative Input to the Internal Sense
Amplifier. Must be tied to system load side of the sense
resistor, either directly or through a resistor.
V+ (Pin 7/Pin 2): Positive Power Supply. Connect to the
more positive side of the sense resistor. A minimum capacitance of 0.1µF is required from V+ to V–.
+IN (Pin 8/Pin 1): Positive Input to the Internal Sense
Amplifier. The internal sense amplifier will drive +IN to the
same potential as –IN. A resistor, R+IN, tied from V+ to +IN
sets the IOUT and IMON output currents as defined in the
the IOUT and IMON pin functions description.
Exposed Pad (Pin 9, DFN Only): V–. Must be soldered
to the PCB.
RS (Pin 6/Pin 3): Internal Sense Resistor. Connect to the
load to use. Leave open when using an external sense
resistor.
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9
LT6110
Block Diagram
VIN
IN
OUT
+
VREG
RF
REGULATOR
I+IN
ADJ
GND
VSENSE
ILOAD
RWIRE
VLOAD+
0.1µF
RIN
+IN
–
V+
RS
–IN
RSENSE
0.020Ω
1k
NC
+ –
IOUT
RG
IMON
V–
6110 F01
RWIRE
VLOAD–
Figure 1. Block Diagram and Typical Connection
10
6110fa
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LT6110
Applications Information
INTRODUCTION
The LT6110 provides a simple and effective solution to
a common problem in power distribution. When a load
draws current through a long or thin wire, wire resistance
causes an IR drop that reduces the voltage delivered to
the load. A regulator IC cannot detect this drop without a
Kelvin sense at the load, which requires a multi-conductor
wire that is not supported in some applications.
The LT6110 detects the load current and sets a proportional current at an output that can be used to control the
output voltage of an adjustable regulator to compensate
for the drop in the wire.
The accuracy and wide output current range of the LT6110
allow it to compensate for either small or large voltage drops
to a high degree of precision. The LT6110 can sense the
load current with its internal sense resistor or an external
sense resistor can be used to improve accuracy and handle
currents greater than 3A. Resistor-programmable gain
gives substantial flexibility to the compensation circuit. A
signal bandwidth of 180kHz enables fast response time
to load changes and provides good loop characteristics
so that the power supply circuit remains stable.
The LT6110 requires that the resistance of the wire be
known. However, that resistance does not have to be very
accurate for the LT6110 to provide good compensation
since the regulation at the load is the product of the error
due to the wire resistance and the error in the LT6110
compensation circuit.
For example, a 5V regulator circuit has 10% regulation at
the load due to a wire resistance drop of 0.5V. Even if the
wire resistance doubled, causing an error in the LT6110
compensation circuit of 50%, the regulation at the load
is still reduced to 10% • 50% = 5%.
For systems that are better controlled, the load regulation can be improved to far exceed that possible without
the LT6110. As an example, for a known wire resistance,
and with an external 1% sense resistor, the same 10%
load regulation in the previous example can be reduced
to less than 0.5%.
The LT6110 has two output pins, IOUT and IMON. Either
pin may be used to provide a current that is proportional to
the load current. The IOUT pin provides a sinking current
to compensate regulators with a ground referred voltagereference, such as the LT3980. The IMON pin provides a
sourcing current to compensate regulators with an output
referred reference like the LT1083 and current-referenced
regulators like the LT3080. As an added feature, the output
current from either pin can be converted to a voltage via a
simple resistor, creating a voltage that is also proportional
to load current. This voltage may be used to measure
or monitor the load current. Either or both pins may be
used for regulator control, and either or both pins may
be used for monitoring, allowing substantial flexibility in
system design.
THEORY OF OPERATION
The outputs of the LT6110 are proportional to a sense
voltage, VSENSE, developed across an internal or external
sense resistor, RSENSE (see Figure 1).
A sense amplifier loop forces +IN to the same voltage as
–IN. Connecting an external resistor, RIN, between V+ and
+IN forces a voltage across RIN equal to VSENSE, creating
a current into +IN, I+IN , equal to VSENSE/RIN. This current
is precisely mirrored to IOUT. The emitter currents of
the three transistors in the mirror are combined to form
the IMON output current. Ideally, the IOUT sink current
is equal to I+IN and the IMON source current is equal to
three times I+IN.
V+ and V–
The LT6110 is designed to operate with a supply voltage
(V+ to V–) up to 50V. However, when using a supply voltage greater than 36V, additional care must be taken not
to exceed the absolute maximum ratings. The V+ to IOUT
voltage must be kept less than 36V to avoid the breakdown
of internal transistors.
The V+ pin needs to be bypassed with at least a 0.1µF
capacitor placed close to the pin.
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11
LT6110
Applications Information
+IN and –IN
Design Procedure
The +IN and –IN inputs can have a maximum differential
voltage equal to the supply voltage. This protects the
LT6110 if the –IN pin (the remote load side) is accidentally
shorted to ground. In this case, the IOUT current must be
limited to less than 2mA (see the Limiting the Regulator
Boost Voltage section).
The design of an LT6110 compensation circuit is a simple
3-step process. To start, the following parameters must
be known:
RWIRE, total wire resistance to the load
RSENSE, resistor used to sense the load current
RF, feedback resistor of the regulator
The +IN to IOUT voltage must be kept below 36V to avoid
the breakdown of internal transistors.
ILOADMAX, maximum load current
The circuit in Figure 2 shows an adjustable voltage regulator with an LT6110 compensation circuit. The regulator
has an internal ground referred voltage reference to set
its output voltage. There are two wires to the load, one
source (RSWIRE) and one return (RRWIRE). Since it is the
most common configuration it will be used for the following design example. Current referenced regulators and
regulators with an output referred reference are covered
in later sections.
IOUT and IMON
The IOUT to IMON outputs can have a maximum differential voltage of 36V for IOUT above IMON and –0.6V for
IOUT below IMON. A 36V Zener diode can be connected
from IOUT to IMON to prevent damage to the output NPN
transistor in the event of a fault condition. In this case, a
low leakage Zener diode should be used to reduce error
in the IOUT current.
Step 1: Determine the drop in voltage at the load due to
the wire resistance and sense resistor at the maximum
load current.
RS (Internal RSENSE)
The internal sense resistor can reliably carry a continuous
current up to 3A and transient currents of 5A for up to 0.1
seconds. For currents greater than this, an external sense
resistor should be used. The internal sense resistor has a
temperature coefficient similar to copper.
VDROP = (RSWIRE + RRWIRE + RSENSE) • ILOADMAX
VDROP = (0.125Ω + 0.125Ω + 0.02Ω) • 2A = 0.54V
VDROP
ILOAD
VREG
VIN
REGULATOR
RF
3.65k
I+IN
VSENSE
RIN
FB
+
IOUT
IMON
LT6110
ILOADMAX = 2A
–
+IN V+
RSENSE
20mΩ
RG
RSWIRE
0.125Ω
RS
–IN
+
LOAD
CIRCUIT
OR
BATTERY
+ –
V–
VLOAD
–
RRWIRE
0.125Ω
VDROP
6110 F02
Figure 2. 2-Wire Compensation, One Wire Is Connected to the Load and One Wire Is the Ground Return Wire
12
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LT6110
Applications Information
Step 2: Determine the resistor on the +IN pin, RIN, required
to cancel VDROP.
The regulator output voltage will increase as current is
pulled from the IOUT pin through the feedback resistor,
RF, creating a compensation voltage.
VCOMP = IIOUT • RF
To cancel the voltage drop at the load, set VCOMP equal
to VDROP.
VCOMP = IIOUT • RF = VDROP
Since the IOUT current is equal to the current going into
the +IN pin and the current in the +IN pin is equal to the
sense voltage divided by RIN, RIN can be determined by
the following equations:
IIOUT = I+IN =
VSENSE
RIN
Combining the above equations,
RIN = (2A • 0.02Ω) •
In most cases, the internal sense resistor, wire resistance
tolerances and temperature mismatch of the RSENSE and
RWIRE resistances will contribute the largest portion of the
overall compensation circuit error. See the sections on
Error Sources, Copper Wire Information and Temperature
Errors for a comprehensive discussion.
Additional Design Considerations
IOUT Current
The recommended range of IOUT current is 30µA ≤ IIOUT
≤ 300µA for the best precision. For performance outside
of this range, see the Typical Performance Curves to
determine typical errors.
If the IOUT current is less than 30µA, the feedback resistor may need to be adjusted to reduce the error in the
compensation circuit.
where VSENSE = ILOADMAX • RSENSE
RIN = (ILOADMAX • RSENSE ) •
external sense resistor with a tighter tolerance. See the
section on External Current Sense Resistors for more
information.
RF
In the previous example,
VDROP
IIOUT =
3.65k
= 270Ω
0.54V
Step 3: The final step is to consider the errors in the
compensation circuit to determine if the resulting voltage
error at the load meets the desired performance.
For example, the internal RSENSE of the LT6110 has a typical
tolerance of ±7.5%. If the other errors in the compensation
circuit such as VOS, IOUT current error and the resistor
tolerances of RF and RIN add an additional ±2.5% error,
then the total error in the compensation circuit would be
±10% resulting in a voltage error at the load of the following:
VLOADERROR = VCOMP • Compensation Error
VLOADERROR = 0.54V • (±10%) = ±0.054V
VSENSE 0.04
=
= 148µA
RIN
270
Since this is within the recommended range no further
adjustment is needed.
See the section on Compensating a Low Quiescent Current
Design for IOUT current less than 30µA.
Load Regulation
Load regulation is often specified as an error in output
voltage at a given load current, as in the previous example,
but it is also specified as a percentage of the regulator
output voltage. If the output voltage of the regulator circuit
in Figure 2 is 5V, the resulting compensated load regulation, in percent, would be the following:
A 10× improvement.
LoadRegCOMP (%) =
If this is not adequate for the given application, steps can
be taken to reduce the sources of error, such as using an
VLOADERROR
• 100
VREG
LoadRegCOMP (%) =
±0.054V
• 100 = ±1.1%
5V
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13
LT6110
Applications Information
Without the compensation circuit (no RSENSE) the load
regulation in percent would be,
Kelvin Sense Connection to RSENSE
To reduce RSENSE error due to trace resistance, the –IN pin
and RIN resistor should be connected as close to RSENSE
as possible, as reflected in Figure 2.
–0.5V
LoadRegUNCOMP (%) =
• 100 = –10%
5V
The regulator’s output will also change due to its own
load regulation effects (per the regulator’s specification).
In general, this change in voltage is small compared to
the wire-drop, and can be ignored. If it is considered to
be a significant source of error, it can be included as part
of the wire-drop compensation. To include the regulator’s
load regulation effect, simply add the voltage drop due to
the regulator’s load regulation at ILOADMAX to VDROP, when
calculating the compensation circuit parameters.
Compensating a Low Quiescent Current Design
Switching regulator circuits are used for high power efficiency. Many are required to maintain high efficiency at
light or no load conditions. In these cases the quiescent
operating current is minimized by using larger valued
resistors to program the output voltage so very little current is wasted in the feedback network.
A large value for resistor RF could require too low of a
compensating current (<30µA) from IOUT of the LT6110.
In this situation the feedback resistor, RF, can be split
into two resistor values. A small value resistor to conduct
IIOUT from the LT6110 and compensate the output voltage
when the load current is high, and a second, larger valued
resistor, to keep the no-load quiescent current drain low.
With this arrangement, as shown in Figure 3, IIOUT can
be designed for 100µA to preserve VDROP compensation
accuracy. At no load the quiescent current drawn through
the feedback resistors, IQ, can be kept very low.
PCB Trace Resistance
Printed circuit trace resistance between the output of the
regulator and the load will cause additional voltage drops.
As with the regulator’s load regulation effects, these drops
can be compensated for by adding them to VDROP when
calculating the compensation circuit parameters. This also
allows the use of narrower traces to deliver power to the
load and still retain good load regulation. See the PCB
Copper Resistor section for more information on how to
determine trace resistance.
ILOAD
VREG
VIN
RFA
REGULATOR
FB
IQ
<30µA
I+IN
RWIRE
VLOAD
VSENSE
RIN
+
–
+
+IN V
RFB
RS
–IN
LOAD
20mΩ
RG
IOUT
IMON
LT6110
+ –
V–
6110 F03
Figure 3. Low Quiescent Current Wire Compensation Using Three Regulator Resistors
14
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LT6110
Applications Information
In Figure 3 RF is split into RFA and RFB. VREG is the no-load
quiescent output voltage of the regulator. The design of
these two feedback resistors follows:
RFA =
VDROP
IIOUT
IIOUT can be sized to be 100µA at full load current and
only this resistor creates the VDROP compensation voltage.
RFB =
VREG – VFB
– RFA
IQ
IQ is the no-load quiescent current flowing through the
resistor string.
Figure 4 is a circuit using the LT6110 and a three resistor
voltage setting technique to compensate the voltage loss
due to a 2A load connected through 6 feet of stranded
copper wire (300mΩ of wire resistance). The LT3980 is a
2A buck switching regulator programmed for 5V out with
only 10µA of current, IQ, through the feedback resistor
string when there is no load current. At the full 2A load the
LT6110 uses the internal 20mΩ sense resistor to produce
100µA at IOUT to compensate for the 640mV drop.
VIN
VIN
Compensating a Current Referenced
Regulator Power Source
Figure 5 shows a cable drop compensation circuit using a
current referenced regulator, the LT3080. A precision 10µA
set current, ISET, is sourced through two series connected
resistors to program the output voltage for the remote
load. To compensate for the load connecting cable drop
requires sourcing an additional current into this resistor
pair to increase the output voltage. The LT6110 provides
a sourced current at the IMON pin which is directly proportional to the current flowing to the load. This current
is three times the normal IOUT current. The following
equations are used to design this circuit:
VREG = ISET • (RSET1 + RSET2)
VSENSE = ILOAD • RSENSE
I+IN =
VSENSE
RIN
IIMON = 3 • I+IN
BD
RUN/SS BOOST
LT3980
100k
SW
RT
DA
0.47µF 10µH
DFLS240L
97.6k
15k
PGOOD
1.5nF
VC
100pF
GND
FB
VREG
47µF
6.49k
NC
10pF
VFB = 0.79V
422k
80.6k
+IN
LT6110
IOUT
V+
20mΩ
IMON
RS
V–
–IN
402Ω
0.1µF
RWIRE
0.3Ω
100µF
5V
2A
6110 F04
Figure 4. LT3980 Buck Regulator with LT6110 Cable Drop Compensation Circuit
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15
LT6110
Applications Information
RWIRE
0.5Ω
RIN
200Ω
LT3080
IN
VIN
VLOAD
3V
1A
+IN V+
ISET
RS
–IN
LOAD
20mΩ
+
–
VREG
LT6110
IOUT
+ –
SET
RSET1
301k
IMON
RSET2
1.69k
IMON
V–
6110 F05
IMON = 3 IIN+
Figure 5. Wire Loss Compensation Using a Current Referenced LDO
To compensate for VDROP at ILOAD(MAX) set:
RSET2 =
VDROP
IIMON
and
RSET1 =
VREG
– RSET2
ISET
As an example, to compensate this 3V regulator for a
500mV cable drop with a 1A load current set I+IN for 100µA
for best accuracy. Then:
RSET1 = 301k and RSET2 = 1.69k using nearest 1%
tolerance standard resistor values.
RIN =
1A • 20mΩ
= 200Ω
100µA
The following equations are used to design this circuit
using an LT1083, 7A adjustable voltage regulator:
VREF = 1.25V between OUT and ADJ pins, IADJ = 75µA typ
V
ISET = REF  IADJ
R1

VLOAD (ILOAD = 0) = (ISET + IADJ) • (R2 + RG) + VREF
VSENSE = ILOAD • RSENSE
Compensating an Output Referred
Adjustable Voltage Regulator
I+IN =
Many adjustable voltage regulators are biased from a
floating voltage reference that sets a voltage between the
output pin and an adjust pin. Three terminal fixed voltage
regulators can also be made adjustable by biasing up
the ground terminal. A feedback resistor string is used
to program the output voltage. The amount of current
through these resistors is scaled to a level to minimize
error caused by any bias current at the adjust pin.
16
As shown in Figure 6, an LT6110 can add cable drop
compensation by using the current sourced from the IMON
pin. To preserve accuracy the voltage at IMON should be
kept within 5V of V–, or ground in this example. By using
two resistors for the bottom resistor in the voltage regulator programming string, the cable drop compensation
voltage can be added to a voltage near ground appearing
at the IMON pin.
VSENSE
RIN
IIMON = 3 • I+IN
As an example, Figure 6 is a 12V regulator for a 5A remotely
connected load with a wire resistance of 250mΩ. For the
higher load current an external 25mΩ sense resistor is
used. The cable drop voltage for such a high current application is significant:
VDROP = ILOAD(MAX) • (RSENSE + RWIRE) = 5A • 275mΩ
= 1.375V
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LT6110
Applications Information
RWIRE
0.25Ω
RSENSE
0.025Ω
RIN
+IN V+
RS
–IN
R1=
20mΩ
VIN
10µF
IN
LT1083
ADJ
OUT
IADJ
VREG
R1
499Ω
IOUT
10µF
RG
1k
+ –
R2 =
RG =
1.62k
ISET
R2
3.16k
LT6110
IMON
VLOAD
12V
5A
1.25V
ISET
R1• (VLOAD – VREF )
IADJ • R1+ VREF
– RG
ILOAD • (RSENSE +R WIRE)
3 •I+IN
VIMON(MAX) = RG • (ISET +IADJ + 3 •I+IN)
V–
6110 F06
IIMON
IIMON = 3 I+IN
Figure 6. Wire Compensation Using a High Current Adjustable Regulator
To program the regulator output voltage and compensate
for VDROP at ILOAD(MAX) the following procedure can be
used:
Make ISET >> IADJ, if ISET = 33.3 • IADJ then ISET = 2.5mA
R1=
VREF 1.25V
=
= 499Ω
ISET 2.5mA
VLOAD – VREF
10.75V
=
= 4.175k
ISET +IADJ
2.575mA
Resistor RG is used to develop the maximum load current
compensation voltage. A smaller value for RG minimizes
the voltage programming error at no load but requires
more current from the LT6110 IMON pin to compensate
for cable drop loss. The IMON pin current is most accurate
over a range from 30µA to 3mA.
RG =
VDROP
IIMON
For 1.375V of compensation, using a convenient value 1k
resistor for RG will require 1.375mA from the IMON pin
which is near the mid range of accurate current levels.
With this selection for RG then:
R2 = 4.175k – 1k = 3.175k
To program the LT6110 compensation current requires
a selection for RIN:
VSENSE VSENSE
=
IIMON
I+IN
3
VSENSE = 5A • 25mΩ = 125mV and
RIN =
For 12V output with no-load current:
(R2+RG ) =
use a 3.16k standard 1% tolerance value to set the no-load
output voltage to 12V.
IIMON 1.375mA
=
= 460µA so
3
3
125mV
RIN =
= 271Ω
460µA
use a 274Ω standard value.
The IOUT pin can be connected to the 12V regulator output. The LT1083 requires a minimum output load current
of 10mA so an additional 1.62k resistor (not required if
ILOAD is always greater than 10mA) is added to the output.
The voltage that appears at the IMON pin can impact the
accuracy of the compensation circuit and should be noted.
In this example the voltage will be a maximum at full load
current and voltage compensation. This voltage is:
VIMON(MAX) = (ISET + IADJ + IIMON) • RG = (2.5mA + 75µA
+ 1.375mA) • 1k = 3.95V.
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LT6110
Applications Information
ERROR SOURCES
this will create a VDROP of 760mV. Without the LT6110
compensator the regulation of the 5V supply at the load
would be 15%.
The LT6110 output current allows for reliable compensation for small or large connection wiring voltage drops.
The voltage regulation at the remote load can be improved
dramatically using the LT6110. With properly designed
cable drop compensation the load voltage variation will
be reduced to only the error in the compensation voltage
created. This error voltage is a combination of several
circuit characteristics.
This example design will use the internal 20mΩ sense
resistor of the LT6110 and will assume that the feedback
resistor network in the voltage regulator cannot be modified
or optimized for compensation. The RF used to develop the
compensation voltage is fixed at 10k and the reference voltage at the feedback node where the compensator connects
is 0.8V. From these parameters the basic compensation
circuit can be easily designed:
The first step in determining the error is to determine the
amount of compensation voltage required. Figure 7 is an
example circuit that indicates the various error terms to be
considered. For this example a 5V regulator will provide
2A maximum to a remote load connected through 6 feet
(~2 meters) of 28AWG (7/36) stranded hook-up wire. Using 28AWG provides the thinnest, low cost wire suitable
for this application. Using wire resistance Table 4, the DC
resistance of 6 ft of 28AWG (7/36) can be determined:
RWIRE = 6ft • 63.3mΩ/ft = 380mΩ. At 2A full load current
VSENSE at full load is 20mΩ • 2A or 40mV
The compensation voltage, VCOMP, required is:
VWIRE + VSENSE, 760mV + 40mV, or 800mV
To create this compensation voltage will require a current
through feedback resistor RF of VCOMP/RF, 800mV/10k
for an IIOUT of 80µA. This is well within the most accurate
range of current (30µA to 300µA) flowing into the IOUT pin.
+ VDROP –
VIN
IN
OUT
REGULATOR
VREG
RF
10k
VFB
+ VSENSE –
+
VCOMP
–
GND
RWIRE
6 FT, AWG 28
7/36 STRANDED
WIRE
≥0.1µF
I+IN
0.8V
RIN
LT6110
+IN
±VOS*
+
–
V+
RS
RSENSE
0.020Ω
–IN
VLOAD
LOAD
1k
NC
I+IN
VIOUT
+ VWIRE –
ILOAD
+ –
IOUT
BIAS
IIOUT
±IOUT
ERROR
IMON
IIMON
±IMON
ERROR
*± VOS = VOS +
V–
6110 F07
VIMON
ΔVOS ΔVOS
ΔVOS
+
+
+ TC VOS • ΔT
ΔI+IN ΔVIOUT ΔVIMON
Figure 7. Cable Drop Compensation Error Sources
18
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LT6110
Applications Information
To create this current at full load requires an RIN value of
VSENSE/IIOUT, 40mV/80µA, or 500Ω. Using the nearest
standard 1% tolerance value of 499Ω will be sufficient.
Without considering any error terms other than this slight
change in value for RIN results in nearly perfect cable drop
compensation. The theoretical load regulation would be
improved from 15% to less than 0.01%.
• ∆VOS/∆VIOUT is a change in the offset voltage caused
by a change in the voltage applied to the IOUT pin
specified in mV/V. The change in VIOUT is relative to
1.2V DC where the LT6110 is trimmed for accuracy.
The single largest source of compensation error comes
from any change in the connecting wire resistance from the
design assumptions. This could be caused by temperature,
aging and possibly corrosion. In the compensator circuit,
component tolerances and errors terms will combine to
deviate from the near perfect designed amount of compensation. Figure 7 shows this simple example design and
indicates the various error sources within the LT6110. All
of the error terms can be determined from the Electrical
Characteristics Table. The error terms for any compensator design include:
• IOUT current error is the accuracy of the internal current
mirror. This is a percent deviation from I+IN.
RSENSE tolerance
RIN tolerance
RF tolerance
VOS, the offset voltage in µV of the internal current
sense amplifier
• ∆VOS/∆I+IN is an error term caused by the finite gain
of the current sense amplifier.
•
•
•
•
This is the change in the offset voltage as the sense
voltage and resulting input current varies from 0 to the
maximum value. It is a factor specified in mV/mA which
is ohms and is accounted for as a small resistance in
series with RIN. The voltage across this small resistance
is included in the total offset voltage term. The change
in I+IN current is relative to 100µA where the LT6110
is trimmed for accuracy.
• ∆VOS/∆VIMON is a change in the offset voltage caused
by a change in the voltage applied to the IMON pin
specified in mV/V.
• IMON current error is the accuracy of the total internal
mirror current sourced to the IMON output. This is a
percent deviation from 3 • I+IN.
• Temperature Related Errors (see Temperature Errors
section)
Table 1 is an example of the stack-up of all error terms in
the design of Figure 7. This table uses typical variances to
be seen at 25°C. It is not a rigorous worst case analysis
over all possible operating conditions, but instead serves
to illustrate what to expect for load regulation improvement
under nominal conditions.
In this example, including all typical error terms, the LT6110
still provides a factor of 10 improvement in voltage regulation at the remote load. To obtain the same level of load
voltage stability without using the LT6110 would require
reducing the amount of cable drop loss. The easiest way
to do so would be to increase the wire gauge used to
connect to the load. For a 76mV change in load voltage
at 2A full load current would require a wire resistance
of only 38mΩ and a 6 foot length 18AWG gauge wire is
required. A larger wire gauge can be significantly more
costly and is less flexible in routing to the load. These are
two significant design compromises to be considered.
6110fa
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19
LT6110
Applications Information
Table 1. Compensation Error Using Typical Variances Expected at 25°C.
FIGURE 7 DESIGN EXAMPLE. TOTAL VDROP TO COMPENSATE = 744mV,
I+IN = 74.6µA
TERM
DESIGN VALUE/SPEC
UNITS
RSENSE
20
mΩ
RIN
499
VOS
0
∆VOS/∆I+IN
0
COMMENT/CALCULATION
FOR MAXIMUM VCOMP
FOR MINIMUM VCOMP
TYPICAL ERROR
VALUE
TYPICAL ERROR
VALUE
7.50%
21.5
–7.50%
18.5
Ω
–0.5%
496.5
0.5%
501.5
µV
–100
–100
100
100
–0.15
–0.15
0.15
0.15
Internal Sense Resistor
mV/mA Relative to I+IN = 100µA
∆VOS/∆VIOUT
0
mV/V
Relative to VIOUT = 1.2V
–0.005
–0.005
0.005
0.005
∆VOS/∆VIMON
0
mV/V
Relative to VIMON = 0V
–0.3
–0.3
0.3
0.3
Total VOS
VOS + ∆VOS /∆I+IN (100µA – 80µA) + ∆VOS /∆VIOUT (1.2V – 0.8V) + ∆VOS /∆VIMON •0V
µV
–105
105
IIOUT Error
0
%
% IOUT Current Error Relative to I+IN
0.5
0.5
–0.5
–0.5
IIMON Error
0
%
% IMON Current Error Relative to 3 • I+IN
1.5
1.5
–1.5
–1.5
Summary of Terms
40
mV
ILOAD(MAX) • RSENSE
43
37
I+IN
80.2
µA
(VSENSE – Total VOS)/RIN
86.8
73.6
IIOUT
80.2
µA
I+IN • (1 + IIOUT Error)
87.2
73.2
IIMON
VSENSE
240.6
µA
3 • I+IN • (1 + IIMON Error)
RF
10
kΩ
Fixed Resistor Value in Power Source
VCOMP
802
mV
IIOUT • RF
0
%
2
mV
0.03
%
VCOMP Error
264.4
0.5%
10.05
219.6
–0.5%
9.95
876
728
9.2
–9.2
With Compensation
VLOAD_ERROR
Load Regulation
VCOMP – VDROP
FREQUENCY RESPONSE AND TRANSIENTS
The LT6110 has a –3dB bandwidth of 180kHz. This
smooth frequency response is shown in Figure 8. This
defines the response time from the sensed input voltage
to the compensation output currents. Power sources will
typically have a large output capacitance making their
loop response bandwidth much slower than the LT6110.
The cable drop compensation loop is much faster than
the power source so there should be little impact on loop
stability in driving a remote load.
For fast or step change variations in load current some
transients will be observed at the power source output
and at the remote load due to the finite reaction time of
the compensation loop. The amount of voltage transient
seen will depend mostly on the size and quality of the
supply bypass capacitors used at each end of the load
connecting wire. An example of these transients is shown
20
76
–72
1.52
–1.44
in Figure 9. Any ringing while settling out can be smoothed
by additional filtering components in the control loop. A
small feedback capacitor across the regulator feedback
resistor, RF, can provide effective smoothing of transients.
Specific values to use depend on the particular application
component values.
One important consideration for transients is a sudden
open or removal of the load current from a high current
condition. There is a risk of overvoltage at the load before
the LT6110 can reduce the compensation voltage. A good
solution to this potential issue is to bypass the remote
load with a capacitance greater than the capacitance at the
output of the regulator or power source. Figure 10 shows
a load removal transient using a 100µF load. Fortunately
the amount of compensation in most applications should
not be so large as to cause a serious overvoltage risk but
should always be considered.
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LT6110
Applications Information
0dB
R+IN
–3dB
R–IN
1k
C1
0
–60
–90
1
IIOUT = 100µA
10
100
FREQUENCY (kHz)
PHASE (DEG)
–30
+IN V+
RS
–IN
20mΩ
–120
1000
LT6110
IOUT
6110 F08
+ –
Figure 8. LT6110 Frequency Response
IMON
VREG
500mV/DIV
V–
6110 F11
Figure 11. LT6110 Frequency Compensation
VLOAD
500mV/DIV
EXTERNAL CURRENT SENSE RESISTORS
2A
1A
100µs/DIV
6110 F09
Figure 9. VLOAD Compensated
VREG
500mV/DIV
VLOAD
500mV/DIV
2A
The LT6110 internal current sense resistor, RSENSE, is
provided for convenient use in many applications with a
maximum load current less than 3A. For higher current
or greater precision wire loss compensation an external
sense resistor can be used. The external RSENSE resistor
can be a low valued current sense or shunt resistor, the
DC resistance (DCR) of an inductor, or the resistance of
a printed circuit board trace. Figure 12 shows an LT6110
circuit configuration using an external sense resistor. The
internal resistor at the RS pin is left open circuited.
0A
CLOAD = 100µF
10ms/DIV
6110 F10
EXTERNAL
RSENSE
TO VREG
Figure 10. Removing Load
TO LOAD
RIN
In addition to using a regulator capacitor to adjust the loop
response, an RC pole in the LT6110 circuit can provide
frequency compensation. Figure 11 shows an LT6110 with
an input RC filter. Using the input RC filter introduces a
second pole to the LT6110 one pole response (Figure 9).
The LT6110 poles become a zero in the regulator’s openloop response that includes the LT6110 in its feedback
path (providing the same function as the regulator’s RF
with a shunt capacitor).
Loop compensation with an LT6110 RC filter is not
required if the regulator’s loop is compensated with a
zero in the feedback divider (refer to the Regulator Loop
Stability section).
+IN V+
RS –IN
20mΩ
IOUT
IMON
LT6110
V–
+ –
6110 F12
Figure 12. Using an External RSENSE
(Resistor, Inductor or PCB Trace)
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LT6110
Applications Information
The value of the external RSENSE determines the VSENSE
voltage. If IIOUT is 100µA then a VSENSE of 50mV is large
enough to minimize the compensating IOUT current error
due to VOS to less than 1% (see Figure 13).
IOUT CURRENT ERROR (%)
100
0.4V ≤ VIOUT ≤ V+ – 1.5V
VIMON = V– = 0V
VOS(MAX)
10
1
0.1
Precision Current Shunt Resistor
A precision, very low VLOAD error, compensation circuit
can be implemented with an LT6110 and a precision external RSENSE. A ±1% to ±5% tolerance or better RSENSE
resistor significantly reduces IIOUT compensation current
error due to part to part variations. In addition, the low
temperature coefficient (TCR of typically ±100ppm/°C) of
an external sense resistor greatly reduces the contribution
of RSENSE to the total voltage drop loss at higher operating
temperatures. Figure 14 shows a 5V, 3.5A buck regulator
with an LT6110 using an external RSENSE. Table 2 is a list
of typical current sense resistors.
IIOUT = 300µA
IIOUT = 100µA
IIOUT = 30µA
0
10 20 30 40 50 60 70 80 90 100
VSENSE (mV)
6110 F13
Figure 13. VSENSE
VIN
8V TO 36V
22µF
BD
VIN
4.7µF
BOOST
RUN/SS
0.047µF
6.8µH
SW
VC
47µF
15k
LT3972
9.53k
47pF
NC
MBRA340
1nF
FB
RT
63.4k
SYNC
0.79V
100k
IMON
V
GND
0.1µF
LT6110
–
866Ω
V+
IOUT
523k
100pF
+IN
RS
–IN
EXTERNAL
RSENSE
RWIRE
25mΩ
0.25Ω
±5%
10 FT
24AWG
I = 600kHz
VLOAD
LOAD 5V
3.5A
6110 F14
Figure 14. LT6110 with an External RSENSE and LT3972 Buck Regulator
Table 2. Surface Mount RSENSE Resistors
PART NUMBER THICK FILM
VALUE RANGE
TOLERANCE
TCR
POWER
SIZE
IRC LRC-LRF-2512
2mΩ to 1Ω
1% to 5%
100ppm
2W
2512
Stackpole Electronics CSR2512
10mΩ to 1Ω
1% to 5%
200ppm
2W
2512
Vishay RCWE2512
33mΩ to 51Ω
1% to 5%
200ppm
2W
2512
Panasonic ERJM1W
Susumu PRL1632
Susumu PRL3264
22
1mΩ to 20mΩ
1% to 5%
100ppm
2W
2512
10mΩ to 100mΩ
10mΩ to 100mΩ
1% to 2%
1% to 2%
100ppm (20mΩ to 51mΩ)
100ppm (20mΩ to 51mΩ)
1W
2W
1206
2512
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LT6110
Applications Information
Copper Resistor Made from an RF Inductor
24
Table 3. Coilcraft Air Core Inductors for External RSENSE
COILCRAFT PART
NUMBER
INDUCTANCE
(nH)*
DCR NOMINAL (mΩ)
(±6% TYPICAL)
IRMS
(A)
0908SQ-27N
27
8.5
4.4
2222SQ-221
221
9.8
5
1010 US-141
146
3.1
14
20
PCB TRACE CURRENT (A)
An inductor made of copper wire will have a small DC
resistance, DCR or RCOIL, 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 operating environment and subject to an increase in temperature,
the resistance increase in RSENSE will increase both VSENSE
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
RSENSE resistors.
22
PCB Copper Resistor
In a high load current application without a high precision load regulation specification, the cost of an external
RSENSE resistor can be eliminated using the resistance of
a printed circuit board, PCB, trace as a sense resistor. The
resistance, RPCB, is a function of copper resistivity (ρ), PCB
copper thickness (T), trace width (W) and trace length (L),
RPCB = ρ (L/(T • W)). The typical manufacturing of PCB
fabrication limits the trace resistance tolerance to ±15%.
A simplified RPCB 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 copper 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).
2oz COPPER
16
14
12
10
1oz COPPER
8
6
4
2
0
0
50 100 150 200 250 300 350 400 450 500
PCB TRACE WIDTH (MILS)
6110 F15
Figure 15. PCB Trace Current vs Trace Width
(<10°C Temperature Rise)
Example: Design a 2oz copper PCB trace sense resistor to
compensate for wire voltage drop for an ILOAD(MAX) of 10A.
A VSENSE of 60mV is large enough to minimize the compensating IOUT current error due to the input offset voltage
of the LT6110.
RPCB =
*Inductance is not relevant for current sense.
18
VSENSE
ILOAD(MAX)
=
60mV
= 6mΩ
10A
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
RPCB. Figure 16 shows a serpentine layout for a 6mΩ PCB
sense resistor and the VSENSE connections to the LT6110.
The corners of the serpentine resistor count as 3/4 of a
square. In Figure 16, RPCB consists of six 3.5 square rectangular traces (two whole squares and two 3/4 squares).
The RPCB 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
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LT6110
Applications Information
5.4mΩ ±15% AT 25°C PCB RESISTOR
21 2oz COPPER SQUARES
TO
REGULATOR
ONE SQUARE
0.15 INCH × 0.15 INCH
TO
LOAD
A
3/4 CORNER SQUARES
0.15 INCH × 0.15 INCH
B
A 3.5-SQUARE COLUMN
RIN
— 3/4 SQUARE
21 SQUARES (6 COLUMNS)
— ONE SQUARE
–IN
— 3/4 SQUARE
6110 F16
V–
IMON RS
V+
IOUT
NC
+IN
— ONE SQUARE
A
B
Figure 16. LT6110 and PCB Trace Resistor Layout
traces at the top and bottom corner squares. There are five
connecting traces and their total resistance is 0.125mΩ
([0.015 inch/0.15 inch] • 0.25mΩ • 5).
Temperature Errors
In addition to the initial errors at 25°C the errors due to
a temperature variation must be included. The ambient
temperature variation of the LT6110 and the wire can
have the following cases: The LT6110 and wire are at
the same temperature, the LT6110 and wire are at much
different temperatures or the temperature of the LT6110
circuit is known and the wire temperature can only be approximated. The design procedure targets a load voltage
equal to VREG(NOM) at maximum load current and cancels
VDROP by setting IIOUT • RF = VDROP. If, over the specified
temperature range, {IIOUT • RF – VDROP} is not zero volts,
then there will be an error to the expected load voltage
at maximum load current (for example, if VLOAD = 5V at
25°C and at 75°C {IIOUT • RF – VDROP} is 5mV then the
VLOAD error is 100 • (5mV/5V) = 0.1%).
Since IIOUT = VSENSE/RIN, the temperature errors must
include the errors due to RIN, RSENSE and VOS.
24
The error sources due to temperature of an LT6110
circuit are:
The IOUT current error vs temperature coefficient is
–50ppm/°C
The VOS temperature coefficient is ±1µV/°C
The RIN and RF resistors temperature coefficient is
±100ppm/°C
The internal RSENSE resistor temperature coefficient is
+3400ppm/°C
An additional temperature error is due to RWIRE. The
copper wire temperature coefficient is +3900ppm/°C
The IOUT current, VOS, RIN and RF errors are small compared to the errors of the internal RSENSE and RWIRE. For
a 50°C temperature rise the IOUT current, VOS, RIN and RF
resistor error is 0.25%, 50µV and 0.5% respectively and
the internal RSENSE and RWIRE error is 17% and 19.5%
respectively.
Using the example of VLOAD = 5V, ILOAD = 2A, IIOUT =
71.2µA, RF = 10k, RIN = 562Ω and RWIRE = 0.336Ω the
VLOAD error due to the following three example cases is
calculated:
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LT6110
Applications Information
Case 1: LT6110 and the wire are at 75°C and the VLOAD
error is –0.36%. If the RSENSE temperature coefficient
matches the wire’s temperature coefficient of 3900ppm/°C
then the VLOAD error is reduced. Using the copper wire
resistance of an inductor as an RSENSE external the VLOAD
error is reduced to –0.025%.
Case 2: The LT6110 is at 75°C, the wire is at 25°C and the
VLOAD error is 2.3%. The 2.3% error is mostly due to the
internal RSENSE temperature coefficient. Using an external
±100ppm/°C RSENSE reduces the VLOAD error to ±0.05%.
In addition, using a thermistor across RIN to increase the
IOUT current as the temperature increases can reduce the
temperature induced VLOAD error.
Case 3: The LT6110 is at 25°C, the wire is at 75°C and the
VLOAD error is –2.6%. The error is due only to the copper
wire resistance increase vs temperature. The Case 3 error
can be reduced by designing for the maximum RWIRE at
a specified temperature. Copper wire specifications from
a reliable manufacturer are required.
The maximum current per wire is a function of the wire
temperature rise due to current, the maximum wire insulation temperature and the number of cable wires (refer to
the Copper Wire Information section).
Table 4 is a random list of AWG wire resistance versus
current based on lab measurements.
Copper Wire Information
The wire used in the power distribution of electronic systems is annealed (heated and cooled) copper wire and is
specified for its resistance per unit length, weight per unit
mass and current capacity. In the American Wire Gauge
standard, AWG is the gauge number and corresponds to
the diameter of a solid wire (as the gauge number increases
the wire diameter decreases, the wire resistance increases
and the current capacity decreases). Stranded copper
wire is an insulated bundle of packed and twisted bare
solid strands and its resistance, weight or cost depends
on the type of coating (tin, silver or nickel) and stranding
options (how the strands are grouped and twisted). The
stranded wire’s flexibility is useful for building and routing wire harness. The current capacity of copper wire is
inversely proportional to its gauge number, number of
wire conductors and operating temperature (increasing
gauge, conductors and temperature, decreases current
capacity). In addition the wire insulation temperature rating determines the maximum operating current (typical
insulation ratings range from 80°C to 200°C).
Copper wire resistance increases directly with operating
temperature. The temperature coefficient of copper α is
equal to 0.0039/°C at 20°C (a useful linear approximation from 0°C to 100°C). If RLOW is the resistance at a
TLOW temperature and RHIGH is the resistance at a THIGH
Table 4. A Random List of Wire Resistance vs Current at 20°C
AWG 18
AWG 20
AWG 22
AWG 24
AWG 26
AWG 28
AWG 30
STRANDS/GAUGE STRANDS/GAUGE STRANDS/GAUGE STRANDS/GAUGE STRANDS/GAUGE STRANDS/GAUGE STRANDS/GAUGE
16/30
7/28
7/30
19/36
19/38
7/36
7/38
Current
(AMPS)
RWIRE
(mΩ/ft)
RWIRE
(mΩ/ft)
RWIRE
(mΩ/ft)
RWIRE
(mΩ/ft)
RWIRE
(mΩ/ft)
RWIRE
(mΩ/ft)
RWIRE
(mΩ/ft)
1
6.53
9.61
15.42
22.47
37.97
62.31
102.36
2
6.54
9.63
15.51
22.66
38.41
63.32
109.14
3
6.56
9.68
15.66
22.99
39.08
65.23
4
6.59
9.73
15.84
23.38
40.21
5
6.62
9.82
15.99
23.78
6
6.65
9.90
16.32
7
6.71
10.02
8
6.79
10.15
9
6.83
10
6.91
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25
LT6110
Applications Information
temperature then the wire’s resistance vs temperature is:
RHIGH = RLOW • (1 + α • (THIGH – TLOW)).
An approximation to the temperature rise in a wire due
to current can be derived from the wire’s resistance vs
temperature equation using the wire’s resistance increase
vs safe operating current. If RLOW is the wire resistance at
a low current and RHIGH is the wire resistance at a higher
current and TRISE is equal to THIGH – TLOW then the temperature rise in a wire is:
TRISE (°C) = 256.4 • (RHIGH/RLOW – 1).
Table 4 is a list of measured copper wire resistance versus current at 20°C for an arbitrary group of 18AWG to
30AWG wires.
Example: Find the wire temperature rise for 3A flowing in
a 28AWG wire. The 28AWG wire on Table 4 has 62.31mΩ/
ft RLOW resistance at 1A and 65.23mΩ/ft RHIGH resistance
at 3A.
TRISE for 3A is equal to 256.4 • (65.23/62.31 – 1) = 12°C.
Example: Find the weight of one hundred thousand feet of
18AWG wire and compare it to the weight of a 24AWG wire:
Table 4 shows 6.5mΩ/ft for 18AWG and 22.43mΩ/ft for
24AWG.
The weight of the 18AWG wire is:
(31.39 • 10–6) • [(100000)2/(6.5 • 10–3 • 100000)] =
483 pounds.
The weight of the 24AWG wire is:
(31.39 • 10–6) • [(100000)2/ (22.43 • 10–3 • 100000)]
= 141 pounds.
The weight of the 18AWG is 3.4× the weight of the 24AWG.
Using an LT6110 simplifies wire drop compensation and
provides the option to specify the smallest size and lowest
cost of copper wire.
The US Department of Commerce, National Bureau of
Standards Handbook 100 is a comprehensive source of
copper wire information.
An LT6110 wire drop compensation design requires reliable information of wire resistance and current capacity.
Published copper wire tables are a convenient quick-start
guide to copper wire information. However accurate copper
wire data is obtained by actual measurements of samples
of copper wire to be used from a reputable manufacturer.
A statistically small sample of copper wire is sufficient for
measurements (the average measured mass resistivity
deviation of a large sample of copper wire is only ±0.26%).
The LT6110 power dissipation is at a minimum for I+IN
100µA or less. If the I+IN current is at its specified maximum
of 1mA or greater then the maximum power dissipation and
operating temperature must be considered. The LT6110
power dissipation is the sum of three components:
The International Annealed Copper Standard of mass
resistivity is:
ILOAD2 • RSENSE (if the internal RSENSE is used)
153.28 • 10–6(Ω-kg)/m2 in Metric and
VIOUT • IIOUT,
VREG • (I+IN + ISUPPLY) and
Example of an extreme power dissipation case:
VREG = 50V, I+IN = 1mA.
31.39 • 10–6(Ω-lb)/ft2 in English units.
Mass resistivity is the product of Resistance/Length and
Mass/Length and is useful for estimating the weight of
copper wire required and its cost (the cost of copper wire
depends on its weight and the price fluctuation of copper
in the commodities market).
The weight of copper wire is:
153.28 • 10–6(Length in m2)/(Resistance in Ω) in kilograms
or 31.39 • 10–6(Length in ft2)/(Resistance in Ω) in pounds.
26
Power Dissipation
VIOUT = 36V, IIOUT = 1mA,
ISUPPLY = 2.7mA (ISUPPLY is a function of I+IN. See the ISUPPLY
vs I+IN plot under Typical Performance Characteristics).
ILOAD = 2A and RSENSE = 20mΩ
Calculate LT6110 power dissipation:
Power = 36 • 0.001 + 50 • (0.001 + 0.0027) + 22 • 0.02
Power = 0.301 Watts
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LT6110
Applications Information
ILOAD
VREG
VIN
I+IN
SWITCHING
REGULATOR
FB
IOUT
ISUPPLY
RIN
+IN V+
RS
SWITCHING
REGULATOR
IMON
RFA
FB
–IN
IIOUT
+IN V+
RS
VLOAD
LOAD
–IN
20mΩ
RIOUT
+ –
RWIRE
VDROP
RIN
RFB
20mΩ
VIOUT IOUT
ILOAD
VREG
VIN
+ –
RG
IOUT
V–
LT6110
IMON
V–
LT6110
6110 F18
6110 F17
Figure 17. LT6110 Power Dissipation
The maximum operating ambient temperature TAMAX is
equal to TJMAX – θJA • Power.
TJMAX is 150°C and θJA is 195°C/W for a TSOT-23 package and
TJMAX is 150°C and θJA is 80.6°C/W for a DFN package.
Figure 18. Limiting Regulator Voltage Boost (VREGMAX)
2. Calculate RIOUT:
RIOUT
⎛
⎞
R
⎜ VLOAD – FA • VFB ⎟ • RFA
RG
⎝
⎠
=
VREGMAX – VLOAD
Example: Limit the output of a 5V regulator to less than 6V.
TAMAX = 150°C – 0.301W • 195°C/W = 91°C for the TSOT23 package and
VLOAD = 5V, ILOADMAX = 2A and IIOUT = 100µA.
TAMAX = 150°C – 0.301W • 80.6°C/W = 126°C for the
DFN package.
RFA = 6.49k, RFB = 422k and RG = 80.6k, RIN = 402Ω, VFB
= 0.79V (Figure 4).
Limiting the Regulator Boost Voltage (VREGMAX)
Calculate RIOUT:
In some wire drop compensation applications it may be
necessary to limit the maximum voltage at the regulator
output to ensure the safe operation of all load circuitry.
Adding a resistor, RIOUT, in series with the output pin limits
the maximum compensation current. This in turn limits
the maximum voltage boosting at the regulator output,
VREGMAX. The increasing IIOUT current through RIOUT
drops the voltage at the IOUT pin to a minimum level and
limits the maximum IOUT current (refer to the Minimum
IOUT to IMON Voltage vs Temperature graph under Typical
Performance Characteristics). If the limited IOUT current
is greater than 1mA, a 0.1µF capacitor should be placed
from the IOUT pin to ground to ensure stable operation.
The RIOUT resistor limits the regulator’s voltage to an
arbitrary value higher than VLOAD + RFA • IIOUT.
Design Procedure:
1. Select a VREGMAX voltage > VLOAD + RFA • IIOUT.
RIOUT
⎛
⎞
6490
• 0.79 ⎟ • 6490
⎜5 –
⎝ 80600
⎠
=
6–5
RIOUT = 32k and 5.649V ≤ VREGMAX ≤ 6V.
Limiting VIOUT
The absolute maximum voltage at the IOUT pin (VIOUT) is
36V. If VIOUT is greater than 36V then a Zener diode from
the IOUT pin to the regulator resistors and a resistor from
the IOUT pin to V– can limit the VIOUT voltage to ≤36V.
The Zener diode voltage, VZENER, is typically specified as
a nominal voltage with a minimum and a maximum. For
limiting VIOUT, use the minimum Zener voltage rating,
VZENERMIN. VZENERMIN is typically specified at a current
of 2mA to 5mA and at the low LT6110 IIOUT currents
(≤1mA), the actual VZENERMIN can be up to 2V less than
the minimum voltage listed in a diode data sheet. Therefore
select a Zener diode with a minimum voltage at least 2V
6110fa
For more information www.linear.com/LT6110
27
LT6110
Applications Information
ILOAD
VREG
VIN
SWITCHING
REGULATOR
RFA
RWIRE
VDROP
RIN
FB
+IN V+
RFB
RS
VLOAD
LOAD
SWITCHING
REGULATOR
VZENER
IMON
–
V
LT6110
+IN V+
RG
IOUT
IMON
Example: Limit VIOUT to 20V.
VLOAD = 48V and ILOADMAX = 2A, RWIRE = 1Ω.
RSENSE = 20mΩ, RFA = 20.5k, RFB = 453k, RG = 12.4k, RIN
= 402Ω, VFB = 1.223V, IIOUT = 100µA.
Calculate VREGMAX = 48 + 2(0.02 + 1) = 50.04V.
Calculate VZENERMIN:
⎞
⎛
⎜20 + 100 • 10 –6 • ⎟
⎟
⎜
⎟
⎜
VZENERMIN ≥ 50.04 – ⎜ 20.5 • 103 +
⎟
⎟
⎜
3
⎜ 20.5 • 10 • 1.223 ⎟
⎟
⎜
⎠
⎝ 12.4 • 103
)
)
VZENERMIN = 26V.
The minimum Zener diode voltage must be ≥28V.
Setting the Wire Compensation Threshold
With light load currents, wire drop compensation may not
be desirable. An additional resistor, RIN2, from the +IN
pin to ground provides the option to set a load current
V–
LT6110
VREG
VREGMAX
ILOADCOMP
ILOADMAX
WIRE DROP
COMPENSATION
THRESHOLD
VREGMAX = VLOAD + ILOADMAX • (RSENSE + RWIRE).
28
–IN
6110 F19
⎛ VIOUT +IIOUT • RFA +⎞
⎟
⎜
– ⎜ RFA
⎟
• VFB
⎟
⎜
R
⎠
⎝ G
(
RS
+ –
VIOUT
greater than the calculated VZENERMIN voltage.
(
LOAD
20mΩ
Figure 19. Limiting the Voltage at the IOUT Pin (VOUT ≤ 36V)
VZENERMIN ≥ VREGMAX
VLOAD
RIN2
IOUT
RZ
10M
RWIRE
VDROP
RIN1
IOUT
RFB
+ –
VIOUT
RFA
FB
–IN
20mΩ
RG
ILOAD
VREG
VIN
ILOAD
6110 F20
Figure 20. Setting the Wire Drop Compensation Threshold
threshold, ILOADCOMP, for the start of wire drop compensation. When the load current is equal to ILOADCOMP the
maximum error in voltage at the load occurs. For ILOAD
greater than ILOADCOMP the error in voltage at the load
decreases to zero at ILOADMAX.
Design Procedure:
1. Choose a threshold current.
2. Calculate RIN1 and RIN2:
RIN1 =
ILOADMAX • RSENSE
IIOUT
VLOAD +ILOADMAX • RWIRE
IIOUT
–
VLOAD
–1
ILOADCOMP • RSENSE
⎛
⎞
VLOAD
RIN2 = ⎜
– 1⎟ • RIN1
⎝ ILOADCOMP • RSENSE ⎠
Example: Design the start of wire drop compensation at 1A.
VLOAD = 5V, ILOADMAX = 3.5A, RWIRE = 0.25Ω, RSENSE =
25mΩ and IIOUT = 100µA.
6110fa
For more information www.linear.com/LT6110
LT6110
Applications Information
VIN
VREG
µModule
REGULATOR
RINT
RSENSE
RF
+
LOAD VLOAD
RIN
IOUT
VFB
ILOAD 1/2R
WIRE
+IN V+
RS
–
–IN
1/2RWIRE
RG
+ –
V–
LT6110
6110 F21
Figure 21. An LT6110 with a µModule Regulator
1. ILOADCOMP = 1A.
2. Calculate RIN1 and RIN2: RIN1 = 576Ω and RIN2 = 115k.
At ILOAD = 1A VLOAD = 4.75V and at ILOAD = 3.5A VLOAD = 5V.
Wire Drop Compensation Using a µModule Regulator
Typically a µModule regulator contains a resistor (RINT)
from the regulator’s output to the error amplifier’s input.
The µModule resistor is inaccessible and is in parallel to
the external feedback resistor (RF) required for wire drop
compensation with an LT6110 (the RINT value is listed in
the µModule regulator data sheet).
®
Design Procedure:
1. Choose the compensation current IIOUT (100µA typically).
2. Calculate RF, RG and RIN.
RF =
ILOAD
• (RSENSE +R WIRE )
IIOUT
I
– LOAD • (RSENSE +R WIRE )
IIOUT
RINT •
RINT
RG =
RF • RINT
VFB
•
(RF +RINT ) ( VREG – VFB )
RIN =
ILOAD • RSENSE
IIOUT
The RSENSE resistor is a 6mΩ PCB trace.
ILOAD = 10A and set IIOUT = 100µA.
Calculate RF, RG and RIN.
For IIOUT = 100µA, RIN = (10 • 0.006)/0.0001 = 600Ω and
to the nearest 1% resistor RIN = 604Ω.
IOUT
IMON
The RWIRE of 24ft, 18AWG is 0.15Ω.
If RIN = 604Ω then IIOUT = 99.34µA [IIOUT = (ILOAD •
RSENSE/RIN)].
10
• 0.006 + 0.15)
–6 (
99.34
•
10
RF =
10
105 –
• (0.006 + 0.15)
99.34 • 10 –6
105 •
RF = 18.7k (to the nearest 1% value).
(18.7 • 10 ) • 10 • 0.6
=
(18.7 • 10 + 10 ) • (3 – 0.6)
3
RG
3
5
5
RG = 3.92k (to the nearest 1% value).
Regulator Loop Stability
A regulator’s control loop response is optimized for a
variety of load, input voltage and temperature conditions.
Adding an LT6110 to a regulator circuit does not disturb
control loop stability. However an LT6110 adds a pole
that reduces the loop’s phase margin. The effect of the
LT6110 pole in the loop is easily compensated by a zero
in the feedback divider.
Figure 22 shows a small-signal model for a current mode
buck regulator with an LT6110 in the control loop. The open
loop transfer function from the error amplifier output (VC),
to the modulator output (VREG), to the feedback divider
output (VFB) is: (VREG/VC) (VFB/VREG) (VC/VFB).
Example: Use 24ft, 18AWG wire to regulate a 3V, 10A load,
using an LTM4600 µModule regulator.
RINT of LTM4600 is 100k and the feedback voltage VFB
= 0.6V.
The loop’s DC gain is equal to the product of the modulator
gain (gm • RLOAD), the error amplifier gain (ge • Re) and
the feedback ratio (VREF/VREG).
The overall regulator control loop frequency response
is determined by a combination of several poles and
zeros. Loop compensation is provided by the R1 and
C1 zero at the error amplifier’s output. This zero adds a
6110fa
For more information www.linear.com/LT6110
29
LT6110
Applications Information
BUCK REGULATOR MODEL
VIN
–
+
MODULATOR
gm IS THE
MODULATOR
TRANSCONDUCTANCE
SLOPE COMP
R
OSCILLATOR
Q
S
L
LT6110 MODEL
VREG
CO
RFA
CCOMP
RESR
C2
R1
Re
C1
+
–
VREF
RFB
VFB
VLOAD
RLOAD
RG
ERROR
AMPLIFIER
ge IS THE ERROR
AMPLIFIER
TRANSCONDUCTANCE
RWIRE
RSENSE
+ –
VC
RIN
IIOUT
LT6110
6110 F22
Figure 22. A Small-Signal Model: Current Mode Buck Regulator with an LT6110
positive-going phase near the loop’s crossover frequency
and is adjusted for an optimum phase margin. Regulator
loop compensation, transient response and stability are
covered in depth in AN76.
An LT6110 in the control loop introduces a pole near
160kHz (from the Typical Performance Curves) and this
pole reduces the loop’s optimized phase margin resulting
in load transient overshoot and possibly ringing. Adding
a capacitor, CCOMP in parallel with the regulator’s feedback resistance, RFA introduces a zero to compensate the
VIN
VIN
effects of the LT6110 pole. The frequency of the RFA and
CCOMP zero is best adjusted during a load transient test.
Start with a CCOMP value for a zero equal to or less than
160kHz (the LT6110 pole), then increase CCOMP for a load
transient that settles with minimal overshoot or ringing.
Figure 23 shows an LT3980 buck regulator with an LT6110
circuit used for transient response testing and with the
added zero to restore the loop’s phase margin. During
the circuit’s load transient testing, a CCOMP value of 1nF
BD
RUN/SS BOOST
100k
97.6k
15k
LT3980
PGOOD
SW
VREG
47µF
DA
RT
1.5nF
VC
100pF
0.47µF 10µH
GND
FB
RFA
6.49k
RFB
412k
RG
80.6k
CCOMP
1nF
NC
+IN
LT6110
IOUT
V+
20mΩ
IMON
RS
V–
–IN
RIN
402Ω
0.1µF
RWIRE
0.3Ω
VLOAD
5Ω
10µF
1.6A
1A
8Ω
2k
180pF
6110 F23
Figure 23. Load Transient Response Test Circuit Using an LT3980 Buck Regulator with an LT6110
30
6110fa
For more information www.linear.com/LT6110
LT6110
Applications Information
produces a load transient that settles without overshoot or
ringing (a 10% CCOMP tolerance is adequate). An optional
connection for CCOMP is in parallel with RFA and RFB (from
VREG to VFB) to reduce the CCOMP value for the smallest
capacitor size.
Figures 24a through 24c illustrate a typical loop optimization procedure when an LT6110 is included in the
regulator’s loop.
Figure 24a shows a load transient response of the LT3980
buck regulator with an optimum phase margin without
line drop compensation. The load transient settles without
overshoot.
Figure 24b shows a load transient response of the LT3980
buck regulator with LT6110 line drop compensated load
voltage. The load transient has an overshoot due to the
LT6110 decreasing the phase margin.
Figure 24c shows a load transient response of the LT3980
buck regulator with an LT6110 and with a CCOMP capacitor
added to compensate for the LT6110 in the loop. The load
transient settles without overshoot as the phase margin
is restored.
VREG
200mV/DIV
VLOAD
200mV/DIV
VLOAD
200mV/DIV
1.6A
1A
VREG
200mV/DIV
1.6A
ILOAD
1A
6110 F24a
200µs/DIV
ILOAD
200µs/DIV
Figure 24a. Transient Response of Buck Regulator without
LT6110 Line Drop Compensation
6110 F24b
Figure 24b. Transient Response Buck Regulator with an LT6110
in the Loop
VREG
200mV/DIV
VLOAD
200mV/DIV
1.6A
1A
ILOAD
200µs/DIV
6110 F24c
Figure 24c. Capacitor CCOMP Compensates for the LT6110 in the
Regulator’s Loop
6110fa
For more information www.linear.com/LT6110
31
LT6110
Typical Applications
LT6110 with External RSENSE and LT3690 Buck Regulator at 3.3V
VIN
6.5V TO 25V
10µF
VIN
EN
BST
0.68µF
UVLO
4.7µH
SS
1000pF
VC
SW
LT3690
0.47µF
FB
VCCINT
SYNC
47pF
100µF
PG
22k
680pF
10.2k
BIAS
2
301k
0.8V
3
100k
GND
RT
1
4
32.4k
600kHz
NC
+IN
V+
IOUT
LT6110
IMON
RS
V–
–IN
8
RIN
340Ω
7
0.1µF
6
5
RSENSE*
8.5mΩ RWIRE
0.25Ω
*THE CURRENT SENSE RESISTOR
IS THE DCR OF A LOW COST INDUCTOR.
COILCRAFT 0908SQ-27N (27nH)
VLOAD
LOAD 3.3V
4A
6110 TA02
WIRE DROP COMPENSATION: VLOAD = 3.3V, ILOADMAX = 4A, USING 10ft, 24AWG WIRE.
MEASURED VLOAD REGULATION FOR 0 ≤ ILOAD ≤ 4A AT 25°C:
WITHOUT COMPENSATION: ∆VLOAD = 1000mV (250mV/A)
WITH COMPENSATION: ∆VLOAD = 16mV (4mV/A)
LT6110 with External RSENSE and LT3690 Buck Regulator at 5V
VIN
8.5V TO 36V
10µF
VIN
EN
BST
0.68µF
UVLO
10µH
SS
1000pF
VC
SW
LT3690
0.47µF
FB
VCCINT
SYNC
47µF
PG
22k
680pF
20.5k
BIAS
511k
0.8V
RT
1
2
3
100k
GND
47pF
32.4k
600kHz
4
NC
+IN
V+
IOUT
LT6110
IMON
RS
V–
–IN
8
RIN
340Ω
7
6
5
0.1µF
RSENSE*
8.5mΩ RWIRE
0.5Ω
*THE CURRENT SENSE RESISTOR
IS THE DCR OF A LOW COST INDUCTOR.
COILCRAFT 0908SQ-27N (27nH)
VLOAD
LOAD 5V
4A
6110 TA03
WIRE DROP COMPENSATION: VLOAD = 5V, ILOADMAX = 4A, USING 20ft, 24AWG WIRE.
MEASURED VLOAD REGULATION FOR 0 ≤ ILOAD ≤ 4A AT 25°C:
WITHOUT COMPENSATION: ∆VLOAD = 2000mV (500mV/A)
WITH COMPENSATION: ∆VLOAD = 24mV (6mV/A)
32
6110fa
For more information www.linear.com/LT6110
LT6110
Typical Applications
LT6110 with External PCB RSENSE and LTM4600 µRegulator at 3V
VIN
6V TO 24V
+
150µF
22µF
22µF
VIN
VREG
VOUT
22µF
100µF
100µF
LTM4600HV
1M
RUN/SS
0.1µF
FCB SGND PGND
100k
±0.5% 0.6V
VOSET
1000pF
RF
18.7k
RG
3.92k
1
2
3
4
NC
+IN
IOUT
V+
LT6110
IMON
RS
V–
–IN
8
RIN
523Ω
7
0.1µF
6
5
PCB
RSENSE
RWIRE
6mΩ
0.075Ω
±20%
+
RWIRE
0.075Ω
WIRE DROP COMPENSATION: VLOAD = 3.0V, ILOADMAX = 10A, USING 20ft, 18AWG WIRE WITH GROUND RETURN.
MEASURED VLOAD REGULATION FOR 0 ≤ ILOAD ≤ 10A AT 25°C:
WITHOUT COMPENSATION: ∆VLOAD = 1500mV (150mV/A)
WITH COMPENSATION: ∆VLOAD = 50mV (5mV/A)
100µF
+
100µF
+
VLOAD
LOAD 3V
10A
–
6110 TA04
Wire Drop Compensation Circuit if V+ of LT6110 is <2V
VIN
6V TO 24V
+
150µF
22µF
22µF
VIN
VOUT
VREG
22µF
100µF
100µF
LTM4600HV
1M
RUN/SS
0.1µF
FCB SGND PGND
1000pF
100k
±0.5% 0.6V
VOSET
11k
10k
1
2
3
4
22µF
VIN RANGE OF –5V INVERTER, 1V TO 6V
MURATA
LQH3C220K
22µH
+
15µF
1µF
5
1
VIN
SW
LT1617-1
4
SHDN
IOUT
IMON
RS
V–
–IN
RIN
1k
7
6
5
0.1µF
PCB
RSENSE
RWIRE
10mΩ
90mΩ
±5%
12 ft.
18AWG
100Ω
+
V+
LT6110
8
+
100µF + 100µF
+
VLOAD
LOAD 1.2V
10A
–
–5V
15µF
1µF
0.1µF
30.1k
NFB
GND
+IN
0.1µF
MURATA
LQH3C220K
22µH
MBR0520
NC
3
10k
6110 TA05
WIRE DROP COMPENSATION: VLOAD = 1.2V, ILOADMAX = 10A, USING 12ft, 18AWG WIRE.
(ONE WIRE TO A LOAD SHARING THE REGULATOR’S GROUND).
MEASURED VLOAD REGULATION FOR 0 ≤ ILOAD ≤ 10A AT 25°C:
WITHOUT COMPENSATION: ∆VLOAD = 1000mV (100mV/A)
WITH COMPENSATION: ∆VLOAD = 80mV (8mV/A)
6110fa
For more information www.linear.com/LT6110
33
LT6110
Typical Applications
LT6110 with External RSENSE and LTC3789 Buck-Boost Regulator at 12V
VIN
5V TO 36V
INTVCC
100k
1
2
1nF
0.01µF
0.01µF
3
4
14.7k
5
6
7
121k
8
400kHz
VIN
VREG
9
10
11
12
13
PGOOD
VFB
SW1
TG1
SS
BOOST1
SENSE+
PGND
SENSE–
BG1
ITH
SGND
LTC3789
VIN
MODE/PLLIN
INTVCC
FREQ
EXTVCC
RUN
VINSNS
VOUTSNS
BG2
BOOST2
27
2.2µF 14 I
–
OSENSE
TG2
SW2
TRIM
+
270µF
390pF
5.6Ω
10Ω
26
QA
Si7848BDP
25
24
DA
DFLS160
23
1µF
22
L
4.7µH
QB
Si7848BDP
10µF
21
D1
B240A
10mΩ
20
DB
DFLS160
19
QC
SiR496DP
18
ILIM
IOSENSE+
CA
0.22µF
17
VREG
BZT52C6V2S
16
CB
0.22µF
VREG
100pF
QD
SiR496DP
D2
B240A
15
5.62k
133k
1
2
3
100Ω
100Ω
1k
1k
10k
10mΩ
2.2µF
+
4
NC
+IN
LT6110
IMON
RS
V–
–IN
RIN
619Ω
7
6
5
0.1µF
EXTERNAL
RSENSE
RWIRE
25mΩ
0.1Ω
±5%
+
330µF
0.8V
WIRE DROP COMPENSATION: VLOAD = 12V, ILOADMAX = 5A, USING 20ft, 20AWG WIRE WITH GROUND RETURN.
MEASURED VLOAD REGULATION FOR 0 ≤ ILOAD ≤ 5A AT 25°C:
WITHOUT COMPENSATION: ∆VLOAD = 1000mV (250mV/A)
WITH COMPENSATION: ∆VLOAD = 25mV (5mV/A)
34
V+
IOUT
8
RWIRE
0.1Ω
LOAD
–
12V
5A
6110 TA06
6110fa
For more information www.linear.com/LT6110
LT6110
Package Description
Please refer to http://www.linear.com/designtools/packaging/ for the most recent package drawings.
TS8 Package
8-Lead Plastic TSOT-23
(Reference LTC DWG # 05-08-1637 Rev A)
0.40
MAX
2.90 BSC
(NOTE 4)
0.65
REF
1.22 REF
1.4 MIN
3.85 MAX 2.62 REF
2.80 BSC
1.50 – 1.75
(NOTE 4)
PIN ONE ID
RECOMMENDED SOLDER PAD LAYOUT
PER IPC CALCULATOR
0.22 – 0.36
8 PLCS (NOTE 3)
0.65 BSC
0.80 – 0.90
0.20 BSC
0.01 – 0.10
1.00 MAX
DATUM ‘A’
0.30 – 0.50 REF
0.09 – 0.20
(NOTE 3)
1.95 BSC
TS8 TSOT-23 0710 REV A
NOTE:
1. DIMENSIONS ARE IN MILLIMETERS
2. DRAWING NOT TO SCALE
3. DIMENSIONS ARE INCLUSIVE OF PLATING
4. DIMENSIONS ARE EXCLUSIVE OF MOLD FLASH AND METAL BURR
5. MOLD FLASH SHALL NOT EXCEED 0.254mm
6. JEDEC PACKAGE REFERENCE IS MO-193
6110fa
For more information www.linear.com/LT6110
35
LT6110
Package Description
Please refer to http://www.linear.com/designtools/packaging/ for the most recent package drawings.
DC8 Package
8-Lead Plastic DFN (2mm × 2mm)
(Reference LTC DWG # 05-08-1719 Rev A)
0.70 ±0.05
2.55 ±0.05
1.15 ±0.05 0.64 ±0.05
(2 SIDES)
PACKAGE
OUTLINE
0.25 ±0.05
0.45 BSC
1.37 ±0.05
(2 SIDES)
RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS
APPLY SOLDER MASK TO AREAS THAT ARE NOT SOLDERED
R = 0.05
TYP
2.00 ±0.10
(4 SIDES)
PIN 1 BAR
TOP MARK
(SEE NOTE 6)
R = 0.115
TYP
5
8
0.40 ±0.10
0.64 ±0.10
(2 SIDES)
PIN 1 NOTCH
R = 0.20 OR
0.25 × 45°
CHAMFER
(DC8) DFN 0409 REVA
4
0.200 REF
1
0.23 ±0.05
0.45 BSC
0.75 ±0.05
1.37 ±0.10
(2 SIDES)
0.00 – 0.05
BOTTOM VIEW—EXPOSED PAD
NOTE:
1. DRAWING IS NOT A JEDEC PACKAGE OUTLINE
2. DRAWING NOT TO SCALE
3. ALL DIMENSIONS ARE IN MILLIMETERS
4. DIMENSIONS OF EXPOSED PAD ON BOTTOM OF PACKAGE DO NOT INCLUDE
MOLD FLASH. MOLD FLASH, IF PRESENT, SHALL NOT EXCEED 0.15mm ON ANY SIDE
5. EXPOSED PAD SHALL BE SOLDER PLATED
6. SHADED AREA IS ONLY A REFERENCE FOR PIN 1 LOCATION ON THE
TOP AND BOTTOM OF PACKAGE
36
6110fa
For more information www.linear.com/LT6110
LT6110
Revision History
REV
DATE
DESCRIPTION
A
11/13
Maximum Amplifier Bias Current changed from 200nA to 100nA
PAGE NUMBER
3
Addition of Minimum Input Voltage graph
7
Edits to Compensating an Output Referred Adjustable Voltage Regulator section
16
Edits to Error Sources section
18, 19, 20
Title added – Wire Drop Compensation Using a Micromodule Regulator
29
Edits to schematic LT6110 with External PCB RSENSE and LTM4600 µModule Regulator at 3V
33
Replaced schematic LT6110 with External PCB RSENSE and LTM4600 µModule Regulator at 1.8V with new circuit,
Wire Drop Compensation Circuit if V+ of LT6110 < 2V
33
Edits to schematic LT6110 with Internal RSENSE and LT3975 Buck Regulator at 3.3V
38
6110fa
Information furnished by Linear Technology Corporation is believed to be accurate and reliable.
However, no responsibility is assumed for its use. Linear Technology Corporation makes no representation that the interconnection
of its circuits
as described
herein will not infringe on existing patent rights.
For more
information
www.linear.com/LT6110
37
LT6110
Typical Application
LT6110 with Internal RSENSE and LT3975 Buck Regulator at 3.3V
VIN
7V TO 42V
100µF
B00ST
VIN
10µF
0.47µF
EN
4.7µH
SW
PDS360
LT3975
VREG
OUT
100µF
SS
10nF
RT
78.7k
f = 600kHz
SYNC
GND
FB
1.197V
1µF
16.5k
10pF
1M
1
2
3
576k
4
NC
+IN
V+
IOUT
LT6110
IMON
RS
V–
–IN
8
RIN
499Ω
7
0.1µF
RWIRE
0.32Ω
6
5
+
WIRE DROP COMPENSATION: VLOAD = 3.3V, ILOADMAX = 2.5A, USING 6ft, 30AWG WIRE WITH GROUND RETURN.
MEASURED VLOAD REGULATION FOR 0 ≤ ILOAD ≤ 2.5A AT 25°C:
WITHOUT COMPENSATION: ∆VLOAD = 1600mV (640mV/A)
WITH COMPENSATION: ∆VLOAD = 25mV (10mV/A)
VLOAD
LOAD 3.3V
RWIRE
2.5A
–
0.32Ω
6110 TA07
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38 Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
For more information www.linear.com/LT6110
(408) 432-1900 ● FAX: (408) 434-0507
●
www.linear.com/LT6110
6110fa
LT 1113 REV A • PRINTED IN USA
 LINEAR TECHNOLOGY CORPORATION 2013