LINER LT3080-1 800ma single resistor rugged linear regulator Datasheet

LT3088
800mA Single Resistor
Rugged Linear Regulator
FEATURES
DESCRIPTION
Extended Safe Operating Area
n Maximum Output Current: 800mA
n Stable with or without Input/Output Capacitors
n Wide Input Voltage Range: 1.2V to 36V
n Single Resistor Sets Output Voltage
n Output Adjustable to 0V
n 50µA SET Pin Current: 1% Initial Accuracy
n Output Voltage Noise: 27µV
RMS
n Parallel Multiple Devices for Higher Current, Heat
Spreading and Lower Noise
nn Pin Compatible Upgrade to LT1117
n Reverse-Battery and Reverse-Current Protection
n <1mV Typical Load Regulation Independent of V
OUT
n <0.001%/V Typical Line Regulation
n 3-Lead SOT-223, 3-Lead DD-Pak, 8-Lead 3mm ×
3mm DFN Packages
The LT®3088 is an 800mA low dropout linear regulator
designed for rugged industrial applications. A key feature
of the IC is the extended safe operating area (SOA). The
LT3088 can be paralleled for higher output current or heat
spreading. The device withstands reverse input and reverse
output-to-input voltages without reverse current flow.
n
APPLICATIONS
n
n
n
n
n
The LT3088’s precision 50µA reference current source
allows a single resistor to program output voltage to
any level between zero and 34.5V. The current reference
architecture makes load regulation independent of output
voltage. The LT3088 is stable with or without input and
output capacitors.
Internal protection circuitry includes reverse-battery and
reverse-current protection, current limiting and thermal
limiting. The LT3088 is offered in the 3-lead SOT-223,
3-lead DD-Pak, and an 8-lead 3mm × 3mm DFN package.
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.
All Surface Mount Power Supply
Rugged Industrial Power Supply
Post Regulator for Switching Supplies
Low Output Voltage Supply
Intrinsic Safety Applications
TYPICAL APPLICATION
SET Pin Current
Wide Safe Operating Area Supply
51.0
50.8
VIN
50µA
+
–
SET
OUT
10µF*
30.1k
750Ω*
3088 TA01a
*OPTIONAL
IOUT
1.5V
800mA
SET PIN CURRENT (µA)
50.6
IN
LT3088
ILOAD = 2mA
50.4
50.2
50.0
49.8
49.6
49.4
49.2
49.0
–75 –50 –25 0 25 50 75 100 125 150 175
TEMPERATURE (°C)
3088 G01
3088fb
For more information www.linear.com/LT3088
1
LT3088
ABSOLUTE MAXIMUM RATINGS
(Note 1) All voltages Relative to VOUT
IN Pin to OUT Pin Differential Voltage......................±40V
SET Pin Current (Note 6)......................................±25mA
SET Pin Voltage (Relative to OUT, Note 6)............... ±10V
Output Short-Circuit Duration........................... Indefinite
Operating Junction Temperature Range (Note 2)
E-, I-Grades........................................ –40°C to 125°C
H-Grade.............................................. –40°C to 150°C
MP-Grade........................................... –55°C to 150°C
Storage Temperature Range................... –65°C to 150°C
Lead Temperature (Soldering, 10 sec)
M, ST Packages Only......................................... 300°C
PIN CONFIGURATION
TOP VIEW
FRONT VIEW
FRONT VIEW
OUT 1
OUT 2
OUT 3
SET 4
9
OUT
8
IN
7
IN
6
NC
5
NC
3
TAB IS
OUT
IN
2
OUT
1
SET
TAB IS
OUT
3
IN
2
OUT
1
SET
DD PACKAGE
8-LEAD (3mm × 3mm) PLASTIC DFN
ST PACKAGE
3-LEAD PLASTIC SOT-223
M PACKAGE
3-LEAD PLASTIC DD-PAK
TJMAX = 150°C, θJA = 28°C/W, θJC = 5.3°C/W
EXPOSED PAD (PIN 9) IS OUT, MUST BE SOLDERED TO PCB
TJMAX = 150°C, θJA = 25°C/W, θJC = 15°C/W
TJMAX = 150°C, θJA = 14°C/W, θJC = 3°C/W
ORDER INFORMATION
LEAD FREE FINISH
TAPE AND REEL
PART MARKING*
PACKAGE DESCRIPTION
TEMPERATURE RANGE
LT3088EDD#PBF
LT3088EDD#TRPBF
LGSZ
8-Lead (3mm × 3mm) Plastic DFN
–40°C to 125°C
LT3088IDD#PBF
LT3088IDD#TRPBF
LGSZ
8-Lead (3mm × 3mm) Plastic DFN
–40°C to 125°C
LT3088HDD#PBF
LT3088HDD#TRPBF
LGSZ
8-Lead (3mm × 3mm) Plastic DFN
–40°C to 150°C
LT3088EST#PBF
LT3088EST#TRPBF
3088
3-Lead Plastic SOT-223
–40°C to 125°C
LT3088IST#PBF
LT3088IST#TRPBF
3088
3-Lead Plastic SOT-223
–40°C to 125°C
LT3088HST#PBF
LT3088HST#TRPBF
3088
3-Lead Plastic SOT-223
–40°C to 150°C
LT3088MPST#PBF
LT3088MPST#TRPBF
3088
3-Lead Plastic SOT-223
–55°C to 150°C
LT3088EM#PBF
LT3088EM#TRPBF
LT3088M
3-Lead Plastic DD-Pak
–40°C to 125°C
LT3088IM#PBF
LT3088IM#TRPBF
LT3088M
3-Lead Plastic DD-Pak
–40°C to 125°C
LT3088HM#PBF
LT3088HM#TRPBF
LT3088M
3-Lead Plastic DD-Pak
–40°C to 150°C
LT3088MPM#PBF
LT3088MPM#TRPBF
LT3088M
3-Lead Plastic DD-Pak
–55°C to 150°C
Consult LTC Marketing for parts specified with wider operating temperature ranges. *The temperature grade is identified by a label on the shipping container.
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/. Some packages are available in 500 unit reels through
designated sales channels with #TRMPBF suffix.
2
3088fb
For more information www.linear.com/LT3088
LT3088
ELECTRICAL CHARACTERISTICS
The l denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TJ = 25°C. (Note 2)
PARAMETER
CONDITIONS
ISET
SET Pin Current
VOS
Offset Voltage
(VOUT – VSET)
ISET Load Regulation
VOS Load Regulation
(Note 7)
MIN
TYP
MAX
UNITS
VIN = 2V, ILOAD = 2mA
2V ≤ VIN ≤ 36V, 2mA ≤ ILOAD ≤ 800mA
l
49.5
49
50
50
50.5
51
µA
µA
VIN = 2V, ILOAD = 2mA
VIN = 2V, ILOAD = 2mA
l
–1.5
–3.5
0
0
1.5
3.5
mV
mV
∆ILOAD = 2mA to 800mA
∆ILOAD = 2mA to 800mA
Line Regulation
∆ISET
∆VOS
–0.1
nA
DD Package
l
–0.5
–3
mV
M, ST Packages
l
–1.5
–4
mV
∆VIN = 2V to 36V, ILOAD = 2mA
∆VIN = 2V to 36V, ILOAD = 2mA
1.5
0.001
nA/V
mV/V
Minimum Load Current (Note 3)
2V ≤ VIN ≤ 36V
l
0.4
2
Dropout Voltage (Note 4)
ILOAD = 100mA
ILOAD = 800mA
l
1.21
1.35
1.6
l
V
V
Current Limit
VIN = 5V, VSET = 0V, VOUT = –0.1V
1.2
A
Reference Current RMS Output Noise (Note 5)
10Hz ≤ f ≤ 100kHz
5.7
nARMS
Error Amplifier RMS Output Noise (Note 5)
ILOAD = 800mA, 10Hz ≤ f ≤ 100kHz,
COUT = 0µF, CSET = 0.1µF
27
µVRMS
Ripple Rejection
VRIPPLE = 0.5VP-P, ILOAD = 0.1A, CSET = 0.1µF,
COUT=10µF, VIN = VOUT(NOMINAL) + 3V
f = 120Hz
f = 10kHz
f = 1MHz
90
75
20
dB
dB
dB
Thermal Regulation, ISET
10ms Pulse
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.
Note 2: Unless otherwise specified, all voltages are with respect to VOUT.
The LT3088 is tested and specified under pulse load conditions such
that TJ ≈ TA. The LT3088E is tested at TA = 25°C and performance is
guaranteed from 0°C to 125°C. Performance of the LT3088E over the
full –40°C and 125°C operating temperature range is assured by design,
characterization, and correlation with statistical process controls. The
LT3088I is guaranteed over the full –40°C to 125°C operating junction
temperature range. The LT3088MP is 100% tested and guaranteed
over the –55°C to 150°C operating junction temperature range. The
LT3088H is tested at 150°C operating junction temperature. High junction
temperatures degrade operating lifetimes. Operating lifetime is degraded at
junction temperatures greater than 125°C.
Note 3: Minimum load current is equivalent to the quiescent current of
the part. Since all quiescent and drive current is delivered to the output
of the part, the minimum load current is the minimum current required to
maintain regulation.
0.8
mA
75
0.003
%/W
Note 4: For the LT3088, dropout is specified as the minimum input-tooutput voltage differential required supplying a given output current.
Note 5: Adding a small capacitor across the reference current resistor
lowers output noise. Adding this capacitor bypasses the resistor shot
noise and reference current noise; output noise is then equal to error
amplifier noise (see Applications Information section).
Note 6: Diodes with series 400Ω resistors clamp the SET pin to the
OUT pin. These diodes and resistors only carry current under transient
overloads. During normal operation, keep the OUT-to-SET differential
voltage below 2V.
Note 7: Load regulation is Kelvin sensed at the package.
Note 8: This IC includes overtemperature protection that protects the
device during momentary overload conditions. Junction temperature
exceeds the maximum operating junction temperature when
overtemperature protection is active. Continuous operation above the
specified maximum operating junction temperature may impair device
reliability.
3088fb
For more information www.linear.com/LT3088
3
LT3088
TYPICAL PERFORMANCE CHARACTERISTICS
SET Pin Current
51.0
50.8
TJ = 25°C unless otherwise specified.
SET Pin Current
Offset Voltage (VOUT – VSET)
1.0
N = 2994
ILOAD = 2mA
0.8
0.6
OFFSET VOLTAGE (mV)
SET PIN CURRENT (µA)
50.6
ILOAD = 2mA
50.4
50.2
50.0
49.8
49.6
49.4
0.4
0.2
0.0
–0.2
–0.4
–0.6
49.2
–0.8
49.0
–75 –50 –25 0 25 50 75 100 125 150 175
TEMPERATURE (°C)
–1.0
–75 –50 –25 0 25 50 75 100 125 150 175
TEMPERATURE (°C)
49
50.5
49.5
50
SET PIN CURRENT DISTRIBUTION (µA)
51
3088 G02
3088 G01
Offset Voltage
3088 G03
Offset Voltage (VOUT – VSET)
1.0
N = 2994
Offset Voltage (VOUT – VSET)
0.2
ILOAD = 2mA
0.8
0.0
OFFSET VOLTAGE (mV)
OFFSET VOLTAGE (mV)
0.6
0.4
0.2
0.0
–0.2
–0.4
TJ = 25°C
–0.2
TJ = 125°C
–0.4
–0.6
–0.6
–0.8
1
–1
0
VOS DISTRIBUTION (mV)
2
6
18
24
30
12
INPUT-TO-OUTPUT DIFFERENTIAL (V)
0
3088 G04
Load Regulation
0.1
0.2
0.3
0.4
0.5
0.6
–1
–0.2
–2
–0.3
–3
–0.4
–4
–0.5
–5
–0.6
–6
–0.7
–7
–0.8
–8
–0.9
–9
∆ILOAD = 2mA TO 0.8A
–10
–1.0
–75 –50 –25 0 25 50 75 100 125 150 175
TEMPERATURE (°C)
Dropout Voltage
1.6
700
1.5
600
500
400
300
0.8
3088 G06
800
MINIMUM LOAD CURRENT (µA)
–0.1
0.7
LOAD CURRENT (A)
Minimum Load Current
0
SET PIN CURRENT LOAD REGULATION (nA)
OFFSET VOLTAGE LOAD REGULATION (mV)
0
3088 G05
0.0
VIN – VOUT = 36V
VIN – VOUT = 2V
200
1.4
TJ = –50°C
1.3
TJ = 25°C
1.2
TJ = 125°C
1.1
100
0
–75 –50 –25 0 25 50 75 100 125 150 175
TEMPERATURE (°C)
3088 G07
4
–0.8
36
DROPOUT VOLTAGE (V)
–2
–1.0
3088 G08
1.0
0
0.1
0.2 0.3 0.4 0.5 0.6
LOAD CURRENT (A)
0.7
0.8
3088 G09
3088fb
For more information www.linear.com/LT3088
LT3088
TYPICAL PERFORMANCE CHARACTERISTICS
Dropout Voltage
Current Limit
1.6
2.0
Current Limit
1.6
VIN = 7V
VOUT = 0V
1.8
1.5
1.4
1.6
ILOAD = 800mA
1.3
1.2
ILOAD = 2mA
CURRENT LIMIT (A)
1.4
1.2
1.4
CURRENT LIMIT (A)
DROPOUT VOLTAGE (V)
TJ = 25°C unless otherwise specified.
1.2
1.0
0.8
0.6
1.0
0.8
0.6
0.4
0.4
1.1
0.2
0.2
1.0
–75 –50 –25 0 25 50 75 100 125 150 175
TEMPERATURE (°C)
3088 G10
∆ILOAD =5mA TO 100mA
100
0
20 40 60 80 100 120 140 160 180 200
TIME (µs)
COUT = 2.2µF
100
0
–100
0.5
0.0
∆ILOAD = 100mA TO 800mA
–0.3
–0.4
OUTPUT VOLTAGE
DEVIATION (mV)
LOAD
CURRENT (A)
tr = tf = 1µs
∆ILOAD = 100mA TO 800mA
0
5
10 15 20 25 30 35 40 45 50
TIME (µs)
3088 G16
10 15 20 25 30 35 40 45 50
TIME (µs)
6
Current Source
Line Transient Response
RSET = 20k
RLOAD = 1.25Ω
COUT = 2.2µF
CSET = 0.1µF
5
4
3
20
0
–20
–40
6
RSET = 6.04k
ROUT = 3.01Ω
COUT = 0
CSET = 30pF
5
4
3
100mA CURRENT SOURCE CONFIGURATION
120
40
0.8
5
3088 G15
INPUT VOLTAGE (V)
0.0
0
3088 G14
INPUT VOLTAGE (V)
OUTPUT VOLTAGE
DEVIATION (V)
COUT = 0
tr = tf = 1µs
0
–100
20 40 60 80 100 120 140 160 180 200
TIME (µs)
0
∆ILOAD = 5mA TO 100mA
100
Linear Regulator
Line Transient Response
0.3
0.0
0
–200
1.0
COUT = 0
50
–50
Linear Regulator
Load Transient Response
0.4
VIN = 3V
100 VOUT = 1V
CSET = 30pF
–100
8088 G13
VIN = 3V
0.6 VOUT = 1V
CSET = 30pF
150
OUTPUT VOLTAGE
DEVIATION (mV)
VIN = 3V
200 VOUT = 1V
CSET = 0.1µF
LOAD
CURRENT (mA)
–50
0
Linear Regulator
Load Transient Response
0
5
10 15 20 25 30 35 40 45 50
TIME (µs)
3088 G17
OUTPUT CURRENT (mA)
LOAD
CURRENT (mA)
OUTPUT VOLTAGE
DEVIATION (mV)
0
–100
300
COUT = 2.2µF
50
–100
3082 G12
Linear Regulator
Load Transient Response
LOAD
CURRENT (A)
OUTPUT VOLTAGE
DEVIATION (mV)
VIN = 3V
100 VOUT = 1V
CSET = 0.1µF
36
6
18
24
0
30
12
INPUT-TO-OUTPUT DIFFERENTIAL VOLTAGE (V)
3088 G11
Linear Regulator
Load Transient Response
150
0.0
0.0
–75 –50 –25 0 25 50 75 100 125 150 175
TEMPERATURE (°C)
110
100
90
80
0
10 20 30 40 50 60 70 80 90 100
TIME (µs)
3088 G18
3088fb
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5
LT3088
TYPICAL PERFORMANCE CHARACTERISTICS
3
500mA CURRENT SOURCE CONFIGURATION
600
500
450
10 20 30 40 50 60 70 80 90 100
TIME (µs)
0
3
RSET = 20k
RLOAD = 1.25Ω
COUT = 2.2µF CERAMIC
CSET = 0
2
1
0
550
400
1.0
0.5
0.0
–0.5
10 20 30 40 50 60 70 80 90 100
TIME (µs)
0
100
0
80
60
RSET = 6.04k
ROUT = 3.01Ω
COUT = 0
CSET = 30pF
600
1
500
0
400
300
RSET = 6.04k
ROUT = 0.6Ω
COUT = 0
CSET = 30pF
200
100
0
0
30
80
1k
10k 100k
FREQUENCY (Hz)
60
50
40
30
VIN = VOUT + 5V
VIN = VOUT + 2V
VIN = VOUT + 1.5V
10
1M
10M
3088 G25
2
4
6
8
10 12 14 16 18 20
TIME (ms)
VIN = 36V
400
VIN = 5V
300
200
SET PIN = 0V
VIN
0
VOUT
RTEST
0
500
1000
1500
0
10
100
1k
10k 100k
FREQUENCY (Hz)
2000
RTEST (Ω)
3088 G24
Output Impedance
70
20
ILOAD = 100mA
ILOAD = 500mA
ILOAD = 800mA
100
0
100
10M
CURRENT SOURCE CONFIGURATION
1M
OUTPUT IMPEDANCE (Ω)
40
10
–0.5
500
COUT = 2.2µF CERAMIC
CSET = 0.1µF
ILOAD = 100mA
90
RIPPLE REJECTION (dB)
RIPPLE REJECTION (dB)
50
6
100
COUT = 2.2µF CERAMIC
CSET = 0.1µF
VIN = VOUT(NOMINAL) + 2V
60
0
0.0
Ripple Rejection
70
10
0.5
3088 G23
Ripple Rejection
20
1.0
600
10 20 30 40 50 60 70 80 90 100
TIME (µs)
3088 G22
80
3
2
10 20 30 40 50 60 140 80 90 100
TIME (µs)
90
1
Residual Output Voltage with
Less Than Minimum Load
500mA CURRENT SOURCE CONFIGURATION
OUTPUT CURRENT (mA)
OUTPUT CURRENT (mA)
1
100
RSET = 20k
RLOAD = 1.25Ω
COUT = 2.2µF CERAMIC
CSET = 0.1µF
3088 G21
INPUT VOLTAGE (V)
120
INPUT VOLTAGE (V)
3
2
0
2
Current Source
Turn-On Response
100mA CURRENT SOURCE CONFIGURATION
0
3
3080 G20
Current Source
Turn-On Response
20
4
0
3088 G19
40
INPUT VOLTAGE (V)
4
4
OUTPUT VOLTAGE (V)
5
INPUT VOLTAGE (V)
RSET = 6.04k
ROUT = 0.6Ω
COUT = 0
CSET = 30pF
Linear Regulator
Turn-On Response
OUTPUT VOLTAGE (mV)
6
Linear Regulator
Turn-On Response
OUTPUT VOLTAGE (V)
OUTPUT CURRENT (mA)
INPUT VOLTAGE (V)
Current Source
Line Transient Response
TJ = 25°C unless otherwise specified.
100k
10k
1k
100
ISOURCE = 10mA
ISOURCE = 100mA
ISOURCE = 500mA
10
1M
10M
3088 G26
1
10
100
1k
10k 100k
FREQUENCY (Hz)
1M
10M
3088 G27
3088fb
For more information www.linear.com/LT3088
LT3088
TYPICAL PERFORMANCE CHARACTERISTICS
TJ = 25°C unless otherwise specified.
Ripple Rejection (120Hz)
Ripple Rejection (10kHz)
90
70
88
68
RIPPLE REJECTION (dB)
RIPPLE REJECTION (dB)
86
84
82
80
78
VIN = VOUT(NOMINAL) + 2V
RIPPLE = 500mVP-P
f = 120Hz
ILOAD = 0.1A
COUT = 2.2µF
CSET = 0.1µF
76
74
72
66
64
62
60
56
54
52
VIN = VOUT(NOMINAL) + 2V
RIPPLE = 500mVP-P
f = 10kHz
ILOAD = 0.1A
COUT = 2.2µF
CSET = 0.1µF
50
–75 –50 –25 0 25 50 75 100 125 150 175
TEMPERATURE (°C)
70
–75 –50 –25 0 25 50 75 100 125 150 175
TEMPERATURE (°C)
3088 G29
3088 G28
Ripple
Ripple Rejection
Rejection
60
48
55
46
RIPPLE REJECTION (dB)
RIPPLE REJECTION (dB)
Ripple Rejection (1MHz)
50
44
42
40
38 VIN = VOUT(NOMINAL) + 2V
RIPPLE = 200mVP-P
36
f = 1MHz
34 ILOAD = 0.1A
COUT = 2.2µF
32 C
SET = 0.1µF
ILOAD = 800mA
COUT = 2.2µF
50
45
40
35
30
10kHz
100kHz
1MHz
25
20
1.5
30
–75 –50 –25 0 25 50 75 100 125 150 175
TEMPERATURE (°C)
2
2.5
3
3.5
4
4.5
IN–TO–OUT DIFFERENTIAL (V)
5
3088 G31
3088 G30
10Hz to 100kHz
Output Voltage Noise
Noise Spectral Density
1000
10
100
10
10
100
1k
10k
FREQUENCY (Hz)
1
100k
REFERENCE CURRENT NOISE
SPECTRAL DENSITY (pA/√Hz)
ERROR AMPLIFIER NOISE
SPECTRAL DENSITY (nV/√Hz)
100
CSET = 0.1µF
COUT = 4.7µF
ILOAD = 800mA
VOUT
50µV/DIV
NOISE INDEPENDENT
OF OUTPUT VOLTAGE
TIME 1ms/DIV
3088 G33
3988 G32
3088fb
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7
LT3088
PIN FUNCTIONS
IN: Input. This pin supplies power to regulate internal
circuitry and supply output load current. For the device
to operate properly and regulate, the voltage on this pin
must be between the dropout voltage and 36V above the
OUT pin (depending on output load current, see Dropout
Voltage Specifications).
Exposed Pad/Tab: Output. The exposed pad of the DD
package and the tab of the M and ST packages are tied
internally to OUT. As such, tie them directly to OUT at the
PCB. The amount of copper area and planes connected
to OUT determine the effective thermal resistance of the
packages.
OUT: Output. This is the power output of the device. The
LT3088 requires a 2mA minimum load current for proper
output regulation.
NC: (DD Package Only) No Connection. No connect pins
have no connection to internal circuitry and may be tied
to IN, OUT, GND or floated.
SET: Set. This pin is the error amplifier’s noninverting
input and also sets the operating bias point of the circuit.
A fixed 50µA current source flows out of this pin. A single
external resistor programs VOUT. Output voltage range
is 0V to 34.5V.
BLOCK DIAGRAM
IN
50µA
+
–
SET
8
OUT
3088 BD
3088fb
For more information www.linear.com/LT3088
LT3088
APPLICATIONS INFORMATION
Introduction
Programming Linear Regulator Output Voltage
The LT3088 regulator is easy to use and has all the protection features expected in high performance regulators.
Included are short-circuit protection, reverse-input protection and safe operating area protection, as well as thermal
shutdown with hysteresis. Safe operating area (SOA) for
the LT3088 is extended, allowing for use in harsh industrial and automotive environments where sudden spikes
in input voltage lead to high power dissipation.
The LT3088 generates a 50µA reference current that flows
out of the SET pin. Connecting a resistor from SET to
ground generates a voltage that becomes the reference
point for the error amplifier (see Figure 1). The reference
voltage equals 50µA multiplied by the value of the SET
pin resistor (Ohm’s Law). Any voltage can be generated
and there is no minimum output voltage for the regulator.
The LT3088 fits well in applications needing multiple rails.
This new architecture adjusts down to zero with a single
resistor, handling modern low voltage digital ICs as well
as allowing easy parallel operation and thermal management without heat sinks. Adjusting to zero output allows
shutting off the powered circuitry.
A precision “0” TC 50µA reference current source connects
to the noninverting input of a power operational amplifier.
The power operational amplifier provides a low impedance
buffered output to the voltage on the noninverting input.
A single resistor from the noninverting input to ground
sets the output voltage. If this resistor is set to 0Ω, zero
output voltage results. Therefore, any output voltage can
be obtained between zero and the maximum defined by
the input power supply is obtainable.
The benefit of using a true internal current source as the
reference, as opposed to a bootstrapped reference in older
regulators, is not so obvious in this architecture. A true
reference current source allows the regulator to have gain
and frequency response independent of the impedance on
the positive input. On older adjustable regulators, such as
the LT1086 loop gain changes with output voltage and
bandwidth changes if the adjustment pin is bypassed to
ground. For the LT3088, the loop gain is unchanged with
output voltage changes or bypassing. Output regulation
is not a fixed percentage of output voltage, but is a fixed
fraction of millivolts. Use of a true current source allows
all of the gain in the buffer amplifier to provide regulation,
and none of that gain is needed to amplify up the reference
to a higher output voltage.
LT3088
IN
CIN
50µA
+
–
SET
OUT
VOUT = 50µA • RSET
CSET
RSET
COUT
RLOAD
3088 F01
Figure 1. Basic Adjustable Regulator
Table 1 lists many common output voltages and the closest standard 1% resistor values used to generate that
output voltage.
Regulation of the output voltage requires a minimum load
current of 2mA. For true zero voltage output operation,
return this 2mA load current to a negative output voltage.
Table 1. 1% Resistors for Common Output Voltages
VOUT (V)
RSET (kΩ)
1
20
1.2
24.3
1.5
30.1
1.8
35.7
2.5
49.9
3.3
66.5
5
100
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LT3088
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With the 50µA current source used to generate the reference
voltage, leakage paths to or from the SET pin can create
errors in the reference and output voltages. High quality
insulation should be used (e.g., Teflon, Kel-F); cleaning of
all insulating surfaces to remove fluxes and other residues
is required. Surface coating may be necessary to provide
a moisture barrier in high humidity environments.
Minimize board leakage by encircling the SET pin and
circuitry with a guard ring operated at a potential close
to itself. Tie the guard ring to the OUT pin. Guarding both
sides of the circuit board is required. Bulk leakage reduction
depends on the guard ring width. 50nA of leakage into or
out of the SET pin and its associated circuitry creates a
0.1% reference voltage error. Leakages of this magnitude,
coupled with other sources of leakage, can cause significant offset voltage and reference drift, especially over the
possible operating temperature range. Figure 2 depicts an
example guard ring layout.
If guard ring techniques are used, this bootstraps any
stray capacitance at the SET pin. Since the SET pin is
a high impedance node, unwanted signals may couple
into the SET pin and cause erratic behavior. This will
be most noticeable when operating with minimum
output capacitors at full load current. The easiest way
to remedy this is to bypass the SET pin with a small
amount of capacitance from SET to ground, 10pF to
20pF is sufficient.
Using the LT3088 as a Replacement for the LT1117
The LT3088 can be used as an upgrade or replacement
for the LT1117 regulator. The LT3088 offers superior
performance over the LT1117, including extended input
voltage range, lower output voltage capability, extended
safe operating area, and protection features such as reverse
voltage/current protection. Figure 3 shows how the LT1117
is used as a basic adjustable regulator. Two methods are
shown in Figures 4 and 5 to change from the LT1117 to
the LT3088. The first method (shown in Figure 4) requires
no changes to existing board layouts: replace the LT1117
with the LT3088, change resistor R2 to set the desired
output voltage, and do not stuff resistor R1 (the minimum
load requirement of 2mA for the LT3088 must still be met).
The second method is shown in Figure 5: a 25k resistor is
LT1117
VIN
+
IN
VOUT
OUT
ADJ
IADJ
50µA
VREF
1.25V
( )
R2
VOUT = VREF 1 + — + IADJ R2
R1
R1
R2
3088 F03
Figure 3. LT1117 Basic Adjustable Regulator
LT3088
VIN
+
IN
VOUT
OUT
SET
R1*
ISET
50µA
OUT
SET
GND
R2
VOUT = ISET • R2
*DO NOT STUFF R1
3088 F04
Figure 4. Upgrade to LT1117 Requires No Layout Changes
LT3088
VIN
3088 F02
Figure 2. Guard Ring Layout Example of DD Package
+
IN
ISET
50µA
VOUT
OUT
SET
25k
( )
R2
VOUT = 1.25V 1 +—
R1
1.25V
+ ISET R2
R1
R2
3088 F05
Figure 5. Resistor in Series with SET Pin Matches
LT1117 Operation
10
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added in series with the SET pin of the LT3088 and uses
the same existing resistor divider. This technique can be
used to easily satisfy the LT3088’s 2mA minimum load
current requirement.
Configuring the LT3088 as a Current Source
Setting the LT3088 to operate as a 2-terminal current
source is a simple matter. The 50µA reference current from
the SET pin is used with one resistor to generate a small
voltage, usually in the range of 100mV to 1V (200mV is a
level that rejects offset voltage, line regulation, and other
errors without being excessively large). This voltage is
then applied across a second resistor that connects from
OUT to the first resistor. Figure 6 shows connections and
formulas to calculate a basic current source configuration.
Again, the lower current levels used in the LT3088 necessitate attention to board leakages as error sources (see the
Programming Linear Regulator Output Voltage section).
Selecting RSET and ROUT in Current Source Applications
In Figure 6, both resistors RSET and ROUT program the
value of the output current. The question now arises: the
ratio of these resistors is known, but what value should
each resistor be?
IN
LT3088
+
–
VSET = 50µA • RSET
SET
OUT
+
VSET
IOUT =
VSET 50µA • RSET
=
ROUT
ROUT
3088 F06
RSET
ROUT
–
From this point, selecting ROUT is easy, as it is a straightforward calculation from RSET. Take note, however, resistor
errors must be accounted for as well. While larger voltage
drops across RSET minimize the error due to offset, they
also increase the required operating headroom.
Obtaining the best temperature coefficient does not require
the use of expensive resistors with low ppm temperature
coefficients. Instead, since the output current of the LT3088
is determined by the ratio of RSET to ROUT, those resistors should have matching temperature characteristics.
Less expensive resistors made from the same material
provide matching temperature coefficients. See resistor
manufacturers’ data sheets for more details.
Higher output currents necessitate the use of higher wattage resistors for ROUT. There may be a difference between
the resistors used for ROUT and RSET. A better method to
maintain consistency in resistors is to use multiple resistors in parallel to create ROUT, allowing the same wattage
and type of resistor as RSET.
Stability and Input Capacitance
IOUT ≥ 2mA
50µA
The first resistor to select is RSET. The value selected should
generate enough voltage to minimize the error caused by
the offset between the SET and OUT pins. A reasonable
starting level is ~200mV of voltage across RSET (RSET equal
to 4.02k). Resultant errors due to offset voltage are a few
percent. The lower the voltage across RSET becomes, the
higher the error term due to the offset.
IOUT
Figure 6. Using the LT3088 as a Current Source
The LT3088 does not require an input capacitor to maintain stability. Input capacitors are recommended in linear
regulator configurations to provide a low impedance input
source to the LT3088. If using an input capacitor, low
ESR, ceramic input bypass capacitors are acceptable for
applications without long input leads. However, applications connecting a power supply to an LT3088 circuit’s
IN and GND pins with long input wires combined with
low ESR, ceramic input capacitors are prone to voltage
spikes, reliability concerns and application-specific board
oscillations. The input wire inductance found in many
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LT3088
battery-powered applications, combined with the low ESR
ceramic input capacitor, forms a high Q LC resonant tank
circuit. In some instances this resonant frequency beats
against the output current dependent LDO bandwidth and
interferes with proper operation. Simple circuit modifications/solutions are then required. This behavior is not
indicative of LT3088 instability, but is a common ceramic
input bypass capacitor application issue.
The self-inductance, or isolated inductance, of a wire is
directly proportional to its length. Wire diameter is not a
major factor on its self-inductance. For example, the selfinductance of a 2-AWG isolated wire (diameter = 0.26") is
about half the self-inductance of a 30-AWG wire (diameter
= 0.01"). One foot of 30-AWG wire has about 465nH of
self inductance.
One of two ways reduces a wire’s self-inductance. One
method divides the current flowing towards the LT3088
between two parallel conductors. In this case, the farther
apart the wires are from each other, the more the selfinductance is reduced; up to a 50% reduction when placed
a few inches apart. Splitting the wires basically connects
two equal inductors in parallel, but placing them in close
proximity gives the wires mutual inductance adding to
the self-inductance. The second and most effective way
to reduce overall inductance is to place both forward and
return current conductors (the input and GND wires) in
very close proximity. Two 30-AWG wires separated by
only 0.02", used as forward and return current conductors, reduce the overall self-inductance to approximately
one-fifth that of a single isolated wire.
If wiring modifications are not permissible for the applications, including series resistance between the power supply
and the input of the LT3088 also stabilizes the application.
As little as 0.1Ω to 0.5Ω, often less, is effective in damping the LC resonance. If the added impedance between
the power supply and the input is unacceptable, adding
ESR to the input capacitor also provides the necessary
damping of the LC resonance. However, the required ESR
is generally higher than the series impedance required.
12
Stability and Frequency Compensation for Linear
Regulator Configurations
The LT3088 does not require an output capacitor for
stability. LTC recommends an output capacitor of 10µF
with an ESR of 0.5Ω or less to provide good transient
performance in linear regulator configurations. Larger
values of output capacitance decrease peak deviations and
provide improved transient response for larger load current
changes. Bypass capacitors, used to decouple individual
components powered by the LT3088, increase the effective output capacitor value. For improvement in transient
performance, place a capacitor across the voltage setting
resistor. Capacitors up to 1µF can be used. This bypass
capacitor reduces system noise as well, but start-up time
is proportional to the time constant of the voltage setting
resistor (RSET in Figure 1) and SET pin bypass capacitor.
Stability and Frequency Compensation for Current Source
Configurations
The LT3088 does not require input or output capacitors
for stability in many current-source applications. Clean,
tight PCB layouts provide a low reactance, well controlled
operating environment for the LT3088 without requiring
capacitors to frequency compensate the circuit. Figure 6
highlights the simplicity of using the LT3088 as a current
source.
Some current source applications use a capacitor connected in parallel with the SET pin resistor to lower the
current source’s noise. This capacitor also provides a
soft-start function for the current source. See Quieting the
Noise section for further details. When operating without
output capacitors, the high impedance nature of the SET
pin as the input of the error amplifier allows signal from
the output to couple in, showing as high frequency ringing during transients. Bypassing the SET resistor with a
capacitor in the range of 20pF to 30pF dampens the ringing.
Depending on the pole introduced by a capacitor or other
complex impedances presented to the LT3088, external
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compensation may be required for stability. Techniques
are discussed to achieve this in the following paragraphs.
Linear Technology strongly recommends testing stability
in situ with final components before beginning production.
Although the LT3088’s design strives to be stable without
capacitors over a wide variety of operating conditions, it is
not possible to test for all possible combinations of input
and output impedances that the LT3088 will encounter.
These impedances may include resistive, capacitive, and
inductive components and may be complex distributed
networks. In addition, the current source’s value will differ between applications and its connection may be GND
referenced, power supply referenced, or floating in a signal
line path. Linear Technology strongly recommends that
stability be tested in situ for any LT3088 application.
In LT3088 applications with long wires or PCB traces, the
inductive reactance may cause instability. In some cases,
adding series resistance to the input and output lines (as
shown in Figure 7) may sufficiently dampen these possible
high-Q lines and provide stability. The user must evaluate
the required resistor values against the design’s headroom
constraints. In general, operation at low output current
levels (<20mA) automatically requires higher values of
programming resistors and may provide the necessary
damping without additional series impedance.
If the line impedances in series with the LT3088 are
complex enough such that series damping resistors are
not sufficient, a frequency compensation network may be
necessary. Several options may be considered.
Figure 8 depicts the simplest frequency compensation
networks as a single capacitor across the two terminals
of the current source. Some applications may use the
capacitance to stand off DC voltage but allow the transfer
of data down a signal line.
IN
LT3088
RCOMP
50µA
LONG LINE
REACTANCE/INDUCTANCE
RSERIES
SET
RSET
IN
LT3088
CCOMP OR
+
–
CCOMP
OUT
ROUT
3088 F08
50µA
+
–
Figure 8. Compensation from Input to Output
of Current Source Provides Stability
SET
OUT
RSET
ROUT
3088 F07
RSERIES
LONG LINE
REACTANCE/INDUCTANCE
Figure 7. Adding Series Resistance Decouples
and Dampens Long Line Reactances
For some applications, pure capacitance may be unacceptable or present a design constraint. One circuit example
typifying this is an “intrinsically-safe” circuit in which an
overload or fault condition potentially allows the capacitor’s stored energy to create a spark or arc. For applications where a single capacitor is unacceptable, Figure 8
alternately shows a series RC network connected across
the two terminals of the current source. This network has
the added benefit of limiting the discharge current of the
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13
LT3088
APPLICATIONS INFORMATION
capacitor under a fault condition, preventing sparks or
arcs. In many instances, a series RC network is the best
solution for stabilizing the application circuit. Typical resistor values will range from 100Ω to 5k. Once again, Linear
Technology strongly recommends testing stability in situ
for any LT3088 application across all operating conditions,
especially ones that present complex impedance networks
at the input and output of the current source.
If an application refers the bottom of the LT3088 current
source to GND, it may be necessary to bypass the top
of the current source with a capacitor to GND. In some
cases, this capacitor may already exist and no additional
capacitance is required. For example, if the LT3088 is
used as a variable current source on the output of a power
supply, the output bypass capacitance would suffice to
provide LT3088 stability. Other applications may require
the addition of a bypass capacitor. A series RC network
may also be used as necessary, and depends on the application requirements.
In some extreme cases, capacitors or series RC networks
may be required on both the LT3088’s input and output to
stabilize the circuit. Figure 9 depicts a general application
using input and output capacitor networks rather than
an input-to-output capacitor. As the input of the current
source tends to be high impedance, placing a capacitor
on the input does not have the same effect as placing a
capacitor on the lower impedance output. Capacitors in the
range of 0.1µF to 1µF usually provide sufficient bypassing
on the input, and the value of input capacitance may be
increased without limit. Pay careful attention to using low
ESR input capacitors with long input lines (see the Stability and Input Capacitance section for more information).
Using Ceramic Capacitors
Give extra consideration to the use of ceramic capacitors.
Ceramic capacitors are manufactured with a variety of dielectrics, each with different behavior across temperature
and applied voltage. The most common dielectrics used
14
VIN
RIN
IN
LT3088
CIN
50µA
+
–
SET
RSET
OUT
ROUT
IOUT
COUT OR
ROUT
COUT
3088 F09
Figure 9. Input and/or Output Capacitors May
Be Used for Compensation
are specified with EIA temperature characteristic codes of
Z5U, Y5V, X5R and X7R. The Z5U and Y5V dielectrics are
good for providing high capacitances in a small package,
but they tend to have strong voltage and temperature coefficients as shown in Figures 10 and 11. When used with
a 5V regulator, a 16V 10µF Y5V capacitor can exhibit an
effective value as low as 1µF to 2µF for the DC bias voltage
applied and over the operating temperature range. The X5R
and X7R dielectrics result in more stable characteristics
and are more suitable for use as the output capacitor.
The X7R type has better stability across temperature,
while the X5R is less expensive and is available in higher
values. Care still must be exercised when using X5R and
X7R capacitors. The X5R and X7R codes only specify
operating temperature range and maximum capacitance
change over temperature. Capacitance change due to DC
bias with X5R and X7R capacitors is better than Y5V and
Z5U capacitors, but can still be significant enough to drop
capacitor values below appropriate levels. Capacitor DC
bias characteristics tend to improve as component case
size increases, but expected capacitance at operating
voltage should be verified.
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to promote equal current sharing. PC trace resistance in
milliohms/inch is shown in Table 2. Ballasting requires
only a tiny area on the PCB.
40
CHANGE IN VALUE (%)
20
X5R
0
Table 2. PC Board Trace Resistance
–20
–40
WEIGHT (oz)
10mil WIDTH
1
54.3
2
27.1
Trace resistance is measured in mΩ/in.
Y5V
–60
–80
BOTH CAPACITORS ARE 16V,
1210 CASE SIZE, 10µF
–100
–50 –25
50
25
75
0
TEMPERATURE (°C)
125
100
3088 F10
Figure 10. Ceramic Capacitor Temperature Characteristics
20
BOTH CAPACITORS ARE 16V,
1210 CASE SIZE, 10µF
CHANGE IN VALUE (%)
0
X5R
–20
–40
–60
Y5V
–80
–100
0
2
4
8
6
10 12
DC BIAS VOLTAGE (V)
14
The worst-case room temperature offset, only ±1.5mV
between the SET pin and the OUT pin, allows the use of
very small ballast resistors.
As shown in Figure 12, each LT3088 has a small 10mΩ
ballast resistor, which at full output current gives better
than 80% equalized sharing of the current. The external
resistance of 10mΩ (5mΩ for the two devices in parallel)
only adds about 8mV of output regulation drop at an output
of 1.6A. Even with an output voltage as low as 1V, this
only adds 0.8% to the regulation. Of course, paralleling
more than two LT3088s yields even higher output current.
Spreading the devices on the PC board also spreads the
heat. Series input resistors can further spread the heat if
the input-to-output difference is high.
LT3088
IN
16
3088 F11
50µA
+
–
Figure 11. Ceramic Capacitor DC Bias Characteristics
Voltage and temperature coefficients are not the only
sources of problems. Some ceramic capacitors have a
piezoelectric response. A piezoelectric device generates
voltage across its terminals due to mechanical stress. In a
ceramic capacitor, the stress can be induced by vibrations
in the system or thermal transients.
20mil WIDTH
27.1
13.6
SET
OUT
LT3088
IN
VIN
4.8V TO 36V
10mΩ
50µA
+
–
1µF
Paralleling Devices
SET
Higher output current is obtained by paralleling multiple
LT3088s together. Tie the individual SET pins together and
tie the individual IN pins together. Connect the outputs in
common using small pieces of PC trace as ballast resistors
OUT
33k
10mΩ
10µF
VOUT
3.3V
1.6A
3088 F12
Figure 12. Parallel Devices
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LT3088
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Quieting the Noise
The LT3088 offers numerous noise performance advantages. Every linear regulator has its sources of noise. In
general, a linear regulator’s critical noise source is the
reference. In addition, consider the error amplifier’s noise
contribution along with the resistor divider’s noise gain.
Many traditional low noise regulators bond out the voltage
reference to an external pin (usually through a large value
resistor) to allow for bypassing and noise reduction. The
LT3088 does not use a traditional voltage reference like
other linear regulators. Instead, it uses a 50µA reference
current. The 50µA current source generates noise current
levels of 18pA/√Hz (5.7nARMS over a 10Hz to 100kHz
bandwidth). The equivalent voltage noise equals the RMS
noise current multiplied by the resistor value.
The SET pin resistor generates spot noise equal to √4kTR
(k = Boltzmann’s constant, 1.38 • 10–23J/°K, and T is absolute temperature) which is RMS summed with the voltage
noise. If the application requires lower noise performance,
bypass the voltage setting resistor with a capacitor to GND.
Note that this noise-reduction capacitor increases start-up
time as a factor of the RC time constant.
The LT3088 uses a unity-gain follower from the SET pin
to the OUT pin. Therefore, multiple possibilities exist
(besides a SET pin resistor) to set output voltage. For
example, using a high accuracy voltage reference from
SET to GND removes the errors in output voltage due to
reference current tolerance and resistor tolerance. Active
driving of the SET pin is acceptable.
The typical noise scenario for a linear regulator is that the
output voltage setting resistor divider gains up the reference
noise, especially if VOUT is much greater than VREF. The
LT3088’s noise advantage is that the unity-gain follower
presents no noise gain whatsoever from the SET pin to the
output. Thus, noise figures do not increase accordingly.
Error amplifier noise is typical 85nV/√Hz(27µVRMS over
a 10Hz to 100kHz bandwidth). The error amplifier’s noise
is RMS summed with the other noise terms to give a final
noise figure for the regulator.
16
Paralleling of regulators adds the benefit that output noise
is reduced. For n regulators in parallel, the output noise
drops by a factor of √n.
Curves in the Typical Performance Characteristics section show noise spectral density and peak-to-peak noise
characteristics for both the reference current and error
amplifier over a 10Hz to 100kHz bandwidth.
Load Voltage Regulation
The LT3088 is a floating device. No ground pin exists on
the packages. Thus, the IC delivers all quiescent current
and drive current to the load. Therefore, it is not possible
to provide true remote load sensing. The connection resistance between the regulator and the load determines
load regulation performance. The data sheet’s load
regulation specification is Kelvin sensed at the package’s
pins. Negative-side sensing is a true Kelvin connection by
returning the bottom of the voltage setting resistor to the
negative side of the load (see Figure 13).
Connected as shown, system load regulation is the sum
of the LT3088’s load regulation and the parasitic line
resistance multiplied by the output current. To minimize
load regulation, keep the positive connection between the
regulator and load as short as possible. If possible, use
large diameter wire or wide PC board traces.
LT3088
IN
50µA
+
–
PARASITIC
RESISTANCE
SET
OUT
RSET
RP
RP
LOAD
RP
3088 F13
Figure 13. Connections for Best Load Regulation
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Thermal Considerations
Table 3. DD Package, 8-Lead DFN
The LT3088’s internal power and thermal limiting circuitry
protects itself under overload conditions. For continuous
normal load conditions, do not exceed the 125°C (E- and
I-grades) or 150°C (H- and MP-grades) maximum junction temperature. Carefully consider all sources of thermal
resistance from junction-to-ambient. This includes (but is
not limited to) junction-to-case, case-to-heat sink interface, heat sink resistance or circuit board-to-ambient as
the application dictates. Consider all additional, adjacent
heat generating sources in proximity on the PCB.
Surface mount packages provide the necessary heat
sinking by using the heat spreading capabilities of the
PC board, copper traces and planes. Surface mount heat
sinks, plated through-holes and solder-filled vias can also
spread the heat generated by power devices.
Junction-to-case thermal resistance is specified from the
IC junction to the bottom of the case directly, or the bottom of the pin most directly in the heat path. This is the
lowest thermal resistance path for heat flow. Only proper
device mounting ensures the best possible thermal flow
from this area of the packages to the heat sinking material.
Note that the exposed pad of the DFN package and the
tab of the DD-Pak and SOT-223 packages are electrically
connected to the output (VOUT).
Tables 3 through 5 list thermal resistance as a function
of copper areas on a fixed board size. All measurements
were taken in still air on a 4-layer FR-4 board with 1oz
solid internal planes and 2oz external trace planes with a
total finished board thickness of 1.6mm.
COPPER AREA
THERMAL RESISTANCE
(JUNCTION-TO-AMBIENT)
TOPSIDE*
BACKSIDE
BOARD AREA
2500mm2
2500mm2
2500mm2
26°C/W
1000mm2
2500mm2
2500mm2
26°C/W
225mm2
2500mm2
2500mm2
28°C/W
100mm2
2500mm2
2500mm2
31°C/W
*Device is mounted on topside
Table 4. ST Package, 3-Lead SOT-223
COPPER AREA
THERMAL RESISTANCE
(JUNCTION-TO-AMBIENT)
TOPSIDE*
BACKSIDE
BOARD AREA
2500mm2
2500mm2
2500mm2
23°C/W
1000mm2
2500mm2
2500mm2
23°C/W
225mm2
2500mm2
2500mm2
25°C/W
100mm2
2500mm2
2500mm2
27°C/W
*Device is mounted on topside
Table 5. M Package, 3-Lead DD-Pak
COPPER AREA
TOPSIDE*
BACKSIDE
BOARD AREA
THERMAL RESISTANCE
(JUNCTION-TO-AMBIENT)
2500mm2
2500mm2
2500mm2
13°C/W
1000mm2
2500mm2
2500mm2
14°C/W
225mm2
2500mm2
2500mm2
16°C/W
*Device is mounted on topside
For further information on thermal resistance and using
thermal information, refer to JEDEC standard JESD51,
notably JESD51-12.
PCB layers, copper weight, board layout and thermal vias
affect the resultant thermal resistance. Tables 3 through 5
provide thermal resistance numbers for best-case 4-layer
boards with 1oz internal and 2oz external copper. Modern,
multilayer PCBs may not be able to achieve quite the same
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LT3088
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level performance as found in these tables. Demo circuit
DC2279A’s board layout using multiple inner VOUT planes
and multiple thermal vias achieves 18°C/W performance
for the DD package.
Calculating Junction Temperature
Example: Given an output voltage of 0.9V, an IN voltage
of 2.5V ±5%, output current range from 10mA to 0.5A
and a maximum ambient temperature of 50°C, what is
the maximum junction temperature for the DD-Pak on a
2500mm2 board with topside copper of 1000mm2?
The power in the circuit equals:
PTOTAL = (VIN – VOUT)(IOUT)
The current delivered to the SET pin is negligible and can
be ignored.
VIN(MAX_CONTINUOUS) = 2.625V (2.5V + 5%)
Reducing Power Dissipation
In some applications it may be necessary to reduce the
power dissipation in the LT3088 package without sacrificing
output current capability. Two techniques are available. The
first technique, illustrated in Figure 14, employs a resistor in series with the regulator’s input. The voltage drop
across RS decreases the LT3088’s IN-to-OUT differential
voltage and correspondingly decreases the LT3088’s
power dissipation.
As an example, assume: VIN = 7V, VOUT = 3.3V and
IOUT(MAX) = 0.8A. Use the formulas from the Calculating
Junction Temperature section previously discussed.
Without series resistor RS, power dissipation in the
LT3088 equals:
PTOTAL = (7V – 3.3V) • 0.8A = 2.96W
If the voltage differential (VDIFF) across the LT3088 is
chosen as 1.5V, then RS equals:
VOUT = 0.9V, IOUT = 0.5A, TA = 50°C
Power dissipation under these conditions equals:
RS =
PTOTAL = (VIN – VOUT)(IOUT)
7V – 3.3V – 1.5V
= 2.75Ω
0.8A
Power dissipation in the LT3088 now equals:
PTOTAL = (2.625V – 0.9V)(0.5A) = 0.87W
PTOTAL = 1.5V • 0.8A = 1.2W
Junction Temperature equals:
TJ = TA + PTOTAL • θJA (using tables)
TJ = 50°C + 0.87W • 14°C/W = 62°C
In this case, the junction temperature is below the maximum rating, ensuring reliable operation.
The LT3088’s power dissipation is now only 40% compared
to no series resistor. RS dissipates 1.75W of power. Choose
appropriate wattage resistors or use multiple resistors in
parallel to handle and dissipate the power properly.
VIN
RS
VIN′
C1
IN
LT3088
50µA
+
–
SET
RSET
OUT
C2
VOUT
3088 F14
Figure 14. Reducing Power Dissipation Using a Series Resistor
18
3088fb
For more information www.linear.com/LT3088
LT3088
APPLICATIONS INFORMATION
The second technique for reducing power dissipation,
shown in Figure 15, uses a resistor in parallel with the
LT3088. This resistor provides a parallel path for current
flow, reducing the current flowing through the LT3088.
This technique works well if input voltage is reasonably
constant and output load current changes are small. This
technique also increases the maximum available output
current at the expense of minimum load requirements.
RP dissipates 0.58W of power. As with the first technique,
choose appropriate wattage resistors to handle and dissipate the power properly. With this configuration, the
LT3088 supplies only 0.55A. Therefore, load current can
increase by 0.25A to a total output current of 1.05A while
keeping the LT3088 in its normal operating range.
As an example, assume: VIN = 5V, VIN(MAX) = 5.5V, VOUT
= 3.3V, VOUT(MIN) = 3.2V, IOUT(MAX) = 0.8A and IOUT(MIN)
= 0.3A. Also, assuming that RP carries no more than 90%
of IOUT(MIN) = 270mA.
Care must be taken when designing the LT3088H/
LT3088MP applications to operate at high ambient temperatures. The LT3088H/LT3088MP operates at high
temperatures, but erratic operation can occur due to unforeseen variations in external components. Some tantalum
capacitors are available for high temperature operation, but
ESR is often several ohms; capacitor ESR above 0.5Ω is
unsuitable for use with the LT3088H/LT3088MP. Multiple
ceramic capacitor manufacturers now offer ceramic capacitors that are rated to 150°C using an X8R dielectric. Check
each passive component for absolute value and voltage
ratings over the operating temperature range.
Calculating RP yields:
5.5V – 3.2V
= 8.52Ω
0.27A
(5% Standard value = 9.1Ω)
RP =
The maximum total power dissipation is:
(5.5V – 3.2V) • 0.8A = 1.84W
Leakages in capacitors or from solder flux left after insufficient board cleaning adversely affects low current nodes,
such as the SET pins. Consider junction temperature increase due to power dissipation in both the junction and
nearby components to ensure maximum specifications
are not violated for the LT3088H/LT3088MP or external
components.
However, the LT3088 supplies only:
0.8A –
High Temperature Operation
5.5V – 3.2V
= 0.55A
9.1Ω
Therefore, the LT3088’s power dissipation is only:
PDISS = (5.5V – 3.2V) • 0.55A = 1.26W
VIN
C1
IN
LT3088
50µA
RP
+
–
SET
RSET
OUT
VOUT
C2
3088 F15
Figure 15. Reducing Power Dissipation Using a Parallel Resistor
3088fb
For more information www.linear.com/LT3088
19
LT3088
APPLICATIONS INFORMATION
Protection Features
The LT3088 incorporates several protection features ideal
for harsh industrial and automotive environments, among
other applications. In addition to normal monolithic regulator protection features such as current limiting and thermal
limiting, the LT3088 protects itself against reverse-input
voltages, reverse-output voltages, and large OUT-to-SET
pin voltages.
Current limit protection and thermal overload protection
protect the IC against output current overload conditions.
For normal operation, do not exceed the rated absolute
maximum junction temperature. The thermal shutdown
circuit’s temperature threshold is typically 165°C and
incorporates about 5°C of hysteresis.
The LT3088’s IN pin withstands ±40V voltages with respect
to the OUT and SET pins. Reverse current flow, if OUT is
greater than IN, is less than 1mA (typically under 100µA),
protecting the LT3088 and sensitive loads.
Clamping diodes and 400Ω limiting resistors protect the
LT3088’s SET pin relative to the OUT pin voltage. These
protection components typically only carry current under
transient overload conditions. These devices are sized to
handle ±10V differential voltages and ±25mA crosspin
current flow without damage. Relative to these application concerns, note the following two scenarios. The first
scenario employs a noise-reducing SET pin bypass capacitor while OUT is instantaneously shorted to GND. The
second scenario follows improper shutdown techniques
in which the SET pin is reset to GND quickly while OUT
is held up by a large output capacitance with light load.
During normal operation, keep OUT-to-SET differential
voltages below 2V.
TYPICAL APPLICATIONS
Paralleling Regulators
Boosting Fixed Output Regulators
VIN
IN
LT3088
IN
LT3088
ISET
50µA
ISET
50µA
+
–
+
–
OUT
VOUT
3V
1.6A
10mΩ
SET
SET
3088 TA03
5V
IN
LT3088
SET
20mΩ
LT1963-3.3
10µF
ISET
50µA
+
–
OUT 20mΩ
8.2Ω*
3.3VOUT
2.3A
47µF
6.2k
OUT
10mΩ
*4mV DROP ENSURES LT3088 IS OFF WITH NO LOAD
3088 TA02
MULTIPLE LT3088s CAN BE USED
RSET
30.1k
20
3088fb
For more information www.linear.com/LT3088
LT3088
TYPICAL APPLICATIONS
Reference Buffer
Adding Soft-Start
VIN
VIN
4.8V TO 38V
10µF
IN
LT3088
ISET
50µA
+
–
LT1019
GND
+
–
IN4148
OUT
VOUT
SET
INPUT
IN
LT3088
ISET
50µA
SET
OUTPUT
47µF
*
1µF
OUT
VOUT
3.3V
10µF 0.8A
3088 TA05
0.1µF
3088 TA04
66.5k
*MIN LOAD 2mA
Using a Lower Value Set Resistor
VIN
12V
4.7µF
IN
LT3088
ISET
50µA
+
–
OUT
VOUT
0.2V TO 10V
4.7µF
SET
4.02k
RSET
2k
40.2Ω
3088 TA06
VOUT = 0.2V + 5mA • RSET
Using an External Reference Current
VIN
1µF
LT3092
ISET
50µA
10µA
+
–
SET
20k
IN
LT3088
IN
+
–
OUT
SET
205Ω
1mA
OUT
1µF
VOUT
0V TO 20V
3088 TA07
20k
3088fb
For more information www.linear.com/LT3088
21
LT3088
PACKAGE DESCRIPTION
Please refer to http://www.linear.com/product/LT3088#packaging for the most recent package drawings.
DD Package
8-Lead Plastic DFN (3mm × 3mm)
(Reference LTC DWG # 05-08-1698 Rev C)
0.70 ±0.05
3.5 ±0.05
1.65 ±0.05
2.10 ±0.05 (2 SIDES)
PACKAGE
OUTLINE
0.25 ±0.05
0.50
BSC
2.38 ±0.05
RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS
APPLY SOLDER MASK TO AREAS THAT ARE NOT SOLDERED
PIN 1
TOP MARK
(NOTE 6)
0.200 REF
3.00 ±0.10
(4 SIDES)
R = 0.125
TYP
5
0.40 ±0.10
8
1.65 ±0.10
(2 SIDES)
0.75 ±0.05
4
0.25 ±0.05
1
(DD8) DFN 0509 REV C
0.50 BSC
2.38 ±0.10
0.00 – 0.05
BOTTOM VIEW—EXPOSED PAD
NOTE:
1. DRAWING TO BE MADE A JEDEC PACKAGE OUTLINE M0-229 VARIATION OF (WEED-1)
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 TOP AND BOTTOM OF PACKAGE
22
3088fb
For more information www.linear.com/LT3088
LT3088
PACKAGE DESCRIPTION
Please refer to http://www.linear.com/product/LT3088#packaging for the most recent package drawings.
ST Package
3-Lead Plastic SOT-223
(Reference LTC DWG # 05-08-1630)
.248 – .264
(6.30 – 6.71)
.129 MAX
.114 – .124
(2.90 – 3.15)
.059 MAX
.264 – .287
(6.70 – 7.30)
.248 BSC
.130 – .146
(3.30 – 3.71)
.039 MAX
.059 MAX
.181 MAX
.033 – .041
(0.84 – 1.04)
.0905
(2.30)
BSC
RECOMMENDED SOLDER PAD LAYOUT
10° – 16°
.010 – .014
(0.25 – 0.36)
10°
MAX
.071
(1.80)
MAX
.090
BSC
10° – 16°
.024 – .033
(0.60 – 0.84)
.181
(4.60)
BSC
.012
(0.31)
MIN
.0008 – .0040
(0.0203 – 0.1016)
ST3 (SOT-233) 0502
3088fb
For more information www.linear.com/LT3088
23
LT3088
PACKAGE DESCRIPTION
Please refer to http://www.linear.com/product/LT3088#packaging for the most recent package drawings.
M Package
3-Lead Plastic DD Pak
(Reference LTC DWG # 05-08-1460 Rev F)
.256
(6.502)
.060
(1.524)
TYP
.060
(1.524)
.390 – .415
(9.906 – 10.541)
.165 – .180
(4.191 – 4.572)
.045 – .055
(1.143 – 1.397)
15°
.060
(1.524)
.183
(4.648)
.330 – .370
(8.382 – 9.398)
+.008
.004 –.004
+0.203
0.102 –0.102
(
.059
(1.499)
)
.095 – .115
(2.413 – 2.921)
.075
(1.905)
.300
(7.620)
+.012
.143 –.020
+0.305
3.632 –0.508
(
BOTTOM VIEW OF DD PAK
HATCHED AREA IS SOLDER PLATED
COPPER HEAT SINK
)
DETAIL A
.050
(1.270)
.100
(2.54)
BSC
.013 – .023
(0.330 – 0.584)
.050 ±.012
(1.270 ±0.305)
DETAIL A
0° – 7° TYP
.080
.420
.350
0° – 7° TYP
.420
.276
.325
.205
.585
.585
.320
.090
.100
.070
RECOMMENDED SOLDER PAD LAYOUT
NOTE:
1. DIMENSIONS IN INCH/(MILLIMETER)
2. DRAWING NOT TO SCALE
24
.090
.100
.070
M (DD3) 0212 REV F
RECOMMENDED SOLDER PAD LAYOUT
FOR THICKER SOLDER PASTE APPLICATIONS
3088fb
For more information www.linear.com/LT3088
LT3088
REVISION HISTORY
REV
DATE
DESCRIPTION
A
9/15
Corrected Load Regulation conditions
B
10/15
PAGE NUMBER
3
Corrected graph scales
5
Corrected thermal numbers on parallel resistor use
19
Corrected pin numbers on SOT-223 package
2
3088fb
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
its circuits
as described
herein will not infringe on existing patent rights.
For of
more
information
www.linear.com/LT3088
25
LT3088
TYPICAL APPLICATION
High Efficiency Adjustable Supply
VIN
6.3V TO
36V
15k
63.4k
1000pF
VIN
BD
RUN/SS BOOST
VC
RT
PG
LT3680
SYNC GND
0.47µF 6.8µH
IN
SW
MBRA340T3
47µF
6V
590k
OUT
22µF
LT3088
VOUT
0V TO
25V,
800mA
SET
MTD2955
FB
500k
15k
0.1µF
10k
1µF
1k
2N3904
3088 TA08
1µF
CMDSH-4E
RELATED PARTS
PART NUMBER DESCRIPTION
COMMENTS
LT1185
3A Negative Low Dropout Regulator VIN: –4.5V to –35V, 0.8V Dropout Voltage, DD-Pak and TO-220 Packages
LT1764/
LT1764A
3A, Fast Transient Response,
Low Noise LDO
340mV Dropout Voltage, Low Noise: 40µVRMS, VIN = 2.7V to 20V, TO-220, TSSOP and DD-Pak,
LT1764A Version Stable Also with Ceramic Capacitors
LT1963/
LT1963A
1.5A Low Noise, Fast Transient
Response LDO
340mV Dropout Voltage, Low Noise: 40µVRMS, VIN = 2.5V to 20V, LT1963A Version Stable with
Ceramic Capacitors, TO-220, DD, TSSOP, SOT-223 and SO-8 Packages
LT1965
1.1A, Low Noise, Low Dropout
Linear Regulator
290mV Dropout Voltage, Low Noise: 40µVRMS, VIN: 1.8V to 20V, VOUT: 1.2V to 19.5V, Stable with
Ceramic Capacitors, TO-220, DD-Pak, MSOP and 3mm × 3mm DFN Packages
LT3022
1A, Low Voltage, VLDO Linear
Regulator
VIN: 0.9V to 10V, Dropout Voltage: 145mV Typical, Adjustable Output (VREF = VOUT(MIN) = 200mV),
Stable with Low ESR, Ceramic Output Capacitors, 16-Pin DFN (5mm × 3mm) and 16-Lead
MSOP Packages
LT3070
5A, Low Noise, Programmable
VOUT, 85mV Dropout Linear
Regulator with Digital Margining
Dropout Voltage: 85mV, Digitally Programmable VOUT: 0.8V to 1.8V, Digital Output Margining: ±1%,
±3% or ±5%, Low Output Noise: 25µVRMS (10Hz to 100kHz), Parallelable: Use Two for a 10A Output,
Stable with Low ESR Ceramic Output Capacitors (15µF Minimum), 28-Lead 4mm × 5mm QFN Package
LT3071
5A, Low Noise, Programmable
Dropout Voltage: 85mV, Digitally Programmable VOUT: 0.8V to 1.8V, Analog Margining: ±10%,
Low Output Noise: 25µVRMS (10Hz to 100kHz), Parallelable: Use Two for a 10A Output, IMON Output
VOUT, 85mV Dropout Linear
Regulator with Analog Margining Current Monitor, Stable with Low ESR Ceramic Output Capacitors (15µF Minimum) 28-Lead
4mm × 5mm QFN Package
LT3080/
LT3080-1
1.1A, Parallelable, Low Noise,
Low Dropout Linear Regulator
300mV Dropout Voltage (2-Supply Operation), Low Noise: 40µVRMS, VIN: 1.2V to 36V, VOUT: 0V to 35.7V,
Current-Based Reference with 1-Resistor VOUT Set; Directly Parallelable (No Op Amp Required),
Stable with Ceramic Capacitors, TO-220, DD-Pak, SOT-223, MS8E and 3mm × 3mm DFN-8 Packages;
LT3080-1 Version Has Integrated Internal Ballast Resistor
LT3082
200mA, Parallelable, Single
Resistor, Low Dropout Linear
Regulator
Outputs May Be Paralleled for Higher Output, Current or Heat Spreading, Wide Input Voltage
Range: 1.2V to 40V Low Value Input/Output Capacitors Required: 2.2µF, Single Resistor Sets Output
Voltage 8-Lead SOT-23, 3-Lead SOT-223 and 8-Lead 3mm × 3mm DFN Packages
LT3085
500mA, Parallelable, Low Noise,
Low Dropout Linear Regulator
275mV Dropout Voltage (2-Supply Operation), Low Noise: 40µVRMS, VIN: 1.2V to 36V,
VOUT: 0V to 35.7V, Current-Based Reference with 1-Resistor VOUT Set; Directly Parallelable
(No Op Amp Required), Stable with Ceramic Capacitors, MS8E and 2mm × 3mm DFN-6 Packages
LT3092
200mA 2-Terminal Programmable Programmable 2-Terminal Current Source, Maximum Output Current = 200mA, Wide Input Voltage
Current Source
Range: 1.2V to 40V, Resistor Ratio Sets Output Current, Initial Set Pin Current Accuracy = 1%, Current
Limit and Thermal Shutdown Protection, Reverse-Voltage Protection, Reverse-Current Protection,
8-Lead SOT-23, 3-Lead SOT-223 and 8-Lead 3mm × 3mm DFN Packages.
LT3083
Adjustable 3A Single Resistor
Low Dropout Regulator
26
Low Noise: 40µVRMS, 50µA Set Pin Current, Output Adjustable to 0V, Wide Input Voltage Range: 1.2V to 23V
(DD-Pak and TO-220), Low Dropout Operation: 310mV (2 Supplies)
3088fb
Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
For more information www.linear.com/LT3088
(408) 432-1900 ● FAX: (408) 434-0507
●
www.linear.com/LT3088
LT 1015 REV B • PRINTED IN USA
 LINEAR TECHNOLOGY CORPORATION 2015