LT3085
1
3085fb
TYPICAL APPLICATION
FEATURES
APPLICATIONS
DESCRIPTION
Adjustable 500mA Single
Resistor Low Dropout
Regulator
The LT
®
3085 is a 500mA low dropout linear regulator that
can be paralleled to increase output current or spread
heat on surface mounted boards. Designed as a precision
current source and voltage follower, this new regulator
fi nds use in many applications requiring high current,
adjustability to zero, and no heat sink. The device also
brings out the collector of the pass transistor to allow
low dropout operation—down to 275mV—when used
with a second supply.
A key feature of the LT3085 is the capability to supply a
wide output voltage range. By using a reference current
through a single resistor, the output voltage is programmed
to any level between zero and 36V. The LT3085 is stable
with 2.2μF of capacitance on the output, and the IC uses
small ceramic capacitors that do not require additional
ESR as is common with other regulators.
Internal protection circuitry includes current limiting
and thermal limiting. The LT3085 is offered in the 8-lead
MSOP and a low profi le (0.75mm) 6-lead 2mm × 3mm
DFN package (both with an Exposed Pad for better thermal
characteristics).
Variable Output Voltage 500mA Supply
n Outputs May be Paralleled for Higher Current and
Heat Spreading
n Output Current: 500mA
n Single Resistor Programs Output Voltage
n 1% Initial Accuracy of SET Pin Current
n Output Adjustable to 0V
n Current Limit Constant with Temperature
n Low Output Noise: 40μVRMS (10Hz to 100kHz)
n Wide Input Voltage Range: 1.2V to 36V
n Low Dropout Voltage: 275mV
n < 1mV Load Regulation
n < 0.001%/ V Line Regulation
n Minimum Load Current: 0.5mA
n Stable with Minimum 2.2μF Ceramic Capacitor
n Current Limit with Foldback and Overtemperature
Protected
n 8-Lead MSOP, and 6-Lead 2mm × 3mm DFN Packages
n High Current All Surface Mount Supply
n High Effi ciency Linear Regulator
n Post Regulator for Switching Supplies
n Low Parts Count Variable Voltage Supply
n Low Output Voltage Power Supplies
+
–
LT3085
IN
VIN
1.2V TO 36V
VCONTROL
OUT
3085 TA01a
SET
1μF
2.2μF
RSET
VOUT = RSET • 10μA
VOUT
SET PIN CURRENT DISTRIBUTION (μA)
10.20
3085 TA01b
9.90 10.00 10.10
9.80
N = 1676
L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks and VLDO
and ThinSOT are trademarks of Linear Technology Corporation. All other trademarks are the
property of their respective owners.
LT3085
2
3085fb
ABSOLUTE MAXIMUM RATINGS
VCONTROL Pin Voltage ..................................... 40V, –0.3V
IN Pin Voltage ................................................ 40V, –0.3V
SET Pin Current (Note 7) .................................... ±15mA
SET Pin Voltage (Relative to OUT) ..........................±10V
Output Short-Circuit Duration .......................... Indefi nite
(Note 1) All Voltages Relative to VOUT
TOP VIEW
IN
IN
VCONTROL
OUT
OUT
SET
DCB PACKAGE
6-LEAD (2mm s 3mm) PLASTIC DFN
4
5
7
6
3
2
1
TJMAX = 125°C, θJA = 73°C/W, θJC = 10.6°C/W
EXPOSED PAD (PIN 7) IS OUT, MUST BE SOLDERED TO VOUT ON PCB
SEE THE APPLICATIONS INFORMATION SECTION
1
2
3
4
OUT
OUT
OUT
SET
8
7
6
5
IN
IN
NC
VCONTROL
TOP VIEW
MS8E PACKAGE
8-LEAD PLASTIC MSOP
9
TJMAX = 125°C, θJA = 60°C/W, θJC = 10°C/W
EXPOSED PAD (PIN 9) IS OUT, MUST BE SOLDERED TO VOUT ON PCB
SEE THE APPLICATIONS INFORMATION SECTION
PIN CONFIGURATION
ORDER INFORMATION
LEAD FREE FINISH TAPE AND REEL PART MARKING* PACKAGE DESCRIPTION TEMPERATURE RANGE
LT3085EDCB#PBF LT3085EDCB#TRPBF LDQQ 6-Lead (2mm × 3mm) Plastic DFN –40°C to 125°C
LT3085EMS8E#PBF LT3085EMS8E#TRPBF LTDQP 8-Lead Plastic MSOP –40°C to 125°C
LT3085IDCB#PBF LT3085IDCB#TRPBF LDQQ 6-Lead (2mm × 3mm) Plastic DFN –40°C to 125°C
LT3085IMS8E#PBF LT3085IMS8E#TRPBF LTDQP 8-Lead Plastic MSOP –40°C to 125°C
LT3085MPMS8E#PBF LT3085MPMS8E#TRPBF LTDWQ 8-Lead Plastic MSOP –55°C to 125°C
LEAD BASED FINISH TAPE AND REEL PART MARKING* PACKAGE DESCRIPTION TEMPERATURE RANGE
LT3085EDCB LT3085EDCB#TR LDQQ 6-Lead (2mm × 3mm) Plastic DFN –40°C to 125°C
LT3085EMS8E LT3085EMS8E#TR LTDQP 8-Lead Plastic MSOP –40°C to 125°C
LT3085IDCB LT3085IDCB#TR LDQQ 6-Lead (2mm × 3mm) Plastic DFN –40°C to 125°C
LT3085IMS8E LT3085IMS8E#TR LTDQP 8-Lead Plastic MSOP –40°C to 125°C
LT3085MPMS8E LT3085MPMS8E#TR LTDWQ 8-Lead Plastic MSOP –55°C to 125°C
Consult LTC Marketing for parts specifi ed with wider operating temperature ranges. *The temperature grade is identifi ed 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 specifi cations, go to: http://www.linear.com/tapeandreel/
Operating Junction Temperature Range (Notes 2, 10)
E, I Grade ........................................... –40°C to 125°C
MP Grade ........................................... –55°C to 125°C
Storage Temperature Range ................... –65°C to 150°C
Lead Temperature (Soldering, 10 sec)
MS8E Package Only .......................................... 300°C
LT3085
3
3085fb
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.
Note 2. Unless otherwise specifi ed, all voltages are with respect to VOUT
.
The LT3085 is tested and specifi ed under pulse load conditions such that
TJ ≅ TA. The LT3085E is 100% tested at TA = 25°C. Performance of the
LT3085E over the full –40°C to 125°C operating junction temperature
range is assured by design, characterization, and correlation with
statistical process controls. The LT3085I regulators are guaranteed
over the full –40°C to 125°C operating junction temperature range. The
LT3085 (MP grade) is 100% tested and guaranteed over the –55°C to
125°C operating junction temperature range.
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.
Note 4. For the LT3085, dropout is caused by either minimum control
voltage (VCONTROL) or minimum input voltage (VIN). Both parameters are
specifi ed with respect to the output voltage. The specifi cations represent
the minimum input-to-output differential voltage required to maintain
regulation.
PARAMETER CONDITIONS MIN TYP MAX UNITS
SET Pin Current ISET VIN = 1V, VCONTROL = 2V, ILOAD = 1mA, TJ = 25°C
VIN ≥ 1V, VCONTROL ≥ 2V, 1mA ≤ ILOAD ≤ 500mA (Note 9) l
9.9
9.8
10
10
10.1
10.2
μA
μA
Output Offset Voltage (VOUT – VSET) VOS VIN = 1V, VCONTROL = 2V, ILOAD = 1mA, TJ = 25°C
VIN = 1V, VCONTROL = 2V, ILOAD = 1mA l
–1.5
–3
1.5
3
mV
mV
Load Regulation ΔISET
ΔVOS
ΔILOAD = 1mA to 500mA
ΔILOAD = 1mA to 500mA (Note 8) l
–0.1
–0.6 –1
nA
mV
Line Regulation ΔISET
ΔVOS
ΔVIN = 1V to 36V, ΔVCONTROL = 2V to 36V, ILOAD = 1mA
ΔVIN = 1V to 36V, ΔVCONTROL = 2V to 36V, ILOAD = 1mA
0.1
0.003
0.5 nA/V
mV/V
Minimum Load Current (Notes 3, 9) VIN = VCONTROL = 10V
VIN = VCONTROL = 36V
l
l
300 500
1
μA
mA
VCONTROL Dropout Voltage (Note 4) ILOAD = 100mA
ILOAD = 500mA l
1.2
1.35 1.6
V
V
VIN Dropout Voltage (Note 4) ILOAD = 100mA
ILOAD = 500mA
l
l
85
275
150
450
mV
mV
VCONTROL Pin Current (Note 5) ILOAD = 100mA
ILOAD = 500mA
l
l
3
8
6
15
mA
mA
Current Limit (Note 9) VIN = 5V, VCONTROL = 5V, VSET = 0V, VOUT = –0.1V l500 650 mA
Error Amplifi er RMS Output Noise (Note 6) ILOAD = 500mA, 10Hz ≤ f ≤ 100kHz, COUT = 10μF, CSET = 0.1μF 33 μVRMS
Reference Current RMS Output Noise (Note 6) 10Hz ≤ f≤ 100kHz 0.7 nARMS
Ripple Rejection f = 120Hz, VRIPPLE = 0.5VP-P
, ILOAD = 0.1A, CSET = 0.1μF, COUT = 2.2μF
f=10kHz
f=1MHz
90
75
20
dB
dB
dB
Thermal Regulation, ISET 10ms Pulse 0.003 %/W
The l denotes the specifi cations which apply over the full operating
temperature range, otherwise specifi cations are at TA = 25°C (Note 2).
Note 5. The VCONTROL pin current is the drive current required for the
output transistor. This current will track output current with roughly a 1:60
ratio. The minimum value is equal to the quiescent current of the device.
Note 6. Output noise is lowered by adding a small capacitor across the
voltage setting resistor. Adding this capacitor bypasses the voltage setting
resistor shot noise and reference current noise; output noise is then equal
to error amplifi er noise (see Applications Information section).
Note 7. The SET pin is clamped to the output with diodes through 1k
resistors. These resistors and diodes will only carry current under
transient overloads.
Note 8. Load regulation is Kelvin sensed at the package.
Note 9. Current limit includes foldback protection circuitry. Current limit
decreases at higher input-to-output differential voltages. See the Typical
Performance Characteristics graphs for more information.
Note 10. This IC includes over-temperature protection that is intended
to protect the device during momentary overload conditions. Junction
temperature will exceed the maximum operating junction temperature
when over-temperature protection is active. Continuous operation above
the specifi ed maximum operating junction temperature may impair device
reliability.
LT3085
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TYPICAL PERFORMANCE CHARACTERISTICS
TEMPERATURE (°C)
–50
SET PIN CURRENT (μA)
10.00
10.10
150
3085 G01
9.90
9.80 050 100
–25 25 75 125
10.20
9.95
10.05
9.85
10.15
SET PIN CURRENT DISTRIBUTION (μA)
10.20
3085 G02
9.90 10.00 10.10
9.80
N = 1676
TEMPERATURE (°C)
–50
OFFSET VOLTAGE (mV)
0
1.0
150
3085 G03
–1.0
–2.0 050 100
–25 25 75 125
2.0
–0.5
0.5
–1.5
1.5
VOS DISTRIBUTION (mV)
2
3085 G04
–1 01
–2
N = 1676
INPUT-TO-OUTPUT VOLTAGE (V)
0
OFFSET VOLTAGE (mV)
–0.25
0
0.25
18 30
3085 G05
–0.50
–0.75
–1.00 612 24
0.50
0.75
1.00
36
ILOAD = 1mA
LOAD CURRENT (mA)
0
OFFSET VOLTAGE (mV)
–1.00
–0.75
–0.50
250 300 450
3085 G06
–1.25
–1.50
–1.75 50 100 150 200 350 400
–0.25
0
0.25
500
TJ = 25°C
TJ = 125°C
TEMPERATURE (°C)
–50
CHANGE IN OFFSET VOLTAGE WITH LOAD (mV)
CHANGE IN REFERENCE CURRENT
WITH LOAD (nA)
150
3085 G07
050 100
–25 25 75 125
020
CHANGE IN REFERENCE CURRENT
CHANGE IN OFFSET VOLTAGE
(VOUT – VSET)
–0.4
–0.2
–0.6
–0.8
–0.5
–0.3
–0.7
–0.1
–20
0
–40
–60
–30
–10
–50
10
ΔILOAD = 1mA TO 500mA
VIN – VOUT = 2V
TEMPERATURE (°C)
–50
MINIMUM LOAD CURRENT (mA)
0.4
0.6
150
3085 G08
0.2
0050 100
–25 25 75 125
0.8
0.3
0.5
0.1
0.7 VIN, CONTROL – VOUT = 36V
VIN, CONTROL – VOUT = 1.5V
LOAD CURRENT (mA)
0
MINIMUM IN VOLTAGE (VIN – VOUT) (mV)
150
200
250
3085 G09
100
50
0
300
350
400
0 250 300 45050 100 150 200 350 400 500
TJ = 125°C
TJ = 25°C
Set Pin Current Set Pin Current Distribution Offset Voltage (VOUT – VSET)
Offset Voltage Distribution Offset Voltage Offset Voltage
Load Regulation Minimum Load Current
Dropout Voltage
(Minimum IN Voltage)
LT3085
5
3085fb
TYPICAL PERFORMANCE CHARACTERISTICS
TEMPERATURE (°C)
–50
MINIMUM IN VOLTAGE (VIN – VOUT) (mV)
200
300
150
3085 G10
100
0050 100
–25 25 75 125
400
150
250
50
350
ILOAD = 500mA
ILOAD = 100mA
LOAD CURRENT (mA)
0
MINIMUM CONTROL VOLTAGE
(VCONTROL – VOUT) (V)
0.6
0.8
1.0
3085 G11
0.4
0.2
0
1.2
1.4
1.6
250 300 45050 100 150 200 350 400 500
TJ = 125°C
TJ = 25°C
TJ = –50°C
TEMPERATURE (°C)
–50
MINIMUM CONTROL VOLTAGE
(VCONTROL – VOUT) (V)
0.8
1.2
150
3085 G12
0.4
0050 100
–25 25 75 125
1.6
0.6
1.0
0.2
1.4 ILOAD = 500mA
ILOAD = 100mA
TEMPERATURE (°C)
–50
CURRENT LIMIT (mA)
300
500
150
3085 G13
0050 100
–25 25 75 125
700
200
400
100
600
VIN = 7V
VOUT = 0V
INPUT-TO-OUTPUT DIFFERENTIAL (V)
0
CURRENT LIMIT (mA)
300
400
500
20 30
3085 G14
200
100
051015 25
600
700
35 40
TJ = 25°C
TIME (μs)
0
OUTPUT VOLTAGE
DEVIATION (mV)
LOAD CURRENT
(mA)
–20
60
160
3085 G15
200
–40
0
40
20
100
04020 8060 120 140 180
100 200
VOUT = 1.5V
CSET = 0.1μF
VIN = VCONTROL = 3V
COUT = 10μF CERAMIC
COUT = 2.2μF CERAMIC
TIME (μs)
0
–50
50
150
80
3085 G16
–100
0
100
500
250
02010 4030 60 70 90
50 100
VIN = VCONTROL = 3V
VOUT = 1.5V
CSET = 0.1μF
OUTPUT VOLTAGE
DEVIATION (mV)
LOAD CURRENT
(mA)
COUT = 10μF CERAMIC
COUT = 2.2μF CERAMIC
TIME (μs)
0
IN/CONTROL
VOLTAGE (V)
OUTPUT VOLTAGE
DEVIATION (mV)
–50
100
80
3085 G17
6
4
–100
0
50
0
2
2010 4030 60 70 90
50 100
VOUT = 1.5V
ILOAD = 10mA
COUT = 2.2μF
CERAMIC
CSET = 0.1μF
CERAMIC
TIME (μs)
0
IN/CONTROL
VOLTAGE (V) OUTPUT VOLTAGE (V)
0
1.5
16
3085 G18
6
4
8
0.5
1
0
2
42 86 12 14 18
10 20
COUT = 2.2μF
CERAMIC
RSET = 100k
CSET = 0
RLOAD = 2Ω
Dropout Voltage
(Minimum IN Voltage)
Dropout Voltage (Minimum
VCONTROL Pin Voltage)
Dropout Voltage (Minimum
VCONTROL Pin Voltage)
Current Limit Current Limit Load Transient Response
Load Transient Response Line Transient Response Turn-On Response
LT3085
6
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TYPICAL PERFORMANCE CHARACTERISTICS
INPUT-TO-OUTPUT DIFFERENTIAL (V)
0
0
CONTROL PIN CURRENT (mA)
2
4
6
8
16
14
12
10
20
612 18 24
3085 G19
30 36
18
ILOAD = 1mA
ILOAD = 500mA
DEVICE IN
CURRENT LIMIT
LOAD CURRENT (A)
0 0.1 0.2
CONTROL PIN CURRENT (mA)
8
7
6
5
4
3
2
1
0
3085 G20
0.50.3 0.4
TJ = –50°C
TJ = 25°C
TJ = 125°C
VIN = VCONTROL = 2V
VIN = VOUT = 1V
RTEST (Ω)
0
OUTPUT VOLTAGE (mV)
800
700
600
500
400
300
200
100
0
3085 G21
2k1k
VIN = 10V
SET PIN = 0V
VIN VOUT
RTEST
VIN = 20V
VIN = 5V
FREQUENCY (Hz)
0
RIPPLE REJECTION (dB)
40
100
10k 100k10010 1k 1M
3085 G22
20
60
80
30
90
10
50
70
VIN = VCONTROL = VOUT (NOMINAL) +2V
RIPPLE = 50mVP–P
COUT = 2.2μF CERAMIC
CSET = 0.1μF CERAMIC
ILOAD = 100mA
ILOAD = 500mA
FREQUENCY (Hz)
0
RIPPLE REJE
C
TI
O
N (dB)
40
100
10k 100k10010 1k 1
M
3085
G
23
20
60
80
30
90
10
50
70
VIN = VOUT (NOMINAL) + 1V
VCONTROL = VOUT (NOMINAL) +2V
RIPPLE = 50mVP–P
COUT = 2.2μF CERAMIC
CSET = 0.1μF CERAMIC
ILOAD = 100mA
ILOAD = 500mA
FREQUENCY (Hz)
10
0
RIPPLE REJECTION (dB)
10
30
40
50
100
70
100 10k
3085 G24
20
80
90
60
1k 100k 1M
VIN = VCONTROL +2V
VCONTROL = VOUT (NOMINAL) +2V
RIPPLE = 50mVP–P
COUT = 2.2μF CERAMIC
CSET = 0.1μF CERAMIC
ILOAD = 100mA
ILOAD = 500mA
VCONTROL Pin Current VCONTROL Pin Current
Residual Output Voltage with
Less Than Minimum Load
Ripple Rejection - Single Supply
Ripple Rejection - Dual Supply
- VCONTROL Pin
Ripple Rejection - Dual Supply
- IN Pin
FREQUENCY (Hz)
1
ERROR AMPLIFIER NOISE
SPECTRAL DENSITY (nV/√Hz)
REFERENCE CURRENT NOISE
SPECTRAL DENSITY (pA/ √Hz)
10k
10k 100k10010 1k
3085 G26
100
10
1k
0.1
1k
10
1.0
100
Noise Spectral Density
VOUT
100μV/DIV
TIME 1ms/DIV 3085 G27
VOUT = 1V
RSET = 100k
CSET = O.1μF
COUT = 10μF
ILOAD = 0.5A
Output Voltage Noise
FREQUENCY (Hz)
77
78
RIPPLE REJECTION (dB)
85
84
100 125 1500–50 –25 25 50 75
3085 G25
81
82
80
79
83
SINGLE SUPPLY OPERATION
VIN = VOUT (NOMINAL) +2V
RIPPLE = 50mVP–P, f = 120Hz
ILOAD = 0.1A
COUT = 2.2μF, CSET = 0.1μF
Ripple Rejection (120Hz)
LT3085
7
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TYPICAL PERFORMANCE CHARACTERISTICS
Error Amplifi er Gain and Phase
FREQUENCY (Hz)
10
GAIN (dB)
PHASE (deg)
9
15
21
100k
3085 G28
3
–3
6
12
18
0
–6
–9
–72
72
216
–216
–360
–144
0
144
–288
–432
–504
100 1k 10k 1M
ILOAD = 500mA
ILOAD = 100mA
ILOAD = 100mA
ILOAD = 500mA
Ripple Rejection - SET Pin Current
FREQUENCY (Hz)
0
RIPPLE REJECTION (dB)
60
150
10k 100k10010 1k 1M
3085 G29
30
90
120
45
135
15
75
105
CSET = 0.1μF
CSET = 0
RSET = 100k
VIN = VCONTROL = VOUT (NOMINAL) +2V
RIPPLE = 50mVP–P
PIN FUNCTIONS
VCONTROL (Pin 4/Pin 5): This pin is the supply pin for the
control circuitry of the device. The current fl ow into this
pin is about 1.7% of the output current. For the device to
regulate, this voltage must be more than 1.2V to 1.35V
greater than the output voltage (see VCONTROL Dropout
Voltage in the Electrical Characteristics table and graphs
in the Typical Performance Characteristics). The LT3085
requires a bypass capacitor at VCONTROL if more than six
inches away from the main input fi lter capacitor. The output
impedance of a battery rises with frequency, so include
a bypass capacitor in battery-powered circuits. A bypass
capacitor in the range of 1μF to 10μF suffi ces.
IN (Pins 5, 6/Pins 7, 8): This is the collector to the power
device of the LT3085. The output load current is supplied
through this pin. For the device to regulate, the voltage at
this pin must be more than 0.1V to 0.5V greater than the
output voltage (see VIN Dropout Voltage in the Electrical
Characteristics table and graphs in the Typical Perfor-
mance Characteristics). The LT3085 requires a bypass
capacitor at IN if more than six inches away from the main
input fi lter capacitor. The output impedance of a battery
rises with frequency, so include a bypass capacitor in
battery-powered circuits. A bypass capacitor in the range
of 1μF to 10μF suffi ces.
NC (NA/Pin 6): No Connection. The No Connect pin has
no connection to internal circuitry and may be tied to VIN,
VCONTROL, VOUT, GND, or fl oated.
OUT (Pins 1, 2/Pins 1, 2, 3): This is the power output
of the device. There must be a minimum load current of
1mA or the output may not regulate. A minimum 2.2μF
output capacitor is required for stability.
SET (Pin 3/Pin 4): This pin is the non-inverting input to the
error amplifi er and the regulation set point for the device.
A fi xed current of 10μA fl ows out of this pin through a
single external resistor, which programs the output voltage
of the device. Output voltage range is zero to the absolute
maximum rated output voltage. Transient performance can
be improved and output noise can be decreased by adding
a small capacitor from the SET pin to ground.
Exposed Pad (Pin 7/Pin 9): OUT. Tie directly to Pins 1, 2/
Pins 2, 3 directly at the PCB.
(DCB/MS8E)
LT3085
8
3085fb
BLOCK DIAGRAM
–
+
VCONTROL
IN
10μA
3085 BD
OUTSET
APPLICATIONS INFORMATION
The LT3085 regulator is easy to use and has all the pro-
tection features expected in high performance regulators.
Included are short-circuit protection and safe operating
area protection, as well as thermal shutdown.
The LT3085 is especially well suited to applications needing
multiple rails. The new architecture adjusts down to zero
with a single resistor, handling modern low voltage digital
IC’s as well as allowing easy parallel operation and thermal
management without heat sinks. Adjusting to “zero” output
allows shutting off the powered circuitry and when the
input is pre-regulated – such as a 5V or 3.3V input supply
– external resistors can help spread the heat.
A precision “0” TC 10μA internal current source is connected
to the non-inverting input of a power operational amplifi er.
The power operational amplifi er provides a low impedance
buffered output to the voltage on the non-inverting input.
A single resistor from the non-inverting input to ground
sets the output voltage and if this resistor is set to zero,
zero output results. As can be seen, any output voltage
can be obtained from zero up to the maximum defi ned by
the input power supply.
What is not so obvious from this architecture are the ben-
efi ts of using a true internal current source as the reference
as opposed to a bootstrapped reference in older regulators.
A true current source allows the regulator to have gain
and frequency response independent of the impedance on
the positive input. Older adjustable regulators, such as the
LT1086, have a change in loop gain with output voltage
as well as bandwidth changes when the adjustment pin
is bypassed to ground. For the LT3085, the loop gain is
unchanged by changing the output voltage or bypassing.
Output regulation is not fi xed at a percentage of the output
voltage but is a fi xed fraction of millivolts. Use of a true
current source allows all the gain in the buffer amplifi er
to provide regulation and none of that gain is needed to
amplify up the reference to a higher output voltage.
The LT3085 has the collector of the output transistor
connected to a separate pin from the control input. Since the
dropout on the collector (IN pin) is only 275mV, two supplies
can be used to power the LT3085 to reduce dissipation: a
higher voltage supply for the control circuitry and a lower
voltage supply for the collector. This increases effi ciency and
reduces dissipation. To further spread the heat, a resistor
can be inserted in series with the collector to move some
of the heat out of the IC and spread it on the PC board.
LT3085
9
3085fb
The LT3085 can be operated in two modes. Three terminal
mode has the control pin connected to the power input pin
which gives a limitation of 1.35V dropout. Alternatively, the
“control” pin can be tied to a higher voltage and the power
IN pin to a lower voltage giving 275mV dropout on the
IN pin and minimizing the power dissipation. This allows
for a 500mA supply regulating from 2.5VIN to 1.8VOUT or
1.8VIN to 1.2VOUT with low dissipation.
Setting Output Voltage
The LT3085 generates a 10μA reference current that fl ows
out of the SET pin. Connecting a resistor from SET to
ground generates a voltage that becomes the reference
point for the error amplifi er (see Figure 1). The reference
voltage is a straight multiplication of the SET pin current
and the value of the resistor. Any voltage can be generated
and there is no minimum output voltage for the regulator.
Table 1 lists many common output voltages and standard
1% resistor values used to generate that output voltage.
A minimum load current of 1mA is required to maintain
regulation regardless of output voltage. For true zero
voltage output operation, this 1mA load current must be
returned to a negative supply voltage.
APPLICATIONS INFORMATION
With the low level current 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., Tefl on, Kel-F); cleaning
of all insulating surfaces to remove fl uxes and other resi-
dues will probably be required. Surface coating may be
necessary to provide a moisture barrier in high humidity
environments.
Table 1. 1% Resistors for Common Output Voltages
VOUT RSET
1V 100k
1.2V 121k
1.5V 150k
1.8V 182k
2.5V 249k
3.3V 332k
5V 499k
Board leakage can be minimized by encircling the SET
pin and circuitry with a guardring operated at a potential
close to itself; the guardring should be tied to the OUT pin.
Guarding both sides of the circuit board is required. Bulk
leakage reduction depends on the guard ring width. Ten
nanoamperes of leakage into or out of the SET pin and
associated circuitry creates a 0.1% error in the reference
voltage. Leakages of this magnitude, coupled with other
sources of leakage, can cause signifi cant offset voltage and
reference drift, especially over a wide temperature range.
If guardring 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 suffi cient.
Figure 1. Basic Adjustable Regulator
+
–
LT3085
10μA
IN
VCONTROL
VCONTROL
OUT
3085 F01
SET
COUT
RSET
VOUT
CSET
+
VIN
+
VOUT = RSET • 10μA
LT3085
10
3085fb
APPLICATIONS INFORMATION
Input Capacitance and Stability
The LT3085 is designed to be stable with a minimum
capacitance of 1μF at each input pin. Ceramic capacitors
with low ESR are available for use to bypass these pins,
but in cases where long wires connect the LT3085 inputs
to a power supply (and also from ground of the LT3085
circuitry back to power supply ground), this causes insta-
bilities. This happens due to the wire inductance forming
an LC tank circuit with the input capacitor and not as a
result of instability on the LT3085.
The self-inductance, or isolated inductance, of a wire is
directly proportional to its length. The diameter does not
have a major infl uence on its self-inductance. As an ex-
ample, the self-inductance of a 2-AWG isolated wire with a
diameter of 0.26in. is approximately half the self-inductance
of a 30-AWG wire with a diameter of 0.01in. One foot of
30-AWG wire has 465nH of self-inductance.
The overall self-inductance of a wire is reduced in one of
two ways. One is to divide the current fl owing towards
the LT3085 between two parallel conductors. In this
case, the farther apart the wires are from each other, the
more the self-inductance is reduced, up to a 50% reduc-
tion 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
wire for the input and the wire for ground) in very close
proximity. Two 30-AWG wires separated by only 0.02in.
used as forward- and return-current conductors reduce
the overall self-inductance to approximately one-fi fth that
of a single isolated wire.
If the LT3085 is powered by a battery mounted in close
proximity on the same circuit board, a 2.2μF input capaci-
tor is suffi cient for stability. When powering from distant
supplies, use a larger input capacitor based on a guide-
line of 1μF plus another 1μF per 8 inches of wire length.
As power supply impedance does vary, the amount of
capacitance needed to stabilize your application will also
vary. Extra capacitance placed directly on the output of
the power supply requires an order of magnitude more
capacitance as opposed to placing extra capacitance close
to the LT3085.
Using series resistance between the power supply and
the input of the LT3085 also stabilizes the application.
As little as 0.1Ω to 0.5Ω, often less, is all that is needed
to provide damping in the circuit. If the extra impedance
between the power supply and the input is unacceptable,
placing the resistors in series with the capacitors will pro-
vide damping to prevent the LC resonance from causing
full-blown oscillation.
Stability and Output Capacitance
The LT3085 requires an output capacitor for stability. It
is designed to be stable with most low ESR capacitors
(typically ceramic, tantalum or low ESR electrolytic). A
minimum output capacitor of 2.2μF with an ESR of 0.5Ω
or less is recommended to prevent oscillations. 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 LT3085, increase
the effective output capacitor value.
For improvement in transient performance, place a capaci-
tor 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.
Extra consideration must be given 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 are specifi ed with EIA temperature
characteristic codes of Z5U, Y5V, X5R and X7R. The Z5U and
Y5V dielectrics are good for providing high capacitances
LT3085
11
3085fb
APPLICATIONS INFORMATION
DC BIAS VOLTAGE (V)
CHANGE IN VALUE (%)
3085 F02
20
0
–20
–40
–60
–80
–100 04810
26 12 14
X5R
Y5V
16
BOTH CAPACITORS ARE 16V,
1210 CASE SIZE, 10μF
Figure 2. Ceramic Capacitor DC Bias Characteristics
TEMPERATURE (°C)
–50
40
20
0
–20
–40
–60
–80
–100 25 75
3085 F03
–25 0 50 100 125
Y5V
CHANGE IN VALUE (%)
X5R
BOTH CAPACITORS ARE 16V,
1210 CASE SIZE, 10μF
Figure 3. Ceramic Capacitor Temperature Characteristics
in a small package, but they tend to have strong voltage
and temperature coeffi cients as shown in Figures 2
and 3. 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 signifi cant 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 verifi ed.
Voltage and temperature coeffi cients 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,
ceramic capacitor the stress can be induced by vibrations
in the system or thermal transients.
Paralleling Devices
LT3085’s may be paralleled with other LT308X devices to
obtain higher output current. The SET pins are tied together
and the IN pins are tied together. This is the same whether
it’s in three terminal mode or has separate input supplies.
The outputs are connected in common using a small piece
of PC trace as a ballast resistor to equalize the currents.
PC trace resistance in milliohms/inch is shown in Table
1. Only a tiny area is needed for ballasting.
Table 1. PC Board Trace Resistance
WEIGHT (oz) 10 mil WIDTH 20 mil WIDTH
1 54.3 27.1
2 27.1 13.6
Trace resistance is measured in mΩ/in
LT3085
12
3085fb
+
–
LT3080
VIN
VCONTROL
OUT
SET
10mΩ
+
–
LT3085
VIN
VIN
4.8V TO 28V
VOUT
3.3V
1.5A
VCONTROL
OUT
10μF
1μF
SET
165k
3085 F04
20mΩ
Figure 4. Parallel Devices
APPLICATIONS INFORMATION
The worst-case offset between the SET pin and the output
of only ±1.5mV allows very small ballast resistors to be
used. As shown in Figure 4, the two devices have a small
10mΩ and 20mΩ ballast resistors, which at full output
current gives better than 80% equalized sharing of the
current. The external resistance of 20mΩ (6.6mΩ for the
two devices in parallel) only adds about 10mV of output
regulation drop at an output of 1.5A. Even with an output
voltage as low as 1V, this only adds 1% to the regulation.
Of course, more than two LT308X’s can be paralleled for
even higher output current. They are spread out on the
PC board, spreading the heat. Input resistors can further
spread the heat if the input-to-output difference is high.
Thermal Performance
In this example, two LT3085 2mm × 3mm DFN devices
are mounted on a 1oz copper 4-layer PC board. They are
placed approximately 1.5 inches apart and the board is
mounted vertically for convection cooling. Two tests were
set up to measure the cooling performance and current
sharing of these devices.
The first test was done with approximately 1.6V
input- to-output and 0.5A per device. This gave a 800mW
dissipation in each device and a 1A output current. The
temperature rise above ambient is approximately 28°C
and both devices were within plus or minus 1°C. Both the
thermal and electrical sharing of these devices is excel-
lent. The thermograph in Figure 5 shows the temperature
distribution between these devices and the PC board
reaches ambient temperature within about a half an inch
from the devices.
The power is then increased with 3.4V across each device.
This gives 1.7 watts dissipation in each device and a device
temperature of about 90°C, about 65°C above ambient
as shown in Figure 6. Again, the temperature matching
Figure 6. Temperature Rise at 1.7W Dissipation
Figure 5. Temperature Rise at 800mW Dissipation
LT3085
13
3085fb
APPLICATIONS INFORMATION
The LT3085 uses a unity-gain follower from the SET pin
to drive the output, and there is no requirement to use
a resistor to set the output voltage. Use a high accuracy
voltage reference placed at the SET pin to remove the er-
rors in output voltage due to reference current tolerance
and resistor tolerance. Active driving of the SET pin is
acceptable; the limitations are the creativity and ingenuity
of the circuit designer.
One problem that a normal linear regulator sees with refer-
ence voltage noise is that noise is gained up along with the
output when using a resistor divider to operate at levels
higher than the normal reference voltage. With the LT3085,
the unity-gain follower presents no gain whatsoever from
the SET pin to the output, so noise fi gures do not increase
accordingly. Error amplifi er noise is typically 100nV/√Hz
(33μVRMS over the 10Hz to 100kHz bandwidth); this is
another factor that is RMS summed in to give a fi nal noise
fi gure for the regulator.
Curves in the Typical Performance Characteristics show
noise spectral density and peak-to-peak noise character-
istics for both the reference current and error amplifi er
over the 10Hz to 100kHz bandwidth.
Overload Recovery
Like many IC power regulators, the LT3085 has safe operat-
ing area (SOA) protection. The SOA protection decreases
current limit as the input-to-output voltage increases and
keeps the power dissipation at safe levels for all values
of input-to-output voltage. The LT3085 provides some
output current at all values of input-to-output voltage up
to the device breakdown. See the Current Limit curve in
the Typical Performance Characteristics.
When power is fi rst turned on, the input voltage rises and
the output follows the input, allowing the regulator to start
into very heavy loads. During start-up, as the input voltage
is rising, the input-to-output voltage differential is small,
allowing the regulator to supply large output currents.
With a high input voltage, a problem can occur wherein
removal of an output short will not allow the output volt-
age to recover. Other regulators, such as the LT1085 and
LT1764A, also exhibit this phenomenon so it is not unique
to the LT3085.
between the devices is within 2°C, showing excellent
tracking between the devices. The board temperature has
reached approximately 40°C within about 0.75 inches of
each device.
While 90°C is an acceptable operating temperature for these
devices, this is in 25°C ambient. For higher ambients, the
temperature must be controlled to prevent device tempera-
ture from exceeding 125°C. A 3-meter-per-second airfl ow
across the devices will decrease the device temperature
about 20°C providing a margin for higher operating ambi-
ent temperatures.
Both at low power and relatively high power levels de-
vices can be paralleled for higher output current. Current
sharing and thermal sharing is excellent, showing that
acceptable operation can be had while keeping the peak
temperatures below excessive operating temperatures on
a board. This technique allows higher operating current
linear regulation to be used in systems where it could
never be used before.
Quieting the Noise
The LT3085 offers numerous advantages when it comes
to dealing with noise. There are several sources of noise
in a linear regulator. The most critical noise source for any
LDO is the reference; from there, the noise contribution
from the error amplifi er must be considered, and the gain
created by using a resistor divider cannot be forgotten.
Traditional low noise regulators bring the voltage refer-
ence out to an external pin (usually through a large value
resistor) to allow for bypassing and noise reduction of
reference noise. The LT3085 does not use a traditional
voltage reference like other linear regulators, but instead
uses a reference current. That current operates with typi-
cal noise current levels of 2.3pA/√Hz (0.7nARMS over the
10Hz to 100kHz bandwidth). The voltage noise of this is
equal to the noise current multiplied by the resistor value.
The resistor generates spot noise equal to√4kTR (k =
Boltzmann’s constant, 1.38 • 10-23 J/°K, and T is absolute
temperature) which is RMS summed with the reference
current noise. To lower reference noise, the voltage set-
ting resistor may be bypassed with a capacitor, though
this causes start-up time to increase as a factor of the RC
time constant.
LT3085
14
3085fb
APPLICATIONS INFORMATION
The problem occurs with a heavy output load when the
input voltage is high and the output voltage is low. Com-
mon situations are immediately after the removal of a
short circuit. The load line for such a load may intersect
the output current curve at two points. If this happens,
there are two stable operating points for the regulator.
With this double intersection, the input power supply may
need to be cycled down to zero and brought up again to
make the output recover.
Load Regulation
Because the LT3085 is a fl oating device (there is no ground
pin on the part, all quiescent and drive current is delivered
to the load), it is not possible to provide true remote load
sensing. Load regulation will be limited by the resistance
of the connections between the regulator and the load.
The data sheet specifi cation for load regulation is Kelvin
sensed at the pins of the package. Negative side sensing
is a true Kelvin connection, with the bottom of the voltage
setting resistor returned to the negative side of the load
(see Figure 7). Connected as shown, system load regulation
will be the sum of the LT3085 load regulation and the
parasitic line resistance multiplied by the output current.
It is important to keep the positive connection between
the regulator and load as short as possible and use large
wire or PC board traces.
Internal Parasitic Diodes and Protection Diodes
In normal operation, the LT3085 does not require protection
diodes. Older three-terminal regulators require protection
diodes between the VOUT pin and the input pin or between
the ADJ pin and the VOUT pin to prevent die overstress.
Figure 7. Connections for Best Load Regulation
+
–
LT3085
IN
VCONTROL
OUT
3085 F07
SET RSET
RP
PARASITIC
RESISTANCE
RP
RP
LOAD
On the LT3085, internal resistors and diodes limit current
paths on the SET pin. Even with bypass capacitors on the
SET pin, no protection diode is needed to ensure device
safety under short-circuit conditions. The SET pin handles
±10V (either transient or DC) with respect to OUT without
any device degradation.
Internal parasitic diodes exist between OUT and the two
inputs. Negative input voltages are transferred to the output
and may damage sensitive loads. Reverse-biasing either
input to OUT will turn on these parasitic diodes and allow
current fl ow. This current fl ow will bias internal nodes
of the LT3085 to levels that possibly cause errors when
suddenly returning to normal operating conditions and
expecting the device to start and operate. Prediction of
results of a bias fault is impossible, immediate return to
normal operating conditions can be just as diffi cult after
a bias fault. Suffi ce it to say that extra wait time, power
cycling, or protection diodes may be needed to allow the
LT3085 to return to a normal operating mode as quickly
as possible.
Protection diodes are not otherwise needed between
the OUT pin and IN pin. The internal diodes can handle
microsecond surge currents of up to 50A. Even with
large output capacitors, obtaining surge currents of those
magnitudes is diffi cult in normal operation. Only with large
output capacitors, such as 1000μF to 5000μF, and with
IN instantaneously shorted to ground will damage occur.
A crowbar circuit at IN is capable of generating those
levels of currents, and then protection diodes from OUT
to IN are recommended. Normal power supply cycling or
system “hot plugging and unplugging” does not do any
damage.
A protection diode between OUT and VCONTROL is usually
not needed. The internal parasitic diode on VCONTROL of
the LT3085 handles microsecond surge currents of 1A to
10A. Again, this only occurs when using crowbar circuits
with large value output capacitors. Since the VCONTROL
pin is usually a low current supply, this is unlikely. Still,
a protection diode is recommended if VCONTROL can be
instantaneously shorted to ground. Normal power supply
cycling or system “hot plugging and unplugging” does
not do any damage.
LT3085
15
3085fb
If the LT3085 is confi gured as a three-terminal (single supply)
regulator with IN and VCONTROL shorted together, the internal
diode of the IN pin will protect the VCONTROL pin.
Like any other regulator, exceeding the maximum input-
to-output differential causes internal transistors to break
down and then none of the internal protection circuitry
is functional.
Thermal Considerations
The LT3085 has internal power and thermal limiting cir-
cuitry designed to protect it under overload conditions.
For continuous normal load conditions, maximum junc-
tion temperature must not be exceeded. It is important
to give consideration to all sources of thermal resistance
from junction to ambient. This includes junction-to-case,
case-to-heat sink interface, heat sink resistance or circuit
board-to-ambient as the application dictates. Additional
heat sources nearby must also be considered.
For surface mount devices, heat sinking is accomplished
by using the heat spreading capabilities of the PC board
and its copper traces. Surface mount heat sinks and
plated through-holes can also be used to spread the heat
generated by power devices. Boards specifi ed in thermal
resistance tables have no vias on plated through-holes
from topside to backside.
Junction-to-case thermal resistance is specifi ed from
the IC junction to the bottom of the case directly below
the die. This is the lowest resistance path for heat fl ow.
Proper mounting is required to ensure the best possible
thermal fl ow from this area of the package to the heat
sinking material. Note that the Exposed Pad is electrically
connected to the output.
The following tables list thermal resistance for several
different copper areas given a fi xed board size. All mea-
surements were taken in still air on two-sided 1/16” FR-4
board with one ounce copper.
PCB layers, copper weight, board layout and thermal vias
affect the resultant thermal resistance. Although Tables
2 and 3 provide thermal resistance numbers for 2-layer
board with 1 ounce copper, modern multi-layer PCBs
APPLICATIONS INFORMATION
provide better performance than found in these tables.
For example, a 4-layer, 1 ounce copper PCB board with
5 thermal vias from the DFN or MSOP exposed backside
pad to inner layers (connected to V
OUT
) achieves 40
°C/W
thermal resistance. Demo circuit 1401A’s board layout
achieves this
40
°C/W performance. This is approximately
a 45% improvement over the numbers shown in Tables
2 and 3.
Table 2. MSE Package, 8-Lead MSOP
COPPER AREA THERMAL RESISTANCE
(JUNCTION-TO-AMBIENT)
TOPSIDE* BACKSIDE BOARD AREA
2500mm22500mm22500mm255°C/W
1000mm22500mm22500mm257°C/W
225mm22500mm22500mm260°C/W
100mm22500mm22500mm265°C/W
*Device is mounted on topside
Table 3. DCB Package, 6-Lead DFN
COPPER AREA THERMAL RESISTANCE
(JUNCTION-TO-AMBIENT)
TOPSIDE* BACKSIDE BOARD AREA
2500mm22500mm22500mm268°C/W
1000mm22500mm22500mm270°C/W
225mm22500mm22500mm273°C/W
100mm22500mm22500mm278°C/W
*Device is mounted on topside
For future information on the thermal resistance and using thermal
information, refer to JEDEC standard JESD51, notably JESD51-12.
Calculating Junction Temperature
Example: Given an output voltage of 0.9V, a VCONTROL
voltage of 3.3V ±10%, an IN voltage of 1.5V ±5%, output
current range from 1mA to 0.5A and a maximum ambi-
ent temperature of 50°C, what will the maximum junction
temperature be for the DFN package on a 2500mm2 board
with topside copper area of 500mm2?
The power in the drive circuit equals:
P
DRIVE = (VCONTROL – VOUT)(ICONTROL)
where ICONTROL is equal to IOUT/60. ICONTROL is a function
of output current. A curve of ICONTROL vs IOUT can be found
in the Typical Performance Characteristics curves.
LT3085
16
3085fb
APPLICATIONS INFORMATION
Figure 8. Reducing Power Dissipation Using a Series Resistor
+
–
LT3085 IN
VCONTROL
OUT VOUT
VINa
VIN
C2
3085 F08
SET
RSET
RS
C1
The power in the output transistor equals:
P
OUTPUT = (VIN – VOUT)(IOUT)
The total power equals:
P
TOTAL = PDRIVE + POUTPUT
The current delivered to the SET pin is negligible and can
be ignored.
VCONTROL(MAX CONTINUOUS) = 3.630V (3.3V + 10%)
VIN(MAX CONTINUOUS) = 1.575V (1.5V + 5%)
VOUT = 0.9V, IOUT = 0.5A, TA = 50°C
Power dissipation under these conditions is equal to:
P
DRIVE = (VCONTROL – VOUT)(ICONTROL)
ICONTROL =IOUT
60 =0.5A
60 =8.3mA
P
DRIVE = (3.630V – 0.9V)(8.3mA) = 23mW
P
OUTPUT = (VIN – VOUT)(IOUT)
P
OUTPUT = (1.575V – 0.9V)(0.5A) = 337mW
Total Power Dissipation = 360mW
Junction Temperature will be equal to:
T
J = TA + PTOTAL • θJA (approximated using tables)
T
J = 50°C + 360mW • 73°C/W = 76°C
In this case, the junction temperature is below the maximum
rating, ensuring reliable operation.
Reducing Power Dissipation
In some applications it may be necessary to reduce
the power dissipation in the LT3085 package without
sacrifi cing output current capability. Two techniques are
available. The fi rst technique, illustrated in Figure 8, em-
ploys a resistor in series with the regulator’s input. The
voltage drop across RS decreases the LT3085’s IN-to-OUT
differential voltage and correspondingly decreases the
LT3085’s power dissipation.
As an example, assume: VIN = VCONTROL = 5V, VOUT = 3.3V
and IOUT(MAX) = 0.5A. Use the formulas from the Calculating
Junction Temperature section previously discussed.
LT3085
17
3085fb
Without series resistor RS, power dissipation in the LT3085
equals:
PTOTAL =5V – 3.3V
()
•0.5A
60 +5V – 3.3V
()
• 0.5A
=0.86W
If the voltage differential (VDIFF) across the NPN pass
transistor is chosen as 0.5V, then RS equals:
RS=5V – 3.3V −0.5V
0.5A =2.4Ω
Power dissipation in the LT3085 now equals:
PTOTAL =5V – 3.3V
()
•0.5A
60 +0.5V
()
• 0.5A =0.26W
The LT3085’s power dissipation is now only 30% compared
to no series resistor. RS dissipates 0.6W of power. Choose
appropriate wattage resistors to handle and dissipate the
power properly.
The second technique for reducing power dissipation,
shown in Figure 9, uses a resistor in parallel with the
LT3085. This resistor provides a parallel path for current
fl ow, reducing the current fl owing through the LT3085.
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.
As an example, assume: VIN = VCONTROL = 5V, VIN(MAX) =
5.5V, VOUT = 3.3V, VOUT(MIN) = 3.2V, IOUT(MAX) = 0.5A and
IOUT(MIN) = 0.35A. Also, assuming that RP carries no more
than 90% of IOUT(MIN) = 630mA.
Calculating RP yields:
RP=5.5V – 3.2V
315mA =7.30Ω
(5% Standard value = 7.Ω)
The maximum total power dissipation is (5.5V – 3.2V) •
0.5A = 1.2W. However the LT3085 supplies only:
0.5A – 5.5V – 3.2V
7.5Ω=0.193A
Therefore, the LT3085’s power dissipation is only:
P
DIS = (5.5V – 3.2V) • 0.193A = 0.44W
RP dissipates 0.71W of power. As with the fi rst technique,
choose appropriate wattage resistors to handle and dis-
sipate the power properly. With this confi guration, the
LT3085 supplies only 0.36A. Therefore, load current can
increase by 0.3A to 0.143A while keeping the LT3085 in
its normal operating range.
Figure 9. Reducing Power Dissipation Using a Parallel Resistor
+
–
LT3085 IN
VCONTROL
OUT VOUT
VIN
C2
3085 F09
SET
RSET
RP
C1
APPLICATIONS INFORMATION
LT3085
18
3085fb
TYPICAL APPLICATIONS
Higher Output Current
+
–
LT3085
IN
50Ω
MJ4502
VCONTROL
OUT
3085 TA02
SET 4.7μF
332k
VOUT
3.3V
5A
+
1μF
100μF
+
100μF
VIN
6V
Current Source
+
–
LT3085
IN
VCONTROL
OUT 2Ω
0.5W
100k
3085 TA03
SET
IOUT
0A TO 0.5A
4.7μF
VIN
10V
1μF
Power Oscillator
+
–
LT3085
IN
VIN
VCONTROL
OUT VOUT
400Hz
4VACP-P
3085 TA22
10μF
SET
499k
8.45k
8.45k
47nF
4.7μF
2.21k
47nF
220n 121Ω
6.3V, 150mA
LIGHT BULB #47
20Ω
LT3085
19
3085fb
TYPICAL APPLICATIONS
Adding Shutdown Low Dropout Voltage LED Driver
+
–
LT3085 IN
100mA
D1
VCONTROL
OUT
VIN
3085 TA05
SET
R1
24.9k
R2
2.49Ω
C1
+
–
LT3085
IN
VIN
VCONTROL
OUT VOUT
3085 TA04
SET
R1
ON OFF
SHUTDOWN
Q1
VN2222LL
Q2*
VN2222LL
Q2 INSURES ZERO OUTPUT
IN THE ABSENCE OF ANY
OUTPUT LOAD.
*
Using a Lower Value SET Resistor
+
–
LT3085
IN
1mA
VIN
12V
VCONTROL
OUT
COUT
4.7μF
VOUT
0.5V TO 10V
3085 TA06
SET
R1
49.9k
1%
RSET
10k
R2
499Ω
1%
C1
1μF
VOUT = 0.5V + 1mA • RSET
LT3085
20
3085fb
TYPICAL APPLICATIONS
Adding Soft-Start
Coincident Tracking
+
–
LT3085
IN
VCONTROL
OUT
4.7μF
VOUT3
5V
0.5A
SET
C3
4.7μF
+
–
LT3085
IN
VCONTROL
OUT VOUT2
3.3V
0.5A 3085 TA08
SET
R2
80.6k
169k
C2
4.7μF
C1
1.5μF
+
–
LT3085
IN
VCONTROL
VIN
7V TO 28V
OUT
SET
R1
249k
VOUT1
2.5V
0.5A
+
–
LT3085
IN
VIN
4.8V to 28V
VCONTROL
OUT VOUT
3.3V
0.5A
COUT
4.7μF
3085 TA07
SET
R1
332k
C2
0.01μF
C1
1μF
D1
1N4148
LT3085
21
3085fb
TYPICAL APPLICATIONS
High Voltage Regulator
Ramp Generator
+
–
LT3085
6.1V
IN
1N4148
VIN
50V
VCONTROL
OUT VOUT
0.5A VOUT = 20V
VOUT = 10μA • RSET
3085 TA10
SET
RSET
2M
4.7μF
15μF
10μF
BUZ11
10k
+
+
+
–
LT3085
IN
VIN
5V
VCONTROL
OUT VOUT
3085 TA12
SET
VN2222LL VN2222LL 4.7μF
1μF
1μF
+
–
LT3085
+
–
LT3085 ININ
VIN
12V TO 18V
VCONTROL
VCONTROL
OUTOUT
4.7μF 100μF
VOUT
0V TO 10V
3085 TA09
SETSET +
15μF R4
1M
1Ω
0.25W
50k
0A TO 0.5A
+
15μF
+
Lab Supply
LT3085
22
3085fb
TYPICAL APPLICATIONS
Ground Clamp
Reference Buffer
+
–
LT3085
IN
VIN
VCONTROL
OUT
4.7μF
VOUT
VEXT
3085 TA13
1N4148
SET
5k
20Ω
1μF
+
–
LT3085
IN
VIN
VCONTROL
OUT VOUT*
3085 TA11
SET
OUTPUT
INPUT
C1
1μF
GND
C2
4.7μF
LT1019
*MIN LOAD 0.5mA
3085 TA20
20mΩ
20mΩ
42Ω* 47μF
3.3VOUT
2A
33k
*4mV DROP ENSURES LT3085 IS
OFF WITH NO LOAD
MULTIPLE LT3085’S CAN BE USED
+
–
LT3085
10μF
5V
OUT
SET
LT1963-3.3
IN
VCONTROL
Boosting Fixed Output Regulators
LT3085
23
3085fb
TYPICAL APPLICATIONS
Low Voltage, High Current Adjustable High Effi ciency Regulator*
2.7V TO 5.5V†
100μF
×2 2.2MEG 100k 470pF
10k
1000pF
100μF
×2
294k
12.1k
0.47μH
78.7k
100k
124k
PVIN SW
2N3906
SVIN ITH
RT
VFB
SYNC/MODE
PGOOD
RUN/SS
SGND PGND
LTC3414
+
–
LT3085
IN
VCONTROL
OUT
SET
+
–
LT3085
IN
VCONTROL
OUT 0V TO 4V†
2A
SET
+
–
LT3085
IN
VCONTROL
OUT
SET
3085 TA18
+
–
LT3085
IN
VCONTROL
OUT
100μF
SET
+
+
+
*DIFFERENTIAL VOLTAGE ON LT3085
IS 0.6V SET BY THE VBE OF THE 2N3906 PNP.
20mΩ
20mΩ
20mΩ
20mΩ
†MAXIMUM OUTPUT VOLTAGE IS 1.5V
BELOW INPUT VOLTAGE
LT3085
24
3085fb
TYPICAL APPLICATIONS
Adjustable High Effi ciency Regulator*
3085 TA19
4.5V TO 25V†
10μF 100k
0.1μF 68μF
10μH
MBRM140
10k
10k
1μF
VIN BOOST
SW
FB
SHDN
GND
LT3493
CMDSH-4E
0.1μF
TP0610L
+
–
LT3085
IN
VCONTROL
OUT
SET 4.7μF
0V TO 10V†
0.5A
*DIFFERENTIAL VOLTAGE ON LT3085
≈ 1.4V SET BY THE TPO610L P-CHANNEL THRESHOLD.
1MEG
†MAXIMUM OUTPUT VOLTAGE IS 2V
BELOW INPUT VOLTAGE
200k
3085 TA21
R1
100k
+
–
LT3085
CCOMP*
IN
VCONTROL
SET
OUT
*CCOMP
R1 ≤ 10Ω 10μF
R1 ≥ 10Ω 2.2μF
IOUT = 1V
R1
2 Terminal Current Source
LT3085
25
3085fb
PACKAGE DESCRIPTION
3.00 p0.10
(2 SIDES)
2.00 p0.10
(2 SIDES)
NOTE:
1. DRAWING TO BE MADE A JEDEC PACKAGE OUTLINE M0-229 VARIATION OF (TBD)
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
0.40 p0.10
BOTTOM VIEW—EXPOSED PAD
1.65 p0.10
(2 SIDES)
0.75 p0.05
R = 0.115
TYP
R = 0.05
TYP
1.35 p0.10
(2 SIDES)
1
3
64
PIN 1 BAR
TOP MARK
(SEE NOTE 6)
0.200 REF
0.00 – 0.05
(DCB6) DFN 0405
0.25 p0.05
0.50 BSC
PIN 1 NOTCH
R0.20 OR 0.25
s 45° CHAMFER
0.25 p0.05
1.35 p0.05
(2 SIDES)
RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS
1.65 p0.05
(2 SIDES)
2.15 p0.05
0.70 p0.05
3.55 p0.05
PACKAGE
OUTLINE
0.50 BSC
DCB Package
6-Lead Plastic DFN (2mm × 3mm)
(Reference LTC DWG # 05-08-1715 Rev A)
LT3085
26
3085fb
PACKAGE DESCRIPTION
MSOP (MS8E) 0210 REV F
0.53 p 0.152
(.021 p .006)
SEATING
PLANE
NOTE:
1. DIMENSIONS IN MILLIMETER/(INCH)
2. DRAWING NOT TO SCALE
3. DIMENSION DOES NOT INCLUDE MOLD FLASH, PROTRUSIONS OR GATE BURRS.
MOLD FLASH, PROTRUSIONS OR GATE BURRS SHALL NOT EXCEED 0.152mm (.006") PER SIDE
4. DIMENSION DOES NOT INCLUDE INTERLEAD FLASH OR PROTRUSIONS.
INTERLEAD FLASH OR PROTRUSIONS SHALL NOT EXCEED 0.152mm (.006") PER SIDE
5. LEAD COPLANARITY (BOTTOM OF LEADS AFTER FORMING) SHALL BE 0.102mm (.004") MAX
6. EXPOSED PAD DIMENSION DOES NOT INCLUDE MOLD FLASH. MOLD FLASH ON E-PAD
SHALL NOT EXCEED 0.254mm (.010") PER SIDE.
0.18
(.007)
0.254
(.010)
1.10
(.043)
MAX
0.22 – 0.38
(.009 – .015)
TYP
0.86
(.034)
REF
0.65
(.0256)
BSC
0o – 6o TYP
DETAIL “A”
DETAIL “A”
GAUGE PLANE
12
34
4.90 p 0.152
(.193 p .006)
8
8
1
BOTTOM VIEW OF
EXPOSED PAD OPTION
765
3.00 p 0.102
(.118 p .004)
(NOTE 3)
3.00 p 0.102
(.118 p .004)
(NOTE 4)
0.52
(.0205)
REF
1.68
(.066)
1.88
(.074)
5.23
(.206)
MIN
3.20 – 3.45
(.126 – .136)
1.68 p 0.102
(.066 p .004)
1.88 p 0.102
(.074 p .004)
0.889 p 0.127
(.035 p .005)
RECOMMENDED SOLDER PAD LAYOUT
0.42 p 0.038
(.0165 p .0015)
TYP
0.65
(.0256)
BSC
0.1016 p 0.0508
(.004 p .002)
DETAIL “B”
DETAIL “B”
CORNER TAIL IS PART OF
THE LEADFRAME FEATURE.
FOR REFERENCE ONLY
NO MEASUREMENT PURPOSE
0.05 REF
0.29
REF
MS8E Package
8-Lead Plastic MSOP, Exposed Die Pad
(Reference LTC DWG # 05-08-1662 Rev F)
LT3085
27
3085fb
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 representa-
tion that the interconnection of its circuits as described herein will not infringe on existing patent rights.
REVISION HISTORY
REV DATE DESCRIPTION PAGE NUMBER
B 6/10 Updated trademarks
Revised Conditions in Electrical Characteristics table
Changed ILOAD value on curve G27 in Typical Performance Characteristics section
Revised Figure 1
Added 200k resistor to drawing 3085 TA19 in Typical Applications section
Updated Package Description drawings
1
3
6
9
24
25, 26
(Revision history begins at Rev B)
LT3085
28
3085fb
Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com
© LINEAR TECHNOLOGY CORPORATION 2008
LT 0610 REV B • PRINTED IN USA
RELATED PARTS
TYPICAL APPLICATION
PART NUMBER DESCRIPTION COMMENTS
LDOs
LT1086 1.5A Low Dropout Regulator Fixed 2.85V, 3.3V, 3.6V, 5V and 12V Output
LT1763 500mA, Low Noise LDO 300mV Dropout Voltage, Low Noise = 20μVRMS, VIN: 1.8V to 20V, SO-8 Package
LT3021 500mA VLDO Regulator VIN: 0.9V to 10V, Dropout Voltage = 190mV, VADJ = 200mV, 5mm × 5mm DFN-16,
SO-8 Packages
LT3080 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, SOT-223, MSOP and
3mm × 3mm DFN Packages
LT3080-1 Parallelable 1.1A Adjustable Single
Resistor Low Dropout Regulator
(with Internal Ballast R)
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, SOT-223, MSOP and
3mm × 3mm DFN Packages. LT3080-1 Version Has Integrated Ballast Resistor
LT1963A 1.5A Low Noise, Fast Transient
Response LDO
340mV Dropout Voltage, Low Noise = 40μVRMS, VIN: 2.5V to 20V, TO-220, DD, SOT-223
and SO-8 Packages
LT1965 1.1A Low Noise LDO 290mV Dropout Voltage, Low Noise = 40μVRMS, VIN: 1.8V to 20V, VOUT: 1.2V to 19.5V,
Stable with Ceramic Caps TO-220, DDPak, MSOP and 3mm × 3mm DFN Packages
LTC
®
3026 1.5A Low Input Voltage VLDO
TM
Regulator
VIN: 1.14V to 3.5V (Boost Enabled), 1.14V to 5.5V (with External 5V), VDO = 0.1V,
IQ = 950μA, Stable with 10μF Ceramic Capacitors, 10-Lead MSOP and DFN Packages
Switching Regulators
LT1976 High Voltage, 1.5A Step-Down
Switching Regulator
f = 200kHz, IQ = 100μA, TSSOP-16E Package
LTC3414 4A (IOUT), 4MHz Synchronous
Step-Down DC/DC Converter
95% Effi ciency, VIN: 2.25V to 5.5V, VOUT(MIN) = 0.8V, TSSOP Package
LTC3406/LTC3406B 600mA (IOUT), 1.5MHz Synchronous
Step-Down DC/DC Converter
95% Effi ciency, VIN: 2.5V to 5.5V, VOUT(MIN) = 0.6V, IQ = 20μA, ISD < 1μA,
ThinSOT
TM
Package
LTC3411 1.25A (IOUT), 4MHz Synchronous
Step-Down DC/DC Converter
95% Effi ciency, VIN: 2.5V to 5.5V, VOUT(MIN) = 0.8V, IQ = 60μA, ISD < 1μA, 10-Lead MS or
DFN Packages
Paralleling Regulators
+
–
LT3085
IN
VIN
4.8V TO 36V
VCONTROL
OUT 20mΩ
10μF
VOUT
3.3V
1.5A
3085 TA14
165k
SET
1μF
+
–
LT3080
IN
VCONTROL
OUT 10mΩ
SET
