LM1084
5A Low Dropout Positive Regulators
General Description
The LM1084 is a series of low dropout voltage positive
regulators with a maximum dropout of 1.5V at 5A of load
current. It has the same pin-out as National Semiconductor's
industry standard LM317.
The LM1084 is available in an adjustable version, which can
set the output voltage with only two external resistors. It is
also available in three fixed voltages: 3.3V, 5.0V and 12.0V.
The fixed versions intergrate the adjust resistors.
The LM1084 circuit includes a zener trimmed bandgap ref-
erence, current limiting and thermal shutdown.
The LM1084 series is available in TO-220 and TO-263 pack-
ages. Refer to the LM1085 for the 3A version, and the
LM1086 for the 1.5A version.
Features
n
Available in 3.3V, 5.0V, 12V and Adjustable Versions
n
Current Limiting and Thermal Protection
n
Output Current
5A
n
Industrial Temperature Range
-40C to 125C
n
Line Regulation
0.015% (typical)
n
Load Regulation
0.1% (typical)
Applications
n
Post Regulator for Switching DC/DC Conveter
n
High Efficiency Linear Regulators
n
Battery Charger
Connection Diagrams
TO-220
10094636
Top View
TO-263
10094635
Top View
Basic Functional Diagram, Adjustable Version
10094665
Application Circuit
10094652
1.2V to 15V Adjustable Regulator
June 2005
LM1084
5A
Low
Dropout
Positive
Regulators
2005 National Semiconductor Corporation
DS100946
www.national.com
Ordering Information
Package
Temperature Range
Part Number
Transport Media
NSC Drawing
3-lead TO-263
-40C to +125C
LM1084IS-ADJ
Rails
TS3B
LM1084ISX-ADJ
Tape and Reel
LM1084IS-12
Rails
LM1084ISX-12
Tape and Reel
LM1084IS-3.3
Rails
LM1084ISX-3.3
Tape and Reel
LM1084IS-5.0
Rails
LM1084ISX-5.0
Tape and Reel
3-lead TO-220
-40C to + 125C
LM1084IT-ADJ
Rails
T03B
LM1084IT-12
Rails
LM1084IT-3.3
Rails
LM1084IT-5.0
Rails
Simplified Schematic
10094634
LM1084
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2
Absolute Maximum Ratings
(Note 1)
If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales Office/
Distributors for availability and specifications.
Maximum Input to Output Voltage Differential
LM1084-ADJ
29V
LM1084-12
18V
LM1084-3.3
27V
LM1084-5.0
25V
Power Dissipation (Note 2)
Internally Limited
Junction Temperature (T
J
)(Note 3)
150C
Storage Temperature Range
-65C to 150C
Lead Temperature
260C, to 10 sec
ESD Tolerance (Note 4)
2000V
Operating Ratings
(Note 1)
Junction Temperature Range (T
J
) (Note 3)
Control Section
-40C to 125C
Output Section
-40C to 150C
Electrical Characteristics
Typicals and limits appearing in normal type apply for T
J
= 25C. Limits appearing in Boldface type apply over the entire junc-
tion temperature range for operation.
Symbol
Parameter
Conditions
Min
(Note 6)
Typ
(Note 5)
Max
(Note 6)
Units
V
REF
Reference Voltage
LM1084-ADJ
I
OUT
= 10mA, V
IN
-V
OUT
= 3V
10mA
I
OUT
I
FULL LOAD
,1.5V
(V
IN
-V
OUT
)
25V
(Note 7)
1.238
1.225
1.250
1.250
1.262
1.270
V
V
V
OUT
Output Voltage
(Note 7)
LM1084-3.3
I
OUT
= 0mA, V
IN
= 8V
0
I
OUT
I
FULL LOAD
, 4.8V
V
IN
15V
3.270
3.235
3.300
3.300
3.330
3.365
V
V
LM1084-5.0
I
OUT
= 0mA, V
IN
= 8V
0
I
OUT
I
FULL LOAD
, 6.5V
V
IN
20V
4.950
4.900
5.000
5.000
5.050
5.100
V
V
LM1084-12
I
OUT
= 0mA, V
IN
= 15V
0
I
OUT
I
FULL LOAD
, 13.5V
V
IN
25V
11.880
11.760
12.000
12.000
12.120
12.240
V
V
V
OUT
Line Regulation
(Note 8)
LM1084-ADJ
I
OUT
=10mA, 1.5V
(V
IN
-V
OUT
)
15V
0.015
0.035
0.2
0.2
%
%
LM1084-3.3
I
OUT
= 0mA, 4.8V
V
IN
15V
0.5
1.0
6
6
mV
mV
LM1084-5.0
I
OUT
= 0mA, 6.5V
V
IN
20V
0.5
1.0
10
10
mV
mV
LM1084-12
I
OUT
=0mA, 13.5V
V
IN
25V
1.0
2.0
25
25
mV
mV
V
OUT
Load Regulation
(Note 8)
LM1084-ADJ
(V
IN
-V
OUT
) = 3V, 10mA
I
OUT
I
FULL LOAD
0.1
0.2
0.3
0.4
%
%
LM1084-3.3
V
IN
= 5V, 0
I
OUT
I
FULL LOAD
3
7
15
20
mV
mV
LM1084-5.0
V
IN
= 8V, 0
I
OUT
I
FULL LOAD
5
10
20
35
mV
mV
LM1084-12
V
IN
= 15V, 0
I
OUT
I
FULL LOAD
12
24
36
72
mV
mV
Dropout Voltage
(Note 9)
LM1084-ADJ, 3.3, 5, 12
V
REF
,
V
OUT
= 1%, I
OUT
= 5A
1.3
1.5
V
LM1084
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3
Electrical Characteristics
(Continued)
Typicals and limits appearing in normal type apply for T
J
= 25C. Limits appearing in Boldface type apply over the entire junc-
tion temperature range for operation.
Symbol
Parameter
Conditions
Min
(Note 6)
Typ
(Note 5)
Max
(Note 6)
Units
I
LIMIT
Current Limit
LM1084-ADJ
V
IN
-V
OUT
= 5V
V
IN
-V
OUT
= 25V
5.5
0.3
8.0
0.6
A
A
LM1084-3.3
V
IN
= 8V
5.5
8.0
A
LM1084-5.0
V
IN
= 10V
5.5
8.0
A
LM1084-12
V
IN
= 17V
5.5
8.0
A
Minimum Load
Current (Note 10)
LM1084-ADJ
V
IN
-V
OUT
= 25V
5
10.0
mA
Quiescent Current
LM1084-3.3
V
IN
= 18V
5.0
10.0
mA
LM1084-5.0
V
IN
20V
5.0
10.0
mA
LM1084-12
V
IN
25V
5.0
10.0
mA
Thermal Regulation
T
A
= 25C, 30ms Pulse
0.003
0.015
%/W
Ripple Rejection
f
RIPPLE
= 120Hz, = C
OUT
= 25F Tantalum,
I
OUT
= 5A
60
75
dB
LM1084-ADJ, C
ADJ
, = 25F, (V
IN
-V
O
) = 3V
LM1084-3.3, V
IN
= 6.3V
60
72
dB
LM1084-5.0, V
IN
= 8V
60
68
dB
LM1084-12 V
IN
= 15V
54
60
dB
Adjust Pin Current
LM1084
55
120
A
Adjust Pin Current
Change
10mA
I
OUT
I
FULL LOAD
,
1.5V
V
IN
-V
OUT
25V
0.2
5
A
Temperature
Stability
0.5
%
Long Term Stability
T
A
=125C, 1000Hrs
0.3
1.0
%
RMS Output Noise
(% of V
OUT
)
10Hz
f 10kHz
0.003
%
Thermal Resistance
Junction-to-Case
3-Lead TO-263: Control Section/Output Section
3-Lead TO-220: Control Section/Output Section
0.65/2.7
0.65/2.7
C/W
C/W
Note 1: Absolute Maximum Ratings indicate limits beyond which damage to the device may occur. Operating Ratings indicate conditions for which the device is
intended to be functional, but specific performance is not guaranteed. For guaranteed specifications and the test conditions, see the Electrical Characteristics.
Note 2: Power dissipation is kept in a safe range by current limiting circuitry. Refer to Overload Recovery in Application Notes.
Note 3: The maximum power dissipation is a function of T
J(max)
,
JA
, and T
A
. The maximum allowable power dissipation at any ambient temperature
is P
D
= (T
J(max)
T
A
)/
JA
. All numbers apply for packages soldered directly into a PC board. Refer to Thermal Considerations in the Application Notes.
Note 4: For testing purposes, ESD was applied using human body model, 1.5k
in series with 100pF.
Note 5: Typical Values represent the most likely parametric norm.
Note 6: All limits are guaranteed by testing or statistical analysis.
Note 7: I
FULLLOAD
is defined in the current limit curves. The I
FULLLOAD
Curve defines the current limit as a function of input-to-output voltage. Note that 30W power
dissipation for the LM1084 is only achievable over a limited range of input-to-output voltage.
Note 8: Load and line regulation are measured at constant junction temperature, and are guaranteed up to the maximum power dissipation of 30W. Power
dissipation is determined by the input/output differential and the output current. Guaranteed maximum power dissipation will not be available over the full input/output
range.
Note 9: Dropout voltage is specified over the full output current range of the device.
Note 10: The minimum output current required to maintain regulation.
LM1084
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4
Typical Performance Characteristics
Dropout Voltage (V
IN
-V
OUT
)
Short-Circuit Current
10094663
10094671
Load Regulation
LM1084-ADJ Ripple Rejection
10094638
10094643
LM1084-ADJ Ripple Rejection vs Current
Temperature Stability
10094690
10094625
LM1084
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5
Typical Performance Characteristics
(Continued)
Adjust Pin Current
LM1084-ADJ Load Transient Response
10094626
10094669
LM1084-ADJ LineTransient Response
Maximum Power Dissipation
10094670
10094668
LM1084
www.national.com
6
Application Note
GENERAL
Figure 1 shows a basic functional diagram for the LM1084-
Adj (excluding protection circuitry) . The topology is basically
that of the LM317 except for the pass transistor. Instead of a
Darlingtion NPN with its two diode voltage drop, the LM1084
uses a single NPN. This results in a lower dropout voltage.
The structure of the pass transistor is also known as a quasi
LDO. The advantage a quasi LDO over a PNP LDO is its
inherently lower quiescent current. The LM1084 is guaran-
teed to provide a minimum dropout voltage 1.5V over tem-
perature, at full load.
OUTPUT VOLTAGE
The LM1084 adjustable version develops at 1.25V reference
voltage, (V
REF
), between the output and the adjust terminal.
As shown in figure 2, this voltage is applied across resistor
R1 to generate a constant current I1. This constant current
then flows through R2. The resulting voltage drop across R2
adds to the reference voltage to sets the desired output
voltage.
The current I
ADJ
from the adjustment terminal introduces an
output error . But since it is small (120uA max), it becomes
negligible when R1 is in the 100
range.
For fixed voltage devices, R1 and R2 are integrated inside
the devices.
STABILITY CONSIDERATION
Stability consideration primarily concern the phase response
of the feedback loop. In order for stable operation, the loop
must maintain negative feedback. The LM1084 requires a
certain amount series resistance with capacitive loads. This
series resistance introduces a zero within the loop to in-
crease phase margin and thus increase stability. The equiva-
lent series resistance (ESR) of solid tantalum or aluminum
electrolytic capacitors is used to provide the appropriate zero
(approximately 500 kHz).
The Aluminum electrolytic are less expensive than tantal-
ums, but their ESR varies exponentially at cold tempera-
tures; therefore requiring close examination when choosing
the desired transient response over temperature. Tantalums
are a convenient choice because their ESR varies less than
2:1 over temperature.
The recommended load/decoupling capacitance is a 10uF
tantalum or a 50uF aluminum. These values will assure
stability for the majority of applications.
The adjustable versions allows an additional capacitor to be
used at the ADJ pin to increase ripple rejection. If this is done
the output capacitor should be increased to 22uF for tantal-
ums or to 150uF for aluminum.
Capacitors other than tantalum or aluminum can be used at
the adjust pin and the input pin. A 10uF capacitor is a
reasonable value at the input. See Ripple Rejection section
regarding the value for the adjust pin capacitor.
It is desirable to have large output capacitance for applica-
tions that entail large changes in load current (microproces-
sors for example). The higher the capacitance, the larger the
available charge per demand. It is also desirable to provide
low ESR to reduce the change in output voltage:
V = I x ESR
It is common practice to use several tantalum and ceramic
capacitors in parallel to reduce this change in the output
voltage by reducing the overall ESR.
Output capacitance can be increased indefinitely to improve
transient response and stability.
RIPPLE REJECTION
Ripple rejection is a function of the open loop gain within the
feed-back loop (refer to Figure 1 and Figure 2). The LM1084
exhibits 75dB of ripple rejection (typ.). When adjusted for
voltages higher than V
REF
, the ripple rejection decreases as
function of adjustment gain: (1+R1/R2) or V
O
/V
REF
. There-
fore a 5V adjustment decreases ripple rejection by a factor of
four (-12dB); Output ripple increases as adjustment voltage
increases.
However, the adjustable version allows this degradation of
ripple rejection to be compensated. The adjust terminal can
be bypassed to ground with a capacitor (C
ADJ
). The imped-
ance of the C
ADJ
should be equal to or less than R1 at the
desired ripple frequency. This bypass capacitor prevents
ripple from being amplified as the output voltage is in-
creased.
1/(2
*f
RIPPLE
*C
ADJ
)
R
1
LOAD REGULATION
The LM1084 regulates the voltage that appears between its
output and ground pins, or between its output and adjust
pins. In some cases, line resistances can introduce errors to
the voltage across the load. To obtain the best load regula-
tion, a few precautions are needed.
10094665
FIGURE 1. Basic Functional Diagram for the LM1084,
excluding Protection circuitry
10094617
FIGURE 2. Basic Adjustable Regulator
LM1084
www.national.com
7
Application Note
(Continued)
Figure 3 shows a typical application using a fixed output
regulator. Rt1 and Rt2 are the line resistances. V
LOAD
is less
than the V
OUT
by the sum of the voltage drops along the line
resistances. In this case, the load regulation seen at the
R
LOAD
would be degraded from the data sheet specification.
To improve this, the load should be tied directly to the output
terminal on the positive side and directly tied to the ground
terminal on the negative side.
When the adjustable regulator is used (Figure 4), the best
performance is obtained with the positive side of the resistor
R1 tied directly to the output terminal of the regulator rather
than near the load. This eliminates line drops from appearing
effectively in series with the reference and degrading regu-
lation. For example, a 5V regulator with 0.05
resistance
between the regulator and load will have a load regulation
due to line resistance of 0.05
x I
L
. If R1 (=125
) is con-
nected near the load the effective line resistance will be
0.05
(1 + R2/R1) or in this case, it is 4 times worse. In
addition, the ground side of the resistor R2 can be returned
near the ground of the load to provide remote ground sens-
ing and improve load regulation.
PROTECTION DIODES
Under normal operation, the LM1084 regulator does not
need any protection diode. With the adjustable device, the
internal resistance between the adjustment and output ter-
minals limits the current. No diode is needed to divert the
current around the regulator even with a capacitor on the
adjustment terminal. The adjust pin can take a transient
signal of
25V with respect to the output voltage without
damaging the device.
When an output capacitor is connected to a regulator and
the input is shorted, the output capacitor will discharge into
the output of the regulator. The discharge current depends
on the value of the capacitor, the output voltage of the
regulator, and rate of decrease of V
IN
. In the LM1084 regu-
lator, the internal diode between the output and input pins
can withstand microsecond surge currents of 10A to 20A.
With an extremely large output capacitor (
1000 f), and
with input instantaneously shorted to ground, the regulator
could be damaged. In this case, an external diode is recom-
mended between the output and input pins to protect the
regulator, shown in Figure 5.
OVERLOAD RECOVERY
Overload recovery refers to regulator's ability to recover from
a short circuited output. A key factor in the recovery process
is the current limiting used to protect the output from drawing
too much power. The current limiting circuit reduces the
output current as the input to output differential increases.
Refer to short circuit curve in the curve section.
During normal start-up, the input to output differential is
small since the output follows the input. But, if the output is
shorted, then the recovery involves a large input to output
differential. Sometimes during this condition the current lim-
iting circuit is slow in recovering. If the limited current is too
low to develop a voltage at the output, the voltage will
stabilize at a lower level. Under these conditions it may be
necessary to recycle the power of the regulator in order to
get the smaller differential voltage and thus adequate start
up conditions. Refer to curve section for the short circuit
current vs. input differential voltage.
THERMAL CONSIDERATIONS
ICs heats up when in operation, and power consumption is
one factor in how hot it gets. The other factor is how well the
heat is dissipated. Heat dissipation is predictable by knowing
the thermal resistance between the IC and ambient (
JA
).
Thermal resistance has units of temperature per power (C/
W). The higher the thermal resistance, the hotter the IC.
The LM1084 specifies the thermal resistance for each pack-
age as junction to case (
JC
). In order to get the total
resistance to ambient (
JA
), two other thermal resistance
10094618
FIGURE 3. Typical Application using Fixed Output
Regulator
10094619
FIGURE 4. Best Load Regulation using Adjustable
Output Regulator
10094615
FIGURE 5. Regulator with Protection Diode
LM1084
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8
Application Note
(Continued)
must be added, one for case to heat-sink (
CH
) and one for
heatsink to ambient (
HA
). The junction temperature can be
predicted as follows:
T
J
= T
A
+ P
D
(
JC
+
CH
+
HA
) = T
A
+ P
D
JA
T
J
is junction temperature, T
A
is ambient temperature, and
P
D
is the power consumption of the device. Device power
consumption is calculated as follows:
I
IN
= I
L
+ I
G
P
D
= (V
IN
-V
OUT
) I
L
+ V
IN
I
G
Figure 6 shows the voltages and currents which are present
in the circuit.
Once the devices power is determined, the maximum allow-
able (
JA (max)
) is calculated as:
JA (max)
= T
R(max)
/P
D
= T
J(max)
- T
A(max)
/P
D
The LM1084 has different temperature specifications for two
different sections of the IC: the control section and the output
section. The Electrical Characteristics table shows the junc-
tion to case thermal resistances for each of these sections,
while the maximum junction temperatures (T
J(max)
) for each
section is listed in the Absolute Maximum section of the
datasheet. T
J(max)
is 125C for the control section, while
T
J(max)
is 150C for the output section.
JA (max)
should be calculated separately for each section as
follows:
JA
(max, CONTROL SECTION) = (125C - T
A(max)
)/P
D
JA
(max, OUTPUT SECTION) = (150C - T
A(max)
)/P
D
The required heat sink is determined by calculating its re-
quired thermal resistance (
HA (max)
).
HA (max)
=
JA (max)
- (
JC
+
CH
)
(
HA (max)
) should also be calculated twice as follows:
(
HA (max)
) =
JA
(max, CONTROL SECTION) - (
JC
(CON-
TROL SECTION) +
CH
)
(
HA (max)
) =
JA
(max, OUTPUT SECTION) - (
JC
(OUTPUT
SECTION) +
CH
)
If thermal compound is used,
CH
can be estimated at 0.2
C/W. If the case is soldered to the heat sink, then a
CH
can
be estimated as 0 C/W.
After,
HA (max)
is calculated for each section, choose the
lower of the two
HA (max)
values to determine the appropri-
ate heat sink.
If PC board copper is going to be used as a heat sink, then
Figure 7 can be used to determine the appropriate area
(size) of copper foil required.
10094616
FIGURE 6. Power Dissipation Diagram
10094664
FIGURE 7. Heat sink thermal Resistance vs Area
LM1084
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9
Typical Applications
10094667
5V to 3.3V, 5A Regulator
10094650
Adjustable
@
5V
10094652
1.2V to 15V Adjustable Regulator
10094653
5V Regulator with Shutdown
10094654
Battery Charger
10094655
Adjustable Fixed Regulator
10094656
Regulator with Reference
10094657
High Current Lamp Driver Protection
LM1084
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10
Typical Applications
(Continued)
10094659
Battery Backup Regulated Supply
10094660
Ripple Rejection Enhancement
10094661
Automatic Light control
10094662
Generating Negative Supply voltage
10094658
Remote Sensing
LM1084
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11
Physical Dimensions
inches (millimeters) unless otherwise noted
3-Lead TO-263
NS Package Number TS3B
3-Lead TO-220
NS Package Number T03B
LM1084
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12
Notes
National does not assume any responsibility for use of any circuitry described, no circuit patent licenses are implied and National reserves
the right at any time without notice to change said circuitry and specifications.
For the most current product information visit us at www.national.com.
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LM1084
5A
Low
Dropout
Positive
Regulators
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