June 2005
LM1085
3A Low Dropout Positive Regulators
General Description
Features
n Available in 3.3V, 5.0V, 12V and Adjustable Versions
n Current Limiting and Thermal Protection
n Output Current
The LM1085 is a series of low dropout positive voltage
regulators with a maximum dropout of 1.5V at 3A of load
current. It has the same pin-out as National Semiconductor’s
industry standard LM317.
3A
n Line Regulation
n Load Regulation
0.015% (typical)
0.1% (typical)
The LM1085 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 integrate the adjust resistors.
Applications
n High Efficiency Linear Regulators
n Battery Charger
The LM1085 circuit includes a zener trimmed bandgap ref-
erence, current limiting and thermal shutdown.
n Post Regulation for Switching Supplies
n Constant Current Regulator
n Microprocessor Supply
The LM1085 series is available in TO-220 and TO-263 pack-
ages. Refer to the LM1084 for the 5A version, and the
LM1086 for the 1.5A version.
Connection Diagrams
Application Circuit
TO-220
10094702
Top View
TO-263
10094752
1.2V to 15V Adjustable Regulator
10094704
Top View
Basic Functional Diagram,
Adjustable Version
10094765
© 2005 National Semiconductor Corporation
DS100947
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Absolute Maximum Ratings (Note 1)
If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales Office/
Distributors for availability and specifications.
Junction Temperature (TJ)(Note 3)
Storage Temperature Range
Lead Temperature
150˚C
-65˚C to 150˚C
260˚C, to 10 sec
2000V
ESD Tolerance (Note 4)
Maximum Input to Output Voltage Differential
LM1085-ADJ
LM1085-12
29V
Operating Ratings (Note 1)
Junction Temperature Range (TJ) (Note 3)
18V
27V
LM1085-3.3
Control Section
−40˚C to 125˚C
−40˚C to 150˚C
LM1085-5.0
25V
Output Section
Power Dissipation (Note 2)
Internally Limited
Electrical Characteristics
Typicals and limits appearing in normal type apply for TJ = 25˚C. Limits appearing in Boldface type apply over the entire junc-
tion temperature range for operation.
Min
(Note 6)
Typ
(Note 5)
Max
(Note 6)
Symbol
Parameter
Conditions
Units
VREF
Reference Voltage
LM1085-ADJ
IOUT = 10mA, VIN−VOUT = 3V
10mA ≤IOUT ≤ IFULL LOAD,1.5V ≤ (VIN−VOUT) ≤ 15V
(Note 7)
1.238
1.250
1.262
V
V
1.225
1.250
1.270
VOUT
Output Voltage
(Note 7)
LM1085-3.3
3.270
3.300
3.330
V
V
IOUT = 0mA, VIN = 5V
3.235
3.300
3.365
0 ≤ IOUT ≤IFULL LOAD, 4.8V≤ VIN ≤15V
LM1085-5.0
4.950
5.000
5.050
V
V
IOUT = 0mA, VIN = 8V
4.900
5.000
5.100
0 ≤ IOUT ≤ IFULL LOAD, 6.5V ≤ VIN ≤ 20V
LM1085-12
11.880
12.000
12.120
V
V
IOUT = 0mA, VIN = 15V
0 ≤ IOUT ≤ IFULL LOAD, 13.5V ≤ VIN ≤ 25V
LM1085-ADJ
11.760
12.000
12.240
∆VOUT
Line Regulation
(Note 8)
0.015
0.035
0.5
1.0
0.5
1.0
1.0
2.0
0.1
0.2
3
0.2
0.2
6
%
IOUT =10mA, 1.5V≤ (VIN-VOUT) ≤ 15V
LM1085-3.3
%
mV
mV
mV
mV
mV
mV
%
IOUT = 0mA, 4.8V ≤ VIN ≤ 15V
LM1085-5.0
6
10
10
25
25
0.3
0.4
15
20
20
35
36
72
IOUT = 0mA, 6.5V ≤ VIN ≤ 20V
LM1085-12
I
=0mA, 13.5V ≤ VIN ≤ 25V
OUT
∆VOUT
Load Regulation
(Note 8)
LM1085-ADJ
(VIN-V OUT) = 3V, 10mA ≤ IOUT ≤ IFULL LOAD
LM1085-3.3
%
mV
mV
mV
mV
mV
mV
VIN = 5V, 0 ≤ IOUT ≤ IFULL LOAD
LM1085-5.0
7
5
VIN = 8V, 0 ≤ IOUT ≤ IFULL LOAD
LM1085-12
10
12
VIN = 15V, 0 ≤ IOUT ≤ IFULL LOAD
LM1085-ADJ, 3.3, 5, 12
∆VREF, ∆VOUT = 1%, IOUT = 3A
24
Dropout Voltage
(Note 9)
1.3
1.5
V
3
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Electrical Characteristics (Continued)
Typicals and limits appearing in normal type apply for TJ = 25˚C. Limits appearing in Boldface type apply over the entire junc-
tion temperature range for operation.
Min
Typ
Max
Symbol
Parameter
Current Limit
Conditions
Units
(Note 6) (Note 5) (Note 6)
ILIMIT
LM1085-ADJ
VIN−VOUT = 5V
VIN−VOUT = 25V
LM1085-3.3
VIN = 8V
3.2
0.2
5.5
0.5
A
A
3.2
3.2
3.2
5.5
5.5
5.5
5.0
5.0
5.0
A
A
LM1085-5.0
VIN = 10V
LM1085-12
VIN = 17V
A
Minimum Load
LM1085-ADJ
VIN −VOUT = 25V
LM1085-3.3
VIN ≤ 18V
Current (Note 10)
Quiescent Current
10.0
10.0
10.0
mA
mA
mA
LM1085-5.0
VIN ≤ 20V
LM1085-12
VIN ≤ 25V
5.0
10.0
mA
Thermal Regulation TA = 25˚C, 30ms Pulse
.004
0.02
%/W
Ripple Rejection
fRIPPLE = 120Hz, COUT = 25µF Tantalum, IOUT = 3A
LM1085-ADJ, CADJ = 25µF, (VIN−VO) = 3V
LM1085-3.3, VIN = 6.3V
60
60
60
54
75
72
68
60
55
dB
dB
dB
dB
µA
LM1085-5.0, VIN = 8V
LM1085-12 VIN = 15V
Adjust Pin Current
Adjust Pin Current
Change
LM1085
120
5
10mA ≤ IOUT ≤ IFULL LOAD, 1.5V ≤ VIN−VOUT ≤ 25V
0.2
0.5
µA
%
Temperature
Stability
Long Term Stability TA=125˚C, 1000Hrs
0.3
1.0
%
%
RMS Output Noise 10Hz ≤ f≤ 10kHz
0.003
(% of VOUT
Thermal Resistance 3-Lead TO-263: Control Section/Output Section
Junction-to-Case 3-Lead TO-220: Control Section/Output Section
)
0.7/3.0
0.7/3.0
˚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
, θ , and T . The maximum allowable power dissipation at any ambient temperature is
JA
J(max)
A
P
= (T
–T )/θ . All numbers apply for packages soldered directly into a PC board. Refer to Thermal Considerations in the Application Notes.
D
J(max) JA
A
Note 4: For testing purposes, ESD was applied using human body model, 1.5kΩ 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
is defined in the current limit curves. The I
Curve defines the current limit as a function of input-to-output voltage. Note that 30W power
FULL LOAD
FULL LOAD
dissipation for the LM1085 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.
4
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Typical Performance Characteristics
Dropout Voltage vs. Output Current
Short-Circuit Current vs. Input/Output Difference
10094768
10094763
Percent Change in Output Voltage vs. Temperature
Adjust Pin Current vs. Temperature
10094799
10094798
Maximum Power Dissipation vs. Temperature
Ripple Rejection vs. Frequency (LM1085-Adj.)
10094743
10094770
5
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Typical Performance Characteristics (Continued)
Ripple Rejection vs. Output Current (LM1085-Adj.)
Line Transient Response
10094744
10094772
Load Transient Response
10094771
6
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STABILITY CONSIDERATION
Application Note
Stability consideration primarily concern the phase response
of the feedback loop. In order for stable operation, the loop
must maintain negative feedback. The LM1085 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).
GENERAL
Figure 1 shows a basic functional diagram for the LM1085-
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 LM1085
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 LM1085 is guaran-
teed to provide a minimum dropout voltage 1.5V over tem-
perature, at full load.
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.
10094765
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:
FIGURE 1. Basic Functional Diagram for the LM1085,
excluding Protection circuitry
OUTPUT VOLTAGE
∆V = ∆I x ESR
The LM1085 adjustable version develops at 1.25V reference
voltage, (VREF), 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.
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
The current IADJ from the adjustment terminal introduces an
output error . But since it is small (120uA max), it becomes
negligible when R1 is in the 100Ω range.
For fixed voltage devices, R1 and R2 are integrated inside
the devices.
Ripple rejection is a function of the open loop gain within the
feed-back loop (refer to Figure 1 and Figure 2). The LM1085
exhibits 75dB of ripple rejection (typ.). When adjusted for
voltages higher than VREF, the ripple rejection decreases as
function of adjustment gain: (1+R1/R2) or VO/VREF. 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 (CADJ). The imped-
ance of the CADJ 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/(2π*fRIPPLE*CADJ) ≤ R1
LOAD REGULATION
10094717
The LM1085 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.
FIGURE 2. Basic Adjustable Regulator
7
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adjustment terminal. The adjust pin can take a transient
signal of 25V with respect to the output voltage without
damaging the device.
Application Note (Continued)
Figure 3 shows a typical application using a fixed output
regulator. Rt1 and Rt2 are the line resistances. VLOAD is less
than the VOUT by the sum of the voltage drops along the line
resistances. In this case, the load regulation seen at the
RLOAD 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 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 VIN. In the LM1085 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 µf), 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.
10094718
FIGURE 3. Typical Application using Fixed Output
Regulator
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 0.05Ω resistance
between the regulator and load will have a load regulation
due to line resistance of 0.05Ω x IL. If R1 (= 125Ω) is
connected near the load the effective line resistance will be
0.05Ω (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.
10094715
FIGURE 5. Regulator with Protection Diode
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.
10094719
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 (θJA).
Thermal resistance has units of temperature per power (C/
W). The higher the thermal resistance, the hotter the IC.
FIGURE 4. Best Load Regulation using Adjustable
Output Regulator
PROTECTION DIODES
Under normal operation, the LM1085 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
The LM1085 specifies the thermal resistance for each pack-
age as junction to case (θJC). In order to get the total
resistance to ambient (θJA), two other thermal resistance
8
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IIN = IL + IG
Application Note (Continued)
must be added, one for case to heat-sink (θCH) and one for
heatsink to ambient (θHA). The junction temperature can be
predicted as follows:
PD = (VIN−VOUT) IL + VIN G
I
Figure 6 shows the voltages and currents which are present
in the circuit.
TJ = TA + PD (θJC + θCH + θHA) = TA + PD θJA
TJ is junction temperature, TA is ambient temperature, and
PD is the power consumption of the device. Device power
consumption is calculated as follows:
10094716
FIGURE 6. Power Dissipation Diagram
Once the devices power is determined, the maximum allow-
able (θJA(max)) is calculated as:
θJA(max) = TR(max)/PD = TJ(max − TA(max))/PD
The LM1085 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 (TJ(max)) for each
section is listed in the Absolute Maximum section of the
datasheet. TJ(max) is 125˚C for the control section, while
TJ(max) is 150˚C for the output section.
θHA(max) = θJA(max) − (θJC + θCH)
θHA(max) should also be calculated twice as follows:
θHA(max) = θJA (max, CONTROL SECTION) - (θJC (CON-
TROL SECTION) + θCH
HA(max)=θJA(max, OUTPUT SECTION)
SECTION) + θCH
)
θ
-
(θJC(OUTPUT
)
If thermal compound is used, θCH can be estimated at 0.2
C/W. If the case is soldered to the heat sink, then a θCH can
be estimated as 0 C/W.
After, θHA(max) is calculated for each section, choose the
lower of the two θHA(max) values to determine the appropriate
heat sink.
θJA(max) should be calculated separately for each section as
follows:
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.
θJA (max, CONTROL SECTION) = (125˚C - TA(max))/PD
θ
JA(max, OUTPUT SECTION) = (150˚C - TA(max))/PD
The required heat sink is determined by calculating its re-
quired thermal resistance (θHA(max)).
10094764
FIGURE 7. Heat sink thermal Resistance vs Area
9
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Typical Applications
10094767
5V to 3.3V, 1.5A Regulator
10094754
Battery Charger
10094750
@
Adjustable 5V
10094755
Adjustable Fixed Regulator
10094756
Regulator with Reference
10094752
1.2V to 15V Adjustable Regulator
10094757
High Current Lamp Driver Protection
10094753
5V Regulator with Shutdown
10
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Typical Applications (Continued)
10094761
Automatic Light control
10094759
Battery Backup Regulated Supply
10094762
Generating Negative Supply voltage
10094760
Ripple Rejection Enhancement
10094758
Remote Sensing
11
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Physical Dimensions inches (millimeters) unless otherwise noted
3-Lead TO-263
NS Package Number TS3B
3-Lead TO-220
NS Package Number T03B
12
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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.
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