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LM1085
3A Low Dropout Positive Regulators
LM1085 3A Low Dropout Positive Regulators
June 2005
General Description
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.
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.
The LM1085 circuit includes a zener trimmed bandgap reference, current limiting and thermal shutdown.
The LM1085 series is available in TO-220 and TO-263 packages. Refer to the LM1084 for the 5A version, and the
LM1086 for the 1.5A version.
Connection Diagrams
TO-220
10094702
Top View
TO-263
Features
n Available in 3.3V, 5.0V, 12V and Adjustable Versions
n Current Limiting and Thermal Protection
n Output Current 3A
n Line Regulation 0.015% (typical)
n Load Regulation 0.1% (typical)
Applications
n High Efficiency Linear Regulators
n Battery Charger
n Post Regulation for Switching Supplies
n Constant Current Regulator
n Microprocessor Supply
Application Circuit
1.2V to 15V Adjustable Regulator
10094704
Top View
10094752
Basic Functional Diagram, Adjustable Version
10094765
© 2005 National Semiconductor Corporation DS100947 www.national.com
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Ordering Information
LM1085
3-lead TO-263 −40˚C to +125˚C LM1085IS-ADJ Rails
3-lead TO-220 −40˚C to + 125˚C LM1085IT-ADJ Rails
Package Temperature Range Part Number Transport Media NSC Drawing
LM1085ISX-ADJ Tape and Reel
LM1085IS-12 Rails
LM1085ISX-12 Tape and Reel
LM1085IS-3.3 Rails
LM1085ISX-3.3 Tape and Reel
LM1085IS-5.0 Rails
LM1085ISX-5.0 Tape and Reel
LM1085IT-12 Rails
LM1085IT-3.3 Rails
LM1085IT-5.0 Rails
Simplified Schematic
TS3B
T03B
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10094734
Page 3
LM1085
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) 150˚C
Storage Temperature Range -65˚C to 150˚C
Lead Temperature 260˚C, to 10 sec
ESD Tolerance (Note 4) 2000V
Maximum Input to Output Voltage Differential
LM1085-ADJ 29V
Operating Ratings (Note 1)
LM1085-12 18V
LM1085-3.3 27V
LM1085-5.0 25V
Power Dissipation (Note 2) Internally Limited
Junction Temperature Range (T
Control Section −40˚C to 125˚C
Output Section −40˚C to 150˚C
) (Note 3)
J
Electrical Characteristics
Typicals and limits appearing in normal type apply for TJ= 25˚C. Limits appearing in Boldface type apply over the entire junction temperature range for operation.
Symbol Parameter Conditions
V
REF
Reference Voltage LM1085-ADJ
= 10mA, VIN−V
I
OUT
10mA ≤I
OUT
≤ I
FULL LOAD
=3V
OUT
,1.5V ≤ (VIN−V
(Note 7)
V
OUT
Output Voltage
(Note 7)
LM1085-3.3
= 0mA, VIN=5V
I
OUT
0 ≤ I
OUT≤IFULL LOAD
, 4.8V≤ VIN≤15V
LM1085-5.0
= 0mA, VIN=8V
I
OUT
0 ≤ I
OUT
≤ I
FULL LOAD
, 6.5V ≤ VIN≤ 20V
LM1085-12
= 0mA, VIN= 15V
I
OUT
∆V
OUT
Line Regulation
(Note 8)
OUT
≤ I
FULL LOAD
0 ≤ I
LM1085-ADJ
=10mA, 1.5V≤ (VIN-V
I
OUT
, 13.5V ≤ VIN≤ 25V
OUT
LM1085-3.3
= 0mA, 4.8V ≤ VIN≤ 15V
I
OUT
LM1085-5.0
I
= 0mA, 6.5V ≤ VIN≤ 20V
OUT
LM1085-12
=0mA, 13.5V ≤ VIN≤ 25V
I
OUT
∆V
OUT
Load Regulation
(Note 8)
LM1085-ADJ
(V
IN-VOUT
) = 3V, 10mA ≤ I
OUT
LM1085-3.3
= 5V, 0 ≤ I
V
IN
OUT
≤ I
FULL LOAD
LM1085-5.0
= 8V, 0 ≤ I
V
IN
OUT
≤ I
FULL LOAD
LM1085-12
Dropout Voltage
(Note 9)
= 15V, 0 ≤ I
V
IN
LM1085-ADJ, 3.3, 5, 12
, ∆V
∆V
REF
OUT
OUT
= 1%, I
≤ I
FULL LOAD
=3A 1.3 1.5 V
OUT
) ≤ 15V
≤ I
FULL LOAD
OUT
) ≤ 15V
Min
(Note 6)
1.238
1.225
3.270
3.235
4.950
4.900
11.880
11.760
Typ
(Note 5)
1.250
1.250
3.300
3.300
5.000
5.000
12.000
12.000
0.015
0.035
0.5
1.0
0.5
1.0
1.0
2.0
0.1
0.2
3
7
5
10
12
24
Max
(Note 6)
1.262
1.270
3.330
3.365
5.050
5.100
12.120
12.240
0.2
0.2
6
6
10
10
25
25
0.3
0.4
15
20
20
35
36
72
Units
V
V
V
V
V
V
V
V
%
%
mV
mV
mV
mV
mV
mV
%
%
mV
mV
mV
mV
mV
mV
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Page 4
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-
LM1085
tion temperature range for operation.
Symbol Parameter Conditions
I
LIMIT
Current Limit LM1085-ADJ
V
IN−VOUT
V
IN−VOUT
=5V
= 25V
LM1085-3.3
VIN=8V 3.2 5.5 A
LM1085-5.0
= 10V 3.2 5.5 A
V
IN
LM1085-12
VIN= 17V 3.2 5.5 A
Minimum Load
Current (Note 10)
LM1085-ADJ
V
IN−VOUT
= 25V 5.0 10.0 mA
Quiescent Current LM1085-3.3
VIN≤ 18V 5.0 10.0 mA
LM1085-5.0
≤ 20V 5.0 10.0 mA
V
IN
LM1085-12
≤ 25V 5.0 10.0 mA
V
IN
Thermal Regulation T
Ripple Rejection f
= 25˚C, 30ms Pulse .004 0.02 %/W
A
= 120Hz, C
RIPPLE
LM1085-3.3, V
LM1085-5.0, V
LM1085-12 V
IN
IN
IN
= 25µF Tantalum, I
OUT
= 25µF, (VIN−VO)=3V
ADJ
= 6.3V 60 72 dB
=8V 60 68 dB
= 15V 54 60 dB
Adjust Pin Current LM1085 55 120 µA
Adjust Pin Current
10mA ≤ I
OUT
≤ I
FULL LOAD
, 1.5V ≤ VIN−V
Change
Temperature
Stability
Long Term Stability T
RMS Output Noise
OUT
)
(% of V
Thermal Resistance
Junction-to-Case
=125˚C, 1000Hrs 0.3 1.0 %
A
10Hz ≤ f≤ 10kHz 0.003 %
3-Lead TO-263: Control Section/Output Section
3-Lead TO-220: Control Section/Output Section
OUT
OUT
Min
(Note 6)
3.2
0.2
=3A
60 75 dBLM1085-ADJ, C
≤ 25V
Typ
(Note 5)
(Note 6)
5.5
0.5
0.2 5 µA
0.5 %
0.7/3.0
0.7/3.0
Max
Units
A
A
˚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
P
=(T
D
J(max)–TA
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
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.
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)/θJA. All numbers apply for packages soldered directly into a PC board. Refer to Thermal Considerations in the Application Notes.
FULL LOAD
is defined in the current limit curves. The I
, θJA, and TA. The maximum allowable power dissipation at any ambient temperature is
J(max)
FULL LOAD
Curve defines the current limit as a function of input-to-output voltage. Note that 30W power
Page 5
Typical Performance Characteristics
Dropout Voltage vs. Output Current Short-Circuit Current vs. Input/Output Difference
LM1085
10094763
10094768
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.)
10094770
10094743
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Page 6
Typical Performance Characteristics (Continued)
LM1085
Ripple Rejection vs. Output Current (LM1085-Adj.) Line Transient Response
10094744
Load Transient Response
10094772
10094771
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Page 7
Application Note
GENERAL
Figure 1 shows a basic functional diagram for the LM1085Adj (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 guaranteed to provide a minimum dropout voltage 1.5V over temperature, at full load.
10094765
FIGURE 1. Basic Functional Diagram for the LM1085,
excluding Protection circuitry
OUTPUT VOLTAGE
The LM1085 adjustable version develops at 1.25V reference
voltage, (V
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
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.
), between the output and the adjust terminal.
REF
from the adjustment terminal introduces an
ADJ
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 LM1085 requires a
certain amount series resistance with capacitive loads. This
series resistance introduces a zero within the loop to increase phase margin and thus increase stability. The equivalent 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 tantalums, but their ESR varies exponentially at cold temperatures; 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 tantalums 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 applications that entail large changes in load current (microprocessors 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 LM1085
exhibits 75dB of ripple rejection (typ.). When adjusted for
voltages higher than V
function of adjustment gain: (1+R1/R2) or V
, the ripple rejection decreases as
REF
O/VREF
. Therefore 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
ance of the C
should be equal to or less than R1 at the
ADJ
). The imped-
ADJ
desired ripple frequency. This bypass capacitor prevents
ripple from being amplified as the output voltage is increased.
1/(2π*f
RIPPLE*CADJ
) ≤ R
1
LM1085
FIGURE 2. Basic Adjustable Regulator
10094717
LOAD REGULATION
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 regulation, a few precautions are needed.
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Page 8
Application Note (Continued)
Figure 3 shows a typical application using a fixed output
LM1085
regulator. Rt1 and Rt2 are the line resistances. V
than the V
resistances. In this case, the load regulation seen at the
R
LOAD
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.
FIGURE 3. Typical Application using Fixed Output
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 regulation. 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 xI
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 sensing and improve load regulation.
FIGURE 4. Best Load Regulation using Adjustable
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 terminals limits the current. No diode is needed to divert the
current around the regulator even with a capacitor on the
by the sum of the voltage drops along the line
OUT
would be degraded from the data sheet specification.
Regulator
.IfR1(=125Ω)is
L
Output Regulator
LOAD
10094718
10094719
is less
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 the LM1085 regu-
IN
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 recommended between the output and input pins to protect the
regulator, shown in Figure 5.
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 limiting 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 (θ
JA
Thermal resistance has units of temperature per power (C/
W). The higher the thermal resistance, the hotter the IC.
The LM1085 specifies the thermal resistance for each package as junction to case (θ
resistance to ambient (θ
). In order to get the total
JC
), two other thermal resistance
JA
).
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Page 9
Application Note (Continued)
must be added, one for case to heat-sink (θ
heatsink to ambient (θ
). The junction temperature can be
HA
predicted as follows:
T
J=TA+PD(θJC
+ θCH+ θHA)=TA+PDθ
TJis junction temperature, TAis ambient temperature, and
is the power consumption of the device. Device power
P
D
consumption is calculated as follows:
) and one for
CH
JA
FIGURE 6. Power Dissipation Diagram
I
IN=IL+IG
PD=(VIN−V
OUT)IL+VINIG
Figure 6 shows the voltages and currents which are present
in the circuit.
10094716
LM1085
Once the devices power is determined, the maximum allowable (θ
θ
JA(max)=TR(max)/PD=TJ(max−TA(max)
) is calculated as:
JA(max)
)/P
D
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 junction 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
is 150˚C for the output section.
T
J(max)
θ
should be calculated separately for each section as
JA(max)
is 125˚C for the control section, while
J(max)
follows:
(max, CONTROL SECTION) = (125˚C - T
θ
JA
θJA(max, OUTPUT SECTION) = (150˚C - T
A(max)
A(max)
)/P
)/P
D
D
The required heat sink is determined by calculating its required thermal resistance (θ
HA(max)
).
θ
θ
θ
TROL SECTION) + θ
θ
SECTION) + θ
If thermal compound is used, θ
C/W. If the case is soldered to the heat sink, then a θ
= θ
HA(max)
should also be calculated twice as follows:
HA(max)
= θJA(max, CONTROL SECTION) - (θJC(CON-
HA(max)
HA(max)=θJA
−(θJC+ θCH)
JA(max)
)
CH
(max, OUTPUT SECTION) - (θJC(OUTPUT
)
CH
can be estimated at 0.2
CH
can
CH
be estimated as 0 C/W.
After, θ
lower of the two θ
is calculated for each section, choose the
HA(max)
values to determine the appropriate
HA(max)
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.
10094764
FIGURE 7. Heat sink thermal Resistance vs Area
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Page 10
Typical Applications
LM1085
5V to 3.3V, 1.5A Regulator
Adjustable@5V
10094767
10094754
Battery Charger
10094750
10094755
Adjustable Fixed Regulator
10094756
Regulator with Reference
1.2V to 15V Adjustable Regulator
10094752
10094753
5V Regulator with Shutdown
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High Current Lamp Driver Protection
10094757
Page 11
Typical Applications (Continued)
LM1085
Battery Backup Regulated Supply
Ripple Rejection Enhancement
Automatic Light control
10094761
10094759
10094762
Generating Negative Supply voltage
10094760
Remote Sensing
10094758
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Page 12
Physical Dimensions inches (millimeters) unless otherwise noted
LM1085
3-Lead TO-263
NS Package Number TS3B
3-Lead TO-220
NS Package Number T03B
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Page 13
Notes
LM1085 3A Low Dropout Positive Regulators
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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