LM1085
LOW DROPOUT POSITIVE VOLTAGE REGULATOR
DESCRIPTION
The LM1085 series of positive adjustable and fixed regulators are designed to provide 3A with high efficiency. All internal
circuitry is designed to operate down to 1.3V input to output differential. On-chip trimming adjusts the reference voltage to
1%.
APPLICATIONS
High Efficiency Linear Regulators
Post Regulators for Switching Supplies
Adjustable Power Supply
FEATURES
Adjustable or Fixed Output
Output Current of 3A
Low Dropout, 1.5V max at 3A Output Current
0.04% Line Regulation
0.2 % Load Regulation
100% Thermal Limit Burn-In
Fast Transient Response
TYPICAL APPLICATION DATA
4.8V
4.95V
LM1085-3.3
AMS1085
LM1085
PIN DESCRIPTION
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LM1085
ABSOLUTE MAXIMUM RATINGS
SYMBOL
PARAMETER
V
CC
Power Dissipation
V
IN
Input Voltage
Operation Junction Temperature Range
T
J
Control Section
Power Transistor
T
STG
Storage Temperature Range
T
LEAD
Lead Temperature (Soldering 10 sec)
VALUE
Internally Limited
15
-40 to 125
-40 to 150
-65 to +150
300
UNIT
W
V
0
0
0
C
C
C
ELECTRICAL CHARACTERISTICS
I
LOAD
= 0mA and T
J
= +25
0
C (unless otherwise noted)
PARAMETER
Reference Voltage Note 1
DEVICE
LM1085
TEST CONDITIONS
V
IN
=5V, I
LOAD
= 10mA
V
IN
– V
OUT
= 1.5V to 10V,
I
LOAD
= 10mA to 3A
V
IN
– V
OUT
= 1.5V,
Variator from nominal V
OUT
V
IN
– V
OUT
= 1.5V to 10V
I
LOAD
= 0mA to 3A,
Variator from nominal V
OUT
I
LOAD
= 10mA,
V
IN
– V
OUT
= 1.5V to 10V
V
IN
– V
OUT
= 1.5V
I
LOAD
= 10mA to 3A
V
IN
=5V, V
ADJ
= 0V
V
IN
– V
OUT
= 1.5V
I
LOAD
= 10mA to 3A
V
IN
– V
OUT
= 1.5V to 10V
I
LOAD
= 10mA
V
IN
– V
OUT
= 1.5V
V
IN
– V
OUT
= 3V
I
LOAD
= 3A
I
LOAD
= 3A
V
IN
– V
OUT
= 1.5V,
I
LOAD
= 10mA
MIN.
1.238
*
1.225
-1
*
*
*
*
*
*
*
*
*
*
-2
-
-
-
-
-
3
60
-
-
TYP.
1.250
1.250
-
-
0.04
0.2
3
7
40
4.5
65
1.3
0.005
MAX.
1.262
1.275
+1
%
+2
0.20
%
0.40
7
10
90
-
-
1.5
-
mA
mA
µA
A
dB
V
%/
0
C
V
UNIT
Output Voltage Note 1
All fixed
versions
Line Regulation Note 1
Load Regulation Note 1
Minimum Load Current
Ground Pin Current
Adjust Pin Current
Current Limit
Ripple Rejection Note 2
Dropout Voltage Note 1,3
Temperature Coefficient
All
All
LM1085
All fixed
versions
LM1085
All
All
All
All
The
*
denotes the specifications which apply over the full temperature range (see previous table,
T
J
)
NOTES:
1:
Low duty pulse testing with Kelvin connections required.
2:
120Hz input ripple (C
ADJ
for ADJ = 25µF, C
OUT
= 25µF)
3:
V
OUT
,
V
REF =
1%
APPLICATION INFORMATION
The LM1085 series of adjustable and fixed regulators are easy to use and have all the protection features expected in high
performance voltage regulators: short circuit protection and thermal shut-down.
Pin compatible with older three terminal adjustable regulators, these devices offer the advantage of a lower dropout voltage,
more precise reference tolerance and improved reference stability with temperature.
STABILITY
The circuit design used in the LM1085 series requires the use of an output capacitor as part of the device frequency
compensation.
The addition of 150µF aluminum electrolytic or a 22µF solid tantalum on the output will ensure stability
for all operating conditions.
When the adjustment terminal is bypassed with a capacitor to improve the ripple rejection, the requirement
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for an output capacitor increases. The value of 22µF tantalum or 150µF aluminum covers all cases of bypassing the
adjustment terminal. Without bypassing the adjustment terminal smaller capacitors can be used with equally good results.
To ensure good transient response with heavy load current changes capacitor values on the order of 100µF are used in the
output of many regulators. To further improve stability and transient response of these devices larger values of output
capacitor can be used.
PROTECTION DIODES
Unlike older regulators, the LM1085 family does not need any protection diodes
between the adjustment pin and the output and from the output to the input to
prevent over-stressing the die.
Internal resistors are limiting the internal current paths on the LM1085 adjustment
pin, therefore even with capacitors on the adjustment pin no protection diode is
needed to ensure device safety under short-circuit conditions.
Diodes between the input and output are not usually needed.
Microsecond surge currents of 50A to 100A can be handled by the internal diode between the input and output pins of the
device. In normal operations it is difficult to get those values of surge currents even with the use of large output
capacitances. If high value output capacitors are used, such as 1000µF to 5000µF and the input pin is instantaneously
shorted to ground, damage can occur. A diode from output to input is recommended, when a crowbar circuit at the input of
the LM1085 is used. Normal power supply cycling or even plugging and unplugging in the system will not generate current
large enough to do any damage.
The adjustment pin can be driven on a transient basis ±25V, with respect to the output without any device degradation. As
with any IC regulator, none the protection circuitry will be functional and the internal transistors will break down if the
maximum input to output voltage differential is exceeded.
LM1085
RIPPLE REJECTION
The ripple rejection values are measured with the adjustment pin bypassed. The impedance of the adjust pin capacitor at the
ripple frequency should be less than the value of R1 (normally 100 to 120) for a proper bypassing and ripple rejection
approaching the values shown. The size of the required adjust pin capacitor is a function of the input ripple frequency. If
R1=100
at 120Hz the adjust pin capacitor should be 25µF. At 10
kHz only 0.22
µF is needed.
The ripple rejection will be a function of output voltage, in circuits without an adjust pin bypass capacitor. The output ripple
will increase directly as a ratio of the output voltage to the reference voltage (V
OUT
/ V
REF
).
OUTPUT VOLTAGE
LM1085
The LM1085 series develops a 1.25V reference voltage between the output and
the adjust terminal. Placing a resistor between these two terminals causes a
constant current to flow through R1 and down through R2 to set the overall output
voltage.
This current is normally the specified minimum load current of 10mA. Because I
ADJ
is very small and constant it represents a small error and it can usually be ignored.
LOAD REGULATION
LM1085
True remote load sensing it is not possible to provide, because the LM1085 is a
three terminal device. The resistance of the wire connecting the regulator to the
load will limit the load regulation.
The data sheet specification for load regulation is measured at the bottom of the
package. Negative side sensing is a true Kelvin connection, with the bottom of the
output divider returned to the negative side of the load.
The best load regulation is obtained when the top of the resistor divider R1 is
connected directly to the case not to the load. If R1 were connected to the load, the
effective resistance between the regulator and the load would be:
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Connected as shown Fig.3, R
is not multiplied by the divider ratio. Using 16-gauge wire the parasitic line resistance is about
0.004 per foot, translating to 4mV/ft at 1A load current. It is important to keep the positive lead between regulator and load
as short as possible and use large wire or PC board traces.
THERMAL CONSIDERATIONS
The LM1085 series have internal power and thermal limiting circuitry designed to protect the device under overload
conditions. However maximum junction temperature ratings should not be exceeded under continuous normal load
conditions.
Careful consideration must be given to all sources of thermal resistance from junction to ambient, including junction-to-case,
case-to-heat sink interface and heat sink resistance itself. To ensure safe operating temperatures and reflect more
accurately the device temperature, new thermal resistance specifications have been developed. Unlike older regulators with
a single junction-to-case thermal resistance specification, the data section for these new regulators provides a separate
thermal resistance and maximum junction temperature for both the Control Section and the Power Transistor. Calculations
for both temperatures under certain conditions of ambient temperature and heat sink resistance and to ensure that both
thermal limits are met.
Junction-to-case thermal resistance is specified from the IC junction to the bottom of the case directly below the die. This is
the lowest resistance path for the heat flow. In order to ensure the best possible thermal flow from this area of the package
to the heat sink proper mounting is required. Thermal compound at the case-to-heat sink interface is recommended. A
thermally conductive spacer can be used, if the case of the device must be electrically isolated, but its added contribution to
thermal resistance has to be considered.
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LM1085
Important statement:
Huaguan Semiconductor Co,Ltd. reserves the right to change
the products and services provided without notice. Customers
should obtain the latest relevant information before ordering,
and verify the timeliness and accuracy of this information.
Customers are responsible for complying with safety
standards and taking safety measures when using our
products for system design and machine manufacturing to
avoid potential risks that may result in personal injury or
property damage.
Our products are not licensed for applications in life support,
military, aerospace, etc., so we do not bear the consequences
of the application of these products in these fields.
Our documentation is only permitted to be copied without
any tampering with the content, so we do not accept any
responsibility or liability for the altered documents.
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