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LT1185
1185fe
Low Dropout Regulator
s
Low Resistance Pass Transistor: 0.25
s
Dropout Voltage: 0.75V at 3A
s
1% Reference Voltage
s
Accurate Programmable Current Limit
s
Shutdown Capability
s
Internal Reference Available
s
Full Remote Sense
s
Low Quiescent Current: 2.5mA
s
Good High Frequency Ripple Rejection
s
Available in 5-Lead TO-220 and DD Packages
The LT1185 uses a saturation-limited NPN transistor as
the pass element. This device gives the linear dropout
characteristics of a FET pass element with significantly
less die area. High efficiency is maintained by using special
anti-saturation circuitry that adjusts base drive to track
load current. The "on resistance" is typically 0.25
.
Accurate current limit is programmed with a single 1/8W
external resistor, with a range of zero to three amperes. A
second, fixed internal limit circuit prevents destructive
currents if the programming current is accidentally over-
ranged. Shutdown of the regulator output is guaranteed
when the program current is less than 1
A, allowing
external logic control of output voltage.
The LT1185 has all the protection features of previous
LTC regulators, including power limiting and thermal
shutdown.
, LTC and LT are registered trademarks of Linear Technology Corporation.
5V, 3A Regulator with 3.5A Current Limit
Dropout Voltage
+
R1
2.37k
R2
2.67k
C1
2
F
TANT
REF
GND
FB
V
OUT
V
IN
LT1185
C2
2
F
TANT
R
LIM
*
4.3k
+
V
IN
6V TO 16V
V
OUT
5V AT 3A
LT1185 TA01
*CURRENT LIMIT = 15k/R
LIM
= 3.5A
+
+
LOAD CURRENT (A)
0
V
IN
V
OUT
(V)
0.8
1.0
1.2
4
LT1185 TA02
0.6
0.4
0
1
2
3
0.2
1.6
1.4
T
J
= 125
C
T
J
= 55
C
T
J
= 25
C
The LT
1185 is a 3A low dropout regulator with adjustable
current limit and remote sense capability. It can be used as
a positive output regulator with floating input or as a
standard negative regulator with grounded input. The
output voltage range is 2.5V to 25V, with
1% accuracy on
the internal reference voltage.
FEATURES
DESCRIPTIO
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TYPICAL APPLICATIO
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LT1185
1185fe
2
4
1
3
GND
FB
REF
V
IN
(CASE)
V
OUT
K PACKAGE
4-LEAD TO-3 METAL CAN
BOTTOM VIEW
ORDER PART
NUMBER
LT1185MK
ORDER PART
NUMBER
LT1185CT
LT1185IT
JC MAX
= 2.5
C/ W,
JA
= 50
C/W
T PACKAGE
5-LEAD PLASTIC TO-220
REF
V
OUT
V
IN
FB
GND
FRONT VIEW
TAB IS V
IN
5
4
3
2
1
JC MAX
= 2.5
C/ W,
JA
= 35
C/W
Input Voltage .......................................................... 35V
Input-Output Differential ......................................... 30V
FB Voltage ................................................................ 7V
REF Voltage .............................................................. 7V
Output Voltage ........................................................ 30V
Output Reverse Voltage ............................................ 2V
Operating Ambient Temperature Range
LT1185C ............................................... 0
C to 70
C
LT1185M (OBSOLETE) .................... 55
C to 125
C
*See Application Section for details on calculating Operation Junction Temperature
ABSOLUTE AXI U RATI GS
W
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PACKAGE/ORDER I FOR ATIO
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Operating Junction Temperature Range*
Control Section
LT1185C ............................................. 0
C to 125
C
LT1185I .......................................... 40
C to 125
C
LT1185M (OBSOLETE) ................... 55
C to 150
C
Power Transistor Section
LT1185C ............................................. 0
C to 150
C
LT1185I .......................................... 40
C to 150
C
LT1185M (OBSOLETE) ................... 55
C to 175
C
Storage Temperature Range ................ 65
C to 150
C
Lead Temperature (Soldering, 10 sec)................ 300
C
(Note 1)
OBSOLETE PACKAGE
ORDER PART
NUMBER
LT1185CQ
T
JMAX
= 150
C,
JA
= 30
C/W
Q PACKAGE
5-LEAD PLASTIC DD
FRONT VIEW
TAB
IS
V
IN
REF
V
OUT
V
IN
FB
GND
5
4
3
2
1
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
Reference Voltage (At FB Pin)
2.37
V
Reference Voltage Tolerance (At FB Pin) (Note 2)
V
IN
V
OUT
= 5V, V
OUT
= V
REF
0.3
1
%
1mA
I
OUT
3A
q
1
2.5
%
V
IN
V
OUT
= 1.2V to V
IN
= 30V
P
25W (Note 6), V
OUT
= 5V
T
MIN
T
J
T
MAX
(Note 9)
Feedback Pin Bias Current
V
OUT
= V
REF
q
0.7
2
A
Droput Voltage (Note 3)
I
OUT
= 0.5A, V
OUT
= 5V
0.20
0.37
V
I
OUT
= 3A, V
OUT
= 5V
0.67
1.00
V
The
q
denotes specifications which apply over the operating temperature range, otherwise specifications are at T
A
= 25
C.
Adjustable version, V
IN
= 7.4V, V
OUT
= 5V, I
OUT
= 1mA, R
LIM
= 4.02k, unless otherwise noted.
ELECTRICAL CHARACTERISTICS
Consult LTC Marketing for parts specified with wider operating temperature ranges.
3
LT1185
1185fe
The
q
denotes specifications which apply over the operating temperature range, otherwise specifications are at T
A
= 25
C.
Adjustable version, V
IN
= 7.4V, V
OUT
= 5V, I
OUT
= 1mA, R
LIM
= 4.02k, unless otherwise noted.
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
Load Regulation (Note 7)
I
OUT
= 5mA to 3A
0.05
0.3
%
V
IN
V
OUT
= 1.5V to 10V, V
OUT
= 5V
Line Regulation (Note 7)
V
IN
V
OUT
= 1V to 20V, V
OUT
= 5V
0.002
0.01
%/V
Minimum Input Voltage
I
OUT
= 1A (Note 4), V
OUT
= V
REF
4.0
V
I
OUT
= 3A
4.3
V
Internal Current Limit (See Graph for
1.5V
V
IN
V
OUT
10V
3.3
3.6
4.2
A
Guaranteed Curve) (Note 12)
q
3.1
4.4
A
V
IN
V
OUT
= 15V
q
2.0
3.0
4.2
A
V
IN
V
OUT
= 20V
q
1.0
1.7
2.6
A
V
IN
V
OUT
= 30V
q
0.2
0.4
1.0
A
External Current Limit
5k
R
LIM
15k, V
OUT
= 1V
q
15k
A
Programming Constant
(Note 11)
External Current Limit Error
1A
I
LIM
3A
0.02 I
LIM
0.06 I
LIM
+ 0.03
A
R
LIM
= 15k A/I
LIM
q
0.04 I
LIM
0.09 I
LIM
+ 0.05
A
Quiescent Supply Current
I
OUT
= 5mA, V
OUT
= V
REF
q
2.5
3.5
mA
4V
V
IN
25V (Note 5)
Supply Current Change with Load
V
IN
V
OUT
= V
SAT
(Note 10)
q
25
40
mA/A
V
IN
V
OUT
2V
q
10
25
mA/A
REF Pin Shutoff Current
q
0.4
2
7
A
Thermal Regulation (See Applications
V
IN
V
OUT
= 10V
0.005
0.014
%/W
Information)
I
OUT
= 5mA to 2A
Reference Voltage Temperature Coefficient
(Note 8)
0.003
0.01
%/
C
Thermal Resistance Junction to Case
TO-3 Control Area
1
C/W
Power Transistor
3
C/W
TO-220 Control Area
1
C/W
Power Transistor
3
C/W
Note 1: Absolute Maximum Ratings are those values beyond which the life
of a device may be impaired.
Note 2: Reference voltage is guaranteed both at nominal conditions (no
load, 25
C) and at worst-case conditions of load, line, power and
temperature. An intermediate value can be calculated by adding the effects
of these variables in the actual application. See the Applications
Information section of this data sheet.
Note 3: Dropout voltage is tested by reducing input voltage until the
output drops 1% below its nominal value. Tests are done at 0.5A and 3A.
The power transistor looks basically like a pure resistance in this range so
that minimum differential at any intermediate current can be calculated by
interpolation; V
DROPOUT
= 0.25V + 0.25
I
OUT
. For load current less than
0.5A, see graph.
Note 4: "Minimum input voltage" is limited by base emitter voltage drive
of the power transistor section, not saturation as measured in Note 3. For
output voltages below 4V, "minimum input voltage" specification may limit
dropout voltage before transistor saturation limitation.
Note 5: Supply current is measured on the ground pin, and does not
include load current, R
LIM
, or output divider current.
Note 6: The 25W power level is guaranteed for an input-output voltage of
8.3V to 17V. At lower voltages the 3A limit applies, and at higher voltages
the internal power limiting may restrict regulator power below 25W. See
graphs.
Note 7: Line and load regulation are measured on a pulse basis with a
pulse width of
2ms, to minimize heating. DC regulation will be affected
by thermal regulation and temperature coefficient of the reference. See
Applications Information section for details.
Note 8: Guaranteed by design and correlation to other tests, but not
tested.
Note 9: T
JMIN
= 0
C for the LT1185C, 40
C for LT1185I, and 55
C for
the LT1185M. Power transistor area and control circuit area have different
maximum junction temperatures. Control area limits are T
JMAX
= 125
C for
the LT1185C and LT1185I and 150
C for the LT1185M. Power area limits
are 150
C for LT1185C and LT1185I and 175
C for LT1185M.
Note 10: V
SAT
is the maximum specified dropout voltage;
0.25V + 0.25 I
OUT
.
Note 11: Current limit is programmed with a resistor from REF pin to GND
pin. The value is 15k/I
LIM
.
Note 12: For V
IN
V
OUT
= 1.5V; V
IN
= 5V, V
OUT
= 3.5V. V
OUT
= 1V for all
other current limit tests.
ELECTRICAL CHARACTERISTICS
4
LT1185
1185fe
Ripple Rejection vs Frequency
Ground Pin Current
INPUT-OUTPUT DIFFERENTIAL (V)
0
0
OUTPUT CURRNT (A)
1
2
3
5
10
15
20
LT1185 TPC01
25
4
5
30
TYPICAL
TEST POINTS
GUARANTEED
LIMIT
GUARANTEED
LIMIT
Internal Current Limit
INPUT VOLTAGE (V)
0
GROUND PIN CURRENT (mA)
8
10
12
15
25
LT1185 TPC02
6
4
5
10
20
30
35
2
0
V
OUT
= 5V
*DOES NOT INCLUDE REF CURRENT
OR OUTPUT DIVIDER CURRENT
I
LOAD
= 0
T
J
= 25
C
Quiescent Ground Pin Current*
JUNCTION TEMPERATURE (
C)
50
VOLTAGE (V)
2.37
2.38
2.39
150
LT1185 TPC03
2.36
2.35
2.33
0
50
100
2.34
2.41
2.40
25
25
75
125
Feedback Pin Voltage
Temperature Drift
LOAD CURRENT (A)
0
CURRENT (mA)
80
100
120
4
LT1185 TPC04
60
40
0
1
2
3
20
160
140
T
J
= 25
C
REGULATOR JUST AT
DROPOUT POINT
V
IN
V
OUT
= 5V
FREQUENCY (Hz)
20
RATIO V
OUT
/V
IN
(dB)
40
60
80
100
100
10k
100k
1M
LT1185 TPC05
0
1k
ALL OUTPUT
VOLTAGES
WITH 0.05
F
ACROSS R2
V
OUT
= 5V
V
IN
V
OUT
= 1.5V
Load Transient Response
TIME (
s)
0
4
8
10
LT1185 TPC06
2
6
12
14
16
0.1A t
r
,
f
100ns
C
OUT
= 2.2
F, ESR = 1
C
OUT
= 2.2
F, ESR = 2
V
OUT
= 5V
I
OUT
= 1A
100mV
I
LOAD
Output Impedance
FREQUENCY (Hz)
0.01
IMPEDANCE (
)
0.1
1
10
1M
10k
100k
LT1183 TPC07
0.001
1k
OUTPUT IMPEDANCE IS
SET BY OUTPUT CAPACITOR
ESR IN THIS REGION
V
OUT
= 5V
I
OUT
= 1A
C
OUT
= 2.2
F
TYPICAL PERFOR A CE CHARACTERISTICS
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LT1185
1185fe
Block Diagram
A simplified block diagram of the LT1185 is shown in
Figure 1. A 2.37V bandgap reference is used to bias the
input of the error amplifier A1, and the reference amplifier
A2. A1 feeds a triple NPN pass transistor stage which has
the two driver collectors tied to ground so that the main
pass transistor can completely saturate. This topology
normally has a problem with unlimited current in Q1 and
Q2 when the input voltage is less than the minimum
required to create a regulated output. The standard "fix"
for this problem is to insert a resistor in series with Q1 and
Q2 collectors, but this resistor must be low enough in
value to supply full base current for Q3 under worst-case
Figure 1. Block Diagram
300mV
I1
2
A
R1
350
200mV
D2
D4
D3
+
A5
+
A4
+
A3
V
IN
R2
0.055
+
A1
+
A2
V
REF
2.37V
V
OUT
FB
GND
R
LIM
(EXTERNAL)
REF
D1
LT1185 BD
Q1
Q2
Q3
Q4
conditions, resulting in very high supply current when the
input voltage is low. To avoid this situation, the LT1185
uses an auxiliary emitter on Q3 to create a drive limiting
feedback loop which automatically adjusts the drive to Q1
so that the base drive to Q3 is just enough to saturate Q3,
but no more. Under saturation conditions, the auxiliary
emitter is acting like a collector to shunt away the output
current of A1. When the input voltage is high enough to
keep Q3 out of saturation, the auxiliary emitter current
drops to zero even when Q3 is conducting full load current.
APPLICATIO S I FOR ATIO
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