LT1185
Low Dropout Regulator
FEATURES
Low Resistance Pass Transistor: 0.25Ω
Dropout Voltage: 0.75V at 3A
±1% Reference Voltage
Accurate Programmable Current Limit
Shutdown Capability
Internal Reference Available
Full Remote Sense
Low Quiescent Current: 2.5mA
Good High Frequency Ripple Rejection
Available in 5-Lead TO-220 and DD Packages
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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 0.25Ω.
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DESCRIPTIO
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.
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 overranged. Shutdown of the regulator output is guaranteed
when the program current is less than 1µA, allowing
external logic control of output voltage.
The LT1185 has all the protection features of previous
LTC regulators, including power limiting and thermal
shutdown.
, LT, LTC and LTM are registered trademarks of Linear Technology Corporation.
All other trademarks are the property of their respective owners.
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TYPICAL APPLICATIO
Dropout Voltage
5V, 3A Regulator with 3.5A Current Limit
+
2µF
TANT
RLIM*
4.3k
1.4
2.37k
1.2
VIN
6V TO 16V
REF
+
GND
FB
–
1.6
+
VIN
LT1185
2µF
TANT
VOUT
5V AT 3A
2.67k
VOUT
LT1185 • TA01
–
VIN – VOUT (V)
+
1.0
TJ = 25°C
0.8
TJ = 125°C
0.6
0.4
TJ = –55°C
0.2
*CURRENT LIMIT = 15k/RLIM = 3.5A
0
0
1
2
3
4
LOAD CURRENT (A)
LT1185 • TA02
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LT1185
W W
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ABSOLUTE
RATI GS
(Note 1)
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
LT1185I ............................................. – 40°C to 85°C
LT1185M (OBSOLETE) .................... – 55°C to 125°C
*See Application Section for details on calculating Operation Junction Temperature
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
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PACKAGE/ORDER I FOR ATIO
BOTTOM VIEW
GND
FB
1
FRONT VIEW
2
VIN (CASE)
4
3
VOUT
TAB
IS
VIN
REF
FRONT VIEW
5
REF
5
REF
4
VOUT
4
VOUT
3
VIN
3
VIN
2
FB
2
FB
1
GND
1
GND
Q PACKAGE
5-LEAD PLASTIC DD
K PACKAGE
4-LEAD TO-3 METAL CAN
θJC MAX = 2.5°C/ W, θJA = 35°C/W
TAB IS VIN
TJMAX = 150°C, θJA = 30°C/W
T PACKAGE
5-LEAD PLASTIC TO-220
θJC MAX = 2.5°C/ W, θJA = 50°C/W
OBSOLETE PACKAGE
ORDER PART
NUMBER
LT1185MK
ORDER PART
NUMBER
LT1185CQ
LT1185IQ
ORDER PART
NUMBER
LT1185CT
LT1185IT
Order Options Tape and Reel: Add #TR
Lead Free: Add #PBF Lead Free Tape and Reel: Add #TRPBF
Lead Free Part Marking: http://www.linear.com/leadfree/
Consult LTC Marketing for parts specified with wider operating temperature ranges.
ELECTRICAL CHARACTERISTICS
The ● denotes specifications which apply over the operating temperature range, otherwise specifications are at TA = 25°C.
Adjustable version, VIN = 7.4V, VOUT = 5V, IOUT = 1mA, RLIM = 4.02k, unless otherwise noted.
PARAMETER
CONDITIONS
MIN
Reference Voltage (At FB Pin)
TYP
MAX
2.37
UNITS
V
0.3
±1
%
1mA ≤ IOUT ≤ 3A
VIN – VOUT = 1.2V to VIN = 30V
P ≤ 25W (Note 6), VOUT = 5V
TMIN ≤ TJ ≤ TMAX (Note 9)
●
1
±2.5
%
Feedback Pin Bias Current
VOUT = VREF
●
0.7
2
µA
Droput Voltage (Note 3)
IOUT = 0.5A, VOUT = 5V
IOUT = 3A, VOUT = 5V
0.20
0.67
0.37
1.00
V
V
Reference Voltage Tolerance (At FB Pin) (Note 2)
VIN – VOUT = 5V, VOUT = VREF
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LT1185
ELECTRICAL CHARACTERISTICS
The ● denotes specifications which apply over the operating temperature range, otherwise specifications are at TA = 25°C.
Adjustable version, VIN = 7.4V, VOUT = 5V, IOUT = 1mA, RLIM = 4.02k, unless otherwise noted.
PARAMETER
CONDITIONS
TYP
MAX
Load Regulation (Note 7)
IOUT = 5mA to 3A
VIN – VOUT = 1.5V to 10V, VOUT = 5V
0.05
0.3
%
Line Regulation (Note 7)
VIN – VOUT = 1V to 20V, VOUT = 5V
0.002
0.01
%/V
Minimum Input Voltage
IOUT = 1A (Note 4), VOUT = VREF
IOUT = 3A
Internal Current Limit (See Graph for
Guaranteed Curve) (Note 12)
1.5V ≤ VIN – VOUT ≤ 10V
VIN – VOUT = 15V
VIN – VOUT = 20V
VIN – VOUT = 30V
External Current Limit
Programming Constant
5k ≤ RLIM ≤ 15k, VOUT = 1V
(Note 11)
External Current Limit Error
1A ≤ ILIM ≤ 3A
RLIM = 15k • A/ILIM
MIN
4.0
4.3
●
●
●
●
3.3
3.1
2.0
1.0
0.2
3.6
3.0
1.7
0.4
UNITS
V
V
4.2
4.4
4.2
2.6
1.0
●
15k
●
0.02 ILIM
0.04 ILIM
0.06 ILIM + 0.03
0.09 ILIM + 0.05
A
A
A
A
A
A•Ω
A
A
Quiescent Supply Current
IOUT = 5mA, VOUT = VREF
4V ≤ VIN ≤ 25V (Note 5)
●
2.5
3.5
mA
Supply Current Change with Load
VIN – VOUT = VSAT (Note 10)
VIN – VOUT ≥ 2V
●
●
25
10
40
25
mA/A
mA/A
●
REF Pin Shutoff Current
7
VIN – VOUT = 10V
IOUT = 5mA to 2A
0.005
0.014
Reference Voltage Temperature Coefficient
(Note 8)
0.003
0.01
%/°C
Thermal Resistance Junction to Case
TO-3 Control Area
Power Transistor
TO-220 Control Area
Power Transistor
1
3
1
3
°C/W
°C/W
°C/W
°C/W
Note 1: Stresses beyond those listed under Absolute Maximum Ratings
may cause permanent damage to the device. Exposure to any Absolute
Maximum Rating condition for extended periods may affect device
reliability and lifetime.
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; VDROPOUT = 0.25V + 0.25Ω • IOUT. 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, RLIM, or output divider current.
0.4
µA
2
Thermal Regulation (See Applications
Information)
%/W
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: TJMIN = 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 TJMAX = 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: VSAT is the maximum specified dropout voltage;
0.25V + 0.25 • IOUT.
Note 11: Current limit is programmed with a resistor from REF pin to GND
pin. The value is 15k/ILIM.
Note 12: For VIN – VOUT = 1.5V; VIN = 5V, VOUT = 3.5V. VOUT = 1V for all
other current limit tests.
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TYPICAL PERFOR A CE CHARACTERISTICS
12
5
3
TYPICAL
2
GUARANTEED
LIMIT
1
2.39
0
*DOES NOT INCLUDE REF CURRENT
OR OUTPUT DIVIDER CURRENT
8
6
4
0
30
2.37
2.36
2.34
2.33
–50 –25 0
25 50 75 100 125 150
JUNCTION TEMPERATURE (°C)
0
25
5
15
20
10
INPUT-OUTPUT DIFFERENTIAL (V)
2.38
2.35
VOUT = 5V
2
TEST POINTS
0
2.40
VOLTAGE (V)
GUARANTEED
LIMIT
2.41
ILOAD = 0
TJ = 25°C
10
GROUND PIN CURRENT (mA)
4
OUTPUT CURRNT (A)
Feedback Pin Voltage
Temperature Drift
Quiescent Ground Pin Current*
Internal Current Limit
5
20
15
25
10
INPUT VOLTAGE (V)
30
35
LT1185 • TPC03
LT1185 • TPC02
LT1185 • TPC01
Ripple Rejection vs Frequency
Ground Pin Current
–100
160
TJ = 25°C
140
–80
RATIO VOUT/VIN (dB)
CURRENT (mA)
120
100
REGULATOR JUST AT
DROPOUT POINT
80
60
40
0
0
2
1
–40
VOUT = 5V
VIN – VOUT = 1.5V
–20
VIN – VOUT = 5V
20
ALL OUTPUT
VOLTAGES
WITH 0.05µF
ACROSS R2
–60
0
100
4
3
LOAD CURRENT (A)
1k
10k
100k
FREQUENCY (Hz)
1M
LT1185 • TPC05
LT1185 • TPC04
Load Transient Response
Output Impedance
10
OUTPUT IMPEDANCE IS
SET BY OUTPUT CAPACITOR
ESR IN THIS REGION
COUT = 2.2µF, ESR = 1Ω
1
IMPEDANCE (Ω)
100mV
COUT = 2.2µF, ESR = 2Ω
VOUT = 5V
IOUT = 1A
0.1
0.01
∆ILOAD
VOUT = 5V
IOUT = 1A
COUT = 2.2µF
0.1A tr,f ≤ 100ns
0
2
4
6
8
10
TIME (µs)
12
14
16
LT1185 • TPC06
0.001
1k
10k
100k
FREQUENCY (Hz)
1M
LT1183 • TPC07
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LT1185
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APPLICATIO S I FOR ATIO
Block Diagram
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.
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
GND
RLIM (EXTERNAL)
VREF
2.37V
REF
FB
+
–
–
A1
+
A2
Q4
VOUT
Q1
D2
D4
D3
Q2
Q3
A5
–
300mV
A4
+
+
I1
2µA
D1
A3
–
R1
350Ω
+
–
R2
0.055Ω
200mV
VIN
LT1185 • BD
Figure 1. Block Diagram
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APPLICATIO S I FOR ATIO
Amplifier A2 is used to generate an internal current through
Q4 when an external resistor is connected from the REF
pin to ground. This current is equal to 2.37V divided by
RLIM. It generates a current limit sense voltage across R1.
The regulator will current limit via A4 when the voltage
across R2 is equal to the voltage across R1. These two
resistors essentially form a current “amplifier” with a gain
of 350/0.055 = 6,360. Good temperature drift is inherent
because R1 and R2 are made from the same diffusions.
Their ratio, not absolute value, determines current limit.
Initial accuracy is enhanced by trimming R1 slightly at
wafer level. Current limit is equal to 15kΩ/RLIM.
D1 and I1 are used to guarantee regulator shutdown when
REF pin current drops below 2µA. A current less than 2µA
through Q4 causes the +input of A5 to go low and shut
down the regulator via D2.
A3 is an internal current limit amplifier which can override
the external current limit. It provides “goof proof” protection for the pass transistor. Although not shown, A3 has
a nonlinear foldback characteristic at input-output voltages above 12V to guarantee safe area protection for Q3.
See the graph, Internal Current Limit in the Typical Performance Characteristics of this data sheet.
Setting Output Voltage
The LT1185 output voltage is set by two external resistors
(see Figure 2). Internal reference voltage is trimmed to
2.37V so that a standard 1% 2.37k resistor (R1) can be
used to set divider current at 1mA. R2 is then selected
from:
R2 =
(VOUT – 2.37) R1
VREF
for R1 = 2.37k and VREF = 2.37V, this reduces to:
R2 = VOUT – 2.37k
suggested values of 1% resistors are shown.
VOUT
R2 WHEN R1 = 2.37k
5V
5.2V
6V
12V
15V
2.67k
2.87k
3.65k
9.76k
12.7k
Output Capacitor
The LT1185 has a collector output NPN pass transistor,
which makes the open-loop output impedance much
higher than an emitter follower. Open-loop gain is a direct
function of load impedance, and causes a main-loop
“pole” to be created by the output capacitor, in addition to
an internal pole in the error amplifier. To ensure loop
stability, the output capacitor must have an ESR (effective
series resistance) which has an upper limit of 2Ω, and a
lower limit of 0.2 divided by the capacitance in µF. A 2µF
output capacitor, for instance, should have a maximum
ESR of 2Ω, and a minimum of 0.2/2 = 0.1Ω. These values
are easily encompassed by standard solid tantalum
capacitors, but occasionally a solid tantalum unit will have
abnormally high ESR, especially at very low temperatures. The suggested 2µF value shown in the circuit
applications should be increased to 4.7µF for – 40°C and
– 55°C designs if the 2µF units cannot be guaranteed to
stay below 2Ω at these temperatures.
Although solid tantalum capacitors are suggested, other
types can be used if they meet the ESR requirements.
Standard aluminum electrolytic capacitors need to be
upward of 25µF in general to hold 2Ω maximum ESR,
especially at low temperatures. Ceramic, plastic film, and
monolithic capacitors have a problem with ESR being too
low. These types should have a 1Ω carbon resistor in
series to guarantee loop stability.
The output capacitor should be located close to the regulator (≤ 3") to avoid excessive impedance due to lead
inductance. A six inch lead length (2 • 3") will generate an
extra 0.8Ω inductive reactance at 1MHz, and unity-gain
frequency can be up to that value.
For remote sense applications, the capacitor should still be
located close to the regulator. Additional capacitance can
be added at the remote sense point, but the remote
capacitor must be at least 2µF solid tantalum. It cannot be
a low ESR type like ceramic or mylar unless a 0.5Ω to 1Ω
carbon resistor is added in series with the capacitor. Logic
boards with multiple low ESR bypass capacitors should
have a solid tantalum unit added in parallel whose value is
approximately five times the combined value of low ESR
capacitors.
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Large output capacitors (electrolytic or solid tantalum)
will not cause the LT1185 to oscillate, but they will cause
a damped “ringing” at light load currents where the ESR
of the capacitor is several orders of magnitude lower than
the load resistance. This ringing only occurs as a result of
transient load or line conditions and normally causes no
problems because of its low amplitude (≤ 25mV).
Heat Sinking
The LT1185 will normally be used with a heat sink. The size
of the heat sink is determined by load current, input and
output voltage, ambient temperature, and the thermal
resistance of the regulator, junction-to-case (θJC). The
LT1185 has two separate values for θJC: one for the power
transistor section, and a second, lower value for the
control section. The reason for two values is that the
power transistor is capable of operating at higher continuous temperature than the control circuitry. At low power
levels, the two areas are at nearly the same temperature,
and maximum temperature is limited by the control area.
At high power levels, the power transistor will be at a
significantly higher temperature than the control area
and its maximum operating temperature will be the
limiting factor.
To calculate heat sink requirements, you must solve a
thermal resistance formula twice, one for the power
transistor and one for the control area. The lowest value
obtained for heat sink thermal resistance must be used. In
these equations, two values for maximum junction temperature and junction-to-case thermal resistance are used,
as given in Electrical Specifications.
(TJMAX – TAMAX)
– θJC – θCHS.
P
θHS = Maximum heat sink thermal resistance
θJC = LT1185 junction-to-case thermal resistance
θCHS = Case-to-heat sink (interface) thermal
resistance, including any insulating washers
TJMAX = LT1185 maximum operating junction
temperature
TAMAX = Maximum ambient temperature in
customers application
P = Device dissipaton
I
= (VIN – VOUT) (IOUT) + OUT (VIN)
40
θHS =
Example: A commercial version of the LT1185 in the
TO-220 package is to be used with a maximum ambient
temperature of 60°C. Output voltage is 5V at 2A. Input
voltage can vary from 6V to 10V. Assume an interface
resistance of 1°C/W.
First solve for control area, where the maximum junction
temperature is 125°C for the TO-220 package, and
θJC = 1°C/W:
P = (10V – 5V) (2A) + 2A (10V) = 10.5W
40
125°C
–
60°C
θHS =
– 1°C/W – 1°C/W = 4.2°C/W
10.5W
Next, solve for power transistor limitation, with
TJMAX = 150°C, θJC = 3°C/W:
θHS =
150 – 60
– 3 – 1 = 4.6°C/W
10.5
The lowest number must be used, so heat sink resistance
must be less than 4.2°C/W.
Some heat sink data sheets show graphs of heat sink
temperature rise vs power dissipation instead of listing a
value for thermal resistance. The formula for θHS can be
rearranged to solve for maximum heat sink temperature
rise:
∆THS = TJMAX – TAMAX – P(θJC + θCHS)
Using numbers from the previous example:
∆THS = 125°C – 60 – 10.5(1 + 1) = 44°C control
section
∆THS = 150°C – 60 – 10.5(3 + 1) = 48°C power
transistor
The smallest rise must be used, so heat sink temperature
rise must be less than 44°C at a power level of 10.5W.
For board level applications, where heat sink size may be
critical, one is often tempted to use a heat sink which
barely meets the requirements. This is permissible if
correct assumptions were made concerning maximum
ambient temperature and power levels. One complicating
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APPLICATIO S I FOR ATIO
factor is that local ambient temperature may be somewhat
higher because of the point source of heat. The consequences of excess junction temperature include poor
reliability, especially for plastic packages, and the possibility of thermal shutdown or degraded electrical characteristics. The final design should be checked in situ with a
thermocouple attached to the regulator case under worstcase conditions of high ambient, high input voltage and
full load.
What About Overloads?
IC regulators with thermal shutdown, like the LT1185,
allow heat sink designs which concentrate on worst-case
“normal” conditions and ignore “fault” conditions. An
output overload or short may force the regulator to exceed
its maximum junction temperature rating, but thermal
shutdown is designed to prevent regulator failure under
these conditions. A word of caution however; thermal
shutdown temperatures are typically 175°C in the control
portion of the die and 180°C to 225°C in the power
transistor section. Extended operation at these temperatures can cause permanent degradation of plastic encapsulation. Designs which may be subjected to extended
periods of overload should either use the hermetic TO-3
package or increase heat sink size. Foldback current
limiting can be implemented to minimize power levels
under fault conditions.
External Current Limit
The LT1185 requires a resistor to set current limit. The
value of this resistor is 15k divided by the desired current
limit (in amps). The resistor for 2A current limit would be
15k/2A = 7.5k. Tolerance over temperature is ±10%, so
current limit is normally set 15% above maximum load
current. Foldback limiting can be employed if short-circuit
current must be lower than full load current (see Typical
Applications).
The LT1185 has internal current limiting which will override external current limit if power in the pass transistor
is excessive. The internal limit is ≈ 3.6A with a foldback
characteristic which is dependent on input-output voltage, not output voltage per se (see Typical Performace
Characteristics).
Ground Pin Current
Ground pin current for the LT1185 is approximately 2mA
plus IOUT/40. At IOUT = 3A, ground pin current is typically
2mA + 3/40 = 77mA. Worst case guarantees on the ratio of
IOUT to ground pin current are contained in the Electrical
Specifications.
Ground pin current can be important for two reasons. It
adds to power dissipation in the regulator and it can affect
load/line regulation if a long line is run from the ground pin
to load ground. The additional power dissipation is found
by multiplying ground pin current by input voltage. In a
typical example, with VIN = 8V, VOUT = 5V and IOUT = 2A, the
LT1185 will dissipate (8V – 5V)(2A) = 6W in the pass
transistor and (2A/40)(8V) = 0.4W in the internal drive
circuitry. This is only a 1.5% efficiency loss, and a 6.7%
increase in regulator power dissipation, but these values
will increase at higher output voltages.
Ground pin current can affect regulation as shown in
Figure 2. Parasitic resistance in the ground pin lead will
create a voltage drop which increases output voltage as
load current is increased. Similarly, output voltage can
decrease as input voltage increases because the “IOUT/40”
component of ground pin current drops significantly at
higher input-output differentials. These effects are small
enough to be ignored for local regulation applications, but
+
+
PARASITIC
LEAD RESISTANCES
– rb +
IGND
ra
RLIM
VIN
REF
R1*
2.37k
GND
LOAD
VOUT
FB
–
R2
VIN
LT1185
–
VOUT
LT1185 • F02
*R1 SHOULD BE CONNECTED DIRECTLY TO GROUND LEAD, NOT TO THE LOAD,
SO THAT ra ≈ 0Ω. THIS LIMITS THE OUTPUT VOLTAGE ERROR TO (IGND)(rb).
ERRORS CREATED BY ra ARE MULTIPLIED BY (1 + R2/R1). NOTE THAT VOUT
INCREASES WITH INCREASING GROUND PIN CURRENT. R2 SHOULD BE CONNECTED
DIRECTLY TO LOAD FOR REMOTE SENSING
Figure 2. Proper Connection of Positive Sense Lead
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for remote sense applications, they may need to be considered. Ground lead resistance of 0.4Ω would cause an
output voltage error of up to (3A/40)(0.4Ω) = 30mV, or
0.6% at VOUT = 5V. Note that if the sense leads are
connected as shown in Figure 2, with ra ≈ 0Ω, this error is
a fixed number of millivolts, and does not increase as a
function of DC output voltage.
Shutdown Techniques
The LT1185 can be shut down by open-circuiting the REF
pin. The current flowing into this pin must be less than
0.4µA to guarantee shutdown. Figure 3 details several
ways to create the “open” condition, with various logic
levels. For variations on these schemes, simply remember
that the voltage on the REF pin is 2.4V negative with
respect to the ground pin.
Output Overshoot
Very high input voltage slew rate during start-up may
cause the LT1185 output to overshoot. Up to 20% overshoot could occur with input voltage ramp-up rate exceeding 1V/µs. This condition cannot occur with normal 50Hz
to 400Hz rectified AC inputs because parasitic resistance
and inductance will limit rate of rise even if the power
switch is closed at the peak of the AC line voltage. This
assumes that the switch is in the AC portion of the circuit.
If instead, a switch is placed directly in the regulator input
so that a large filter capacitor is precharged, fast input slew
rates will occur on switch closure. The output of the
regulator will slew at a rate set by current limit and output
capacitor size; dVdt = ILIM/COUT. With ILIM = 3.6A and COUT
= 2.2µF, the output will slew at 1.6V/µs and overshoot can
occur. This overshoot can be reduced to a few hundred
millivolts or less by increasing the output capacitor to
10µF and/or reducing current limit so that output slew rate
is held below 0.5V/µs.
A second possibility for creating output overshoot is
recovery from an output short. Again, the output slews at
a rate set by current limit and output capacitance. To avoid
overshoot, the ratio ILIM/COUT should be less than
0.5 × 106. Remember that load capacitance can be added
to COUT for this calculation. Many loads will have multiple
supply bypass capacitors that total more than COUT.
5V Logic, Negative Regulated Output
5V Logic, Positive Regulated Output
+
+ VOUT
RLIM†
4k
REF
R1
*
GND
LT1185
“HI” = OUTPUT “OFF”
3 EA 1N4148
+
FB
VIN
5V
5V
R2
Q1
2N3906
Q1
2N3906
R5
300k
R4
33k
VOUT
RLIM
VIN
REF
GND
FB
LT1185 • F3a
R7
2.4k†
–
R6
30k
*CMOS LOGIC
†
FOR HIGHER VALUES OF RLIM, MAKE R7 = (RLIM)(0.6)
VIN
–
VIN
LT1185
VOUT
LT1185 • F03b
Figure 3. Shutdown Techniques
1185ff
9
LT1185
U
W
U U
APPLICATIO S I FOR ATIO
Thermal Regulation
IC regulators have a regulation term not found in discrete
designs because the power transistor is thermally coupled
to the reference. This creates a shift in the output voltage
which is proportional to power dissipation in the regulator.
∆VOUT = P(K1 + K2 θJA)
= (IOUT)(VIN – VOUT)(K1 + K2 θJA)
K1 and K2 are constants. K1 is a fast time constant effect
caused by die temperature gradients which are established within 50ms of a power change. K1 is specified on
the data sheet as thermal regulation, in percent per watt.
K2 is a long time constant term caused by the temperature
drift of the regulator reference voltage. It is also specified,
but in percent per degree centigrade. It must be multiplied
by overall thermal resistance, junction-to-ambient, θJA.
As an example, assume a 5V regulator with an input
voltage of 8V, load current of 2A, and a total thermal
resistance of 4°C/W, including junction-to-case, (use
control area specification), interface, and heat sink resistance. K1 and K2, respectively, from the data sheet are
0.014%/W and 0.01%/°C.
This shift in output voltage could be in either direction
because K1 and K2 can be either positive or negative.
Thermal regulation is already included in the worst case
reference specification.
Output Voltage Reversal
Some IC regulators suffer from a latch-up state when their
output is forced to a reverse voltage of as little as one diode
drop. The latch-up state can be triggered without a fault
condition when the load is connected to an opposite
polarity supply instead of to ground. If the second supply
is turned on first, it will pull the output of the first supply
to a reverse voltage through the load. The first supply may
then latch off when turned on. This problem is particularly
annoying because the diode clamps which should always
be used to protect against polarity reversal do not usually
stop the latch-up problem.
The LT1185 is designed to allow output reverse polarity of
several volts without damage or latch-up, so that a simple
diode clamp can be used.
∆VOUT = (2A)(8V – 5V)(0.014 + 0.01 • 4)
= 0.32%
1185ff
10
LT1185
U
TYPICAL APPLICATIO S
Foldback Current Limiting
+
+
R3
15k
VIN
1.6
R1
2.37k
1.4
Q1
2N3906
2µF
TANT
+
GND
2µF
TANT
VOUT (NORMALIZED)
+
R4
5.36k
VOUT
REF
FB
–
VIN
1.0
0.8
0.6
0.4
R2
2.61k
LT1185
IFULL LOAD = 15k + 10.8k
R4
R3
1.2
0.2
–
VOUT
ISHORT-CIRCUIT = 15k
R3
0
IOUT
LT1185 • TA03b
LT1185 • TA03a
Auxiliary + 12V Low Dropout Regulator for Switching Supply
12V
REGULATED
AUXILIARY
*
R1
2.37k
RLIM
+
REF
GND
+
FB
VIN
R2
9.76k
LT1185
VOUT
PRIMARY
5V
MAIN
OUTPUT
*
+
*DIODE CONNECTION INDICATES A FLYBACK
SWITCHING TOPOLOGY, BUT FORWARD
CONVERTERS MAY ALSO BE USED
5V
CONTROL
LT1185 • TA04
1185ff
11
LT1185
U
TYPICAL APPLICATIO S
Low Input Voltage Monitor Tracks Dropout Characteristics
+
+
+
R3
360k
C2
2.2µF
TANT
VIN
R1
2.37k
4k
REF
R5*
0.01Ω
–
+
GND
R4**
1k
FB
VIN
C1
2.2µF VOUT
TANT
R2
2.6k
LT1185
(
)
TRIP POINT FOR VIN = VOUT 1 + R4 • R7 + IOUT R5 • R7
R6
R3 • R6
FOR VALUES SHOWN, TRIP POINT FOR VIN IS:
VOUT + 0.37V AT IOUT = 0 AND VOUT = 1.18V AT IOUT = 3A
†
DO NOT SUBSTITUTE. OP AMP MUST HAVE COMMON MODE
RANGE EQUAL TO NEGATIVE SUPPLY
–
VOUT
R6**
1k
*3" #26 WIRE
**R4 DETERMINES TRIP POINT AT IOUT = 0
R6 DETERMINES INCREASE OF TRIP POINT AS IOUT INCREASES
R7
27k
OPTIONAL HYSTERESIS
≈2M
3
+
“LOW” FOR LOW INPUT
OUTPUT SWINGS FROM VIN+ TO VIN–
LT1006† +
V
7
– V–
4
2
LT1185 • TA05
Time Delayed Start-Up
Delay Time
+
+
D3
R3**
15k D2
RLIM***
3.5
D1
VIN
+
+
Q1**
C2
2.2µF
–
REF
GND
FB
C3*
VIN
4.0
R1
2.37k
LT1185
C1
2.2µF
TANT
VOUT
R2
–
VOUT
LT1185 • TA06
ALL DIODES 1N4148
*SEE CHART FOR DELAY TIME VERSUS (C3)(R3//RLIM) PRODUCT
**FOR LONG DELAY TIMES, REPLACE D2 WITH 2N3906 TRANSISTOR AND USE R3 ONLY FOR
CALCULATING DELAY TIME. R3 CAN INCREASE TO 100k
***ILIM IS ≈11k/RLIM, INSTEAD OF 15k, BECAUSE OF VOLTAGE DROP IN D1. TEMPERATURE
COEFFICIENT OF ILIM WILL BE ≈0.11%/°C, SO ADEQUATE MARGIN MUST BE ALLOWED
FOR COLD OPERATION
†
D3 PROVIDES FAST RESET OF TIMING. INPUT MUST DROP TO A LOW VALUE TO RESET TIMING
TIME CONSTANTS (t)*
†
3.0
2.5
2.0
1.5
1.0
0.5
0
0
5
10
20
15
INPUT VOLTAGE (V)
(
25
)
30
LT1185 • TA07
R3 • RLIM
*t = (R3//RLIM)(C3) =
(C3)
R3 + RLIM
1185ff
12
Q49
Q3
Q1
Q50
R2
3k
R56
600Ω
R4
520Ω
10k
Q47
Q39
Q6
R54
4k
Q5
C1
10pF
R5
600Ω
Q4
Q2
R55
30k
R3
3k
R1
5.5k
Q48
R50
160Ω
R49
700Ω
Q8
R46
8k
Q40
Q36
Q43
R53
10k
Q11
Q52
Q51
R9
2.7k
R6
750Ω
Q37
Q7
500Ω
Q46
R47
4k
Q44
C2
Q35
R52
10k
Q9
R11
220Ω
Q12
R8
6.5k
R7
500Ω
Q41
Q42
Q33
R45
1.3k
Q34
R48
2k
Q45
REF
R40
1k
R42
50k
Q13 Q14
Q15
R12
2k
R43
50k
C3
30pF
D1
Q30
R39
1k
Q32
R44
5k
Q31
Q16
R13
2k
Q27
R38
400Ω
C4
10pF
Q28
Q29
Q26
R35
20k
Q17
R37
1k
C5
10pF
R34
300Ω
R14
3.2k
R36
20k
Q18
R15
4k
Q25
Q53
R18
2k
R16
1k
R38
20k
R24
6k
R23
80Ω
Q22
Q21
Q19
R17
6k
R31
200Ω
R26
1k
Q23
LT1185 • SD
VIN
R28
0.055Ω
Q24
VOUT
FB
D4
R19
20k
Q20
GND
LT1185
W
W
SCHE ATIC DIAGRA
1185ff
13
LT1185
U
PACKAGE DESCRIPTIO
K Package
4-Lead TO-3 Metal Can
(Reference LTC DWG # 05-08-1311)
1.177 – 1.197
(29.90 – 30.40)
.320 – .350
(8.13 – 8.89)
.760 – .775
(19.30 – 19.69)
.470 TP
P.C.D.
.060 – .135
(1.524 – 3.429)
.655 – .675
(16.64 – 19.05)
.151 – .161
(3.84 – 4.09)
DIA 2 PLC
.420 – .480
(10.67 – 12.19)
.167 – .177
(4.24 – 4.49)
R
.038 – .043
(0.965 – 1.09)
72°
18°
.490 – .510
(12.45 – 12.95)
R
K4(TO-3) 0801
OBSOLETE PACKAGE
T Package
5-Lead Plastic TO-220 (Standard)
(Reference LTC DWG # 05-08-1421)
.390 – .415
(9.906 – 10.541)
.165 – .180
(4.191 – 4.572)
.147 – .155
(3.734 – 3.937)
DIA
.045 – .055
(1.143 – 1.397)
.230 – .270
(5.842 – 6.858)
.460 – .500
(11.684 – 12.700)
.570 – .620
(14.478 – 15.748)
.330 – .370
(8.382 – 9.398)
.620
(15.75)
TYP
.700 – .728
(17.78 – 18.491)
SEATING PLANE
.152 – .202
.260 – .320 (3.861 – 5.131)
(6.60 – 8.13)
.095 – .115
(2.413 – 2.921)
.155 – .195*
(3.937 – 4.953)
.013 – .023
(0.330 – 0.584)
BSC
.067
(1.70)
.028 – .038
(0.711 – 0.965)
.135 – .165
(3.429 – 4.191)
* MEASURED AT THE SEATING PLANE
T5 (TO-220) 0801
1185ff
14
LT1185
U
PACKAGE DESCRIPTIO
Q Package
5-Lead Plastic DD Pak
(Reference LTC DWG # 05-08-1461)
.256
(6.502)
.060
(1.524)
TYP
.060
(1.524)
.390 – .415
(9.906 – 10.541)
.165 – .180
(4.191 – 4.572)
.045 – .055
(1.143 – 1.397)
15° TYP
.060
(1.524)
.183
(4.648)
+.008
.004 –.004
+0.203
0.102 –0.102
.059
(1.499)
TYP
.330 – .370
(8.382 – 9.398)
(
)
.095 – .115
(2.413 – 2.921)
.075
(1.905)
.300
(7.620)
+.012
.143 –.020
+0.305
3.632 –0.508
(
BOTTOM VIEW OF DD PAK
HATCHED AREA IS SOLDER PLATED
COPPER HEAT SINK
.067
(1.702)
.028 – .038 BSC
(0.711 – 0.965)
TYP
)
Q(DD5) 0502
.420
.276
.080
.420
.050 ± .012
(1.270 ± 0.305)
.013 – .023
(0.330 – 0.584)
.325
.350
.205
.565
.565
.320
.090
.090
.067
.042
RECOMMENDED SOLDER PAD LAYOUT
NOTE:
1. DIMENSIONS IN INCH/(MILLIMETER)
2. DRAWING NOT TO SCALE
.067
.042
RECOMMENDED SOLDER PAD LAYOUT
FOR THICKER SOLDER PASTE APPLICATIONS
1185ff
Information furnished by Linear Technology Corporation is believed to be accurate and reliable.
However, no responsibility is assumed for its use. Linear Technology Corporation makes no representation that the interconnection of circuits as described herein will not infringe on existing patent rights.
15
LT1185
U
TYPICAL APPLICATIO S
Logic Controlled 3A Low-Side Switch with Fault Protection
5V
RLIM
4k
REF
FB
GND
1N4001
ADD FOR
INDUCTIVE LOADS
LOAD
VOUT
LT1185
VIN
LT1185 • TA08
Improved High Frequency Ripple Rejection
+
+
+
C2
2.2µF
TANT
R1
2.37k
RLIM
VIN
REF
C1
4.7µF
TANT
GND
FB
–
VIN
R2
LT1185
VOUT
C3
0.05µF
–
VOUT
LT1185 • TA09
NOTE: C3 IMPOVES HIGH FREQUENCY RIPPLE REJECTION BY 6dB AT VOUT = 5V,
AND BY 14dB AT VOUT = 12V. C1 IS INCREASED TO 4.7µF TO ENSURE GOOD STABILTITY
WHEN C3 IS USED
RELATED PARTS
PART NUMBER
DESCRIPTION
COMMENTS
LT1085
7.5A Low Dropout Regulator
1V Dropout Voltage
LT1117
800mA Low Dropout Regulator with Shutdown
Reverse Voltage and Reverse Current Protection
LT1120A
Micropower Regulator with Comparator and Shutdown
20µA Supply Current, 2.5V Reference Output
LT1129
200mA Micropower Low Dropout Regulator
400mV Dropout Voltage, 50µA Supply Current
LT1175
500mA Negative Low Dropout Micropower Regulator
45µA Supply Current, Adjustable Current Limit
LT1585
4.6A Low Dropout Fast Transient Response Regulator
For High Performance Microprocessors
LT1964
200mA, Low Noise Micropower, Negative LDO
VIN: –0.9V to –20V, VOUT(MIN) = –1.21V, VDO = 0.34V, IQ = 30µA,
ISD = 3µA, ThinSOT Package
1185ff
16
Linear Technology Corporation
LT/LWI 0906 REV F • PRINTED IN USA
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(408) 432-1900 ● FAX: (408) 434-0507
●
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