1
LT1521/LT1521-3
LT1521-3.3/LT1521-5
The LT
1521/LT1521-3/LT1521-3.3/LT1521-5 are low
dropout regulators with micropower quiescent current
and shutdown. These devices are capable of supplying
300mA of output current with a dropout voltage of 0.5V.
Designed for use in battery-powered systems, the low
quiescent current, 12
A operating and 6
A in
shutdown, makes them an ideal choice. The quiescent
current is well controlled; it does not rise in dropout as it
does with many other low dropout PNP regulators.
Other features of the LT1521/LT1521-3/LT1521-3.3/
LT1521-5 include the ability to operate with very small
output capacitors. They are stable with only 1.5
F on the
output while most older devices require between 10
F and
100
F for stability. Small ceramic capacitors can be used,
enhancing manufacturability. Also, the input may be con-
nected to voltages lower than the output voltage, including
negative voltages, without reverse current flow from out-
put to input. This makes the LT1521 series ideal for backup
power situations where the output is held high and the
input is low or reversed. Under these conditions only 5
A
will flow from the output pin to ground.
300mA Low Dropout
Regulators with Micropower
Quiescent Current and Shutdown
FEATURES
DESCRIPTIO
N
U
TYPICAL APPLICATIO
N
U
5V Battery-Powered Supply with Shutdown
Dropout Voltage
s
Dropout Voltage: 0.5V
s
Output Current: 300mA
s
Quiescent Current: 12
A
s
No Protection Diodes Needed
s
Adjustable Output from 3.8V to 20V
s
Fixed Output Voltages: 3V, 3.3V, 5V
s
Controlled Quiescent Current in Dropout
s
Shutdown I
Q
= 6
A
s
Reverse Battery Protection
s
No Reverse Current
s
Thermal Limiting
OUTPUT CURRENT (mA)
0
0
DROPOUT VOLTAGE (V)
0.1
0.2
0.3
0.4
0.6
50
100
150
200
LT1521 TA02
250
300
0.5
8
5
1
2
3
+
6V
1.5
F
1
F
5V
300mA
V
SHDN
(PIN 5) OUTPUT
<0.25
OFF
>2.80
ON
NC
ON
LT1521-5
GND
IN
SHDN
OUT
SENSE
LT1521 TA01
, LTC and LT are registered trademarks of Linear Technology Corporation.
APPLICATIO
N
S
U
s
Low Current Regulator
s
Regulator for Battery-Powered Systems
s
Post Regulator for Switching Supplies
2
LT1521/LT1521-3
LT1521-3.3/LT1521-5
ABSOLUTE
M
AXI
M
U
M
RATINGS
W
W
W
U
PACKAGE/ORDER I
N
FOR
M
ATIO
N
W
U
U
1
2
3
FRONT VIEW
TAB IS
GND
OUT
GND
IN
ST PACKAGE
3-LEAD PLASTIC SOT-223
T
JMAX
= 125
C,
JA
= 50
C/ W
SEE THE APPLICATIONS INFORMATION SECTION
ORDER PART NUMBER
ORDER PART NUMBER
1
2
3
4
8
7
6
5
TOP VIEW
IN
GND
GND
SHDN
OUT
SENSE/ADJ*
GND
NC
S8 PACKAGE
8-LEAD PLASTIC SO
*PIN 2 = SENSE FOR LT1521-3/LT1521-3.3/LT1521-5
= ADJ FOR LT1521
T
JMAX
= 125
C,
JA
= 70
C/ W
SEE THE APPLICATIONS INFORMATION SECTION
ORDER PART NUMBER
T
JMAX
= 125
C,
JA
= 125
C/ W
1
2
3
4
OUT
SENSE/ADJ*
NC
GND
8
7
6
5
IN
NC
NC
SHDN
TOP VIEW
MS8 PACKAGE
8-LEAD PLASTIC MSOP
*PIN 2 = SENSE FOR LT1521-3/
LT1521-3.3/LT1521-5
PIN 2 = ADJ FOR LT1521
Input Voltage ......................................................
20V*
Output Pin Reverse Current .................................. 10mA
Adjust Pin Current ................................................ 10mA
Shutdown Pin Input Voltage (Note 1) .......... 6.5V, 0.6V
Shutdown Pin Input Current (Note 1) ..................... 5mA
Output Short-Circuit Duration .......................... Indefinite
Storage Temperature Range ................. 65
C to 150
C
Operating Junction Temperature Range (Note 2)
Commercial ........................................... 0
C to 125
C
Industrial ......................................... 40
C to 125
C
Lead Temperature (Soldering, 10 sec).................. 300
C
*For applications requiring input voltage ratings greater than 20V, contact
the factory.
LT1521CMS8
LT1521CMS8-3
LT1521CMS8-3.3
LT1521CMS8-5
LT1521CS8
LT1521CS8-3
LT1521CS8-3.3
LT1521CS8-5
LT1521IS8
LT1521IS8-3
LT1521IS8-3.3
LT1521IS8-5
LT1521CST-3
LT1521CST-3.3
LT1521CST-5
LT1521IST-3
LT1521IST-3.3
LT1521IST-5
S8 PART MARKING
ST PART MARKING
MS8 PART MARKING
1521
15213
152133
15215
LTEZ
LTFB
LTDU
LTFA
Consult factory for Military grade parts.
1521I
1521I3
1521I33
1521I5
1521I3
1521I33
1521I5
15213
152133
15215
(Note 1)
3
LT1521/LT1521-3
LT1521-3.3/LT1521-5
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
Regulated Output Voltage
LT1521-3
V
IN
= 3.5V, I
OUT
= 1mA, T
J
= 25
C
2.950
3.000
3.050
V
(Note 4)
4V < V
IN
< 20V, 1mA < I
OUT
< 300mA
q
2.900
3.000
3.100
V
LT1521-3.3
V
IN
= 3.8V, I
OUT
= 1mA, T
J
= 25
C
3.250
3.300
3.350
V
4.3V < V
IN
< 20V, 1mA < I
OUT
< 300mA
q
3.200
3.300
3.400
V
LT1521-5
V
IN
= 5.5V, I
OUT
= 1mA, T
J
= 25
C
4.925
5.000
5.075
V
6V < V
IN
< 20V, 1mA < I
OUT
< 300mA
q
4.850
5.000
5.150
V
LT1521 (Note 5)
V
IN
= 4.3V, I
OUT
= 1mA, T
J
= 25
C
3.695
3.750
3.805
V
4.8V < V
IN
< 20V, 1mA < I
OUT
< 300mA
q
3.640
3.750
3.860
V
Line Regulation
LT1521-3
V
IN
= 4.5 to 20V, I
OUT
= 1mA
q
1.5
20
mV
LT1521-3.3
V
IN
= 4.8 to 20V, I
OUT
= 1mA
q
1.5
20
mV
LT1521-5
V
IN
= 5.5 to 20V, I
OUT
= 1mA
q
1.5
20
mV
LT1521 (Note 5)
V
IN
= 4.3 to 20V, I
OUT
= 1mA
q
1.5
20
mV
Load Regulation
LT1521-3
I
LOAD
= 1mA to 300mA, T
J
25
C
20
30
mV
LT1521-3.3
I
LOAD
= 1mA to 300mA, T
J
25
C
20
30
mV
LT1521-5
I
LOAD
= 1mA to 300mA, T
J
25
C
25
45
mV
LT1521 (Note 5)
I
LOAD
= 1mA to 300mA, T
J
25
C
20
30
mV
LT1521-3
I
LOAD
= 1mA to 300mA, T
J
>25
C
20
55
mV
LT1521-3.3
I
LOAD
= 1mA to 300mA, T
J
> 25
C
20
55
mV
LT1521-5
I
LOAD
= 1mA to 300mA, T
J
> 25
C
25
75
mV
LT1521 (Note 5)
I
LOAD
= 1mA to 300mA, T
J
> 25
C
20
55
mV
Dropout Voltage (Note 6)
I
LOAD
= 1mA, T
J
= 25
C
130
170
mV
I
LOAD
= 1mA
q
250
mV
I
LOAD
= 50mA, T
J
= 25
C
290
350
mV
I
LOAD
= 50mA
q
450
mV
I
LOAD
= 100mA, T
J
= 25
C
350
420
mV
I
LOAD
= 100mA
q
550
mV
I
LOAD
= 150mA, T
J
= 25
C
400
470
mV
I
LOAD
= 150mA
q
600
mV
I
LOAD
= 300mA, T
J
= 25
C
500
600
mV
I
LOAD
= 300mA
q
750
mV
Ground Pin Current (Note 7)
I
LOAD
= 0mA
q
12
20
A
I
LOAD
= 1mA
q
65
100
A
I
LOAD
= 10mA
q
300
450
A
I
LOAD
= 50mA
q
0.8
1.5
mA
I
LOAD
= 100mA
q
1.4
2.5
mA
I
LOAD
= 150mA
q
2.2
4.0
mA
I
LOAD
= 300mA
q
6.5
12.0
mA
Adjust Pin Bias Current (Notes 5, 8)
T
J
= 25
C
50
100
nA
Shutdown Threshold
V
OUT
= Off to On
q
1.20
2.80
V
V
OUT
= On to Off
q
0.25
0.75
V
Shutdown Pin Current (Note 9)
V
SHDN
= 0V
q
2.0
5.0
A
Quiescent Current in Shutdown (Note 10)
V
IN
= V
OUT
(NOMINAL) + 1V, V
SHDN
= 0V
q
6
12
A
Ripple Rejection
V
IN
V
OUT
= 1V(Avg), V
RIPPLE
= 0.5V
PP
,
50
58
dB
f
RIPPLE
= 120Hz, I
LOAD
= 150mA
Current Limit
V
IN
V
OUT
= 7V, T
J
= 25
C
400
800
mA
V
IN
= V
OUT
(NOMINAL) + 1.5V,
V
OUT
= 0.1V
q
320
400
mA
ELECTRICAL CHARACTERISTICS
4
LT1521/LT1521-3
LT1521-3.3/LT1521-5
ELECTRICAL CHARACTERISTICS
TEMPERATURE (
C)
50
DROPOUT VOLTAGE (V)
0.7
25
LT1521 TPC02
0.4
0.2
25
0
50
0.1
0
0.8
0.6
0.5
0.3
75
100
125
I
LOAD
= 1mA
I
LOAD
= 50mA
I
LOAD
= 150mA
I
LOAD
= 300mA
I
LOAD
= 100mA
TEMPERATURE (
C)
50
QUIESCENT CURRENT (
A)
15.0
12.5
10.0
7.5
5.0
2.5
0
25
75
LT1521 TPC03
25
0
50
100
125
V
IN
= 6V
R
L
=
I
LOAD
= 0
V
SHDN
= OPEN
V
SHDN
= 0V
Quiescent Current
Guaranteed Dropout Voltage
OUTPUT CURRENT (mA)
0
DROPOUT VOLTAGE (V)
150
250
LT1521 TPC01
50
100
200
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
300
T
J
125
C
T
J
25
C
= TEST POINTS
Dropout Voltage
TYPICAL PERFOR
M
A
N
CE CHARACTERISTICS
U
W
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
Input Reverse Leakage Current
V
IN
= 20V, V
OUT
= 0V
q
1.0
mA
Reverse Output Current (Note 11)
LT1521-3
V
OUT
= 3V, V
IN
< 3V, T
J
= 25
C
5
10
A
LT1521-3.3
V
OUT
= 3.3V, V
IN
< 3.3V, T
J
= 25
C
5
10
A
LT1521-5
V
OUT
= 5V, V
IN
< 5V, T
J
= 25
C
5
10
A
LT1521 (Note 5)
V
OUT
= 3.8V, V
IN
< 3.75V, T
J
= 25
C
5
10
A
The
q
denotes specifications which apply over the full operating
temperature range.
Note 1: Absolute Maximum Ratings are those values beyond which the life
of a device may be impaired.
Note 2: The shutdown pin input voltage rating is required for a low
impedance source. Internal protection devices connected to the shutdown
pin will turn on and clamp the pin to approximately 7V or 0.6V. This
range allows the use of 5V logic devices to drive the pin directly. For high
impedance sources or logic running on supply voltages greater than 5.5V,
the maximum current driven into the shutdown pin must be limited to less
than 5mA.
Note 3: For junction temperatures greater than 110
C, a minimum load of
1mA is recommended. For T
J
> 110
C and I
OUT
< 1mA, output voltage
may increase by 1%.
Note 4: Operating conditions are limited by maximum junction
temperature. The regulated output voltage specification will not apply for
all possible combinations of input voltage and output current. When
operating at maximum input voltage, the output current range must be
limited. When operating at maximum output current, the input voltage
range must be limited.
Note 5: The LT1521 (adjustable version) is tested and specified with the
adjust pin connected to the output pin.
Note 6: Dropout voltage is the minimum input/output voltage required to
maintain regulation at the specified output current. In dropout the output
voltage will be equal to: (V
IN
V
DROPOUT
)
Note 7: Ground pin current is tested with V
IN
= V
OUT
(nominal) and a
current source load. This means the device is tested while operating in its
dropout region. This is the worst-case ground pin current. The ground pin
current will decrease slightly at higher input voltages.
Note 8: Adjust pin bias current flows into the adjust pin.
Note 9: Shutdown pin current at V
SHDN
= 0V flows out of the shutdown
pin.
Note 10: Quiescent current in shutdown is equal to the total sum of the
shutdown pin current (2
A) and the ground pin current (4
A).
Note 11: Reverse output current is tested with the input pin grounded and
the output pin forced to the rated output voltage. This current flows into
the output pin and out of the ground pin.
5
LT1521/LT1521-3
LT1521-3.3/LT1521-5
TYPICAL PERFOR
M
A
N
CE CHARACTERISTICS
U
W
LT1521-3
Quiescent Current
INPUT VOLTAGE (V)
0
QUIESCENT CURRENT (
A)
50
45
40
35
30
25
20
15
10
5
0
8
LT1521 TPC08
2
1
3
5
7
9
4
6
10
T
J
= 25
C
R
L
=
V
SHDN
= OPEN
V
SHDN
= 0V
INPUT VOLTAGE (V)
0
QUIESCENT CURRENT (
A)
30
40
50
45
35
25
15
5
8
LT1521 TPC09
20
10
0
2
1
3
5
7
9
4
6
10
T
J
= 25
C
R
L
=
LT1521-3.3
Quiescent Current
INPUT VOLTAGE (V)
0
QUIESCENT CURRENT (
A)
50
45
40
35
30
25
20
15
10
5
0
8
LT1521 TPC11
2
1
3
5
7
9
4
6
10
T
J
= 25
C
R
L
=
V
SHDN
= OPEN
V
SHDN
= 0V
LT1521-3
Ground Pin Current
INPUT VOLTAGE (V)
0
GROUND PIN CURRENT (
A)
1000
900
800
700
600
500
400
300
200
100
0
8
LT1521 TPC12
2
1
3
5
7
9
4
6
10
T
J
= 25
C
*FOR V
OUT
= 3V
R
LOAD
= 3k
I
LOAD
= 1mA*
R
LOAD
= 300
I
LOAD
= 10mA*
R
LOAD
= 120
I
LOAD
= 25mA*
R
LOAD
= 60
I
LOAD
= 50mA*
LT1521
Quiescent Current
LT1521-3
Output Voltage
LT1521-3.3
Output Voltage
LT1521-5
Output Voltage
TEMPERATURE (
C)
50
OUTPUT VOLTAGE (V)
25
LT1521 TPC04
25
0
50
3.08
3.06
3.04
3.02
3.00
2.98
2.96
2.94
2.92
75
100
125
TEMPERATURE (
C)
50
OUTPUT VOLTAGE (V)
25
LT1521 TPC05
25
0
50
3.38
3.36
3.34
3.32
3.30
3.28
3.26
3.24
3.22
75
100
125
I
OUT
= 1mA
TEMPERATURE (
C)
50
OUTPUT VOLTAGE (V)
25
LT1521 TPC06
25
0
50
5.08
5.06
5.04
5.02
5.00
4.98
4.96
4.94
4.92
75
100
125
LT1521
Adjust Pin Voltage
TEMPERATURE (
C)
50
OUTPUT VOLTAGE (V)
3.81
25
LT1521 TPC07
3.75
3.71
25
0
50
3.69
3.67
3.83
3.79
3.77
3.73
75
100
125
I
LOAD
= 1mA
LT1521-5
Quiescent Current
INPUT VOLTAGE (V)
0
QUIESCENT CURRENT (
A)
50
45
40
35
30
25
20
15
10
5
0
8
LT1521 TPC10
2
1
3
5
7
9
4
6
10
T
J
= 25
C
R
L
=
V
SHDN
= OPEN
V
SHDN
= 0V
6
LT1521/LT1521-3
LT1521-3.3/LT1521-5
TYPICAL PERFOR
M
A
N
CE CHARACTERISTICS
U
W
INPUT VOLTAGE (V)
0
GROUND PIN CURRENT (
A)
1000
900
800
700
600
500
400
300
200
100
0
8
LT1521 TPC18
2
1
3
5
7
9
4
6
10
T
J
= 25
C
R
LOAD
= 75
I
LOAD
= 50mA*
R
LOAD
= 150
I
LOAD
= 25mA*
R
LOAD
= 375
I
LOAD
= 10mA*
R
LOAD
= 3.8k
I
LOAD
= 1mA*
V
OUT
= V
ADJ
*FOR V
OUT
= 3.75V
INPUT VOLTAGE (V)
0
GROUND PIN CURRENT (
A)
1000
900
800
700
600
500
400
300
200
100
0
8
LT1521 TPC16
2
1
3
5
7
9
4
6
10
T
J
= 25
C
*FOR V
OUT
= 5V
R
LOAD
= 100
I
LOAD
= 50mA*
R
LOAD
= 200
I
LOAD
= 25mA*
R
LOAD
= 500
I
LOAD
= 10mA*
R
LOAD
= 5k
I
LOAD
= 1mA*
LT1521-5
Ground Pin Current
INPUT VOLTAGE (V)
0
GROUND PIN CURRENT (mA)
8
7
6
5
4
3
2
1
0
8
LT1521 TPC17
2
4
6
10
7
1
3
5
9
T
J
= 25
C
*FOR V
OUT
= 5V
R
LOAD
= 16.7
I
LOAD
= 300mA*
R
LOAD
= 33.3
I
LOAD
= 150mA*
R
LOAD
= 50
I
LOAD
= 100mA*
LT1521-5
Ground Pin Current
LT1521
Ground Pin Current
LT1521
Ground Pin Current
INPUT VOLTAGE (V)
0
GROUND PIN CURRENT (mA)
8
7
6
5
4
3
2
1
0
8
LT1521 TPC19
2
4
6
10
7
1
3
5
9
T
J
= 25
C
V
OUT
= V
ADJ
*FOR V
OUT
= 3.75V
R
LOAD
= 12.5
I
LOAD
=300mA*
R
LOAD
= 25
I
LOAD
=150mA*
R
LOAD
= 37.5
I
LOAD
=100mA*
Ground Pin Current
Shutdown Pin Threshold
(On-to-Off)
TEMPERATURE (
C)
50
SHUTDOWN PIN THRESHOLD (V)
2.0
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0
0
50
75
LT1521 TPC21
25
25
100
125
I
LOAD
= 1mA
OUTPUT CURRENT (mA)
0
0
GROUND PIN CURRENT (mA)
2
3
4
5
6
7
50
100
150
200
LT1521 TPC20
250
8
9
10
1
300
V
IN
= 3V (LT1521-3)
V
IN
= 3.3V (LT1521-3.3)
V
IN
= 5V (LT1521-5)
V
IN
= 3.75V (LT1521)
DEVICE IS OPERATING
IN DROPOUT
T
J
= 125
C
T
J
= 55
C
T
J
= 25
C
INPUT VOLTAGE (V)
0
GROUND PIN CURRENT (mA)
8
7
6
5
4
3
2
1
0
8
LT1521 TPC13
2
4
6
10
7
1
3
5
9
T
J
= 25
C
*FOR V
OUT
= 3V
R
LOAD
= 10
I
LOAD
= 300mA*
R
LOAD
= 20
I
LOAD
= 150mA*
R
LOAD
= 30
I
LOAD
= 100mA*
INPUT VOLTAGE (V)
0
GROUND PIN CURRENT (
A)
600
800
1000
900
700
500
300
100
8
LT1521 TPC14
400
200
0
2
1
3
5
7
9
4
6
10
T
J
= 25
C
*FOR V
OUT
= 3.3V
R
LOAD
= 66
I
LOAD
= 50mA*
R
LOAD
= 132
I
LOAD
= 25mA*
R
LOAD
= 330
I
LOAD
= 10mA*
R
LOAD
= 3.3k
I
LOAD
= 1mA*
INPUT VOLTAGE (V)
0
GROUND PIN CURRENT (mA)
8
7
6
5
4
3
2
1
0
8
LT1521 TPC15
2
4
6
10
7
1
3
5
9
R
LOAD
= 11
I
LOAD
= 300mA*
R
LOAD
= 22
I
LOAD
= 150mA*
R
LOAD
= 33
I
LOAD
= 100mA*
T
J
= 25
C
*FOR V
OUT
= 3.3V
LT1521-3.3
Ground Pin Current
LT1521-3.3
Ground Pin Current
LT1521-3
Ground Pin Current
7
LT1521/LT1521-3
LT1521-3.3/LT1521-5
TYPICAL PERFOR
M
A
N
CE CHARACTERISTICS
U
W
Current Limit
TEMPERATURE (
C)
50
SHORT-CIRCUIT CURRENT (A)
0.6
0.5
0.4
0.3
0.2
0.1
0
25
75
LT1521 TPC28
25
0
50
100
125
V
IN
= 7V
V
OUT
= 0V
Current Limit
Adjust Pin Bias Current
TEMPERATURE (
C)
50
ADJUST PIN BIAS CURRENT (nA)
25
LT1521 TPC25
25
0
50
200
175
150
125
100
75
50
25
0
75
100
125
Reverse Output Current
OUTPUT VOLTAGE (V)
0
REVERSE OUTPUT CURRENT (
A)
50
45
40
35
30
25
20
15
10
5
0
8
LT1521 TPC26
3
5
7
9
4
6
10
T
J
= 25
C
V
IN
= 0V
CURRENT FLOWS
INTO OUTPUT PIN
V
OUT
= V
SENSE
(LT1521-3/LT1521-3.3
LT1521-5)
V
OUT
= V
ADJ
(LT1521)
LT1521
LT1521-3.3
LT1521-3
LT1521-5
2
1
TEMPERATURE (
C)
50
OUTPUT PIN CURRENT (
A)
25
LT1521 TPC29
25
0
50
8
7
6
5
4
3
2
1
0
75
100
125
V
IN
= 0V
V
OUT
= 3V (LT1521-3)
V
OUT
= 3.3V (LT1521-3.3)
V
OUT
= 5V (LT1521-5)
V
OUT
= 3.75V (LT1521)
Reverse Output Current
Ripple Rejection
TEMPERATURE (
C)
50
66
64
62
60
58
56
54
52
25
75
LT
1521 TPC30
25
0
50
100
125
RIPPLE REJECITON (dB)
V
IN
= V
OUT
(NOMINAL) + 1V + 0.5V
P-P
RIPPLE AT f = 120Hz
I
LOAD
= 150mA
INPUT VOLTAGE (V)
0
SHORT-CIRCUIT CURRENT (A)
0.6
0.5
0.4
0.3
0.2
0.1
0
3
5
LT1521 TPC27
1
2
4
6
7
V
OUT
= 0V
Shutdown Pin Threshold
(Off-to-On)
TEMPERATURE (
C)
50
SHUTDOWN PIN CURRENT (
A)
3.0
2.5
2.0
1.5
1.0
0.5
0
25
75
LT1521 TPC23
25
0
50
100
125
V
SHDN
= 0V
TEMPERATURE (
C)
50
SHUTDOWN PIN THRESHOLD (V)
2.0
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0
0
50
75
LT1521 TPC22
25
25
100
125
I
LOAD
= 1mA
I
LOAD
= 300mA
Shutdown Pin Current
Shutdown Pin Input Current
SHUTDOWN PIN VOLTAGE (V)
0
SHUTDOWN PIN INPUT CURRENT (mA)
25
20
15
10
5
0
2
4
5
9
LT1521 TPC24
1
3
6
7
8
8
LT1521/LT1521-3
LT1521-3.3/LT1521-5
TYPICAL PERFOR
M
A
N
CE CHARACTERISTICS
U
W
OUT (Pin 1): The output pin supplies power to the load. A
minimum output capacitor of 1.5
F is required to prevent
oscillations, but larger values of output capacitor will be
necessary to deal with larger load transients. See the
Applications Information section for more on output
capacitance and reverse output characteristics.
SENSE (Pin 2): For fixed voltage versions of the LT1521
(LT1521-3, LT1521-3.3, LT1521-5), the sense pin is the
input to the error amplifier. Optimum regulation will be
obtained at the point where the sense pin is connected to
PI
N
FU
N
CTIO
N
S
U
U
U
LT1521-5
Transient Response
TIME (
s)
0
OUTPUT VOLTAGE
DEVIATION (V)
LOAD CURRENT
(mA)
0.2
0.1
0
0.1
0.2
150
100
50
400
LT1521 TPC33
100
50
150
250
350
450
200
300
500
V
IN
= 6V
C
IN
= 0.1
F
C
OUT
= 1.5
F
FREQUENCY (Hz)
RIPPLE REJECTION (dB)
100
90
80
70
60
50
40
30
20
10
0
10
1k
10k
1M
LT1521 TPC31
100
100k
I
LOAD
= 150mA
V
IN
= 6V + 50mV
RMS
RIPPLE
C
OUT
= 33
F
SOLID TANTALUM
C
OUT
= 3.3
F
SOLID TANTALUM
Ripple Rejection
Load Regulation
TEMPERATURE (
C)
50
LOAD REGULATION (mV)
25
LT1521 TPC32
25
0
50
0
5
10
15
20
25
30
35
40
75
100
125
I
LOAD
= 1mA TO 300mA
LT1521-3.3
LT1521-3
LT1521*
LT1521-5
V
IN
= V
OUT
(NOMINAL + 1V)
*V
ADJ
= V
OUT
LT1521-5
Transient Response
TIME (ms)
0
OUTPUT VOLTAGE
DEVIATION (V)
LOAD CURRENT
(mA)
0.2
0.1
0
0.1
0.2
300
200
100
4.0
LT1521 TPC34
1.0
0.5
1.5
2.5
3.5
4.5
2.0
3.0
5.0
V
IN
= 6V
C
IN
= 0.1
F
C
OUT
= 33
F
the output pin of the regulator. In critical applications small
voltage drops caused by the resistance (R
P
) of PC traces
between the regulator and the load, which would normally
degrade regulation, may be eliminated by connecting the
sense pin to the output at the load as shown in Figure 1
(Kelvin Sense Connection). Note that the voltage drop
across the external PC traces will add to the dropout
voltage of the regulator. The sense pin bias current is 5
A
at the nominal regulated output voltage. This pin is inter-
nally clamped to 0.6V (one V
BE
).
9
LT1521/LT1521-3
LT1521-3.3/LT1521-5
PI
N
FU
N
CTIO
N
S
U
U
U
V
BE
). This allows the shutdown pin to be driven directly by
5V logic or by open collector logic with a pull-up resistor.
The pull-up resistor is only required to supply the leakage
current of the open collector gate, normally several micro-
amperes. Pull-up current must be limited to a maximum of
5mA. A curve of the shutdown pin input current as a
function of voltage appears in the Typical Performance
Characteristics. If the shutdown pin is not used it can be
left open circuit. The device will be active (output on) if the
shutdown pin is not connected.
IN (Pin 8): Power is supplied to the device through the
input pin. The input pin should be bypassed to ground if
the device is more than six inches away from the main
input filter capacitor. In general, the output impedance of
battery rises with frequency, so it is advisable to include a
bypass capacitor in battery-powered circuits. A bypass
capacitor in the range of 1
F to 10
F is sufficient. The
LT1521 is designed to withstand reverse voltages on the
input pin with respect to ground and the output pin. In the
case of reversed input, which can happen if a battery is
plugged in backwards, the LT1521 will act as if there is a
diode in series with its input. There will be no reverse
current flow into the LT1521 and no reverse voltage will
appear at the load. The device will protect both itself and
the load.
Figure 1. Kelvin Sense Connection
APPLICATIO
N
S I
N
FOR
M
ATIO
N
W
U
U
U
The LT1521 is a 300mA low dropout regulator with
micropower quiescent current and shutdown. The device
is capable of supplying 300mA at a dropout of 0.5V and
operates with very low quiescent current (12
A). In shut-
down, the quiescent current drops to only 6
A. In addition
to the low quiescent current, the LT1521 incorporates
several protection features which make it ideal for use in
battery-powered systems. The device is protected against
both reverse input voltages and reverse output voltages.
In battery backup applications where the output can be
held up by a backup battery when the input is pulled to
ground, the LT1521 acts like it has a diode in series with
its output and prevents reverse current flow.
Adjustable Operation
The adjustable version of the LT1521 has an output
voltage range of 3.75V to 20V. The output voltage is set by
the ratio of two external resistors as shown in Figure 2. The
device servos the output voltage to maintain the voltage at
the adjust pin at 3.75V. The current in R1 is then equal to
3.75V/R1. The current in R2 is equal to the sum of the
current in R1 and the adjust pin bias current. The adjust pin
bias current, 50nA at 25
C, flows through R2 into the
adjust pin. The output voltage can be calculated using the
formula in Figure 2. The value of R1 should be less than
400k to minimize errors in the output voltage caused by
the adjust pin bias current. Note that in shutdown the
5
4
1
2
3
+
LT1521
GND
IN
SHDN
OUT
SENSE
+
V
IN
LOAD
R
P
R
P
LT1521 F01
ADJ (Pin 2): For adjustable LT1521, the adjust pin is the
input to the error amplifier. This pin is internally clamped
to 6V and 0.6V (one V
BE
). It has a bias current of 50nA
which flows into the pin. See Adjust Pin Bias Current vs
Temperature in the Typical Performance Characteristics
section. The adjust pin reference voltage is 3.75V refer-
enced to ground. The output voltage range that can be
produced by this device is 3.75V to 20V.
SHDN (Pin 5): The shutdown pin is used to put the device
into shutdown. In shutdown the output of the device is
turned off. This pin is active low. The device will be shut
down if the shutdown pin is pulled low. The shutdown pin
current with the pin pulled to ground will be 1.7
A. The
shutdown pin is internally clamped to 7V and 0.6V (one
10
LT1521/LT1521-3
LT1521-3.3/LT1521-5
output is turned off and the divider current will be zero.
Curves of Adjust Pin Voltage vs Temperature and Adjust
Pin Bias Current vs Temperature appear in the Typical
Performance Characteristics. The reference voltage at the
adjust pin has a positive temperature coefficient of ap-
proximately 15ppm/
C. The adjust pin bias current has a
negative temperature coefficient. These effects will tend to
cancel each other.
The adjustable device is specified with the adjust pin tied
to the output pin. This sets the output voltage to 3.75V.
Specifications for output voltages greater than 3.75V will
be proportional to the ratio of the desired output voltage
to 3.75V; (V
OUT
/3.75V). For example: load regulation for
an output current change of 1mA to 300mA is 20mV
typical at V
OUT
= 3.75V. At V
OUT
= 12V, load regulation
would be:
(12V/3.75V)( 20mV) = 64mV
Thermal Considerations
The power handling capability of the device will be limited
by the maximum rated junction temperature (125
C). The
power dissipated by the device will be made up of two
components:
1. Output current multiplied by the input/output voltage
differential: I
OUT
(V
IN
V
OUT
), and
2. Ground pin current multiplied by the input voltage:
(I
GND
)(V
IN
)
APPLICATIO
N
S I
N
FOR
M
ATIO
N
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U
U
Figure 2. Adjustable Operation
+
V
IN
V
OUT
LT1521
GND
IN
SHDN
OUT
ADJ
LT1521 F01
V
OUT
= 3.75V 1 + + (I
ADJ
+ R2)
V
ADJ
= 3.75V
I
ADJ
= 50nA AT 25
C
OUTPUT RANGE = 3.75V TO 20V
R2
R1
)
)
R1
R2
The ground pin current can be found by examining the
Ground Pin Current curves in the Typical Performance
Characteristics. Power dissipation will be equal to the sum
of the two components listed above.
The LT1521 series regulators have internal thermal limit-
ing designed to protect the device during overload condi-
tions. For continuous normal load conditions the maxi-
mum junction temperature rating of 125
C must not be
exceeded. It is important to give careful consideration to
all sources of thermal resistance from junction to ambient.
Additional heat sources mounted nearby must also be
considered.
For surface mount devices, heat sinking is accomplished
by using the heat spreading capabilities of the PC board
and its copper traces. Copper board stiffeners and plated
through-holes can also be used to spread the heat gener-
ated by power devices.
The following tables list thermal resistance for each pack-
age. Measured values of thermal resistance for several
different board sizes and copper areas are listed for each
package. All measurements were taken in still air on
3/32" FR-4 board with one ounce copper. All NC leads were
connected to the ground plane.
Table 1. MS8 Package
COPPER AREA
THERMAL RESISTANCE
TOPSIDE**
BACKSIDE
BOARD AREA
(JUNCTION-TO-AMBIENT)
2500mm
2
2500mm
2
2500mm
2
110
C/W
1000mm
2
2500mm
2
2500mm
2
115
C/W
225mm
2
2500mm
2
2500mm
2
120
C/W
100mm
2
2500mm
2
2500mm
2
130
C/W
* Pin 4 is ground. ** Device is mounted on topside.
Table 2. S8 Package*
COPPER AREA
THERMAL RESISTANCE
TOPSIDE** BACKSIDE
BOARD AREA (JUNCTION-TO-AMBIENT)
2500mm
2
2500mm
2
2500mm
2
60
C/W
1000mm
2
2500mm
2
2500mm
2
60
C/W
225mm
2
2500mm
2
2500mm
2
68
C/W
100mm
2
2500mm
2
2500mm
2
74
C/W
* Pins 3, 6, 7 are ground. ** Device is mounted on topside.
11
LT1521/LT1521-3
LT1521-3.3/LT1521-5
APPLICATIO
N
S I
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FOR
M
ATIO
N
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U
U
U
is required to prevent oscillations. The LT1521 is a
micropower device and output transient response will be
a function of output capacitance. See the Transient
Response curves in the Typical Performance Characteris-
tics. Larger values of output capacitance will decrease the
peak deviations and provide improved output transient
response for larger load current deltas. Bypass capaci-
tors, used to decouple individual components powered by
the LT1521, will increase the effective value of the output
capacitor.
Protection Features
The LT1521 incorporates several protection features which
make it ideal for use in battery-powered circuits. In
addition to the normal protection features associated with
monolithic regulators, such as current limiting and ther-
mal limiting, the device is protected against reverse input
voltages, reverse output voltages and reverse voltages
from output to input.
Current limit protection and thermal overload protection
are intended to protect the device against current overload
conditions at the output of the device. For normal opera-
tion, the junction temperatures should not exceed 125
C.
The input of the device will withstand reverse voltages of
20V. Current flow into the device will be limited to less
than 1mA (typically less than 100
A) and no negative
voltage will appear at the output. The device will protect
both itself and the load. This provides protection against
batteries that can be plugged in backward.
For fixed voltage versions of the device, the output can be
pulled below ground without damaging the device. If the
input is left open circuit or grounded, the output can be
pulled below ground by 20V. The output will act like an
open circuit, no current will flow out of the pin. If the input
is powered by voltage source, the output will source the
short-circuit current of the device and will protect itself by
thermal limiting. For the adjustable version of the device,
the output pin is internally clamped at one diode drop
below ground. Reverse current for the adjustable device
must be limited to 5mA.
Table 3. SOT-223 Package
(Thermal Resistance Junction-to-Tab 20
C/W)
COPPER AREA
THERMAL RESISTANCE
TOPSIDE*
BACKSIDE
BOARD AREA
(JUNCTION-TO-AMBIENT)
2500mm
2
2500mm
2
2500mm
2
50
C/W
1000mm
2
2500mm
2
2500mm
2
50
C/W
225mm
2
2500mm
2
2500mm
2
58
C/W
100mm
2
2500mm
2
2500mm
2
64
C/W
1000mm
2
1000mm
2
1000mm
2
57
C/W
1000mm
2
0
1000mm
2
60
C/W
* Tab of device attached to topside copper.
Calculating Junction Temperature
Example: Given an output voltage of 3.3V, an input voltage
range of 4.5V to 7V, an output current range of 0mA to
150mA and a maximum ambient temperature of 50
C,
what will the maximum junction temperature be?
The power dissipated by the device will be equal to:
I
OUT(MAX)
(V
IN(MAX)
V
OUT
) + I
GND
(
V
IN(MAX)
)
Where,
I
OUT(MAX)
= 150mA
V
IN(MAX)
= 7V
I
GND
at (I
OUT
= 150mA, V
IN
= 7V) = 2.1mA
So,
P = 150mA(7V 3.3V) + (2.1mA)(7V) = 0.57W
If we use a SOT-223 package, then the thermal resistance
will be in the range of 50
C/W to 65
C/W depending on the
copper area. So the junction temperature rise above
ambient will be approximately equal to:
0.57W(60
C/W) = 34.2
C
The maximum junction temperature will then be equal to the
maximum junction temperature rise above ambient plus the
maximum ambient temperature or:
T
JMAX
= 50
C + 34.2
C = 84.2
C
Output Capacitance and Transient Performance
The LT1521 is designed to be stable with a wide range of
output capacitors. A minimum output capacitor of 1.5
F
12
LT1521/LT1521-3
LT1521-3.3/LT1521-5
APPLICATIO
N
S I
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FOR
M
ATIO
N
W
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U
U
Figure 3. Reverse Output Current
rectified AC source. If the AC source is removed, then the
input of the LT1521 is effectively left floating. The reverse
output current also follows the curve in Figure 3 if the input
pin is left open. The state of the shutdown pin will have no
effect on the reverse output current when the input pin is
floating.
When the input of the LT1521 is forced to a voltage below
its nominal output voltage and its output is held high, the
output current will follow the curve shown in Figure 3. This
can happen if the input of the LT1521 is connected to a
discharged (low voltage) battery and the output is held up
by either a backup battery or by second regulator circuit.
When the input pin is forced below the output pin or the
output pin is pulled above the input pin, the input current
will typically drop to less than 2
A (see Figure 4). The state
of the shutdown pin will have no effect on the reverse
output current when the output is pulled above the input.
In circuits where a backup battery is required, several
different input/output conditions can occur. The output
voltage may be held up while the input is either pulled to
ground, pulled to some intermediate voltage or is left open
circuit. Current flow back into the output will vary depend-
ing on the conditions. Many battery-powered circuits
incorporate some form of power management. The fol-
lowing information will help optimize battery life. Table 4
summarizes the following information.
The reverse output current will follow the curve in Figure
3 when the input is pulled to ground. This current flows
through the output pin to ground. The state of the shut-
down pin will have no effect on output current when the
input pin is pulled to ground.
In some applications it may be necessary to leave the input
on the LT1521 unconnected when the output is held high.
This can happen when the LT1521 is powered from a
OUTPUT VOLTAGE (V)
0
REVERSE OUTPUT CURRENT (
A)
50
45
40
35
30
25
20
15
10
5
0
8
LT1521 F03
3
5
7
9
4
6
10
T
J
= 25
C
V
IN
= 0V
CURRENT FLOWS
INTO OUTPUT PIN
V
OUT
= V
SENSE
(LT1521-3/LT1521-3.3
LT1521-5)
V
OUT
= V
ADJ
(LT1521)
LT1521
LT1521-3.3
LT1521-3
LT1521-5
2
1
INPUT VOLTAGE (V)
0
INPUT CURRENT (
A)
5
4
3
2
1
0
4
LT1521 F04
1
2
3
5
V
OUT
= 3V (LT1521-3)
V
OUT
= 3.3V (LT1521-3.3)
V
OUT
= 5V (LT1521-5)
LT1521-3
LT1521-3.3
LT1521-5
Figure 4. Input Current
13
LT1521/LT1521-3
LT1521-3.3/LT1521-5
APPLICATIO
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ATIO
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Table 4. Fault Conditions
INPUT PIN
SHDN PIN
OUTPUT/SENSE PINS
RESULTING CONDITIONS
< V
OUT
(Nominal)
Open (High)
Forced to V
OUT
(Nominal)
Reverse Output Current
5
A (See Figure 3)
Input Current
1
A (See Figure 4)
< V
OUT
(Nominal)
Grounded
Forced to V
OUT
(Nominal)
Reverse Output Current
5
A (See Figure 3)
Input Current
1
A (See Figure 4)
Open
Open (High)
> 1V
Reverse Output Current
5
A (See Figure 3)
Open
Grounded
> 1V
Reverse Output Current
5
A (See Figure 3)
0.8V
Open (High)
0V
Output Current = 0
0.8V
Grounded
0V
Output Current = 0
> 1.5V
Open (High)
0V
Output Current = Short-Circuit Current
20V < V
IN
< 20V
Grounded
0V
Output Current = 0
Dimensions in inches (millimeters) unless otherwise noted.
PACKAGE DESCRIPTIO
N
U
MSOP (MS8) 1197
* DIMENSION DOES NOT INCLUDE MOLD FLASH, PROTRUSIONS OR GATE BURRS. MOLD FLASH,
PROTRUSIONS OR GATE BURRS SHALL NOT EXCEED 0.006" (0.152mm) PER SIDE
** DIMENSION DOES NOT INCLUDE INTERLEAD FLASH OR PROTRUSIONS.
INTERLEAD FLASH OR PROTRUSIONS SHALL NOT EXCEED 0.006" (0.152mm) PER SIDE
0.021
0.006
(0.53
0.015)
0
6
TYP
SEATING
PLANE
0.007
(0.18)
0.040
0.006
(1.02
0.15)
0.012
(0.30)
REF
0.006
0.004
(0.15
0.102)
0.034
0.004
(0.86
0.102)
0.0256
(0.65)
TYP
1
2
3
4
0.192
0.004
(4.88
0.10)
8
7 6
5
0.118
0.004*
(3.00
0.102)
0.118
0.004**
(3.00
0.102)
MS8 Package
8-Lead Plastic MSOP
(LTC DWG # 05-08-1660)
14
LT1521/LT1521-3
LT1521-3.3/LT1521-5
Dimensions in inches (millimeters) unless otherwise noted.
PACKAGE DESCRIPTIO
N
U
1
2
3
4
0.150 0.157**
(3.810 3.988)
8
7
6
5
0.189 0.197*
(4.801 5.004)
0.228 0.244
(5.791 6.197)
0.016 0.050
0.406 1.270
0.010 0.020
(0.254 0.508)
45
0
8
TYP
0.008 0.010
(0.203 0.254)
SO8 0996
0.053 0.069
(1.346 1.752)
0.014 0.019
(0.355 0.483)
0.004 0.010
(0.101 0.254)
0.050
(1.270)
TYP
DIMENSION DOES NOT INCLUDE MOLD FLASH. MOLD FLASH
SHALL NOT EXCEED 0.006" (0.152mm) PER SIDE
DIMENSION DOES NOT INCLUDE INTERLEAD FLASH. INTERLEAD
FLASH SHALL NOT EXCEED 0.010" (0.254mm) PER SIDE
*
**
S8 Package
8-Lead Plastic Small Outline (Narrow 0.150)
(LTC DWG # 05-08-1610)
15
LT1521/LT1521-3
LT1521-3.3/LT1521-5
Dimensions in inches (millimeters) unless otherwise noted.
PACKAGE DESCRIPTIO
N
U
0.116 0.124
(2.95 3.15)
0.248 0.264
(6.30 6.71)
0.130 0.146
(3.30 3.71)
0.264 0.287
(6.71 7.29)
0.090
(2.29)
NOM
0.033 0.041
(0.84 1.04)
0.181
(4.60)
NOM
0.025 0.033
(0.64 0.84)
0.071
(1.80)
MAX
10
MAX
0.012
(0.31)
MIN
0.0008 0.0040
(0.0203 0.1016)
10
16
0.010 0.014
(0.25 0.36)
10
16
ST3 (SOT-233) 0792
ST Package
3-Lead Plastic SOT-223
(LTC DWG # 05-08-1630)
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 represen-
tation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
16
LT1521/LT1521-3
LT1521-3.3/LT1521-5
LINEAR TECHNOLOGY CORPORATION 1995
RELATED PARTS
PART NUMBER
DESCRIPTION
COMMENTS
LTC
1174
425mA High Efficiency Step-Down Switching Regulator
>90% Efficiency, SO-8 Package
LT1175
500mA Micropower Low Dropout Negative Linear Regulator
Selectable Current Limit
LT1120A
125mA Micropower Low Dropout Linear Regulator
20
A Quiescent Current, Includes Comparator
LT1304
Micropower Step-Up DC/DC Converter
15
A Quiescent Current, 1.5 Minimum Input
LT1529
3A Micropower Low Dropout Regulator
50
A Quiescent Current
Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
q
FAX: (408) 434-0507
q
www.linear-tech.com
1521335fa LT/TP 0299 REV A 2K PRINTED IN USA
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