LT1121/LT1121-3.3/LT1121-5
1
1121ff
TYPICAL APPLICATION
FEATURES
APPLICATIONS
DESCRIPTION
Micropower Low Dropout
Regulators with Shutdown
The LT
®
1121/LT1121-3.3/LT1121-5 are micropower low
dropout regulators with shutdown. These devices are ca-
pable of supplying
150mA of output current with a dropout
voltage of 0.4V. Designed for use in battery-powered sys-
tems, the low quiescent current, 30μA operating and 16μA
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 LT1121/LT1121-3.3/LT1121-5 include
the ability to operate with very small output capacitors. They
are stable with only 0.33μF on the output while most older
devices require between 1μF and 100μF for stability. Small
ceramic capacitors can be used, enhancing manufactur-
ability. Also the input may be connected to ground or a
reverse voltage without reverse current fl ow from output
to input. This makes the LT1121 series ideal for backup
power situations where the output is held high and the
input is at ground or reversed. Under these conditions only
16μA will fl ow from the output pin to ground.
5V Battery-Powered Supply with Shutdown
n 0.4V Dropout Voltage
n 150mA Output Current
n 30μA Quiescent Current
n No Protection Diodes Needed
n Adjustable Output from 3.75V to 30V
n 3.3V and 5V Fixed Output Voltages
n Controlled Quiescent Current in Dropout
n Shutdown
n 16μA Quiescent Current in Shutdown
n Stable with 0.33μF Output Capacitor
n Reverse Battery Protection
n No Reverse Current with Input Low
n Thermal Limiting
n Available in the 8-Lead SO, 8-Lead PDIP, 3-Lead
SOT-23 and 3-Lead TO-92 Packages
n Low Current Regulator
n Regulator for Battery-Powered Systems
n Post Regulator for Switching Supplies L, LT, LTC and LTM are registered trademarks of Linear Technology Corporation.
All other trademarks are the property of their respective owners.
Dropout Voltage
IN OUT
LT1121-3.3
GND
3.3VOUT
150mA
1μF
SOLID TANTALUM
5V
8
+
5
1
3
LT1121 • TA01
VSHDN (PIN 5)
<0.25
>2.8
NC
OUTPUT
OFF
ON
ON
SHDN
OUTPUT CURRENT (mA)
0
DROPOUT VOLTAGE (V)
0.3
0.4
0.5
60 100 160
0.2
0.1
020 40 80 120 140
LT1121 • TA02
LT1121/LT1121-3.3/LT1121-5
2
1121ff
ABSOLUTE MAXIMUM RATINGS
Input Voltage
LT1121 .............................................................. ± 30V
LT1121HV ............................................... +36V, –30V
Output Pin Reverse Current ................................. 10mA
Adjust Pin Current ................................................ 10mA
Shutdown Pin Input Voltage (Note 2) .......... 6.5V, – 0.6V
Shutdown Pin Input Current (Note 2) .................. 20mA
(Note 1)
1
2
3
4
8
7
6
5
TOP VIEW
OUT
NC/ADJ*
GND
NC
IN
NC**
NC**
SHDN
N8 PACKAGE
8-LEAD PDIP
S8 PACKAGE
8-LEAD PLASTIC SO
PIN 2 = NC FOR LT1121-3.3/LT1121-5
= ADJ FOR LT1121
PINS 6 AND 7 ARE FLOATING (NO
INTERNAL CONNECTION) ON THE
STANDARD S8 PACKAGE.
PINS 6 AND 7 CONNECTED TO GROUND
ON THE A VERSION OF THE LT1121 (S8 ONLY).
CONNECTING PINS 6 AND 7 TO THE
GROUND PLANE WILL REDUCE THERMAL
RESISTANCE. SEE THERMAL RESISTANCE
TABLES IN THE APPLICATIONS INFORMATION
SECTION.
*
**
TJMAX = 150°C, θJA = 120°C/W (N8, S8)
TJMAX = 150°C, θJA = 70°C/W (AS8)
3
2
1
FRONT VIEW
TAB IS
GND
OUTPUT
GND
VIN
ST PACKAGE
3-LEAD PLASTIC SOT-223
TJMAX = 150°C, θJA = 50°C/W
BOTTOM VIEW
IN GND OUT
Z PACKAGE
3-LEAD PLASTIC TO-92
TJMAX = 150°C, θJA = 150°C/W
ORDER PART NUMBER S8 PART MARKING ORDER PART
NUMBER
ST PART
MARKING
ORDER PART
NUMBER
LT1121CN8
LT1121CN8-3.3
LT1121CN8-5
LT1121IN8
LT1121IN8-3.3
LT1121IN8-5
LT1121CS8
LT1121CS8-3.3
LT1121CS8-5
LT1121HVCS8
LT1121IS8
LT1121IS8-3.3
LT1121IS8-5
LT1121HVIS8
LT1121ACS8
LT1121ACS8-3.3
LT1121ACS8-5
LT1121AHVCS8
LT1121AIS8
LT1121AIS8-3.3
LT1121AIS8-5
LT1121AHVIS8
1121
11213
11215
1121HV
1121I
121I3
121I5
121HVI
1121A
121A3
121A5
121AHV
121AI
121AI3
121AI5
21AHVI
LT1121CST-3.3
LT1121IST-3.3
LT1121CST-5
LT1121IST-5
11213
121I3
11215
1121I5
LT1121CZ-3.3
LT121IZ-3.3
LT1121CZ-5
LT1121IZ-5
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 specifi ed with wider operating temperature ranges.
PACKAGE/ORDER INFORMATION
Output Short-Circuit Duration ....................... Indefi nite
Operating Junction Temperature Range (Note 3)
LT1121C-X ............................................ 0°C to 125°C
LT1121I-X ........................................ – 40°C to 125°C
Storage Temperature Range ...................– 65°C to 150°C
Lead Temperature (Soldering, 10 sec) .................. 300°C
LT1121/LT1121-3.3/LT1121-5
3
1121ff
ELECTRICAL CHARACTERISTICS
PARAMETER CONDITIONS MIN TYP MAX UNITS
Regulated Output Voltage (Note 4) LT1121-3.3 VIN = 3.8V, IOUT = 1mA, TJ = 25ºC
4.3V < VIN < 20V, 1mA < IOUT < 150mA l
3.25
3.2
3.3
3.3
3.35
3.4
V
V
LT1121-5 VIN = 5.5V, IOUT = 1mA, TJ = 25ºC
6V < VIN < 20V, 1mA < IOUT < 150mA l
4.925
4.85
5
5
5.075
5.15
V
V
LT1121 (Note 5) VIN = 4.3V, IOUT = 1mA, TJ = 25ºC
4.8V < VIN < 20V, 1mA < IOUT < 150mA l
3.695
3.64
3.75
3.75
3.805
3.86
V
V
Line Regulation LT1121-3.3 ΔVIN = 4.8V TO 20V, IOUT = 1mA
LT1121-5 ΔVIN = 5.5V TO 20V, IOUT = 1mA
LT1121 (Note 5) ΔVIN = 4.3V TO 20V, IOUT = 1mA
l
l
l
1.5
1.5
1.5
10
10
10
mV
mV
mV
Load Regulation LT1121-3.3 ΔILOAD = 1mA to 150mA, TJ = 25ºC
ΔILOAD = 1mA to 150mA
LT1121-5 ΔILOAD = 1mA to 150mA, TJ = 25ºC
ΔILOAD = 1mA to 150mA
LT1121 (Note 5) ΔILOAD = 1mA to 150mA, TJ = 25ºC
ΔILOAD = 1mA to 150mA
l
l
l
–12
–20
–17
–28
–12
–18
–25
–40
–35
–50
–25
–40
mV
mV
mV
mV
mV
mV
Dropout Voltage (Note 6) ILOAD = 1mA, TJ = 25ºC
ILOAD = 1mA l
0.13 0.16
0.25
V
V
ILOAD = 50mA, TJ = 25ºC
ILOAD = 50mA l
0.3 0.35
0.5
V
V
ILOAD = 100mA, TJ = 25ºC
ILOAD = 100mA l
0.37 0.45
0.6
V
V
ILOAD = 150mA, TJ = 25ºC
ILOAD = 150mA l
0.42 0.55
0.7
V
V
Ground Pin Current (Note 7) ILOAD = 0mA
ILOAD = 1mA
ILOAD = 10mA
ILOAD = 50mA
ILOAD = 100mA
ILOAD = 150mA
l
l
l
l
l
l
30
90
350
1.5
4
7
50
120
500
2.5
7
14
μA
μA
μA
mA
mA
mA
Adjust Pin Bias Current (Notes 5, 8) TJ = 25ºC 150 300 nA
Shutdown Threshold VOUT = Off to On
VOUT = On to Off
l
l0.25
1.2
0.75
2.8 V
V
Shutdown Pin Current (Note 9) VSHDN = 0V l610 μA
Quiescent Current in Shutdown (Note 10) VIN = 6V, VSHDN = 0V l16 22 μA
Ripple Rejection VIN – VOUT = 1V (Avg), VRIPPLE = 0.5VP-P, fRIPPLE = 120Hz,
ILOAD = 0.1A
50 58 dB
Current Limit VIN – VOUT = 7V, TJ = 25ºC 200 500 mA
Input Reverse Leakage Current VIN = –20V, VOUT = 0V l1mA
Reverse Output Current (Note 11) LT1121-3.3 VOUT = 3.3V, VIN = 0V
LT1121-5 VOUT = 5V, VIN = 0V
LT1121 (Note 5) VOUT = 3.8V, VIN = 0V
16
16
16
25
25
25
μA
μA
μA
The l denotes the specifi cations which apply over the full operating
temperature range, otherwise specifi cations are at TA = 25°C.
LT1121/LT1121-3.3/LT1121-5
4
1121ff
ELECTRICAL CHARACTERISTICS
Guaranteed Dropout Voltage Dropout Voltage Quiescent Current
The
l denotes the specifi cations which apply over the full operating
temperature range, otherwise specifi cations are at TA = 25°C.
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: 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 20mA.
Note 3: For junction temperatures greater than 110°C, a minimum load
of 1mA is recommended. For TJ > 110°C and IOUT < 1mA, output voltage
may increase by 1%.
Note 4: Operating conditions are limited by maximum junction
temperature. The regulated output voltage specifi cation 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
TYPICAL PERFORMANCE CHARACTERISTICS
limited. When operating at maximum output current the input voltage
range must be limited.
Note 5: The LT1121 (adjustable version) is tested and specifi ed 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 specifi ed output current. In dropout the output
voltage will be equal to: (VIN – VDROPOUT).
Note 7: Ground pin current is tested with VIN = VOUT (nominal) and a
current source load. This means that 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 fl ows into the adjust pin.
Note 9: Shutdown pin current at VSHDN = 0V fl ows out of the shutdown pin.
Note 10: Quiescent current in shutdown is equal to the sum total of the
shutdown pin current (6μA) and the ground pin current (9μA).
Note 11: Reverse output current is tested with the input pin grounded and
the output pin forced to the rated output voltage. This current fl ows into
the output pin and out of the ground pin.
OUTPUT CURRENT (mA)
DROPOUT VOLTAGE (V)
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
40 80 120
1121 G01
20 60 140 160
0100
= TEST POINTS
TJ ≤ 125°C
TJ ≤ 25°C
TEMPERATURE (°C)
–50
DROPOUT VOLTAGE (V)
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
050 75
1121 G02
–25 25 100 125
ILOAD = 150mA
ILOAD = 50mA
ILOAD = 1mA
ILOAD = 100mA
TEMPERATURE (°C)
–50
QUIESCENT CURRENT (μA)
50
40
30
20
10
0
050 75
1121 G03
–25 25 100 125
VIN = 6V
RLOAD = ∞
VSHDN = OPEN
VSHDN = 0V
LT1121/LT1121-3.3/LT1121-5
5
1121ff
TYPICAL PERFORMANCE CHARACTERISTICS
LT1121-3.3
Quiescent Current
LT1121-5
Quiescent Current
LT1121
Quiescent Current
LT1121-3.3
Output Voltage
LT1121-5
Output Voltage
LT1121
Adjust Pin Voltage
LT1121-3.3
Ground Pin Current
LT1121-5
Ground Pin Current
LT1121
Ground Pin Current
INPUT VOLTAGE (V)
0
QUIESCENT CURRENT (μA)
120
100
80
60
40
20
08
1121 G04
24610
13579
VSHDN = 0V
TJ = 25°C
RLOAD = ∞
VSHDN = OPEN
INPUT VOLTAGE (V)
0
QUIESCENT CURRENT (μA)
120
100
80
60
40
20
08
1121 G05
24610
13579
TJ = 25°C
RLOAD = ∞
VSHDN = 0V
VSHDN = OPEN
INPUT VOLTAGE (V)
0
QUIESCENT CURRENT (μA)
120
100
80
60
40
20
08
1121 G06
24610
13579
VSHDN = 0V
TJ = 25°C
RLOAD = ∞
VOUT = VADJ
VSHDN = OPEN
TEMPERATURE (°C)
–50
ADJ PIN VOLTAGE (V)
3.83
3.81
3.79
3.77
3.75
3.73
3.71
3.69
3.67 050 75
1121 G09
–25 25 100 125
IOUT = 1mA
TEMPERATURE (°C)
–50
OUTPUT VOLTAGE (V)
5.08
5.06
5.04
5.02
5.00
4.98
4.96
4.94
4.92 050 75
1121 G08
–25 25 100 125
IOUT = 1mA
TEMPERATURE (°C)
–50
OUTPUT VOLTAGE (V)
3.38
3.36
3.34
3.32
3.30
3.28
3.26
3.24
3.22 050 75
1121 G07
–25 25 100 125
IOUT = 1mA
INPUT VOLTAGE (V)
0
GROUND PIN CURRENT (μA)
800
700
600
500
400
300
200
100
08
1121 G11
24610
13579
TJ = 25°C
RLOAD = 500Ω
ILOAD = 10mA*
RLOAD = 5k
ILOAD = 1mA*
RLOAD = 200Ω
ILOAD = 25mA*
*FOR VOUT = 5V
INPUT VOLTAGE (V)
0
GROUND PIN CURRENT (μA)
800
700
600
500
400
300
200
100
08
1121 G12
24610
13579
TJ = 25°C
VOUT = VADJ RLOAD = 150Ω
ILOAD = 25mA*
RLOAD = 380Ω
ILOAD = 10mA*
RLOAD = 3.8k
ILOAD = 1mA*
*FOR VOUT = 3.75V
INPUT VOLTAGE (V)
0
GROUND PIN CURRENT (μA)
800
700
600
500
400
300
200
100
08
1121 G10
24610
13579
RLOAD = 130Ω
ILOAD = 25mA*
RLOAD = 330Ω
ILOAD = 10mA*
RLOAD = 3.3k
ILOAD = 1mA*
TJ = 25°C
*FOR VOUT = 3.3V
LT1121/LT1121-3.3/LT1121-5
6
1121ff
TYPICAL PERFORMANCE CHARACTERISTICS
LT1121-3.3
Ground Pin Current
LT1121-5
Ground Pin Current
LT1121
Ground Pin Current
Ground Pin Current
Shutdown Pin Threshold
(On-to-Off)
Shutdown Pin Threshold
(Off-to-On)
Shutdown Pin Current Shutdown Pin Input Current
LT1121
Adjust Pin Bias Current
INPUT VOLTAGE (V)
0
GROUND PIN CURRENT (mA)
10
9
8
7
6
5
4
3
2
1
08
1121 G14
24610
13579
TJ = 25°C
RLOAD = 50Ω
ILOAD = 100mA*
RLOAD = 100Ω
ILOAD = 50mA*
RLOAD = 33Ω
ILOAD = 150mA*
*FOR VOUT = 5V
INPUT VOLTAGE (V)
0
GROUND PIN CURRENT (mA)
10
9
8
7
6
5
4
3
2
1
08
1121 G15
24610
13579
TJ = 25°C
VOUT = VADJ
RLOAD = 25Ω
ILOAD = 150mA*
RLOAD = 38Ω
ILOAD = 100mA*
RLOAD = 75Ω
ILOAD = 50mA*
*FOR VOUT = 3.75V
INPUT VOLTAGE (V)
0
GROUND PIN CURRENT (mA)
10
9
8
7
6
5
4
3
2
1
08
1121 G13
24610
13579
TJ = 25°C
RLOAD = 22Ω
ILOAD = 150mA*
RLOAD = 33Ω
ILOAD = 100mA*
RLOAD = 66Ω
ILOAD = 50mA*
*FOR VOUT = 3.3V
OUTPUT CURRENT (mA)
GROUND PIN CURRENT (mA)
14
12
10
8
6
4
2
0
40 80 120
1121 G16
20 60 140 160
0100
TJ= 125°C
TJ= –55°C
TJ= 25°C
VIN = 3.3V (LT1121-3.3)
VIN = 5V (LT1121-5)
VIN = 3.75V (LT1121)
DEVICE IS OPERATING
IN DROPOUT
TEMPERATURE (°C)
–50
SHUTDOWN THRESHOLD (V)
2.0
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0050 75
1121 G17
–25 25 100 125
ILOAD = 1mA
TEMPERATURE (°C)
–50
SHUTDOWN THRESHOLD (V)
2.0
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0050 75
1121 G18
–25 25 100 125
ILOAD = 1mA
ILOAD = 150mA
TEMPERATURE (°C)
–50
SHUTDOWN PIN CURRENT (μA)
10
9
8
7
6
5
4
3
2
1
0050 75
1121 G19
–25 25 100 125
VSHDN = 0V
SHUTDOWN PIN VOLTAGE (V)
0
0
SHUTDOWN PIN INPUT CURRENT (mA)
5
15
20
25
2459
1121 G20
10
13 678
TEMPERATURE (°C)
–50
ADJUST PIN BIAS CURRENT (nA)
400
350
300
250
200
150
100
50
0050 75
1121 G21
–25 25 100 125
LT1121/LT1121-3.3/LT1121-5
7
1121ff
TYPICAL PERFORMANCE CHARACTERISTICS
Reverse Output Current Current Limit Current Limit
Reverse Output Current Ripple Rejection Ripple Rejection
Load Regulation
LT1121-5
Load Transient Response
LT1121-5
Load Transient Response
TEMPERATURE (°C)
–50
OUTPUT PIN CURRENT (μA)
30
25
20
15
10
5
0050 75
1121 G22
–25 25 100 125
VIN = 0V
VOUT = 5V (LT1121-5)
VOUT = 3.3V (LT1121-3.3)
VOUT = 3.8V (LT1121)
INPUT VOLTAGE (V)
0
SHORT-CIRCUIT CURRENT (mA)
400
350
300
250
200
150
100
50
0245
1121 G23
1367
VOUT = 0V
TEMPERATURE (°C)
–50
CURRENT LIMIT (mA)
400
350
300
250
200
150
100
50
0050 75
1121 G24
–25 25 100 125
VIN = 7V
VOUT = 0V
OUTPUT VOLTAGE (V)
0
OUTPUT PIN CURRENT (μA)
100
90
80
70
60
50
40
30
20
10
08
1121 G25
24610
13579
TJ = 25°C
VIN = 0V
CURRENT FLOWS
INTO OUTPUT PIN
LT1121-3.3
LT1121
(VOUT = VADJ)
LT1121-5
TEMPERATURE (°C)
–50
RIPPLE REJECTION (dB)
64
62
60
58
56
54
52
50
050 75
1121 G26
–25 25 100 125
VIN = VOUT (NOMINAL) + 1V
+ 0.5VP-P RIPPLE AT f = 120Hz
IOUT = 100mA
FREQUENCY (Hz)
RIPPLE REJECTION (dB)
100
90
80
70
60
50
40
30
20
10
010 1k 10k 1M
1121 G27
100 100k
COUT = 1μF
SOLID TANTALUM
COUT = 47μF
SOLID TANTALUM
IOUT = 100mA
VIN = 6V + 50mVRMS RIPPLE
TEMPERATURE (°C)
–50
LOAD REGULATION (mV)
0
–5
–10
–15
–20
–25
–30
–35
–40 050 75
1121 G28
–25 25 100 125
LT1121*
LT1121-5
LT1121-3.3
ΔILOAD = 1mA TO 150mA
* ADJ PIN TIED TO
OUTPUT PIN
TIME (ms)
0
OUTPUT VOLTAGE
DEVIATION (V)
0.2
0.1
0
–0.1
–0.2
0.8
1121 G29
0.1 0.4 0.6 1.0
150
100
LOAD CURRENT
(mA)
0.2 0.3 0.5 0.7 0.9
VIN = 6V
CIN = 0.1μF
COUT = 1μF
TIME (ms)
0
OUTPUT VOLTAGE
DEVIATION (V)
0.2
0.1
0
–0.1
–0.2
0.8
1121 G30
0.1 0.4 0.6 1.0
150
100
50
LOAD CURRENT
(mA)
0.2 0.3 0.5 0.7 0.9
VIN = 6V
CIN = 0.1μF
COUT = 3.3μF
LT1121/LT1121-3.3/LT1121-5
8
1121ff
PIN FUNCTIONS
Input Pin: 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 fi lter capacitor. In general the output impedance of a
battery rises with frequency so it is usually adviseable to
include a bypass capacitor in battery-powered circuits. A
bypass capacitor in the range of 0.1μF to 1μF is suffi cient.
The LT1121 is designed to withstand reverse voltages on
the input pin with respect to both ground and the output
pin. In the case of a reversed input, which can happen if
a battery is plugged in backwards, the LT1121 will act as
if there is a diode in series with its input. There will be no
reverse current fl ow into the LT1121 and no reverse volt-
age will appear at the load. The device will protect both
itself and the load.
Output Pin: The output pin supplies power to the load. An
output capacitor is required to prevent oscillations. See
the Applications Information section for recommended
value of output capacitance and information on reverse
output characteristics.
Shutdown Pin: This 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 6μA. The shutdown
pin is internally clamped to 7V and –0.6V (one VBE). 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 microamperes.
Pull-up current must be limited to a maximum of 20mA.
A curve of 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.
Adjust Pin: For the adjustable LT1121, the adjust pin
is the input to the error amplifi er. This pin is internally
clamped to 6V and –0.6V (one VBE). It has a bias current
of 150nA which fl ows into the pin. See Bias Current curve
in the Typical Performance Characteristics. The adjust
pin reference voltage is 3.75V referenced to ground. The
output voltage range that can be produced by this device
is 3.75V to 30V.
LT1121/LT1121-3.3/LT1121-5
9
1121ff
APPLICATIONS INFORMATION
The LT1121 is a micropower low dropout regulator with
shutdown, capable of supplying up to 150mA of output
current at a dropout voltage of 0.4V. The device operates
with very low quiescent current (30μA). In shutdown the
quiescent current drops to only 16μA. In addition to the
low quiescent current the LT1121 incorporates several
protection features which make it ideal for use in bat-
tery-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 LT1121 acts like it has a diode in series with
its output and prevents reverse current fl ow.
Adjustable Operation
The adjustable version of the LT1121 has an output voltage
range of 3.75V to 30V. The output voltage is set by the
ratio of two external resistors as shown in Figure 1. 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, 150nA at 25°C, fl ows through R2 into the
adjust pin. The output voltage can be calculated according
to the formula in Figure 1. 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
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 slight positive temperature coeffi cient of
approximately 15ppm/°C. The adjust pin bias current has
a negative temperature coeffi cient. These effects are small
and will tend to cancel each other.
The adjustable device is specifi ed with the adjust pin tied
to the output pin. This sets the output voltage to 3.75V.
Specifi cations for output voltage greater than 3.75V will
be proportional to the ratio of the desired output voltage
to 3.75V (VOUT/3.75V). For example: load regulation for
an output current change of 1mA to 150mA is –12mV
typical at VOUT = 3.75V. At VOUT = 12V, load regulation
would be:
12V
3.75V
• –12mV
()
=–38mV
()
Thermal Considerations
Power handling capability will be limited by maximum
rated junction temperature (125°C). Power dissipated by
the device will be made up of two components:
1. Output current multiplied by the input/output voltage
differential: IOUT • (VIN – VOUT), and
2. Ground pin current multiplied by the input voltage:
IGND • VIN.
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 LT1121 series regulators have internal thermal limiting
designed to protect the device during overload conditions.
For continuous normal load conditions the maximum junc-
tion 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.
Heat sinking, for surface mount devices, 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. Tables 1 through 5 list thermal
resistances for each package. Measured values of thermal
resistance for several different board sizes and copper
areas are listed for each package. All measurements were
Figure 1. Adjustable Operation
IN
LT1121
GND
SHDN
1121 • F01
ADJ
OUT
R2
R1
VOUT = 3.75V 1 + + IADJ • R2
VADJ = 3.75V
IADJ = 150nA AT 25°C
OUTPUT RANGE = 3.75V TO 30V
R2
R1
()
()
VOUT
+
LT1121/LT1121-3.3/LT1121-5
10
1121ff
APPLICATIONS INFORMATION
Table 1. N8 Package*
COPPER AREA THERMAL RESISTANCE
JUNCTION TO AMBIENTTOPSIDE BACKSIDE BOARD AREA
2500 sq mm 2500 sq mm 2500 sq mm 80ºC/W
1000 sq mm 2500 sq mm 2500 sq mm 80ºC/W
225 sq mm 2500 sq mm 2500 sq mm 85ºC/W
1000 sq mm 1000 sq mm 1000 sq mm 91ºC/W
* Device is mounted on topside. Leads are through hole and are
soldered to both sides of board.
taken in still air, on 3/32" FR-4 board with 1oz copper. All
NC leads were connected to the ground plane.
Table 2. S8 Package
COPPER AREA THERMAL RESISTANCE
JUNCTION TO AMBIENT
TOPSIDE* BACKSIDE BOARD AREA
2500 sq mm 2500 sq mm 2500 sq mm 120ºC/W
1000 sq mm 2500 sq mm 2500 sq mm 120ºC/W
225 sq mm 2500 sq mm 2500 sq mm 125ºC/W
100 sq mm 1000 sq mm 1000 sq mm 131ºC/W
* Device is mounted on topside.
Table 3. AS8 Package*
COPPER AREA THERMAL RESISTANCE
JUNCTION TO AMBIENT
TOPSIDE** BACKSIDE BOARD AREA
2500 sq mm 2500 sq mm 2500 sq mm 60ºC/W
1000 sq mm 2500 sq mm 2500 sq mm 60ºC/W
225 sq mm 2500 sq mm 2500 sq mm 68ºC/W
100 sq mm 2500 sq mm 2500 sq mm 74ºC/W
* Pins 3, 6 and 7 are ground.
** Device is mounted on topside.
Table 4. SOT-223 Package
(Thermal Resistance Junction-to-Tab 20ºC/W)
COPPER AREA THERMAL RESISTANCE
JUNCTION TO AMBIENT
TOPSIDE* BACKSIDE BOARD AREA
2500 sq mm 2500 sq mm 2500 sq mm 50ºC/W
1000 sq mm 2500 sq mm 2500 sq mm 50ºC/W
225 sq mm 2500 sq mm 2500 sq mm 58ºC/W
100 sq mm 2500 sq mm 2500 sq mm 64ºC/W
1000 sq mm 2500 sq mm 1000 sq mm 57ºC/W
1000 sq mm 0 1000 sq mm 60ºC/W
* Tab of device attached to topside copper.
Table 5. TO-92 Package THERMAL RESISTANCE
Package alone 220ºC/W
Package soldered into PC board with plated through
holes only
175ºC/W
Package soldered into PC board with 1/4 sq. inch of
copper trace per lead
145ºC/W
Package soldered into PC board with plated through
holes in board, no extra copper trace, and a clip-on type
heat sink: Thermalloy type 2224B
160ºC/W
Aavid type 5754 135ºC/W
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
100mA, and a maximum ambient temperature of 50°C,
what will the maximum junction temperature be?
Power dissipated by the device will be equal to:
I
OUT MAX • (VIN MAX – VOUT) + (IGND • VIN)
where, IOUT MAX = 100mA
V
IN MAX = 7V
I
GND at (IOUT = 100mA, VIN = 7V) = 5mA
so, P = 100mA • (7V – 3.3V) + (5mA • 7V)
= 0.405W
If we use an SOT-223 package, then the thermal resistance
will be in the range of 50°C/W to 65°C/W depending on
copper area. So the junction temperature rise above ambi-
ent will be less than or equal to:
0.405W • 60°C/W = 24°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 + 24°C = 74°C
Output Capacitance and Transient Performance
The LT1121 is designed to be stable with a wide range of
output capacitors. The minimum recommended value is 1μF
with an ESR of 3Ω or less. For applications where space
is very limited, capacitors as low as 0.33μF can be used if
combined with a small series resistor. Assuming that the
ESR of the capacitor is low (ceramic) the suggested series
LT1121/LT1121-3.3/LT1121-5
11
1121ff
APPLICATIONS INFORMATION
resistor is shown in Table 6. The LT1121 is a micropower
device and output transient response will be a function of
output capacitance. See the Transient Response curves
in the Typical Performance Characteristics. Larger values
of output capacitance will decrease the peak deviations
and provide improved output transient response. Bypass
capacitors, used to decouple individual components
powered by the LT1121, will increase the effective value
of the output capacitor.
Protection Features
The LT1121 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
thermal limiting, the device is protected against reverse
input voltages, reverse output voltages, and reverse volt-
ages 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 operation,
the junction temperature should not exceed 125°C.
The input of the device will withstand reverse voltages
of 30V. Current fl ow into the device will be limited to less
than 1mA (typically less than 100μA) 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 backwards.
For fi xed voltage versions of the device, the output can
be pulled below ground without damaging the device. If
the input is open circuit or grounded the output can be
pulled below ground by 20V. The output will act like an
open circuit, no current will fl ow out of the pin. If the input
is powered by a voltage source, the output will source the
Table 6. Suggested Series Resistor Values
OUTPUT CAPACITANCE SUGGESTED SERIES RESISTOR
0.33μF 2Ω
0.47μF 1Ω
0.68μF 1Ω
>1μF None Needed
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.
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 fl ow back into the output will vary
depending on the conditions. Many battery-powered cir-
cuits incorporate some form of power management. The
following information will help optimize battery life. Table
7 summarizes the following information.
The reverse output current will follow the curve in Figure
2 when the input pin is pulled to ground. This current
fl ows through the output pin to ground. The state of the
shutdown 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 to the LT1121 unconnected when the output is held
high. This can happen when the LT1121 is powered from
a rectifi ed AC source. If the AC source is removed, then
the input of the LT1121 is effectively left fl oating. The
reverse output current also follows the curve in Figure 2
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 fl oating.
Figure 2. Reverse Output Current
OUTPUT VOLTAGE (V)
0
OUTPUT PIN CURRENT (μA)
100
90
80
70
60
50
40
30
20
10
08
1121• F02
24610
13579
TJ = 25°C
VIN < VOUT
CURRENT FLOWS
INTO OUTPUT PIN
TO GROUND
LT1121-3.3
LT1121
(VOUT = VADJ)
LT1121-5
LT1121/LT1121-3.3/LT1121-5
12
1121ff
APPLICATIONS INFORMATION
Table 7. Fault Conditions
INPUT PIN SHDN PIN OUTPUT PIN
<VOUT (Nominal) Open (Hi) Forced to VOUT (Nominal) Reverse Output Current ≈ 15μA (See Figure 2)
Input Current ≈ 1μA (See Figure 3)
<VOUT (Nominal) Grounded Forced to VOUT (Nominal) Reverse Output Current ≈ 15μA (See Figure 2)
Input Current ≈ 1μA (See Figure 3)
Open Open (Hi) Forced to VOUT (Nominal) Reverse Output Current ≈ 15μA (See Figure 2)
Open Grounded Forced to VOUT (Nominal) Reverse Output Current ≈ 15μA (See Figure 2)
≤0.8V Open (Hi) ≤0V Output Current = 0
≤0.8V Grounded ≤0V Output Current = 0
>1.5V Open (Hi) ≤0V Output Current = Short-Circuit Current
–30V < VIN < 30V Grounded ≤0V Output Current = 0
When the input of the LT1121 is forced to a voltage below
its nominal output voltage and its output is held high, the
reverse output current will still follow the curve in Figure
2. This condition can occur if the input of the LT1121 is
connected to a discharged (low voltage) battery and the
output is held up by either a backup battery or by a 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 2μA (see
Figure 3). The state of the shutdown pin will have no effect
on the reverse output current when the output is pulled
above the input.
Figure 3. Input Current
INPUT VOLTAGE (V)
0
INPUT CURRENT (μA)
5
4
3
2
1
04
1121 F03
1235
VOUT = 3.3V (LT1121-3.3)
VOUT = 5V (LT1121-5)
LT1121/LT1121-3.3/LT1121-5
13
1121ff
PACKAGE DESCRIPTION
N8 Package
8-Lead PDIP (Narrow 0.300)
(LTC DWG # 05-08-1510)
S8 Package
8-Lead Plastic Small Outline (Narrow 0.150)
(LTC DWG # 05-08-1610)
N8 1002
.065
(1.651)
TYP
.045 – .065
(1.143 – 1.651)
.130 ± .005
(3.302 ± 0.127)
.020
(0.508)
MIN
.018 ± .003
(0.457 ± 0.076)
.120
(3.048)
MIN
12 34
87 65
.255 ± .015*
(6.477 ± 0.381)
.400*
(10.160)
MAX
.008 – .015
(0.203 – 0.381)
.300 – .325
(7.620 – 8.255)
.325 +.035
–.015
+0.889
–0.381
8.255
()
NOTE:
1. DIMENSIONS ARE INCHES
MILLIMETERS
*THESE DIMENSIONS DO NOT INCLUDE MOLD FLASH OR PROTRUSIONS.
MOLD FLASH OR PROTRUSIONS SHALL NOT EXCEED .010 INCH (0.254mm)
.100
(2.54)
BSC
.016 – .050
(0.406 – 1.270)
.010 – .020
(0.254 – 0.508)× 45°
0°– 8° TYP
.008 – .010
(0.203 – 0.254)
SO8 0303
.053 – .069
(1.346 – 1.752)
.014 – .019
(0.355 – 0.483)
TYP
.004 – .010
(0.101 – 0.254)
.050
(1.270)
BSC
1234
.150 – .157
(3.810 – 3.988)
NOTE 3
8765
.189 – .197
(4.801 – 5.004)
NOTE 3
.228 – .244
(5.791 – 6.197)
.245
MIN .160 ±.005
RECOMMENDED SOLDER PAD LAYOUT
.045 ±.005
.050 BSC
.030 ±.005
TYP
INCHES
(MILLIMETERS)
NOTE:
1. DIMENSIONS IN
2. DRAWING NOT TO SCALE
3. THESE DIMENSIONS DO NOT INCLUDE MOLD FLASH OR PROTRUSIONS.
MOLD FLASH OR PROTRUSIONS SHALL NOT EXCEED .006" (0.15mm)
LT1121/LT1121-3.3/LT1121-5
14
1121ff
PACKAGE DESCRIPTION
ST Package
8-Lead Plastic SOT-223
(LTC DWG # 05-08-1630)
.114 – .124
(2.90 – 3.15)
.248 – .264
(6.30 – 6.71)
.130 – .146
(3.30 – 3.71)
.264 – .287
(6.70 – 7.30)
.0905
(2.30)
BSC
.033 – .041
(0.84 – 1.04)
.181
(4.60)
BSC
.024 – .033
(0.60 – 0.84)
.071
(1.80)
MAX
10°
MAX
.012
(0.31)
MIN
.0008 – .0040
(0.0203 – 0.1016)
10° – 16°
.010 – .014
(0.25 – 0.36)
10° – 16°
RECOMMENDED SOLDER PAD LAYOUT
ST3 (SOT-233) 0502
.129 MAX
.059 MAX
.059 MAX
.181 MAX
.039 MAX
.248 BSC
.090
BSC
LT1121/LT1121-3.3/LT1121-5
15
1121ff
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 representa-
tion that the interconnection of its circuits as described herein will not infringe on existing patent rights.
PACKAGE DESCRIPTION
Z Package
8-Lead Plastic TO-92 (Similar to TO-226)
(LTC DWG # 05-08-1510)
.050
(1.27)
BSC
.060 ± .005
(1.524± 0.127)
DIA
.90
(2.286)
NOM
.180 ± .005
(4.572 ± 0.127)
.180 ± .005
(4.572 ± 0.127)
.500
(12.70)
MIN
.050
(1.270)
MAX
UNCONTROLLED
LEAD DIMENSION
.016 ± .003
(0.406 ± 0.076)
5°
NOM
.015 ± .002
(0.381 ± 0.051)
.060 ± .010
(1.524 ± 0.254)
10° NOM
.140 ± .010
(3.556 ± 0.127)
Z3 (TO-92) 0801
321
.098 +.016/–.04
(2.5 +0.4/–0.1)
2 PLCS
TO-92 TAPE AND REEL
REFER TO TAPE AND REEL SECTION OF
LTC DATA BOOK FOR ADDITIONAL INFORMATION
LT1121/LT1121-3.3/LT1121-5
16
1121ff
Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com
© LINEAR TECHNOLOGY CORPORATION 1994
LT 0907 REV F • PRINTED IN USA
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Burst Mode is a trademark of Linear Technology Corporation.
