LT1121CST-3.3#TRPBF - LINEAR TECHNOLOGY

LT1121/LT1121-3.3/LT1121-5

1121fg

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 manu-

facturability. Also the input may be connected to ground

or a reverse voltage without reverse current flow 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 flow 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

Dropout Voltage

IN OUT

LT1121-3.3

GND

3.3VOUT

150mA

1μF

SOLID TANTALUM

LT1121 • TA01

VSHDN (PIN 5)

<0.25

>2.8

OUTPUT

OFF

SHDN

OUTPUT CURRENT (mA)

DROPOUT VOLTAGE (V)

0.3

0.4

0.5

60 100 160

0.2

0.1

020 40 80 120 140

LT1121 • TA02

L, LT , LT C , LT M , Linear Technology and the Linear logo are registered trademarks of Linear

Technology Corporation. All other trademarks are the property of their respective owners.

LT1121/LT1121-3.3/LT1121-5

1121fg

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)

TOP VIEW

OUT

NC/ADJ*

GND

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)

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

Output Short-Circuit Duration ....................... Indefinite

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

PIN CONFIGURATION

ORDER INFORMATION

LEAD FREE FINISH TAPE AND REEL PART MARKING PACKAGE DESCRIPTION TEMPERATURE RANGE

LT1121CN8#PBF LT1121CN8#TRPBF LT1121CN8 8-Lead Plastic PDIP 0°C to 125°C

LT1121CN8-3.3#PBF LT1121CN8-3.3#TRPBF LT1121CN8-3.3 8-Lead Plastic PDIP 0°C to 125°C

LT1121CN8-5#PBF LT1121CN8-5#TRPBF LT1121CN8-5 8-Lead Plastic PDIP 0°C to 125°C

LT1121IN8#PBF LT1121IN8#TRPBF LT1121IN8 8-Lead Plastic PDIP –40°C to 125°C

LT1121IN8-3.3#PBF LT1121IN8-3.3#TRPBF LT1121IN8-3.3 8-Lead Plastic PDIP –40°C to 125°C

LT1121IN8-5#PBF LT1121IN8-5#TRPBF LT1121IN8-5 8-Lead Plastic PDIP –40°C to 125°C

LT1121CS8#PBF LT1121CS8#TRPBF 1121 8-Lead Plastic S0 0°C to 125°C

LT1121CS8-3.3#PBF LT1121CS8-3.3#TRPBF 11213 8-Lead Plastic S0 0°C to 125°C

LT1121CS8-5#PBF LT1121CS8-5#TRPBF 11215 8-Lead Plastic S0 0°C to 125°C

LT1121HVCS8#PBF LT1121HVCS8#TRPBF 1121HV 8-Lead Plastic S0 –40°C to 125°C

LT1121IS8#PBF LT1121IS8#TRPBF 1121I 8-Lead Plastic S0 –40°C to 125°C

LT1121IS8-3.3#PBF LT1121IS8-3.3#TRPBF 121I3 8-Lead Plastic S0 –40°C to 125°C

LT1121IS8-5#PBF LT1121IS8-5#TRPBF 121I5 8-Lead Plastic S0 –40°C to 125°C

LT1121HVIS8#PBF LT1121HVIS8#TRPBF 121HVI 8-Lead Plastic S0 –40°C to 125°C

LT1121ACS8#PBF LT1121ACS8#TRPBF 1121A 8-Lead Plastic S0 0°C to 125°C

LT1121ACS8-3.3#PBF LT1121ACS8-3.3#TRPBF 1121A3 8-Lead Plastic S0 0°C to 125°C

LT1121ACS8-5#PBF LT1121ACS8-5#TRPBF 1121A5 8-Lead Plastic S0 0°C to 125°C

LT1121/LT1121-3.3/LT1121-5

1121fg

ORDER INFORMATION

LEAD FREE FINISH TAPE AND REEL PART MARKING PACKAGE DESCRIPTION TEMPERATURE RANGE

LT1121AHVCS8#PBF LT1121AHVCS8#TRPBF 121AHV 8-Lead Plastic S0 0°C to 125°C

LT1121AIS8#PBF LT1121AIS8#TRPBF 121AI 8-Lead Plastic S0 –40°C to 125°C

LT1121AIS8-3.3#PBF LT1121AIS8-3.3#TRPBF 121AI3 8-Lead Plastic S0 –40°C to 125°C

LT1121AIS8-5#PBF LT1121AIS8-5#TRPBF 121AI5 8-Lead Plastic S0 –40°C to 125°C

LT1121AHVIS8#PBF LT1121AHVIS8#TRPBF 21AHVI 8-Lead Plastic S0 –40°C to 125°C

LT1121CST-3.3#PBF LT1121CST-3.3#TRPBF 11213 3-Lead Plastic SOT-223 0°C to 125°C

LT1121IST-3.3#PBF LT1121IST-3.3#TRPBF 121I3 3-Lead Plastic SOT-223 –40°C to 125°C

LT1121CST-5#PBF LT1121CST-5#TRPBF 11215 3-Lead Plastic SOT-223 0°C to 125°C

LT1121IST-5#PBF LT1121IST-5#TRPBF 1121I5 3-Lead Plastic SOT-223 –40°C to 125°C

LT1121CZ3-3#PBF LT1121CZ3-3#TRPBF LT1121CZ3-3 3-Lead Plastic TO-92 0°C to 125°C

LT1121IZ3-3#PBF LT1121IZ3-3#TRPBF LT1121IZ3-3 3-Lead Plastic TO-92 –40°C to 125°C

LT1121CZ-5#PBF LT1121CZ-5#TRPBF LT1121CZ-5 3-Lead Plastic TO-92 0°C to 125°C

LT1121IZ-5#PBF LT1121IZ-5#TRPBF LT1121IZ-5 3-Lead Plastic TO-92 –40°C to 125°C

Consult LT C Marketing for parts specified with wider operating temperature ranges. *The temperature grade is identified by a label on the shipping container.

Consult LT C Marketing for information on nonstandard lead based finish parts.

For more information on lead free part marking, go to: http://www.linear.com/leadfree/

For more information on tape and reel specifications, go to: http://www.linear.com/tapeandreel/

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

3.25

3.2

3.3

3.35

3.4

LT1121-5 VIN = 5.5V, IOUT = 1mA, TJ = 25ºC

6V < VIN < 20V, 1mA < IOUT < 150mA

4.925

4.85

5.075

5.15

LT1121 (Note 5) VIN = 4.3V, IOUT = 1mA, TJ = 25ºC

4.8V < VIN < 20V, 1mA < IOUT < 150mA

3.695

3.64

3.75

3.805

3.86

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

1.5

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

–12

–20

–17

–28

–12

–18

–25

–40

–35

–50

–25

–40

Dropout Voltage (Note 6) ILOAD = 1mA, TJ = 25ºC

ILOAD = 1mA

0.13 0.16

0.25

ILOAD = 50mA, TJ = 25ºC

ILOAD = 50mA

0.3 0.35

0.5

ILOAD = 100mA, TJ = 25ºC

ILOAD = 100mA

0.37 0.45

0.6

ILOAD = 150mA, TJ = 25ºC

ILOAD = 150mA

0.42 0.55

0.7

The l denotes the specifications which apply over the full operating

temperature range, otherwise specifications are at TA = 25°C.

LT1121/LT1121-3.3/LT1121-5

1121fg

The l denotes the specifications which apply over the full operating

temperature range, otherwise specifications 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 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 LT1121 (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: (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 flows into the adjust pin.

Note 9: Shutdown pin current at VSHDN = 0V flows 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 flows into

the output pin and out of the ground pin.

ELECTRICAL CHARACTERISTICS

PARAMETER CONDITIONS MIN TYP MAX UNITS

Ground Pin Current (Note 7) ILOAD = 0mA

ILOAD = 1mA

ILOAD = 10mA

ILOAD = 50mA

ILOAD = 100mA

ILOAD = 150mA

350

1.5

120

500

2.5

µA

Adjust Pin Bias Current (Notes 5, 8) TJ = 25ºC 150 300 nA

Shutdown Threshold VOUT = Off to On

VOUT = On to Off

0.25

1.2

0.75

2.8 V

Shutdown Pin Current (Note 9) VSHDN = 0V l6 10 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 l1 mA

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

µA

LT1121/LT1121-3.3/LT1121-5

1121fg

Guaranteed Dropout Voltage

Dropout Voltage

Quiescent Current

TYPICAL PERFORMANCE CHARACTERISTICS

OUTPUT CURRENT (mA)

DROPOUT VOLTAGE (V)

0.7

0.6

0.5

0.4

0.3

0.2

0.1

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

050 75

1121 G02

–25 25 100 125

ILOAD = 150mA

ILOAD = 50mA

ILOAD = 1mA

ILOAD = 100mA

TEMPERATURE (°C)

–50

QUIESCENT CURRENT (μA)

050 75

1121 G03

–25 25 100 125

VIN = 6V

RLOAD = ∞

VSHDN = OPEN

VSHDN = 0V

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

INPUT VOLTAGE (V)

QUIESCENT CURRENT (μA)

120

100

1121 G04

24610

13579

VSHDN = 0V

TJ = 25°C

RLOAD = ∞

VSHDN = OPEN

INPUT VOLTAGE (V)

QUIESCENT CURRENT (μA)

120

100

1121 G05

24610

13579

TJ = 25°C

RLOAD = ∞

VSHDN = 0V

VSHDN = OPEN

INPUT VOLTAGE (V)

QUIESCENT CURRENT (μA)

120

100

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

LT1121/LT1121-3.3/LT1121-5

1121fg

TYPICAL PERFORMANCE CHARACTERISTICS

LT1121-3.3

Ground Pin Current

LT1121-5

Ground Pin Current

LT1121

Ground Pin Current

INPUT VOLTAGE (V)

GROUND PIN CURRENT (μA)

800

700

600

500

400

300

200

100

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)

GROUND PIN CURRENT (μA)

800

700

600

500

400

300

200

100

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)

GROUND PIN CURRENT (μA)

800

700

600

500

400

300

200

100

1121 G10

24610

13579

RLOAD = 130Ω

ILOAD = 25mA*

RLOAD = 330Ω

ILOAD = 10mA*

RLOAD = 3.3k

ILOAD = 1mA*

TJ = 25°C

*FOR VOUT = 3.3V

LT1121-3.3

Ground Pin Current

LT1121-5

Ground Pin Current

LT1121

Ground Pin Current

Shutdown Pin Threshold

(On-to-Off)

Shutdown Pin Threshold

(Off-to-On)

INPUT VOLTAGE (V)

GROUND PIN CURRENT (mA)

1121 G14

24610

1 3 5 7 9

TJ = 25°C

RLOAD = 50Ω

ILOAD = 100mA*

RLOAD = 100Ω

ILOAD = 50mA*

RLOAD = 33Ω

ILOAD = 150mA*

*FOR VOUT = 5V

INPUT VOLTAGE (V)

GROUND PIN CURRENT (mA)

1121 G15

24610

1 3 5 7 9

TJ = 25°C

VOUT = VADJ

RLOAD = 25Ω

ILOAD = 150mA*

RLOAD = 38Ω

ILOAD = 100mA*

RLOAD = 75Ω

ILOAD = 50mA*

*FOR VOUT = 3.75V

INPUT VOLTAGE (V)

GROUND PIN CURRENT (mA)

1121 G13

24610

1 3 5 7 9

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)

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

LT1121/LT1121-3.3/LT1121-5

1121fg

TYPICAL PERFORMANCE CHARACTERISTICS

Shutdown Pin Current

Shutdown Pin Input Current

LT1121

Adjust Pin Bias Current

TEMPERATURE (°C)

–50

SHUTDOWN PIN CURRENT (μA)

0050 75

1121 G19

–25 25 100 125

VSHDN = 0V

SHUTDOWN PIN VOLTAGE (V)

SHUTDOWN PIN INPUT CURRENT (mA)

245 9

1121 G20

1 3 678

TEMPERATURE (°C)

–50

ADJUST PIN BIAS CURRENT (nA)

400

350

300

250

200

150

100

0050 75

1121 G21

–25 25 100 125

Reverse Output Current

Current Limit

Reverse Output Current

Ripple Rejection

TEMPERATURE (°C)

–50

OUTPUT PIN CURRENT (μA)

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)

SHORT-CIRCUIT CURRENT (mA)

400

350

300

250

200

150

100

0245

1121 G23

1367

VOUT = 0V

TEMPERATURE (°C)

–50

CURRENT LIMIT (mA)

400

350

300

250

200

150

100

0050 75

1121 G24

–25 25 100 125

VIN = 7V

VOUT = 0V

OUTPUT VOLTAGE (V)

OUTPUT PIN CURRENT (μA)

100

1121 G25

24610

1 3 5 7 9

TJ = 25°C

VIN = 0V

CURRENT FLOWS

INTO OUTPUT PIN

LT1121-3.3

LT1121

(VOUT = VADJ)

LT1121-5

TEMPERATURE (°C)

–50

RIPPLE REJECTION (dB)

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

010 1k 10k 1M

1121 G27

100

100k

COUT = 1μF

SOLID TANTALUM

COUT = 47μF

SOLID TANTALUM

IOUT = 100mA

VIN = 6V + 50mVRMS RIPPLE

LT1121/LT1121-3.3/LT1121-5

1121fg

TYPICAL PERFORMANCE CHARACTERISTICS

Load Regulation

LT1121-5

Load Transient Response

LT1121-5

Load Transient Response

TEMPERATURE (°C)

–50

LOAD REGULATION (mV)

–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)

OUTPUT VOLTAGE

DEVIATION (V)

0.2

0.1

–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)

OUTPUT VOLTAGE

DEVIATION (V)

0.2

0.1

–0.1

–0.2

0.8

1121 G30

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 = 3.3μF

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 filter 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 sufficient.

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 flow into the LT1121 and no reverse

voltage 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 amplifier. This pin is internally

clamped to 6V and –0.6V (one VBE). It has a bias current

of 150nA which flows 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

1121fg

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 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 LT1121 acts like it has a diode in series with its output

and prevents reverse current flow.

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, flows 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 coefficient of

approximately 15ppm/°C. The adjust pin bias current has

a negative temperature coefficient. These effects are small

and 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 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

taken in still air, on 3/32" FR-4 board with 1oz copper. All

Figure 1. Adjustable Operation

LT1121

GND

SHDN

1121 • F01

ADJ

OUT

VOUT = 3.75V 1 + + IADJ • R2

VADJ = 3.75V

IADJ = 150nA AT 25°C

OUTPUT RANGE = 3.75V TO 30V

( )

VOUT

LT1121/LT1121-3.3/LT1121-5

1121fg

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.

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:

IOUT MAX • (VIN MAX – VOUT) + (IGND • VIN)

where, IOUT MAX = 100mA

VIN MAX = 7V

IGND 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:

TJMAX = 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

1121fg

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

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 opera-

tion, the junction temperature should not exceed 125°C.

The input of the device will withstand reverse voltages of

30V. Current flow 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 fixed 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 flow 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 flow 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

flows 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 rectified AC source. If the AC source is removed, then

the input of the LT1121 is effectively left floating. 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 floating.

Figure 2. Reverse Output Current

OUTPUT VOLTAGE (V)

OUTPUT PIN CURRENT (μA)

100

1121• F02

24610

1 3 5 7 9

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

1121fg

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)

INPUT CURRENT (μA)

1121 F03

1235

VOUT = 3.3V (LT1121-3.3)

VOUT = 5V (LT1121-5)

LT1121/LT1121-3.3/LT1121-5

1121fg

N8 REV I 0711

.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

.008 – .015

(0.203 – 0.381)

.300 – .325

(7.620 – 8.255)

.325 +.035

–.015

+0.889

–0.381

8.255

( )

1 2 34

87 65

.255 ±.015*

(6.477 ±0.381)

.400*

(10.160)

MAX

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

N Package

8-Lead PDIP (Narrow .300 Inch)

(Reference LTC DWG # 05-08-1510 Rev I)

.016 – .050

(0.406 – 1.270)

.010 – .020

(0.254 – 0.508)× 45°

0°– 8° TYP

.008 – .010

(0.203 – 0.254)

SO8 REV G 0212

.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)

4. PIN 1 CAN BE BEVEL EDGE OR A DIMPLE

S8 Package

8-Lead Plastic Small Outline (Narrow .150 Inch)

(Reference LTC DWG # 05-08-1610 Rev G)

PACKAGE DESCRIPTION

Please refer to http://www.linear.com/designtools/packaging/ for the most recent package drawings.

LT1121/LT1121-3.3/LT1121-5

1121fg

.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

.181 MAX

.039 MAX

.248 BSC

.090

BSC

ST Package

3-Lead Plastic SOT-223

(Reference LTC DWG # 05-08-1630)

.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

BULK PACK

.015 ± .002

(0.381 ± 0.051) Z3 (TO-92) 1008 REV C

.060 ± .010

(1.524 ± 0.254)

10° NOM

.140 ± .010

(3.556 ± 0.127)

3 2 1

.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

Z Package

3-Lead Plastic TO-92 (Similar to TO-226)

(Reference LTC DWG # 05-08-1410 Rev C)

PACKAGE DESCRIPTION

Please refer to http://www.linear.com/designtools/packaging/ for the most recent package drawings.

LT1121/LT1121-3.3/LT1121-5

1121fg

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.

REVISION HISTORY

REV DATE DESCRIPTION PAGE NUMBER

G 05/12 Changed Order Information top marking for 8-lead SO 3.3V and 5V options, C-grade 2

(Revision history begins at Rev G)

LT1121/LT1121-3.3/LT1121-5

1121fg

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 0512 REV G • PRINTED IN USA

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