Datasheet LT1121, LT1121-3.3, LT1121-5 Datasheet (LINEAR TECHNOLOGY)

FEATURES

LT1121/LT1121-3.3/LT1121-5

Micropower Low Dropout

Regulators with Shutdown

U

DESCRIPTIO

■

0.4V Dropout Voltage

■

150mA Output Current

■

30µA Quiescent Current

■

No Protection Diodes Needed

■

Adjustable Output from 3.8V to 30V

■

3.3V and 5V Fixed Output Voltages

■

Controlled Quiescent Current in Dropout

■

Shutdown

■

16µA Quiescent Current in Shutdown

■

Stable with 0.33µF Output Capacitor

■

Reverse Battery Protection

■

No Reverse Current with Input Low

■

Thermal Limiting

■

Available in the 8-Lead SO, 8-Lead PDIP, 3-Lead
SOT-23 and 3-Lead TO-92 Packages

U

APPLICATIO S

■

Low Current Regulator

■

Regulator for Battery-Powered Systems

■

Post Regulator for Switching Supplies

The LT®1121/LT1121-3.3/LT1121-5 are micropower low
dropout regulators with shutdown. These devices are
capable of supplying

150mA of output current with a
dropout voltage of 0.4V. Designed for use in battery­powered systems, the low quiescent current, 30µA oper-
ating 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
manufacturability. 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.

, LTC and LT are registered trademarks of Linear Technology Corporation.
All other trademarks are the property of their respective owners.

TYPICAL APPLICATIO

5V Battery-Powered Supply with Shutdown

3

OUTPUT

OUT

OFF

ON
ON

1

8

IN

5

V

SHDN

LT1121-3.3

SHDN

GND

(PIN 5)

<0.25

>2.8

NC

5V

U

+

LT1121 • TA01

3.3V

OUT

150mA

1µF
SOLID TANTALUM

Dropout Voltage

0.5

0.4

0.3

0.2

DROPOUT VOLTAGE (V)

0.1

0

0

20 40

OUTPUT CURRENT (mA)

80

60 100 160

120 140

LT1121 • TA02

1121fc

1

LT1121/LT1121-3.3/LT1121-5

WWWU

ABSOLUTE AXI U RATI GS

(Note 1)

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

PACKAGE

TOP VIEW

OUT

1

NC/ADJ*

2

GND

3

NC

4

N8 PACKAGE
8-LEAD PDIP

S8 PACKAGE

8-LEAD PLASTIC SO

T

= 150°C, θJA ≈ 120°C/W (N8, S8)

JMAX

= 150°C, θJA ≈ 70°C/W (AS8)

T

JMAX

/

O

RDER I FOR ATIO

PIN 2 = NC FOR LT1121-3.3/LT1121-5

*

IN

8

NC**

7

NC**

6

SHDN

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.

WU

U

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

FRONT VIEW

TAB IS

GND

ST PACKAGE

3-LEAD PLASTIC SOT-223

= 150°C, θJA ≈ 50°C/W

T

JMAX

3

2

1

OUTPUT

GND

V

IN

BOTTOM VIEW

IN GND OUT

Z PACKAGE

3-LEAD PLASTIC TO-92

T

= 150°C, θJA ≈ 150°C/ W

JMAX

ORDER PART NUMBER

LT1121CN8
LT1121CN8-3.3
LT1121CN8-5
LT1121IN8
LT1121IN8-3.3
LT1121IN8-5
LT1121CS8
LT1121CS8-3.3
LT1121CS8-5
LT1121HVCS8
LT1121IS8

Consult LTC Marketing for parts specified with wider opearating temperature ranges.

LT1121IS8-3.3
LT1121IS8-5
LT1121HVIS8
LT1121ACS8
LT1121ACS8-3.3
LT1121ACS8-5
LT1121AHVCS8
LT1121AIS8
LT1121AIS8-3.3
LT1121AIS8-5
LT1121AHVIS8

S8 PART

MARKING

1121
11213
11215
1121H
1121I
121I3
121I5
121IH

1121A
121A3
121A5
121AH
121AI
121AI3
121AI5
121AIH

ORDER PART

NUMBER

LT1121CST-3.3
LT1121IST-3.3
LT1121CST-5
LT1121IST-5

ORDER PART

NUMBER

LT1121CZ-3.3
LT1121IZ-3.3
LT1121CZ-5
LT1121IZ-5

2

1121fc

LT1121/LT1121-3.3/LT1121-5

ELECTRICAL CHARACTERISTICS

range, otherwise specifications are at T

= 25°C.

A

The ● denotes specifications which apply over the operating temperature

PARAMETER CONDITIONS MIN TYP MAX UNITS

Regulated Output Voltage LT1121-3.3 VIN = 3.8V, I
(Note 4) 4.3V < V

LT1121-5 VIN = 5.5V, I

6V < V

IN

LT1121 (Note 5) VIN = 4.3V, I

4.8V < V

Line Regulation LT1121-3.3 ∆VIN = 4.8V to 20V, I

LT1121-5 ∆VIN = 5.5V to 20V, I
LT1121 (Note 5) ∆VIN = 4.3V to 20V, I

Load Regulation LT1121-3.3 ∆I

LT1121-5 ∆I

LT1121 (Note 5) ∆I

Dropout Voltage I
(Note 6) I

Ground Pin Current I

(Note 7) I

= 1mA, TJ = 25°C 0.13 0.16 V

LOAD

= 1mA ● 0.25 V

LOAD

I

= 50mA, TJ = 25°C 0.30 0.35 V

LOAD

= 50mA ● 0.50 V

LOAD

I

= 100mA, TJ = 25°C 0.37 0.45 V

LOAD

I

= 100mA ● 0.60 V

LOAD

I

= 150mA, TJ = 25°C 0.42 0.55 V

LOAD

= 150mA ● 0.70 V

I

LOAD

= 0mA ● 30 50 µA

LOAD

= 1mA ● 90 120 µA

LOAD

I

= 10mA ● 350 500 µA

LOAD

I

= 50mA ● 1.5 2.5 mA

LOAD

I

= 100mA ● 4.0 7.0 mA

LOAD

I

= 150mA ● 7.0 14.0 mA

LOAD

= 1mA to 150mA, TJ = 25°C–12–25mV

LOAD

= 1mA to 150mA ● –20 –40 mV

∆I

LOAD

= 1mA to 150mA, TJ = 25°C–17–35mV

LOAD

∆I

= 1mA to 150mA ● –28 –50 mV

LOAD

= 1mA to 150mA, TJ = 25°C–12–25mV

LOAD

= 1mA to 150mA ● –18 –40 mV

∆I

LOAD

= 1mA, TJ = 25°C 3.250 3.300 3.350 V

OUT

< 20V, 1mA < I

IN

= 1mA, TJ = 25°C 4.925 5.000 5.075 V

OUT

< 20V, 1mA < I

= 1mA, TJ = 25°C 3.695 3.750 3.805 V

OUT

< 20V, 1mA < I

IN

< 150mA ● 3.200 3.300 3.400 V

OUT

< 150mA ● 4.850 5.000 5.150 V

OUT

< 150mA ● 3.640 3.750 3.860 V

OUT

= 1mA ● 1.5 10 mV

OUT

= 1mA ● 1.5 10 mV

OUT

= 1mA ● 1.5 10 mV

OUT

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

Shutdown Threshold V

Shutdown Pin Current (Note 9) V

Quiescent Current in Shutdown (Note 10) VIN = 6V, V

Ripple Rejection VIN – V

Current Limit VIN – V

Input Reverse Leakage Current VIN = –20V, V

Reverse Output Current (Note 11) LT1121-3.3 V

= Off to On ● 1.2 2.8 V

OUT

V

= On to Off ● 0.25 0.75 V

OUT

= 0V ● 610 µA

SHDN

= 0V ● 15 22 µA

SHDN

= 1V (Avg), V

OUT

f

= 120Hz, I

RIPPLE

OUT

LOAD

= 7V, TJ = 25°C 200 500 mA

= 0V ● 1.0 mA

OUT

LT1121-5 V
LT1121 (Note 5) V

RIPPLE

= 0.5V

,5058dB

P-P

= 0.1A

= 3.3V, VIN = 0V 16 25 µA

OUT

= 5V, VIN = 0V 16 25 µA

OUT

= 3.8V, VIN = 0V 16 25 µA

OUT

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

1121fc

3

LT1121/LT1121-3.3/LT1121-5

ELECTRICAL CHARACTERISTICS

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 T

> 110°C and I

J

< 1mA, output voltage

OUT

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: (V

Note 7: Ground pin current is tested with VIN = V
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 V
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.

UW

TYPICAL PERFOR A CE CHARACTERISTICS

Guaranteed Dropout Voltage

0.7

0.6

0.5

0.4

0.3

0.2

DROPOUT VOLTAGE (V)

0.1
= TEST POINTS

0

0

TJ ≤ 125°C

TJ ≤ 25°C

60

40

20

OUTPUT CURRENT (mA)

80

100

120

140

160

1121 G27

Dropout Voltage

0.7

0.6

I

0.5

0.4

0.3

0.2

DROPOUT VOLTAGE (V)

0.1

0

–50

LLOAD

–25

= 100mA

0

TEMPERATURE (°C)

I

= 150mA

LOAD

I

= 50mA

LOAD

I

= 1mA

LOAD

50

25

75

100

1121 G14

125

– V

IN

DROPOUT

= 0V flows out of the shutdown pin.

SHDN

Quiescent Current

10

VIN = 6V

=

∞

R

LOAD

40

30

20

QUIESCENT CURRENT (µA)

10

0

–50

–25

).

OUT

V

= OPEN

SHDN

V

=0V

SHDN

25

0

TEMPERATURE (°C)

(nominal) and a

50

75

100

1121 G11

125

LT1121-3.3
Quiescent Current

120

100

80

60

40

QUIESCENT CURRENT (µA)

20

0

13579

0

4

V

= OPEN

SHDN

2

4

INPUT VOLTAGE (V)

V

SHDN

6

= 0V

TJ = 25°C

=

R

LOAD

8

1121 G04

LT1121-5
Quiescent Current

120

∞

100

80

V

SHDN

60

40

QUIESCENT CURRENT (µA)

20

10

0

2

13579

0

4

INPUT VOLTAGE (V)

= OPEN

V

SHDN

6

TJ = 25°C

=

R

LOAD

= 0V

8

1121 G02

∞

10

LT1121
Quiescent Current

120

100

80

60

40

QUIESCENT CURRENT (µA)

20

0

2

13579

0

V

= OPEN

SHDN

V

SHDN

6

4

INPUT VOLTAGE (V)

TJ = 25°C
R

LOAD

V

OUT

= 0V

=

∞

= V

ADJ

8

10

1121 G03

1121fc

LT1121/LT1121-3.3/LT1121-5

UW

TYPICAL PERFOR A CE CHARACTERISTICS

LT1121-3.3
Output Voltage

3.38
I

= 1mA

OUT

3.36

3.34

3.32

3.30

3.28

OUTPUT VOLTAGE (V)

3.26

3.24

3.22

–50

–25

LT1121-3.3
Ground Pin Current

800

TJ = 25°C

700

600

500

400

300

200

GROUND PIN CURRENT (µA)

100

0

0

*FOR V

2

13579

50

25

0

TEMPERATURE (°C)

R
I

= 3.3V

OUT

6

4

INPUT VOLTAGE (V)

LOAD

LOAD

R

LOAD

I

LOAD

R
I

LOAD

75

LOAD

100

1121 G22

= 130Ω

= 25mA*

= 330Ω

= 10mA*

= 3.3k

= 1mA*

8

1121 G10

125

10

LT1121-5
Output Voltage

5.08
I

= 1mA

OUT

5.06

5.04

5.02

5.00

4.98

OUTPUT VOLTAGE (V)

4.96

4.94

4.92

–50

–25

LT1121-5
Ground Pin Current

800

TJ = 25°C

700

600

500

400

300

200

GROUND PIN CURRENT (µA)

100

0

2

13579

0

50

25

0

TEMPERATURE (°C)

R

LOAD

I

LOAD

R

LOAD

I

LOAD

*FOR V

INPUT VOLTAGE (V)

= 5V

OUT

R
I

LOAD

6

4

75

LOAD

100

= 200Ω

= 25mA*

= 500Ω

= 10mA*

= 5k

= 1mA*

8

1121 G23

1121 G06

125

LT1121
Adjust Pin Voltage

3.83
I

= 1mA

OUT

3.81

3.79

3.77

3.75

3.73

OUTPUT VOLTAGE (V)

3.71

3.69

3.67

–50

–25

50

25

0

TEMPERATURE (°C)

75

100

125

1121 G24

LT1121
Ground Pin Current

800

TJ = 25°C

= V

V

OUT

700

600

500

400

300

200

GROUND PIN CURRENT (µA)

100

10

0

ADJ

*FOR V

2

13579

0

4

INPUT VOLTAGE (V)

OUT

R
I

R
I

= 3.75V

6

LOAD

LOAD

LOAD

LOAD

R

LOAD

I

LOAD

= 150Ω

= 25mA*

= 380Ω

= 10mA*

= 3.8k

= 1mA*

8

1121 G08

10

LT1121-3.3
Ground Pin Current

10

TJ = 25°C

9

8

7

6

5

4

3

GROUND PIN CURRENT (mA)

2

1

0

2

0

13579

R
I

*FOR V

4

INPUT VOLTAGE (V)

LOAD

LOAD

OUT

= 22Ω

= 150mA*

R

LOAD

I

LOAD

R
I

LOAD

= 3.3V

6

= 33Ω

= 100mA*

= 66Ω

LOAD

= 50mA*

8

1121 G09

LT1121-5
Ground Pin Current

10

TJ = 25°C

9

8

7

6

5

4

3

GROUND PIN CURRENT (mA)

2

1

10

0

2

0

13579

INPUT VOLTAGE (V)

*FOR V

4

OUT

R
I

6

LOAD

LOAD

R
I

LOAD

R
I

LOAD

= 5V

LOAD

LOAD

= 33Ω

= 150mA*

= 50Ω

= 100mA*

= 100Ω

= 50mA*

8

1121 G05

10

LT1121
Ground Pin Current

10

TJ = 25°C

= V

V

9

OUT

ADJ

8

7

6

5

4

3

GROUND PIN CURRENT (mA)

2

1

0

0

13579

*FOR V

2

4

INPUT VOLTAGE (V)

OUT

R

LOAD

I

LOAD

R

LOAD

I

LOAD

R
I

LOAD

= 3.75V

6

= 25Ω

= 150mA*

= 38Ω

= 100mA*

= 75Ω

LOAD

= 50mA*

8

10

1121 G07

1121fc

5

LT1121/LT1121-3.3/LT1121-5

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

0

0

50

75

1121 G17

–25

25

100

125

I

LOAD

= 1mA

I

LOAD

= 150mA

TEMPERATURE (°C)

–50

CURRENT LIMIT (mA)

400

350

300

250

200

150

100

50

0

0

50

75

1121 G19

–25

25

100

125

VIN = 7V
V

OUT

= 0V

UW

TYPICAL PERFOR A CE CHARACTERISTICS

Ground Pin Current

14

VIN = 3.3V (LT1121-3.3)

= 5V (LT1121-5)

V

IN

12

= 3.75V (LT1121)

V

IN

DEVICE IS OPERATING
IN DROPOUT

10

8

6

4

GROUND PIN CURRENT (mA)

2

0

0

TJ = 25°C

60

40

20

OUTPUT CURRENT (mA)

Shutdown Pin Current

10

V

= 0V

SHDN

9

8

7

6

5

4

3

2

SHUTDOWN PIN CURRENT (µA)

1

0

–50

–25

25

0

TEMPERATURE (°C)

TJ = 125°C

80

100

50

TJ = –55°C

120

140

75

100

1121 G29

1121 G15

160

125

Shutdown Pin Threshold
(On-to-Off)

2.0
I

= 1mA

LOAD

1.8

1.6

1.4

1.2

1.0

0.8

0.6

SHUTDOWN THRESHOLD (V)

0.4

0.2

0

–50

–25

0

TEMPERATURE (°C)

50

25

Shutdown Pin Input Current

25

20

15

10

5

SHUTDOWN PIN INPUT CURRENT (mA)

0

13

0

2

SHUTDOWN PIN VOLTAGE (V)

59

4

Shutdown Pin Threshold
(Off-to-On)

100

125

1121 G16

75

LT1121
Adjust Pin Bias Current

400

350

300

250

200

150

100

ADJUST PIN BIAS CURRENT (nA)

50

0

7

8

6

1121 G28

–50

–25

0

TEMPERATURE (°C)

50

25

75

100

125

1121 G25

Reverse Output Current

30

VIN = 0V
V

OUT

V

25

OUT

V

OUT

20

15

10

OUTPUT PIN CURRENT (µA)

5

0

–50

6

–25

= 5V (LT1121-5)
= 3.3V (LT1121-3.3)
= 3.8V (LT1121)

25

0

TEMPERATURE (°C)

50

Current Limit

400

V

= 0V

OUT

350

300

250

200

150

100

SHORT-CIRCUIT CURRENT (mA)

50

100

125

1121 G13

75

0

0

2

1

INPUT VOLTAGE (V)

4

3

5

6

7

1121 G20

Current Limit

1121fc

LT1121/LT1121-3.3/LT1121-5

FREQUENCY (Hz)

RIPPLE REJECTION (dB)

100

90

80

70

60

50

40

30

20

10

0

10 1k 10k 1M

1121 G26

100 100k

C

OUT

= 1µF

SOLID TANTALUM

C

OUT

= 47µF

SOLID TANTALUM

I

OUT

= 100mA

V

IN

= 6V + 50mV

RMS

RIPPLE

UW

TYPICAL PERFOR A CE CHARACTERISTICS

Reverse Output Current

100

TJ = 25°C

90

= 0V

V

IN

CURRENT FLOWS

80

INTO OUTPUT PIN

70

60

50

40

30

OUTPUT PIN CURRENT (µA)

20

10

0

0

13579

(V

LT1121-3.3

2

OUTPUT VOLTAGE (V)

Load Regulation

0

∆I

= 1mA TO 150mA

LOAD

–5

–10

–15

–20

–25

–30

LOAD REGULATION (mV)

–35

* ADJ PIN TIED TO
OUTPUT PIN

–40

–50

–25

0

TEMPERATURE (°C)

LT1121

= V

OUT

4

LT1121*

25

)

ADJ

LT1121-5

6

50

8

LT1121-3.3

LT1121-5

75

100

1121 G01

1121 G21

10

125

64

VIN = V
+ 0.5V

62

I

60

58

56

54

RIPPLE REJECTION (dB)

52

50

–50

OUT

RIPPLE AT f = 120Hz

P-P

= 100mA

OUT

–25

(NOMINAL) + 1V

0

TEMPERATURE (°C)

LT1121-5
Load Transient Response

VIN = 6V

0.2

= 0.1µF

C

IN

= 1µF

C

OUT

0.1

0

–0.1

DEVIATION (V)

OUTPUT VOLTAGE

–0.2

150

(mA)

100

LOAD CURRENT

0.1

0.2

0.3

0

50

25

0.4

0.5 0.7 0.9

0.6

TIME (ms)

Ripple RejectionRipple Rejection

100

125

1121 G18

75

LT1121-5
Load Transient Response

VIN = 6V

0.2

= 0.1µF

C

IN

= 3.3µF

C

OUT

0.1

0

–0.1

DEVIATION (V)

OUTPUT VOLTAGE

–0.2

150

100

(mA)

50

0.8

1121 G30

1.0

LOAD CURRENT

0.1

0

0.2

0.4

0.5 0.7 0.9

0.3
TIME (ms)

0.6

0.8

1.0

1121 G31

1121fc

7

LT1121/LT1121-3.3/LT1121-5

U

UU

PI FU CTIO S

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 V
allows the shutdown pin to be driven directly by 5V logic
or by open collector logic with a pull-up resistor. The pull­up 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.

). This

BE

8

1121fc

WUUU

APPLICATIO S I FOR ATIO

LT1121/LT1121-3.3/LT1121-5

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 battery­powered 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 20V. 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

IN

LT1121

SHDN

GND

V

= 3.75V 1 + + I

OUT

= 3.75V

V

ADJ

= 150nA AT 25°C

I

ADJ

OUTPUT RANGE = 3.75V

R2

()

R1

OUT

R2

ADJ

R1

• R2

()

ADJ

TO

30V

V

OUT

+

1121 • F01

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 (V

/3.75V). For example: load regulation for an

OUT

output current change of 1mA to 150mA is –12mV typical
at V

= 3.75V. At V

OUT

⎛
⎜

⎝

12

375

.

⎞

V

⎟

V

⎠

12 38

•– –

()

= 12V, load regulation would be:

OUT

=

mV mV

()

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: I

• (VIN – V

OUT

OUT

), and

2. Ground pin current multiplied by the input voltage:
I

• VIN.

GND

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 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.

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 gener­ated 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 wereFigure 1. Adjustable Operation

1121fc

9

LT1121/LT1121-3.3/LT1121-5

WUUU

APPLICATIO S I FOR ATIO

taken in still air, on 3/32" FR-4 board with 1oz copper. All
NC leads were connected to the ground plane.

Table 1. N8 Package*

COPPER AREA

TOPSIDE BACKSIDE BOARD AREA

2500 sq mm 2500 sq. mm 2500 sq. mm 80°C/W

1000 sq mm 2500 sq. mm 2500 sq. mm 80°C/W

225 sq mm 2500 sq. mm 2500 sq. mm 85°C/W

1000 sq mm 1000 sq. mm 1000 sq. mm 91°C/W

* Device is mounted on topside. Leads are through hole and are soldered
to both sides of board.

Table 2. S8 Package

COPPER AREA

TOPSIDE* BACKSIDE BOARD AREA

2500 sq. mm 2500 sq. mm 2500 sq. mm 120°C/W

1000 sq. mm 2500 sq. mm 2500 sq. mm 120°C/W

225 sq. mm 2500 sq. mm 2500 sq. mm 125°C/ W

100 sq. mm 1000 sq. mm 1000 sq. mm 131°C/ W

* Device is mounted on topside.

Table 3. AS8 Package*

COPPER AREA

TOPSIDE** BACKSIDE BOARD AREA

2500 sq. mm 2500 sq. mm 2500 sq. mm 60°C/W

1000 sq. mm 2500 sq. mm 2500 sq. mm 60°C/W

225 sq. mm 2500 sq. mm 2500 sq. mm 68° C/W

100 sq. mm 2500 sq. mm 2500 sq. mm 74° C/W

* Pins 3, 6, and 7 are ground.
** Device is mounted on topside.

Table 4. SOT-223 Package
(Thermal Resistance Junction-to-Tab 20°C/W)

COPPER AREA

TOPSIDE* BACKSIDE BOARD AREA

2500 sq. mm 2500 sq. mm 2500 sq. mm 50°C/ W

1000 sq. mm 2500 sq. mm 2500 sq. mm 50°C/ W

225 sq. mm 2500 sq. mm 2500 sq. mm 58° C/W

100 sq. mm 2500 sq. mm 2500 sq. mm 64° C/W

1000 sq. mm 1000 sq. mm 1000 sq. mm 57°C/ W

1000 sq. mm 0 1000 sq. mm 60°C/ W

* Tab of device attached to topside copper

THERMAL RESISTANCE

(JUNCTION-TO-AMBIENT)

THERMAL RESISTANCE

(JUNCTION-TO-AMBIENT)

THERMAL RESISTANCE

(JUNCTION-TO-AMBIENT)

Table 5. TO-92 Package

Package alone 220°C/W

Package soldered into PC board with plated 175°C/W

through holes only

Package soldered into PC board with 1/4 sq. inch of 145°C/ W

copper trace per lead

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 160°C/W

Aavid type 5754 135°C/W

THERMAL

RESISTANCE

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:

where, I

I

OUT MAX

OUT MAX

V

IN MAX

I

GND

= 7V

at (I

• (V

IN MAX

– V

OUT

) + (I

GND

= 100mA

= 100mA, VIN = 7V) = 5mA

OUT

• VIN)

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
ambient 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

= 50°C + 24°C = 74°C

JMAX

Output Capacitance and Transient Performance

The LTC1121 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

10

1121fc

WUUU

APPLICATIO S I FOR ATIO

LT1121/LT1121-3.3/LT1121-5

that the ESR of the capacitor is low (ceramic) the sug­gested series resistor is shown in Table 5. 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.

Table 5.

SUGGESTED SERIES

OUTPUT CAPACITANCE RESISTOR

0.33µF2Ω

0.47µF1Ω

0.68µF1Ω

>1µF None Needed

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 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 opera­tion, 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

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 cir­cuits incorporate some form of power management. The
following information will help optimize battery life. Table
6 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.

100

TJ = 25°C

90

< V

V

IN

OUT

CURRENT FLOWS

80

INTO OUTPUT PIN
TO GROUND

70

60

50

40

30

OUTPUT PIN CURRENT (µA)

20

10

0

2

0

13579

Figure 2. Reverse Output Current

LT1121

= V

(V

OUT

LT1121-3.3

4

OUTPUT VOLTAGE (V)

)

ADJ

LT1121-5

6

8

10

1121• F02

1121fc

11

LT1121/LT1121-3.3/LT1121-5

WUUU

APPLICATIO S I FOR ATIO

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.

Table 6. Fault Conditions

INPUT PIN SHDN PIN OUTPUT PIN

(Nominal) Open (Hi) Forced to V

<V

OUT

(Nominal) Grounded Forced to V

<V

OUT

Open Open (Hi) Forced to V

Open Grounded Forced to V

≤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

(Nominal) Reverse Output Current ≈ 15µA (See Figure 2)

OUT

(Nominal) Reverse Output Current ≈ 15µA (See Figure 2)

OUT

(Nominal) Reverse Output Current ≈ 15µA (See Figure 2)

OUT

(Nominal) Reverse Output Current ≈ 15µA (See Figure 2)

OUT

Input Current ≈ 1µA (See Figure 3)

Input Current ≈ 1µA (See Figure 3)

INPUT CURRENT (µA)

5

V

= 3.3V (LT1121-3.3)

OUT

= 5V (LT1121-5)

V

OUT

4

3

2

1

0

1

0

2

INPUT VOLTAGE (V)

Figure 3. Input Current

3

4

5

1121 F03

12

1121fc

PACKAGE DESCRIPTIO

U

LT1121/LT1121-3.3/LT1121-5

N8 Package

8-Lead PDIP (Narrow 0.300)

(LTC DWG # 05-08-1510)

87 6

.255 ± .015*

(6.477 ± 0.381)

.400*

(10.160)

MAX

5

12

.300 – .325

(7.620 – 8.255)

.065

(1.651)

.008 – .015

(0.203 – 0.381)

+.035

.325

–.015
+0.889

8.255

()

–0.381

NOTE:

1. DIMENSIONS ARE

*THESE DIMENSIONS DO NOT INCLUDE MOLD FLASH OR PROTRUSIONS.
MOLD FLASH OR PROTRUSIONS SHALL NOT EXCEED .010 INCH (0.254mm)

INCHES

MILLIMETERS

TYP

.045 – .065

(1.143 – 1.651)

.100

(2.54)

BSC

S8 Package

8-Lead Plastic Small Outline (Narrow 0.150)

(LTC DWG # 05-08-1610)

.050 BSC

.045 ±.005

.189 – .197

(4.801 – 5.004)

8

3

NOTE 3

7

4

(3.302 ± 0.127)

.018 ± .003

(0.457 ± 0.076)

5

6

.130 ± .005

.120

(3.048)

MIN

.020

(0.508)

MIN

N8 1002

.245
MIN

.030 ±.005

TYP

RECOMMENDED SOLDER PAD LAYOUT

.010 – .020

(0.254 – 0.508)

.008 – .010

(0.203 – 0.254)

.016 – .050

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)

(0.406 – 1.270)

INCHES

(MILLIMETERS)

× 45°

.160

±.005

0°– 8° TYP

.228 – .244

(5.791 – 6.197)

.053 – .069

(1.346 – 1.752)

.014 – .019

(0.355 – 0.483)

TYP

.150 – .157

(3.810 – 3.988)

NOTE 3

1

3

2

4

.050

(1.270)

BSC

.004 – .010

(0.101 – 0.254)

SO8 0303

1121fc

13

LT1121/LT1121-3.3/LT1121-5

U

PACKAGE DESCRIPTIO

ST Package

3-Lead Plastic SOT-223

(LTC DWG # 05-08-1630)

.264 – .287

(6.70 – 7.30)

.130 – .146

(3.30 – 3.71)

.071

(1.80)

MAX

.0905
(2.30)

BSC

.248 – .264

(6.30 – 6.71)

.114 – .124

(2.90 – 3.15)

.024 – .033

(0.60 – 0.84)

.181

(4.60)

BSC

.033 – .041

(0.84 – 1.04)

.012

(0.31)

MIN

.059 MAX

10°

MAX

.129 MAX

.059 MAX

.181 MAX

RECOMMENDED SOLDER PAD LAYOUT

10° – 16°

.0008 – .0040

(0.0203 – 0.1016)

.248 BSC

.039 MAX

.090
BSC

.010 – .014

(0.25 – 0.36)

10° – 16°

ST3 (SOT-233) 0502

14

1121fc

PACKAGE DESCRIPTIO

LT1121/LT1121-3.3/LT1121-5

U

Z Package

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

(LTC DWG # 05-08-1410)

.060 ± .005

(1.524± 0.127)

DIA

.180 ± .005

(4.572 ± 0.127)

.500

(12.70)

MIN

.050

(1.27)

BSC

321

.180 ± .005

(4.572 ± 0.127)

(2.286)

NOM

.050

UNCONTROLLED
LEAD DIMENSION

(1.270)

MAX

.016 ± .003

(0.406 ± 0.076)

.060 ± .010

(1.524 ± 0.254)

.90

.140 ± .010

(3.556 ± 0.127)

5°

NOM

.015 ± .002

(0.381 ± 0.051)

Z3 (TO-92) 0801

.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

10° NOM

Information furnished by Linear Technology Corporation is believed to be accurate and reliable.
However, no responsibility is assumed for its use. Linear Technology Corporation makes no represen­tation that the interconnection of its circuits as described herein will not infringe on existing patent rights.

1121fc

15

LT1121/LT1121-3.3/LT1121-5

RELATED PARTS

PART NUMBER DESCRIPTION COMMENTS

LT1120 125mA Low Dropout Regulator with 20µA I

LT1129 700mA Micropower Low Dropout Regulator 50µA Quiescent Current

LT1175 500mA Negative Low Dropout Micropower Regulator 45µA IQ, 0.26V Dropout Voltage, SOT-223 Package

LT1521 300mA Low Dropout Micropower Regulator with Shutdown 15µA IQ, Reverse Battery Protection

LT1529 3A Low Dropout Regulator with 50µA I

LT1611 Inverting 1.4MHz Switching Regulator 5V to –5V at 150mA, Low Output Noise, SOT-23 Package

LT1613 1.4MHz Single-Cell Micropower DC/DC Converter SOT-23 Package, Internally Compensated

LTC1627 High Efficiency Synchronous Step-Down Switching Regulator Burst ModeTM Operation, Monolithic, 100% Duty Cycle

LT1682 Doubler Charge Pump with Low Noise Linear Regulator Low Output Noise: 60µV

LT1762 Series 150mA, Low Noise, LDO Micropower Regulator 25µA Quiescent Current, 20µV

LT1763 Series 500mA, Low Noise, LDO Micropower Regulator 30µA Quiescent Current, 20µV

LT1764 Series 3A Fast Transient Response LDO 300mV Dropout, 40µV

LT1962 Series 300mA, Low Noise, LDO Micropower Regulator 30µA Quiescent Current, 20µV

LT1963 Series 1.5A Fast Transient Response LDO 300mV Dropout, 40µV

Burst Mode is a trademark of Linear Technology Corporation.

Q

Q

Includes 2.5V Reference and Comparator

500mV Dropout Voltage

(100kHz BW)

RMS

Noise

RMS

Noise

RMS

Noise

RMS

Noise

RMS

Noise

RMS

16

Linear Technology Corporation

1630 McCarthy Blvd., Milpitas, CA 95035-7417

[phone] ● FAX: [phone]

●

www.linear-tech.com

1121fc

LT/LT 0505 REV C • PRINTED IN USA

© LINEAR TECHNOLOGY CORPORATION 1994