LT1579CGN_技术文档

LT1579

300mA Dual Input Smart

Battery Backup Regulator

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DESCRIPTION

FEATURES

■

Maintains Output Regulation with Dual Inputs

The LT^®1579 is a dual input, single output, low dropout

regulator. This device is designed to provide an

uninterruptibleoutputvoltagefromtwoindependentinput

voltage sources on a priority basis. All of the circuitry

needed to switch smoothly and automatically between

inputs is incorporated.

■

Dropout Voltage: 0.4V

■

Output Current: 300mA

■

50µA Quiescent Current

■

No Protection Diodes Needed

■

Two Low-Battery Comparators

■

Status Flags Aid Power Management

The LT1579 can supply 300mA of output current from

eitherinputatadropoutvoltageof0.4V.Quiescentcurrent

is 50µA, dropping to 7µA in shutdown. Two comparators

are included to monitor input voltage status. Two addi-

tional status flags indicate which input is supplying power

and provide an early warning against loss of output

regulation when both inputs are low. A secondary select

pin is provided so that the user can force the device to

switch from the primary input to the secondary input.

Internal protection circuitry includes reverse-battery pro-

tection, current limiting, thermal limiting and reverse-

current protection.

■

Adjustable Output from 1.5V to 20V

■

Fixed Output Voltages: 3V, 3.3V and 5V

■

7µA Quiescent Current in Shutdown

■

Reverse-Battery Protection

Reverse Current Protection

Remove, Recharge and Replace Batteries Without

■

Daisy-Chained Control Outputs

Loss of Regulation

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APPLICATIONS

■

Dual Battery Systems

■

Battery Backup Systems

The device is available in fixed output voltages of 3V, 3.3V

and 5V, and as an adjustable device with a 1.5V reference

voltage. The LT1579 regulators are available in narrow

16-lead SO and 16-lead SSOP packages with all features,

and in SO-8 with limited features.

■

Automatic Power Management for

Battery-Operated Systems

, LTC and LT are registered trademarks of Linear Technology Corporation.

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TYPICAL APPLICATION

5V Dual Battery Supply

Automatic Input Switching

12

5V

IN1

OUT

V

LOAD

= 10V

IN2

300mA

V

I

10

8

IN1 SWITCHOVER

POINT

+

I

= 50mA

2.7M

1M

1µF

4.7µF

80

60

40

20

0

LBI1

I

6

IN1

IN2

LT1579-5

4

SS

2

SHDN

LBO1

IN2

+

TO

0

2.7M

1M

1µF

POWER

5.05

5.00

4.95

LB02

LBI2

MANAGEMENT

BACKUP

DROPOUT

BIASCOMP

0

2

4

6

8

10 12 14 16 18 20

GND

0.01µF

TIME (ms)

1579 TA01

1578 TA02

1

LT1579

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ABSOLUTE MAXIMUM RATINGS

Power Input Pin Voltage ...................................... ±20V*

BIASCOMP Pin Voltage ...............................6.5V, –0.6V

BIASCOMP Pin Current .......................................... 5mA

Logic Flag Output Voltage............................6.5V, –0.6V

Logic Flag Input Current ......................................... 5mA

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

Storage Temperature Range ................. – 65°C to 150°C

Operating Junction Temperature Range .... 0°C to 125°C

Lead Temperature (Soldering, 10 sec).................. 300°C

Output Pin Voltage

Fixed Devices............................................. 6.5V, –6V

Adjustable Device ............................................ ±20V*

Output Pin Reverse Current .................................... 5mA

ADJ Pin Voltage ..............................................2V, –0.6V

ADJ Pin Current...................................................... 5mA

Control Input Pin Voltage ............................6.5V, –0.6V

Control Input Pin Current ....................................... 5mA

*For applications requiring input voltage ratings greater than 20V,

consult factory.

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PACKAGE RDER I FOR ATIO

ORDER PART

TOP VIEW

NUMBER

GND

1

2

3

4

5

6

7

8

16

15

14

13

12

11

10

9

GND

1

2

3

4

5

6

7

8

16

15

14

13

12

11

10

9

GND

LT1579CGN

LT1579CS

LT1579CGN-3

LT1579CGN-3.3

LT1579CGN-5

LT1579CS-3

LT1579CS-3.3

LT1579CS-5

V

OUT

V

OUT

POWER

INPUTS

IN1

POWER

INPUTS

V

ADJ

V

BACKUP

DROPOUT

LBO1

IN2

SS

SHDN

LBI1

BACKUP

LBO1

SS

SHDN

LBI1

LOGIC

OUTPUTS

LOGIC

OUTPUTS

CONTROL

INPUTS

CONTROL

INPUTS

LBO2

LBI2

BIASCOMP

GND

LBI2

BIASCOMP

GND

GN PACKAGE

S PACKAGE

GN PACKAGE

S PACKAGE

GN PART MARKING

1579

16-LEAD PLASTIC SSOP 16-LEAD PLASTIC SO

SEE APPLICATION INFORMATION SECTION

15793

157933

15795

T_JMAX= 125°C, θ_JA= 95°C/W (GN)

T_JMAX= 125°C, θ_JA= 68°C/W (S)

T_JMAX= 125°C, θ_JA= 95°C/W (GN)

T_JMAX= 125°C, θ_JA= 68°C/W (S)

ORDER PART

NUMBER

ORDER PART

NUMBER

TOP VIEW

V

1

2

3

4

8

7

6

5

OUT

V

1

2

3

4

8

7

6

5

OUT

IN1

POWER

INPUTS

POWER

INPUTS

LT1579CS8

LT1579CS8-3

LT1579CS8-3.3

LT1579CS8-5

BACKUP

DROPOUT

BIASCOMP

ADJ

IN2

LOGIC

OUTPUTS

LOGIC

OUTPUT

CONTROL

INPUT

CONTROL

INPUT

SHDN

GND

SHDN

GND

BACKUP

BIASCOMP

S8 PACKAGE

8-LEAD PLASTIC SO

S8 PACKAGE

8-LEAD PLASTIC SO

S8 PART MARKING

1579

SEE APPLICATION INFORMATION SECTION

_JMAX= 125°C, θ_JA= 90°C/W

15793

157933

15795

T_JMAX= 125°C, θ_JA= 90°C/W

T

Consult factory for Industrial and Military grade parts.

2

LT1579

ELECTRICAL CHARACTERISTICS

PARAMETER

CONDITIONS

MIN

TYP

MAX

UNITS

Regulated Output

Voltage (Note 1)

LT1579-3

V

= V = 3.5V, I

IN1

= 1mA, T = 25°C

IN2

2.950 3.000

2.900 3.000

3.050

3.100

V

IN1

IN2

LOAD

J

4V < V < 20V, 4V < V < 20V, 1mA < I < 300mA

LOAD

●

LT1579-3.3 V = V = 3.8V, I

= 1mA, T = 25°C

3.250 3.300

3.200 3.300

3.350

3.400

V

IN1

IN2

IN1

LOAD

J

4.3V < V < 20V, 4.3V < V < 20V, 1mA < I < 300mA

IN2

LOAD

LT1579-5

LT1579

V

= V = 5.5V, I

= 1mA, T = 25°C

4.925 5.000

4.850 5.000

5.075

5.150

V

IN1

IN2

LOAD

J

6V < V < 20V, 6V < V < 20V, 1mA < I < 300mA

IN1

IN2

LOAD

Adjust Pin Voltage

Line Regulation

V

= V = 3.2V, I

= 1mA, T = 25°C (Note 2)

1.475 1.500

1.450 1.500

1.525

1.550

V

IN1

IN2

LOAD

J

3.7V < V < 20V, 3.7V < V < 20V, 1mA < I < 300mA

●

IN1

IN2

LOAD

LT1579-3 ∆V = 3.5V to 20V, ∆V

= 3.5V to 20V, I

= 1mA

1.5

3

10

mV

IN1

IN2

LOAD

LT1579-3.3 ∆V = 3.8V to 20V, ∆V

= 3.8V to 20V, I

= 5.5V to 20V, I

= 3.2V to 20V, I

= 1mA

IN1

LT1579-5 ∆V = 5.5V to 20V, ∆V

= 1mA

IN1

LT1579

∆V = 3.2V to 20V, ∆V

= 1mA (Note 2)

IN1

Load Regulation

LT1579-3

V

= V = 4V, ∆I

= 1mA to 300mA, T = 25°C

= 1mA to 300mA

12

25

mV

IN1

IN2

LOAD

J

= V = 4V, ∆I

●

IN2

LT1579-3.3 V = V = 4.3V, ∆I

= 1mA to 300mA, T = 25°C

= 1mA to 300mA

3

12

25

mV

IN1

IN2

LOAD

J

V

= V = 4.3V, ∆I

IN2

LT1579-5

LT1579

V

= V = 6V, ∆I

= 1mA to 300mA, T = 25°C

= 1mA to 300mA

5

15

35

mV

IN1

IN2

LOAD

J

= V = 6V, ∆I

IN2

V

= V = 3.7V, ∆I

= 1mA to 300mA, T = 25°C (Note 2)

= 1mA to 300mA

2

10

20

mV

IN1

IN2

LOAD

J

= V = 3.7V, ∆I

IN2

Dropout Voltage

(Notes 3, 4)

I

= 10mA, T = 25°C

= 10mA

0.10

0.18

0.25

0.34

50

0.28

0.39

V

LOAD

J

V

= V

OUT(NOMINAL)

=

IN1

IN2

I

= 50mA, T = 25°C

= 50mA

0.35

0.45

V

LOAD

J

I

= 150mA, T = 25°C

= 150mA

0.47

0.60

V

LOAD

J

I

= 300mA, T = 25°C

= 300mA

0.60

0.75

V

LOAD

J

Ground Pin Current

(Note 5)

I

= 0mA, T = 25°C

= 0mA

100

400

µA

LOAD

J

V

= V

OUT(NOMINAL)

=

IN1

IN2

I

= 1mA, T = 25°C

= 1mA

100

200

500

µA

LOAD

J

+ 1V

●

I

= 50mA

= 150mA

= 300mA

0.7

2

1.5

4

mA

LOAD

5.8

12

Standby Current

(Note 6) I = 0mA

I

: V = 20V, V = V

+ 0.5V, V = Open (HI)

IN2 SS

●

3.3

2.0

7.0

µA

VIN2 IN1

IN2

OUT(NOMINAL)

+ 0.5V, V = 20V, V = 0V

SS

: V = V

LOAD

VIN1 IN1

OUT(NOMINAL)

Shutdown Threshold

V

= Off to On

= On to Off

●

0.9

0.75

2.8

V

OUT

0.25

Shutdown Pin Current

(Note 7)

V

= 0V

●

1.3

5

µA

SHDN

Quiescent Current in

Shutdown (Note 9)

I

: V = 20V, V = 6V, V

= 0V

●

5

3

12

µA

VIN1 IN1

IN2

SHDN

: V = 6V, V = 20V, V

VIN2 IN1

IN2

: V = V = 20V, V = 0V

SRC IN1

IN2

SHDN

3

LT1579

ELECTRICAL CHARACTERISTICS

PARAMETER

CONDITIONS

MIN

TYP

MAX

UNITS

Adjust Pin Bias Current

(Notes 2, 7)

T = 25°C

J

6

30

nA

Minimum Input Voltage

(Note 8)

I

= 0mA

●

2.7

3.2

V

LOAD

Minimum Load Current

LT1579

V

= V = 3.2V

3

µA

IN1

IN2

Secondary Select

Threshold

Switch from V to V

●

1.2

0.75

2.8

V

IN2

IN1

IN2

Switch from V to V

0.25

IN1

Secondary Select Pin

Current (Note 7)

V

SS

= 0V

●

1

1.5

µA

Low-Battery Trip Threshold

V

= V = V

+ 1V, High-to-Low Transition

●

1.440 1.500

18

1.550

30

V

IN1

IN2

OUT(NOMINAL)

Low-Battery Comparator

Hysteresis

= V = 6V, I

= 20µA (Note 11)

LBO

mV

IN1

IN2

Low-Battery Comparator

Bias Current (Notes 7, 10)

V

IN1

= V = 6V, V = 1.4V, T = 25°C

2

5

nA

IN2

LBI

J

Logic Flag Output Voltage

I

= 20µA

= 5mA

●

0.17

0.97

0.45

1.3

V

SINK

Ripple Rejection

V

– V

= V – V

= 1.2V (Avg), V

= 0.5V

P-P

55

70

dB

IN1

OUT

IN2

OUT

RIPPLE

f

= 120Hz, I

= 150mA

RIPPLE

LOAD

Current Limit

V

= V = V

+ 1V, ∆V = –0.1V

OUT

●

320

400

mA

IN1

IN2

OUT(NOMINAL)

Input Reverse Leakage

Current

= V = –20V, V

= 0V

1.0

IN1

IN2

OUT

Reverse Output Current

LT1579-3

LT1579-3.3 V

LT1579-5

V

= 3V, V = V = 0V

3

12

µA

OUT

IN1

IN2

= 3.3V, V = V = 0V

IN1

IN2

V

= 5V, V = V = 0V

IN1 IN2

The

temperature range.

Note 1: Operating conditions are limited by maximum junction

●

denotes specifications which apply over the full operating

Note 6: Standby current is the minimum quiescent current for a given

input while the other input supplies the load and bias currents.

Note 7: Current flow is out of the pin.

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, output current must be limited.

When operating at maximum output current, the input voltage range must

be limited.

Note 2: The LT1579 (adjustable version) is tested and specified with the

adjust pin connected to the output pin and a 3µA DC load.

Note 8: Minimum input voltage is the voltage required on either input to

maintain the 1.5V reference for the error amplifier and low-battery

comparators.

Note 9: Total quiescent current in shutdown will be approximately equal to

I

+ I

– I . Both I

and I

are specified for worst-case

VIN1

VIN2

SRC

VIN1

VIN2

conditions. I

is specified under the condition that V > V

and I

VIN1

IN1

IN2 VIN2

is specified under the condition that V > V . I

is drawn from the

IN2

IN1 SRC

Note 3: Dropout voltage is the minimum input-to-output voltage

differential required to maintain regulation at the specified output current.

highest input voltage only. For normal operating conditions, the quiescent

current of the input with the lowest input voltage will be equal to the

In dropout, the output voltage will be equal to V – V

.

specified quiescent current minus I . For example, if V = 20V, V

=

IN

DROPOUT

SRC

IN1

IN2

6V then I

= 5µA and I

= 5µA – 3µA = 2µA.

VIN1

VIN2

Note 4: To meet the requirements for minimum input voltage, the LT1579

(adjustable version) is connected with an external resistor divider for a

3.3V output voltage (see curve of Minimum Input Voltage vs Temperature

in the Typical Performance Characteristics). For this configuration,

Note 10: The specification applies to both inputs independently

(LBI1, LBI2).

Note 11: Low-battery comparator hysteresis will change as a function of

current in the low-battery comparator output. See the curve of Low-Battery

Comparator Hysteresis vs Sink Current in the Typical Performance

Characteristics.

V

= 3.3V.

OUT(NOMINAL)

Note 5: Ground pin current will rise at T > 75°C. This is due to internal

J

circuitry designed to compensate for leakage currents in the output

transistor at high temperatures. This allows quiescent current to be

minimized at lower temperatures, yet maintain output regulation at high

temperatures with light loads. See the curve of Quiescent Current vs

Temperature in the Typical Performance Characteristics.

4

LT1579

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TYPICAL PERFORMANCE CHARACTERISTICS

Guaranteed Dropout Voltage

Dropout Voltage

Quiescent Current

100

90

80

70

60

50

40

30

20

10

0

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

0

0.7

0.6

= TEST POINTS

A: I

B: I

C: I

D: I

= 300mA

= 150mA

= 100mA

= 50mA

= 10mA

= 1mA

V

R

= 6V

LOAD

IN

L

A

=

∞

(FIXED)

T ≤ 125°C

J

= 500k (ADJUSTABLE)

0.5 E: I

F: I

B

C

OPERATING

QUIESCENT

CURRENT

T = 25°C

J

0.4

0.3

0.2

0.1

D

E

STANDBY

QUIESCENT

CURRENT

F

0

100

150

200

250

300

50

TEMPERATURE (°C)

125

50

TEMPERATURE (°C)

100 125

– 50

–25

0

25

75 100

–50 –25

0

25

75

OUTPUT CURRENT (mA)

1579 G35

1579 G02

1579 G01

Quiescent Current in Shutdown

LT1579-3 Output Voltage

LT1579-3.3 Output Voltage

3.38

7

6

3.08

V

= 20V

= 6V

SHDN

I

= 1mA

I

= 1mA

IN1

IN2

LOAD

3.36

3.34

3.06

3.04

= 0V

5

4

3

2

1

3.32

3.30

2.28

2.26

2.24

3.02

3.00

2.98

2.96

2.94

I

VIN1

VIN2

2.92

2.22

0

–25

0

50

75 100 125

–50 –25

0

25

50

75 100 125

50

100 125

–50

25

–50 –25

0

25

75

TEMPERATURE (°C)

1579 G03

1579 G04

1579 G36

Adjust Pin Voltage

Input Current

LT1579-5 Output Voltage

1.54

60

50

5.12

I

IN2

I

= 1mA

I

= 1mA

LOAD

I

IN1

1.53

1.52

5.09

5.06

V

LOAD

= 5V

= 6V

OUT

IN2

40

30

I

= 0

1.51

1.50

1.49

1.48

1.47

5.03

5.00

4.97

4.94

4.91

20

10

0

1.46

4.88

–25

0

50

75 100 125

–0.2 –0.1 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

– V (V)

–25

0

50

75 100 125

–50

25

–50

25

V

IN1

TEMPERATURE (°C)

OUT

1579 G05

1579 G07

1579 G05

5

LT1579

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TYPICAL PERFORMANCE CHARACTERISTICS

Input and Ground Pin Current

1.2

1.0

300

250

12

10

600

500

I

IN1

IN2

I

IN1

IN2

V

= 5V

= 6V

OUT

IN2

V

LOAD

= 5V

= 6V

OUT

IN2

I

= 1mA

LOAD

I

= 10mA

0.8

0.6

200

150

8

6

400

300

I

GND

I

GND

0.4

0.2

0

100

50

0

4

2

0

200

100

0

–0.2 –0.1 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

– V (V)

–0.2 –0.1 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

– V (V)

V

IN1

OUT

IN1

OUT

1579 G08

1579 G09

Input and Ground Pin Current

60

50

1.2

1.0

120

100

3.0

2.5

I

IN1

I

IN1

IN2

V

LOAD

= 5V

= 6V

OUT

IN2

40

30

0.8

0.6

80

60

I

= 100mA 2.0

I

GND

I

V

LOAD

= 5V

= 6V

= 50mA

GND

OUT

IN2

1.5

I

20

10

0

0.4

0.2

0

40

20

0

1.0

0.5

0

–0.2 –0.1 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

– V (V)

–0.2 –0.1 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

– V (V)

V

IN1

OUT

1579 G10

1579 G11

Input and Ground Pin Current

160

140

120

100

80

4.0

3.5

3.0

2.5

2.0

1.5

1.0

0.5

0

350

300

250

200

150

100

50

14

12

10

8

I

IN1

IN2

I

IN2

IN1

I

GND

I

GND

V

= 5V

= 6V

OUT

IN2

V

6

I

= 150mA

60

LOAD

V

LOAD

= 5V

= 6V

OUT

IN2

4

40

I

= 300mA

2

20

0

–0.2 –0.1 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

V

– V

(V)

OUT

V

IN1

– V

OUT

(V)

IN1

1579 G12

1579 G13

6

LT1579

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TYPICAL PERFORMANCE CHARACTERISTICS

Ground Pin Current

Minimum Input Voltage

Shutdown Pin Threshold

1.0

0.8

0.6

0.4

0.2

0

3.0

2.9

2.8

2.7

2.6

2.5

2.4

2.3

2.2

2.1

2.0

8

7

6

5

V

= V = V

+ 1V

I

= 1mA

IN1

IN2

OUT(NOMINAL)

LOAD

4

3

2

1

0

50

100

200

–50 –25

0

25

50

75

125

0

250

300

–50

0

25

50

75

125

100

150

–25

100

TEMPERATURE (°C)

OUTPUT CURRENT (mA)

1579 G37

1579 G15

1579 G14

Secondary Select Threshold

(Switch to V_IN1

Shutdown Pin Current

(Switch to V_IN2

)

2.5

2.0

1.8

1.6

1.4

1.2

1.0

0.8

0.6

0.4

0.2

0

2.0

1.8

1.6

1.4

1.2

1.0

0.8

0.6

0.4

0.2

0

I

= 1mA

V

SHDN

= 0V

LOAD

I

= 300mA

LOAD

2.0

1.5

1.0

0.5

0

I

= 1mA

LOAD

–50 –25

0

25

50

75 100 125

50

–50

0

25

75 100 125

–50

0

25

75 100 125

–25

TEMPERATURE (°C)

1579 G16

1579 G17

1579 G18

Logic Flag Output Voltage

(Output Low)

Logic Flag Output Voltage

(Output Low)

Secondary Select Pin Current

1.0

0.9

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

0

1.0

0.9

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

0

1.2

V

SS

= 0V

1.0

0.8

0.6

I

= 5mA

SINK

0.4

0.2

0

I

= 20µA

SINK

–50

0

25

50

75 100 125

1µA

10µA

100µA

1mA

10mA

50

100 125

–25

–50 –25

0

25

75

TEMPERATURE (°C)

LOGIC FLAG SINK CURRENT

TEMPERATURE (°C)

1579 G20

1579 G19

1579 G21

7

LT1579

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TYPICAL PERFORMANCE CHARACTERISTICS

Low-Battery Comparator

Hysteresis

Logic Flag Input Current

Control Pin Input Current

(Output High)

20

15

10

5

25

20

25

20

15

10

5

10

5

0

10

20

30

40

50

0

1

2

3

4

5

6

7

8

9

7

0

1

2

3

4

5

6

8

9

LOGIC FLAG VOLTAGE (V)

I

LBO

SINK CURRENT (µA)

CONTROL PIN VOLTAGE (V)

1579 G24

1579 G22

1579 G23

Low-Battery Comparator

Hysteresis

Reverse Output Current

25

20

15

10

5

20

18

16

14

12

10

8

25

20

I

= 50µA

T = 25°C

J

V

= V = 0V

IN2

LBO(SINK)

IN1

V

= V = 0V

IN2

= 3V (LT1579-3)

= 3.3V (LT1579-3.3)

= 5V (LT1579-5)

IN1

OUT

CURRENT FLOWS INTO

OUTPUT PIN

15

10

5

LT1579-3.3

LT1579-3

6

4

2

LT1579-5

0

–50 –25

0

25

50

75 100 125

–50

0

25

50

75 100 125

–25

0

1

2

3

4

5

6

7

8

9

TEMPERATURE (°C)

OUTPUT VOLTAGE (V)

1579 G25

1579 G27

1579 G26

Current Limit

Adjust Pin Input Current

0.6

0.5

0.4

0.3

1.0

0.9

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

0

0.7

0.6

V

= 0V

V

= V = V

IN2

OUT

+ 1V

OUT(NOMINAL)

OUT

T = 25°C

IN1

J

IN1

∆V

= –0.1V

V

= V = 0V

IN2

TYPICAL

0.5

0.4

0.3

0.2

0.1

GUARANTEED

0.2

0.1

0

4

6

7

0

1

2

3

5

50

0

TEMPERATURE (°C)

100 125

0

0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0

ADJUST PIN VOLTAGE (V)

1579 G28

–50 –25

25

75

INPUT VOLTAGE (V)

1579 G29

1579 G38

8

LT1579

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TYPICAL PERFORMANCE CHARACTERISTICS

LT1579-5 “Hot” Plugging and

Unplugging Transient Response

Ripple Rejection

Load Regulation

100

90

0

∆I

= 1mA TO 300mA

LT1579-3

I

= 150mA

LOAD

LT1579

LOAD

6V

5V

V

= 6V + 50mV

RIPPLE

RMS

IN

–2

–4

V_IN1

80

LT1579-5

70

C

= 47µF

OUT

SOLID

TANTALUM

–6

–8

60

50

V_OUT

50mV/DIV

LT1579-3.3

40

30

20

10

0

–10

–12

–14

C

= 4.7µF

OUT

SOLID

TANTALUM

UNPLUG

V_IN1

REPLACE

V_IN1

–16

10

100

1k

10k

100k

1M

–50 –25

0

25

50

75 100 125

FREQUENCY (Hz)

TEMPERATURE (°C)

1579 G30

1579 G40

LT1579-5 Transient Response

100

50

100

50

0

V

C

= 6V

V

C

= 6V

–50

–100

100

75

–50

–100

IN

= 1µF CERAMIC

= 4.7µF TANTALUM

= 22µF TANTALUM

OUT

300

200

100

50

25

0

50 100 150 200 250 300 350 400 450 500

0

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1,0

TIME (µs)

TIME (ms)

1579 G33

1579 G34

U

PIN FUNCTIONS

V_IN1: The primary power source is connected to V_IN1. A

bypass capacitor is required on this pin if the device is

more than six inches away from the main input filter

capacitor. In general, the output impedance of a battery

riseswithfrequency,soitisadvisabletoincludeabypass

capacitorinbattery-poweredcircuits.Abypasscapacitor

in the range of 1µF to 10µF is sufficient.

riseswithfrequency,soitisadvisabletoincludeabypass

capacitorinbattery-poweredcircuits.Abypasscapacitor

in the range of 1µF to 10µF is sufficient.

OUT: The output supplies power to the load. A minimum

output capacitor of 4.7µF is required to prevent oscilla-

tions. Larger output capacitors will be required for appli-

cations with large transient loads to limit peak voltage

transients.

V_IN2: The secondary power source is connected to V_IN2

.

A bypass capacitor is required on this pin if the device is

more than six inches away from the main input filter

capacitor. In general, the output impedance of a battery

ADJ: For the adjustable LT1579, this is the input to the

error amplifier. This pin is internally clamped to 7V and

–0.6V (one V_BE). It has a bias current of 6nA which flows

9

LT1579

U

PIN FUNCTIONS

out of the pin (see curve of Adjust Pin Bias Current vs

TemperatureintheTypicalPerformanceCharacteristics).

A DC load of 3µA is needed on the output of the adjustable

part to maintain regulation. The adjust pin voltage is 1.5V

referenced to ground and the output voltage range is 1.5V

to 20V.

nally clamped to 7V and –0.6V (one V_BE). If unused, this

pincanbeleftopencircuit.Deviceoperationisunaffected

if this pin is not connected.

DROPOUT: The dropout flag is an open collector output

which pulls low when both input voltages drop suffi-

ciently for the LT1579 to enter the dropout region. This

signals that the output is beginning to go unregulated.

The DROPOUT output voltage is 1V when sinking 5mA,

dropping to under 200mV at 20µA (see curve of Logic

Flag Voltage vs Current in the Typical Performance Char-

SHDN: The shutdown pin is used to put the LT1579 into

a low power shutdown state. All functions are disabled if

the shutdown pin is pulled low. The output will be off, all

logic outputs will be high impedance and the voltage

comparators will be off when the shutdown pin is pulled acteristics). This makes the DROPOUT pin equally useful

low. The shutdown pin is internally clamped to 7V and

– 0.6V (one V_BE), allowing the shutdown pin to be driven

either by 5V logic or open collector logic with a pull-up

in driving both bipolar and CMOS logic inputs with the

addition of an external pull-up resistor. It is also capable

of driving higher current devices, such as LEDs. This pin

resistor. The pull-up resistor is only required to supply is internally clamped to 7V and –0.6V (one V_BE). If

the pull-up current of the open collector gate, normally unused, this pin can be left open circuit. Device operation

several microamperes. If unused, the shutdown pin can

be left open circuit. The device is active if the shutdown

pin is not connected.

is unaffected if this pin is not connected.

BIASCOMP: This is a compensation point for the internal

bias circuitry. It must be bypassed with a 0.01µF capaci-

tor for stability during the switch from V_IN1to V_IN2.

SS:ThesecondaryselectpinforcestheLT1579toswitch

power draw to the secondary input (V_IN2). This pin is

active low. The current drawn out of V_IN1is reduced to

3µA when this pin is pulled low. The secondary select pin

isinternallyclampedto7Vand–0.6V(oneV_BE), allowing

the pin to be driven directly by either 5V logic or open

collectorlogicwithapull-upresistor.Thepull-upresistor

is required only to supply the leakage current of the open

collector gate, normally several microamperes. If sec-

ondary select is not used, it can be left open circuit. The

LT1579drawspowerfromtheprimaryfirstifthesecond-

ary select pin is not connected.

LBI1: This is the noninvering input to low-battery com-

parator LB1 which is used to detect a low input/battery

condition. The inverting input is connected to a 1.5V

reference.Thelow-batterycomparatorinputhas18mVof

hysteresis with more than 20µA of sink current on the

output (see Applications Information section). This pin is

internally clamped to 7V and –0.6 (one V_BE). If not used,

this pin can be left open circuit, with no effect on normal

circuit operation. If unconnected, the pin will float to 1.5V

and the logic output of LB1 will be high impedance.

LBI2: This is the noninverting input to low-battery com-

parator LB2 which is used to detect a low input/battery

condition. The inverting input is connected to a 1.5V

reference.Thelow-batterycomparatorinputhas18mVof

hysteresis with more than 20µA of sink current on the

output (see Applications Information section). This pin is

internallyclampedto7Vand–0.6V(oneV_BE).Ifnotused,

this pin can be left open circuit, with no effect on normal

circuit operation. If unconnected, the pin will float to 1.5V

and the logic output of LB2 will be high impedance.

BACKUP: The backup flag is an open collector output

which pulls low when the LT1579 starts drawing power

from the secondary input (V_IN2). The BACKUP output

voltage is 1V when sinking 5mA, dropping to under

200mV at 20µA (see curve of Logic Flag Voltage vs

CurrentintheTypicalPerformanceCharacteristics). This

makes the BACKUP pin equally useful in driving both

bipolar and CMOS logic inputs with the addition of an

external pull-up resistor. It is also capable of driving

higher current devices, such as LEDs. This pin is inter-

10

LT1579

U

PIN FUNCTIONS

LBO1:Thisistheopencollectoroutputofthelow-battery

LBO2: Thisistheopencollectoroutputofthelow-battery

comparator LB1. This output pulls low when the com- comparator LB2. This output pulls low when the com-

parator input drops below the threshold voltage. The parator input drops below the threshold voltage. The

LBO1 output voltage is 1V when sinking 5mA, dropping

to under 200mV at 20µA (see curve of Logic Flag Voltage

LBO2 output voltage is 1V when sinking 5mA, dropping

to under 200mV at 20µA (see curve of Logic Flag Voltage

vs Current in the Typical Performance Characteristics). vs Current in the Typical Performance Characteristics).

This makes the LBO1 pin equally useful in driving both This makes the LBO2 pin equally useful in driving both

bipolar and CMOS logic inputs with the addition of an bipolar and CMOS logic inputs with the addition of an

external pull-up resistor. It is also capable of driving external pull-up resistor. It is also capable of driving

higher current devices, such as LEDs. This pin is inter-

nally clamped to 7V and – 0.6V (one V_BE). If unused, this

pincanbeleftopencircuit.Deviceoperationisunaffected

if this pin is not connected.

higher current devices, such as LEDs. This pin is

internally clamped to 7V and –0.6V (one V_BE). If unused,

this pin can be left open circuit. Device operation is

unaffected if this pin is not connected.

W

BLOCK DIAGRAM

V

IN1

IN2

V

OUT

DROPOUT

DETECT

BIAS CURRENT

CONTROL

INTERNAL

SHDN

RESISTOR DIVIDER

FOR FIXED VOLTAGE

DEVICES ONLY

SS

ADJ

–

+

BIASCOMP

OUTPUT DRIVER

CONTROL

E/A

1.5V REFERENCE

BACKUP

DROPOUT

WARNING

FLAGS

LBI1

LBI2

+

–

LBO1

LB1

+

LBO2

LB2

–

1579 • BD

11

LT1579

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APPLICATIONS INFORMATION

Device Overview

currents and logic functions. In shutdown, quiescent

currents are 2µA from the primary input, 2µA from the

secondary input and an additional 3µA which is drawn

from the higher of the two input voltages.

The LT1579 is a dual input, single output, low dropout

linear regulator. The device is designed to provide an

uninterruptibleoutputvoltagefromtwoindependentinput

voltage sources on a priority basis. All of the circuitry

needed to switch smoothly and automatically between

inputs is incorporated in the device. All power supplied to

the load is drawn from the primary input (V_IN1) until the

device senses that the primary input is failing. At this point

the LT1579 smoothly switches from the primary input to

the secondary input (V_IN2) to maintain output regulation.

The device is capable of providing 300mA from either

input at a dropout voltage of 0.4V. Total quiescent current

when operating from the primary input is 50µA, which is

45µA from the primary input, 2µA from the secondary and

a minimum input current of 3µA which will be drawn from

the higher of the two input voltages.

Adjustable Operation

The adjustable version of the LT1579 has an output

voltage range of 1.5V to 20V. The output voltage is set by

theratiooftwoexternalresistorsasshowninFigure1.The

device servos the output to maintain the voltage at the

adjust pin at 1.5V. The current in R1 is then equal to 1.5V/

R1 and the current in R2 is the current in R1 minus the

adjust pin bias current. The adjust pin bias current, 6nA at

25°C,flowsoutoftheadjustpinthroughR1toground.The

output voltage can now be calculated using the formula:

R2

R1

V

= 1.5V 1+

– I

(

R2

)( )

OUT

ADJ

A single error amplifier controls both output stages so

regulation remains tight regardless of which input is

providing power. Threshold levels for the error amplifier

and low-battery detectors are set by the internal 1.5V

reference. Output voltage is set by an internal resistor

divider for fixed voltage parts and an external divider for

adjustable parts. Internal bias circuitry powers the refer-

ence, error amplifier, output driver controls, logic flags

and low-battery comparators.

The value of R1 should be less than 500k to minimize the

error in the output voltage caused by adjust pin bias

current. With 500k resistors for both R1 and R2, the error

induced by adjust pin bias current at 25°C is 3mV or 0.1%

of the total output voltage. With appropriate value and

tolerance resistors, the error due to adjust pin bias current

may often be ignored. Note that in shutdown, the output is

turned off and the divider current is zero. The parallel

combination of R1 and R2 should be greater than 20k to

allow the error amplifier to start. In applications where the

minimum parallel resistance requirement cannot be met,

a 20k resistor may be placed in series with the adjust pin.

This introduces an error in the reference point for the

resistor divider equal to (I_ADJ)(20k).

The LT1579 aids power management with the use of two

independentlow-batterycomparatorsandtwostatusflags.

The low-battery comparators can be used to monitor the

input voltage levels. The BACKUP flag signals when any

power is being drawn from the secondary input and the

DROPOUT flag provides indication that both input volt-

ages are critically low and the output is unregulated.

Additionally, the switch to the secondary input from the

primary can be forced externally through the use of the

secondary select pin (SS). This active low logic pin, when

pulled below the threshold, will cause power draw to

switch from the primary input to the secondary input.

Current flowing in the primary input is reduced to only a

fewmicroamperes, while allpower draw(loadcurrentand

bias currents) switches to the secondary. The LT1579 has

a low power shutdown state which shuts off all bias

OUT

ADJ

V

OUT

+

C

R2

R1

C

FB

OUT

GND

1579 • F01

Figure 1. Adjustable Operation

12

LT1579

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APPLICATIONS INFORMATION

BIASCOMP Pin Compensation

A small capacitor placed in parallel with the top resistor

(R2) of the output divider is necessary for stability and

transient performance of the adjustable LT1579. The

impedance of C_FBat 10kHz should be less than the value

of R1.

The BIASCOMP pin is a connection to a compensation

point for the internal bias circuitry. It must be bypassed

witha0.01µFcapacitorforstabilityduringtheswitchfrom

V

_IN1to V_IN2.

The adjustable LT1579 is tested and specified with the

output pin tied to the adjust pin and a 3µA load (unless

otherwise noted) for an output voltage of 1.5V. Specifica-

tions for output voltages greater than 1.5V are propor-

tional to the ratio of the desired output voltage to 1.5V;

(V_OUT/1.5V). For example, load regulation for an output

current change of 1mA to 300mA is –2mV typical at

V_OUT= 1.5V. At V_OUT= 12V, load regulation is:

“Hot” Plugging and Unplugging of Inputs

The LT1579 is designed to maintain regulation even if one

of the outputs is instantaneously removed. If the primary

input is supplying load current, removal and insertion of

the secondary input creates no noticeable transient at the

output. In this case, the LT1579 continues to supply

current from the primary; no switching is required. How-

ever, whenloadcurrentisbeingsuppliedfromtheprimary

input and it is removed, load current must be switched

from the primary to the secondary input. In this case, the

LT1579 sees the input capacitor as a rapidly discharging

battery. If it discharges too quickly, the LT1579 does not

have ample time to switch over without a large transient

occurring at the output. The input capacitor must be large

enough to supply load current during the transition from

primary to secondary input. Replacement of the primary

creates a smaller transient on the output because both

inputsarepresentduringthetransition. Fora100mAload,

input and output capacitors of 10µF will limit peak output

deviations to less than 50mV. See the “Hot” Plugging and

Unplugging Transient Response in the Typical Perfor-

mance Characteristics. Proportionally larger values for

input and output capacitors are needed to limit peak

deviations on the output when delivering larger load

currents.

12V

1.5V

–2mV = –16mV

(

)

Output Capacitance and Transient Response

The LT1579 is designed to be stable with a wide range of

output capacitors. The ESR of the output capacitor affects

stability, most notably with small capacitors. A minimum

output capacitor of 4.7µF with an ESR of 3Ω or less is

recommended to prevent oscillations. Smaller value ca-

pacitors may be used, but capacitors which have a low

ESR(i.e.ceramics)mayneedasmallseriesresistoradded

to bring the ESR into the range suggested in Table 1. The

LT1579 is a micropower device and output transient

response is a function of output capacitance. Larger

values of output capacitance decrease the peak deviations

andprovideimprovedoutputtransientresponseforlarger

loadcurrentchanges.Bypasscapacitors,usedtodecouple

individual components powered by the LT1579, will

increase the effective output capacitor value.

Standby Mode

“Standby” mode is where one input draws a minimum

quiescent current when the other input is delivering all

bias and load currents . In this mode, the standby current

isthequiescentcurrentdrawnfromthestandbyinput. The

secondary input will be in standby mode, when the pri-

mary input is delivering all load and bias currents. When

the secondary input is in standby mode the current drawn

fromthesecondaryinputwillbe3µAifV_IN1>V_IN2and5µA

Table 1. Suggested ESR Range

OUTPUT CAPACITANCE

SUGGESTED ESR RANGE

1Ω to 3Ω

1.5µF

2.2µF

0.5Ω to 3Ω

3.3µF

0.2Ω to 3Ω

≥4.7µF

0Ω to 3Ω

13

LT1579

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APPLICATIONS INFORMATION

if V_IN2>V_IN1, so typically only 3µA. The primary input will

automatically go into standby mode as the primary input

dropsbelowtheoutputvoltage.Theprimaryinputcanalso

be forced into standby mode by asserting the SS pin. In

either case, the current drawn from the primary input is

reduced to a maximum of 7µA.

Low-Battery Comparators

Therearetwoindependentlow-batterycomparatorsinthe

LT1579. This allows for individual monitoring of each

input. The inverting inputs of both comparators are con-

nected to an internal 1.5V reference. The low-battery

comparator trip point is set by an external resistor divider

as shown in Figure 3. The current in R1 at the trip point is

1.5V/R1. The current in R2 is equal to the current in R1.

The low-battery comparator input bias current, 2nA flow-

ing out of the pin, is negligible and may be ignored. The

value of R1 should be less than 1.5M in order to minimize

errorsinthetrippoint. ThevalueofR2foragiventrippoint

is calculated using the formulas in Figure 3.

Shutdown

The LT1579 has a low power shutdown state where all

functions of the device are shut off. The device is put into

shutdown mode when the shutdown pin is pulled below

0.7V. The quiescent current in shutdown has three com-

ponents:2µAdrawnfromtheprimary,2µAdrawnfromthe

secondary and 3µA which is drawn from the higher of the

two inputs.

The low-battery comparators have a small amount of

hysteresis built-in. The amount of hysteresis is dependent

upon the output sink current (I_SINK) when the comparator

is tripped low. At no load, comparator hysteresis is zero,

increasing to a maximum of 18mV for sink currents above

20µA. See the curve of Low-Battery Comparator Hyster-

esis in the Typical Performance Characteristics. If larger

amounts of hysteresis are desired, R3 and D1 can be

added. D1 can be any small diode, typically a 1N4148.

CalculatingV_LBOcanbedoneusingaloadlineonthecurve

of Logic Flag Output Voltage vs Sink Current in the Typical

Performance Characteristics.

Protecting Batteries Using Secondary Select (SS)

Some batteries, such as lithium-ion cells, are sensitive to

deep discharge conditions. Discharging these batteries

belowacertainthresholdseverelyshortensbatterylife. To

prevent deep discharge of the primary cells, the LT1579

secondary select (SS) pin can be used to switch power

draw from the primary input to the secondary. When this

pin is pulled low, current out of the primary is reduced to

2µA. A low-battery detector with the trip point set at the

critical discharge point can signal the low battery condi-

tion and force the switchover to the secondary as shown

in Figure 2. The second low-battery comparator can be

usedtosetalatchtoshutdowntheLT1579(seetheTypical

Applications).

V

TRIP

V

OUT

R2

R4

LBI

+

–

LBO

R1

1.5V

I

SINK

V

CC

R

P

D1

R3

LTC1579 • F02

LBO

SS

R1

1.5V

R2 = (V

– 1.5V)

TRIP

(

)

R2

GND

HYSTERESIS = V

1 +

HYST

(

)

R1

FOR ADDED HYSTERESIS

1579 F02

(1.5V + V

– 0.6V – V )(R2)

LBO

HYST

V

R3 =

HYST(ADDED)

≥ 20µA, V = 18mV,

FOR I

Figure 2. Connecting SS to Low-Battery Detector

Output to Prevent Damage to Batteries

SINK

HYST

< 20µA, SEE THE TYPICAL

SINK

PERFORMANCE CHARACTERISTICS

Figure 3. Low-Battery Comparator Operation

14

LT1579

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APPLICATIONS INFORMATION

Example: The low-battery detector must be tripped at a

terminal voltage of 5.5V. There is a 100k pull-up resistor

to 5V on the output of the comparator and 200mV of

hysteresis is needed to prevent chatter. With a 1M resistor

for R1, what other resistor values are needed?

current supplied to the load from each input. Normal

output deviation during transient load conditions (with

sufficient input voltages) will not set the status flags.

Timing Diagram

Thetimingdiagramforthe5Vdualbatterysupplyisshown

in Figure 4. The schematic is the same as the 5V Dual

Battery Supply on the front of the data sheet. All logic flag

outputshave100kpull-upresistorsadded. Notethatthere

isnotimescaleforthetimingdiagram.Thetimingdiagram

is meant as a tool to help in understanding basic operation

of the LT1579. Actual discharge rates will be a function of

the load current and the type of batteries used. The load

current used in the example was 100mA DC.

Using the formulas in Figure 3,

5.5V – 1.5V 1MΩ

(

)(

)

R2 =

= 2.67MΩ

1.5V

Use a standard value of 2.7MΩ. With the 100k pull-up

resistor, this gives a sink current and logic flag voltage of

approximately 45µA at 0.4V. The hysteresis in this case

will be:

A

B

C

D

E

2.7MΩ

1MΩ

6V

Hysteresis = 18mV 1+

= 67mV

V

IN1

An additional 133mV of hysteresis is needed, so a resistor

and diode must be added. The value of R3 will be:

5V

6V

1.5V + 18mV – 0.6V – 0.4V 2.7MΩ

(

)(

)

V

IN2

R3 =

= 10.5MΩ

133mV

5V

A standard value of 10MΩ can be used. The additional

current flowing through R3 into the comparator output is

negligible and can usually be ignored

5V

4.8V

V

OUT

100mA

Logic Flags

I

IN1

The low-battery comparator outputs and the status flags

of the LT1579 are open collector outputs capable of

sinking up to 5mA. See the curve of Logic Flag Output

Voltage vs. Current in the Typical Performance Character-

istics.

0

100mA

I

IN2

0

1

There are two status flags on the LT1579. The BACKUP

flag and the DROPOUT flag provide information on which

inputissupplyingpowertotheloadandgiveearlywarning

of loss of output regulation. The BACKUP flag goes low

when the secondary input begins supplying power to the

load. The DROPOUT flag signals the dropout condition on

both inputs, warning of an impending drop in output

voltage. The conditions that set either status flag are

determined by input to output voltage differentials and

LB01

0

1

BACKUP

LB02

0

1

0

1

DROPOUT

0

LTC1579 • F03

Figure 4. Basic Dual Battery Timing Diagram

15

LT1579

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APPLICATIONS INFORMATION

Five milestones are noted on the timing diagram. Time A

iswheretheprimaryinputvoltagedropsenoughtotripthe

low-battery detector LB1. The trip threshold for LB1 is set

at set at 5.5V, slightly above the dropout voltage of the

primary input. At time B, the BACKUP flag goes low,

signaling the beginning of the transition from the primary

source to the secondary source. Between times B and C,

the input current makes a smooth transition from V_IN1to

1. The output current from each input multiplied by the

respective input to output voltage differential:

(I_OUT)(V_IN– V_OUT) and

2. Ground pin current from the associated inputs multi-

plied by the respective input voltage: (I_GND)(V_IN).

If the primary input is not in dropout, all significant power

dissipationisfromtheprimaryinput.Conversely,ifSShas

been asserted to minimize power draw from the primary,

all significant power dissipation will be from the second-

ary. When the primary input enters dropout, calculation of

power dissipation requires consideration of power dissi-

pation from both inputs. Worst-case power dissipation is

foundusingtheworst-caseinputvoltagefromeitherinput

and the worst-case load current.

V

_IN2. BytimeC, theprimarybatteryhasdroppedbelowthe

point where it can deliver useful current to the output. The

primary input will still deliver a small amount of current to

the load, diminishing as the primary input voltage drops.

By time D, the secondary battery has dropped to a low

enough voltage to trip the second low-battery detector,

LB2. The trip threshold for LB2 is also set at 5.5V, slightly

abovewherethesecondaryinputreachesdropout. Attime

E, bothinputsarelowenoughtocausetheLT1579toenter

dropout, with the DROPOUT flag signaling the impending

loss of output regulation. After time E, the output voltage

drops out of regulation.

Ground pin current is found by examining the Ground Pin

Current curves in the Typical Performance Characteris-

tics. Power dissipation will be equal to the sum of the two

components above for the input supplying power to the

load. Power dissipation from the other input is negligible.

Some interesting things can be noted on the timing

diagram. The amount of current available from a given

input is determined by the input/output voltage differen-

tial. As the differential voltage drops, the amount of

current drawn from the input also drops, which slows the

discharge of the battery. Dropout detection circuitry will

maintainthemaximumcurrentdrawfromtheinputforthe

given input/output voltage differential. In the case shown,

this causes the current drawn from the primary input to

approach zero, though never actually dropping to zero.

Note that the primary begins to supply significant current

againwhentheoutputdropsoutofregulation. Thisoccurs

because the input/output voltage differential of the pri-

mary input increases as the output voltage drops. The

LT1579 will automatically maximize the power drawn

from the inputs to maintain the highest possible output

voltage.

The LT1579 has internal thermal limiting designed to

protect the device during overload conditions. For con-

tinuous 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 nearby must also be considered.

Heating sinking for the device is accomplished by using

the heat spreading capabilities of the PC board and its

coppertraces.Copperboardstiffenersandplatedthrough-

holes can also be used to spread the heat. All ground pins

on the LT1579 are fused to the die paddle for improved

heat spreading capabilities.

The following tables list thermal resistances for each pack-

age. Measured values of thermal resistance for several

different board sizes and copper areas are listed for each

package. All measurements were taken in still air on 3/32”

FR-4 board with one ounce copper. All ground leads were

connected to the ground plane. All packages for the

LT1579 have all ground leads fused to the die attach

paddle to lower thermal resistance. Typical thermal

Thermal Considerations

The power handling capability of the LT1579 is limited by

the maximum rated junction temperature (125°C). Power

dissipated is made up of two components:

16

LT1579

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APPLICATIONS INFORMATION

resistancefromthejunctiontoagroundleadis40°C/Wfor

16-lead SSOP, 32°C/W for 16-lead SO and 35°C/W for

8-lead S0.

Therefore,

P = (150mA)(7V – 5V) + (2mA)(7V) = 0.31W

When switched to the secondary input, current from the

primary input is negligible and worst-case power dissipa-

tion will be:

Table 2. 8-Lead SO Package (S8)

COPPER AREA

THERMAL RESISTANCE

BOARD AREA (JUNCTION-TO-AMBIENT)

TOPSIDE*

BACKSIDE

(I_OUT(MAX))(V_IN2(MAX)– V_OUT) + (I_GND)(V_IN2(MAX)

Where:

_OUT(MAX)= 150mA

V_IN2(MAX)= 10V

_GNDat (I_OUT= 150mA, V_IN2= 10V) = 2mA

Therefore,

P = (150mA)(10V – 5V) + (2mA)(10V) = 0.77W

)

2500 sq mm 2500 sq mm 2500 sq mm

1000 sq mm 2500 sq mm 2500 sq mm

225 sq mm 2500 sq mm 2500 sq mm

73°C/W

75°C/W

80°C/W

90°C/W

I

100 sq mm 2500 sq mm 2500 sq mm

I

*Device is mounted on topside.

Table 3. 16-Lead SO Package (S)

COPPER AREA

THERMAL RESISTANCE

TOPSIDE*

BACKSIDE

BOARD AREA (JUNCTION-TO-AMBIENT)

Usinga16-leadSOpackage, thethermalresistancewillbe

in the range of 55°C/W to 68°C/W dependent upon the

copper area. So the junction temperature rise above

ambient will be approximately equal to:

2500 sq mm 2500 sq mm 2500 sq mm

1000 sq mm 2500 sq mm 2500 sq mm

225 sq mm 2500 sq mm 2500 sq mm

55°C/W

58°C/W

60°C/W

68°C/W

100 sq mm 2500 sq mm 2500 sq mm

(0.77W)(65°C/W) = 50.1°C

*Device is mounted on topside.

Table 4. 16-Lead SSOP Package (GN)

COPPER AREA

The maximum junction temperature will then be equal to

the maximum temperature rise above ambient plus the

maximum ambient temperature or:

THERMAL RESISTANCE

TOPSIDE*

BACKSIDE

BOARD AREA (JUNCTION-TO-AMBIENT)

2500 sq mm 2500 sq mm 2500 sq mm

1000 sq mm 2500 sq mm 2500 sq mm

225 sq mm 2500 sq mm 2500 sq mm

70°C/W

75°C/W

80°C/W

95°C/W

T_JMAX= 50.1°C + 50°C = 100.1°C

Protection Features

100 sq mm 2500 sq mm 2500 sq mm

The LT1579 incorporates several protection features that

make it ideal for use in battery-powered circuits. In addi-

tion to the normal protection features associated with

monolithic regulators, such as current limiting and ther-

mal limiting, the device is protected against reverse input

voltages, reverse output voltages and reverse voltages

from output to input.

*Device is mounted on topside.

Calculating Junction Temperature

Example: Given an output voltage of 5V, an input voltage

range of 5V to 7V for V_IN1and 8V to 10V for V_IN2, with an

output current range of 10mA to 150mA and a maximum

ambient temperature of 50°C, what will the maximum

junction temperature be?

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.

Current limit protection is designed to protect the device

if the output is shorted to ground. With the output shorted

to ground, current will be drawn from the primary input

until it is discharged. The current drawn from V_IN2will not

increase until the primary input is discharged. This pre-

vents a short-circuit on the output from discharging both

inputs simultaneously.

When run from the primary input, current drawn from the

secondary input is negligible and worst-case power dissi-

pation will be:

(I_OUT(MAX))(V_IN1(MAX)– V_OUT) + (I_GND)(V_IN1(MAX)

Where:

)

I_OUT(MAX)= 150mA

V_IN1(MAX)= 7V

I_GNDat (I_OUT= 150mA, V_IN1= 7V) = 2mA

17

LT1579

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APPLICATIONS INFORMATION

flow from one input to another will be limited to less than

1mA. Output voltage will be unaffected. In the case of

reverse inputs, no reverse voltages will appear at the load.

Theinputsofthedevicecanwithstandreversevoltagesup

to 20V. 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 which can be plugged in backwards. Internal

protection circuitry isolates the inputs to prevent current

flow from one input to the other. Even with one input

supplying all bias currents and the other being plugged in

backwards (a maximum total differential of 40V), current

Pulling the SS pin low will cause all load currents to come

from the secondary input. If the secondary input is not

present, the output will be turned off. If the part is put into

current limit with the SS pin pulled low, current limit will

be drawn from the secondary input until it is discharged,

at which point the current limit will drop to zero.

U

PACKAGE DESCRIPTION

Dimensions in inches (millimeters) unless otherwise noted.

GN Package

16-Lead Plastic SSOP (Narrow 0.150)

(LTC DWG # 05-08-1641)

0.189 – 0.196*

(4.801 – 4.978)

16 15 14 13 12 11 10

9

0.229 – 0.244

(5.817 – 6.198)

0.150 – 0.157**

(3.810 – 3.988)

1

2

3

4

5

6

7

8

0.015 ± 0.004

(0.38 ± 0.10)

× 45°

0.053 – 0.068

(1.351 – 1.727)

0.004 – 0.0098

(0.102 – 0.249)

0.007 – 0.0098

(0.178 – 0.249)

0° – 8° TYP

0.016 – 0.050

(0.406 – 1.270)

0.008 – 0.012

(0.203 – 0.305)

0.025

(0.635)

BSC

*

DIMENSION DOES NOT INCLUDE MOLD FLASH. MOLD FLASH

SHALL NOT EXCEED 0.006" (0.152mm) PER SIDE

GN16 (SSOP) 1197

** DIMENSION DOES NOT INCLUDE INTERLEAD FLASH. INTERLEAD

FLASH SHALL NOT EXCEED 0.010" (0.254mm) PER SIDE

18

LT1579

U

PACKAGE DESCRIPTION

Dimensions in inches (millimeters) unless otherwise noted.

S8 Package

8-Lead Plastic Small Outline (Narrow 0.150)

(LTC DWG # 05-08-1610)

0.189 – 0.197*

(4.801 – 5.004)

7

5

8

6

0.150 – 0.157**

(3.810 – 3.988)

0.228 – 0.244

(5.791 – 6.197)

1

0.053 – 0.069

3

4

2

0.010 – 0.020

(0.254 – 0.508)

× 45°

(1.346 – 1.752)

0.004 – 0.010

(0.101 – 0.254)

0.008 – 0.010

(0.203 – 0.254)

0°– 8° TYP

0.016 – 0.050

0.406 – 1.270

0.050

(1.270)

TYP

0.014 – 0.019

(0.355 – 0.483)

*DIMENSION DOES NOT INCLUDE MOLD FLASH. MOLD FLASH

SHALL NOT EXCEED 0.006" (0.152mm) PER SIDE

**DIMENSION DOES NOT INCLUDE INTERLEAD FLASH. INTERLEAD

FLASH SHALL NOT EXCEED 0.010" (0.254mm) PER SIDE

SO8 0996

S Package

16-Lead Plastic Small Outline (Narrow 0.150)

(LTC DWG # 05-08-1610)

0.386 – 0.394*

(9.804 – 10.008)

16

15

14

13

12

11

10

9

0.150 – 0.157**

0.228 – 0.244

(3.810 – 3.988)

(5.791 – 6.197)

5

7

8

1

2

3

4

6

0.010 – 0.020

(0.254 – 0.508)

× 45°

0.053 – 0.069

(1.346 – 1.752)

0.004 – 0.010

(0.101 – 0.254)

0.008 – 0.010

(0.203 – 0.254)

0° – 8° TYP

0.050

(1.270)

TYP

0.014 – 0.019

(0.355 – 0.483)

0.016 – 0.050

0.406 – 1.270

S16 0695

*DIMENSION DOES NOT INCLUDE MOLD FLASH. MOLD FLASH

SHALL NOT EXCEED 0.006" (0.152mm) PER SIDE

**DIMENSION DOES NOT INCLUDE INTERLEAD FLASH. INTERLEAD

FLASH SHALL NOT EXCEED 0.010" (0.254mm) PER SIDE

Information furnished by Linear Technology Corporation is believed to be accurate and reliable.

However, no responsibility is assumed for its use. Linear Technology Corporation makes no represen-

tationthattheinterconnectionofitscircuitsasdescribedhereinwillnotinfringeonexistingpatentrights.

19

LT1579

TYPICAL APPLICATION

U

Additional Logic Forces LT1579 Into Shutdown to Protect Input Batteries

V

OUT

IN1

OUT

5V/300mA

C1

1µF

C3

4.7µF

R2

2.7M

R10

1M

IN1

BACKUP

MAIN GOOD

LBI1

R1

1M

R3

1M

D2

D1

R4

10M

D1 TO D3: 1N4148

LBO1

DROPOUT

NC

SS

IN2

C2

1µF

R6

2.7M

LT1579-5

IN2

D3

LBI2

R5

1M

R7

1M

R8

330k

LBO2

BIASCOMP

D4

C5

0.1µF

V

CC

C4

0.01µF

5.1V

1N751A

1/4

74C02

1/4

74C02

SHDN

GND

1579 TA03

RESET

R9

1.5M

1/4

74C02

RELATED PARTS

PART NUMBER

LT1175

DESCRIPTION

COMMENTS

Adjustable Current Limit, Shutdown Control

Controls Multiple Supplies, 24-Lead SSOP Package

Controls Single Supply, 8-Lead SO Package

Power Path Management for Systems with Multiple Inputs

500mA Negative Low Dropout Micropower Regulator

Hot Swap^TMController

LTC^®1421

LTC1422

Hot Swap Controller

LTC1473

LTC1479

Dual PowerPath^TMSwitch Driver

PowerPath Controller for Dual Battery Systems

Complete Power Path Management for Two Batteries,

DC Power Source, Charger and Backup

LT1521

300mA Low Dropout Micropower Regulator with Shutdown

12µA I , Reverse Battery Protection

Q

Hot Swap and PowerPath are trademarks of Linear Technology Corporation.

1579f LT/TP 0398 4K • PRINTED IN USA

Linear Technology Corporation

●

1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408)432-1900

20

●

LINEAR TECHNOLOGY CORPORATION 1998

FAX: (408) 434-0507 TELEX: 499-3977 www.linear-tech.com


型号：	LT1579CGN
厂家：	Linear
描述：	300mA Dual Input Smart Battery Backup Regulator 300毫安双输入智能电池备份稳压器稳压器电池
文件：	总20页 (文件大小：356K)
中文：	中文翻译
下载：	下载PDF数据表文档文件

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