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FEATURES

n

LT3689/LT3689-5

700mA Step-Down

Regulator with Power-On Reset

and Watchdog Timer

DESCRIPTION

The LT

®

3689 is an adjustable frequency (350kHz to 2.2MHz)

monolithic step-down switching regulator with a power-on

reset and watchdog timer. The regulator operates from

inputs up to 36V and withstands transients up to 60V. Low

ripple Burst Mode

®

operation maintains high efﬁciency at

low output current while keeping output ripple below 15mV

in a typical application, with input quiescent current of just

85μA. Shutdown circuitry reduces input supply current to

less than 1μA while EN/UVLO is pulled low. Using a resis-

tor divider on the EN/UVLO pin provides a programmable

undervoltage lockout. Current limit, frequency foldback

and thermal shutdown provide fault protection.

The reset and watchdog timeout periods are indepen-

dently adjustable using external capacitors. Tight accuracy

speciﬁcations and glitch immunity ensure reliable reset

operation of a system without false triggering. The open

collector

RST

will pull down if output voltage drops 10%

below the programmed value. The watchdog timer is

pin-selectable for window or timeout modes. In timeout

mode,

WDO

pulls low if too long of a period passes before

a watchdog transition is detected. In window mode, the

LT3689 monitors for WDI falling edges grouped too close

together or too far apart.

L,

LT, LTC, LTM, Linear Technology, the Linear logo and Burst Mode are registered trademarks of

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

n

Wide Input Range:

Operation from 3.6V to 36V

Overvoltage Lockout Protects Circuits through

60V Transients

85μA I

Q

at 12V

IN

to 3.3V

OUT

Low Ripple Burst Mode

®

Operation Allows Output

Ripple <15mV

P-P

Programmable, Defeatable Watchdog Timer with

Window or Timeout Control

Programmable Power-On Reset Timer (POR)

Synchronizable, Adjustable 350kHz to 2.2MHz

Switching Frequency

700mA Output Switching Regulator with Internal

Power Switch

Fixed 5V or Adjustable Output Voltage

800mV Feedback Voltage

Programmable Input Undervoltage Lockout with

Hysteresis

16-Pin 3mm

×

3mm QFN and 16-Pin MSOP Packages

APPLICATIONS

n

Automotive Electronic Control Units

Industrial Power Supplies

TYPICAL APPLICATION

3.3V Regulator with Power-On Reset Timer and Watchdog Timer

V

IN

4.5V TO 36V

TRANSIENT TO 60V

μP

I/O

RESET

C

WDT

10nF

t

WDU

= 182ms

t

WDL

= 5.9ms

WDI

WDO

RST

C

WDT

C

POR

68nF

t

RST

= 157ms

C

POR

GND

LT3689

DA

FB

RT

SYNC

f

SW

= 700kHz

3689 TA01

Efﬁciency

90

V

IN

= 12V

80 V

OUT

= 3.3V

f = 700kHz

70 L = 12μH

T

A

= 25°C

60

50

40

30

20

10

0

0.0001

0.001

0.01

0.1

LOAD CURRENT (A)

1

3689 TA01b

1000

2.2μF

V

IN

EN/UVLO

OUT

BST

SW

POWER LOSS (mW)

10pF

316k

3.3V

700mA

EFFICIENCY (%)

0.1μF

12μH

100

10

20.5k

100k

22μF

1

3689fa

1

LT3689/LT3689-5

ABSOLUTE MAXIMUM RATINGS

(Note 1)

V

IN

, EN/UVLO Voltage (Note 2) .................................60V

BST Voltage ..............................................................60V

BST Above SW Voltage .............................................30V

OUT,

WDE

Voltage .....................................................30V

FB, RT, SYNC,

W/T,

WDI,

RST, WDO

Voltage ...............6V

C

WDT

, C

POR

Voltage.....................................................3V

Operating Junction Temperature Range (Note 3)

LT3689E ............................................. –40°C to 125°C

LT3689I .............................................. –40°C to 125°C

LT3689H ............................................ –40°C to 150°C

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

Lead Temperature (Soldering, 10 sec)

MSOP ............................................................... 300°C

PIN CONFIGURATION

TOP VIEW

SYNC

C

POR

12 C

WDT

17

11

WDO

10

RST

9 WDI

5

EN/UVLO

6

FB

7

WDE

8

W

/T

OUT

RT

TOP VIEW

WDO

C

WDT

C

POR

RT

SYNC

OUT

BST

V

IN

1

2

3

4

5

6

7

8

16

15

14

13

12

11

10

9

RST

WDI

W/T

WDE

FB

EN/UVLO

DA

SW

BST 1

V

IN

2

SW 3

DA 4

16 15 14 13

17

MSE PACKAGE

16-LEAD PLASTIC MSOP

θ

JA

= 43°C/W,

θ

JC

= 4.3°C/W

EXPOSED PAD (PIN 17) IS GND, MUST BE SOLDERED TO PCB

UD PACKAGE

16-LEAD (3mm 3mm) PLASTIC QFN

θ

JA

= 68°C/W,

θ

JC

= 4.2°C/W

EXPOSED PAD (PIN 17) IS GND, MUST BE SOLDERED TO PCB

ORDER INFORMATION

LEAD FREE FINISH

LT3689EMSE#PBF

LT3689IMSE#PBF

LT3689HMSE#PBF

LT3689EMSE-5#PBF

LT3689IMSE-5#PBF

LT3689HMSE-5#PBF

LT3689EUD#PBF

LT3689IUD#PBF

LT3689EUD-5#PBF

LT3689IUD-5#PBF

TAPE AND REEL

LT3689EMSE#TRPBF

LT3689IMSE#TRPBF

LT3689HMSE#TRPBF

LT3689EMSE-5#TRPBF

LT3689IMSE-5#TRPBF

LT3689HMSE-5#TRPBF

LT3689EUD#TRPBF

LT3689IUD#TRPBF

LT3689EUD-5#TRPBF

LT3689IUD-5#TRPBF

PART MARKING*

3689

36895

LDND

LFFM

PACKAGE DESCRIPTION

16-Lead Plastic MSOP with Exposed Pad

16-Lead (3mm

×

3mm) Plastic QFN

16-Lead (3mm

×

3mm) Plastic QFN

16-Lead (3mm

×

3mm) Plastic QFN

16-Lead (3mm

×

3mm) Plastic QFN

TEMPERATURE RANGE

–40°C to 125°C

–40°C to 150°C

–40°C to 125°C

–40°C to 150°C

–40°C to 125°C

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

Consult LTC Marketing for information on non-standard 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/

3689fa

2

LT3689/LT3689-5

ELECTRICAL CHARACTERISTICS

SYMBOL

PARAMETER

V

IN

Fixed Undervoltage Lockout

V

IN

Overvoltage Lockout

Quiescent Current from V

IN

V

EN/UVLO

= 0.3V

V

OUT

= 3V, Not Switching

V

OUT

= 0V, Not Switching

V

EN/UVLO

= 0.3V

V

OUT

= 3V, Not Switching (Note 7)

V

OUT

= 0V, Not Switching

V

EN/UVLO

= 0.3V

V

OUT

= 5.5V (Note 8)

V

OUT

= 0V

V

EN/UVLO

= 0.3V

V

OUT

= 5.5V

The

l

denotes the speciﬁcations which apply over the full operating

junction temperature range, otherwise speciﬁcations are at T

A

= 25°C. V

IN

= 12V, V

OUT

= 5V, unless otherwise noted. (Note 3)

CONDITIONS

l

MIN

36

TYP

3.4

38

0.01

50

125

0.01

75

–5

0.01

50

125

8

95

MAX

3.7

40

0.5

95

175

0.5

150

–20

0.5

95

175

16

150

0.812

5.050

5.100

–100

UNITS

V

μA

V

nA

%/V

Quiescent Current from OUT

l

LT3689-5 Quiescent Current from V

IN

l

LT3689-5 Quiescent Current from OUT

LT3689 FB Voltage

LT3689-5 Output Voltage

LT3689 FB Pin Bias Current

LT3689 FB Voltage Line Regulation

LT3689-5 Output Voltage Line Regulation

f

SW

t

SW(OFF)

Switching Frequency

Switch Off-Time

Foldback Frequency

Switch Current Limit (Note 4)

Switch V

CESAT

Switch Leakage Current

DA Current Limit

Boost Schottky Reverse Leakage

Minimum BST Above SW Voltage

BST Pin Current

EN/UVLO Threshold Voltage

EN/UVLO Pin Current

EN/UVLO Pin Current Hysteresis

SYNC Threshold Voltage

V

RST

t

RST

t

WDU

t

WDL

V

OL

V

OH

Reset Threshold as % of V

FB

Reset Timeout Period

Watchdog Upper Boundary

Watchdog Lower Boundary

RST, WDO

Output Voltage Low

RST, WDO

Output Voltage High (Note 6)

l

0.790

0.780

4.950

4.900

0.800

5.000

–30

0.005

V

FB

= 0.800V

5V < V

IN

< 36V

5.5V < V

IN

< 36V

R

T

= 4.02k

R

T

= 31.62k

V

BST

= 12V

R

T

= 4.22k, V

OUT

= 0V

l

1.84

420

2

500

120

250

2.16

540

160

1.95

1

1.5

5

2.5

25

1.350

1

5.5

4.8

1

92

21

785

0.4

0.3

MHz

kHz

ns

kHz

A

mV

μA

A

μA

V

mA

V

μA

V

%

ms

μs

V

l

1.15

1.55

450

0.01

I

SW

= 0.8A

0.85

V

BST

= 12V, V

OUT

= 0V

I

SW

= 0.8A

1.150

V

EN/UVLO

= 1.35V

V

EN/UVLO

= 1.15V

I(V

EN/UVLO

= 1.35V) – I(V

EN/UVLO

= 1.15V)

l

1.2

0.1

1.8

15

1.260

0. 01

4.1

3.8

0.8

90

19

675

0.15

0.05

2.5

2.8

0.4

88

17

610

C

POR

= 8200pF

C

WDT

= 1000pF

C

WDT

= 1000pF

I

SINK

= 2.5mA, V

OUT

= 0V

I

SINK

= 100μA, V

OUT

= 0V

l

V

OUT

–1

3689fa

3

LT3689/LT3689-5

ELECTRICAL CHARACTERISTICS

SYMBOL

t

UV

PARAMETER

UV Detect to

RST

Asserted

WDI Input Threshold

WDI Input Pull-Up Current

WDI Input Pulse Width

W/T

Threshold Voltage

W/T

Input Pull-Down Current

WDE

Threshold Voltage

WDO

Pull-Up Current (Note 6)

RST

Pull-Up Current (Note 6)

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:

Absolute maximum voltage at V

IN

and EN/UVLO pins is 60V for

nonrepetitive 1 second transients, and 36V for continuous operation.

Note 3:

The LT3689E is guaranteed to meet performance speciﬁcations

from 0°C to 125°C junction temperature. Speciﬁcations over the –40°C

to 125°C operating junction temperature range are assured by design,

characterization and correlation with statistical process controls. The

LT3689I is guaranteed over the full –40°C to 125°C operating junction

temperature range. The LT3689H is guaranteed over the full –40°C to

150°C operating junction temperature range. High junction temperatures

degrade operating lifetimes. Operating lifetime is derated at junction

temperatures greater than 125°C.

l

The

l

denotes the speciﬁcations which apply over the full operating

junction temperature range, otherwise speciﬁcations are at T

A

= 25°C. V

IN

= 12V, V

OUT

= 5V, unless otherwise noted. (Note 3)

CONDITIONS

Step V

FB

from 0.9V to 0.5V

l

MIN

10

0.4

300

0.4

–0.6

TYP

30

0.95

–2

0.8

2.6

0.8

–0.85

MAX

65

1.15

UNITS

μs

V

μA

ns

1

V

μA

V

μA

Note 4:

Current limit is guaranteed by design and/or correlation to static

test. Slope compensation reduces current limit at higher duty cycles.

Note 5:

This IC includes overtemperature protection that is intended

to protect the device during momentary overload conditions. Junction

temperature will exceed 150°C when overtemperature protection is active.

Continuous operation above the speciﬁed maximum operating junction

temperature may impair device reliability.

Note 6:

The output of

RST

and

WDO

has a weak pull-up to V

OUT

of

typically 1μA. However, external pull-up resistors may be used when faster

rise times are required or for V

OH

higher than V

OUT

.

Note 7:

Polarity speciﬁcation for all currents into pins is positive. All

voltages are referenced to GND unless otherwise speciﬁed.

Note 8:

For the LT3689-5, V

OUT

is set to 5.5V to ensure the switching.

TYPICAL PERFORMANCE CHARACTERISTICS

Efﬁciency, V

OUT

= 5V

95

90

EFFICIENCY (%)

85

80

75

V

IN

= 32V

70

65

0

0.1

0.2 0.3 0.4 0.5

LOAD CURRENT (A)

0.6

0.7

3689 G01

Efﬁciency, V

OUT

= 3.3V

90

T

A

= 25°C

f = 700kHz

V

IN

= 12V

EFFICIENCY (%)

V

IN

= 24V

80

V

IN

= 32V

80

75

70

65

60

55

50

0

0.1

0.2 0.3 0.4 0.5

LOAD CURRENT (A)

0.6

0.7

Efﬁciency, V

OUT

= 1.8V

T

A

= 25°C

f = 1MHz

V

IN

= 5V

T

A

= 25°C

f = 1MHz

V

IN

= 12V

V

IN

= 24V

85

EFFICIENCY (%)

V

IN

= 12V

75

70

65

3689 G02

0

0.1

0.2 0.3 0.4 0.5

LOAD CURRENT (A)

0.6

0.7

3689 G03

3689fa

4

LT3689/LT3689-5

TYPICAL PERFORMANCE CHARACTERISTICS

No-Load Supply Current vs V

IN

140

120

SUPPLY CURRENT (μA)

100

80

60

40

20

0

5

10

15

20

25

30

35

40

INPUT VOLTAGE (V)

3689 G04

No-Load Supply Current

800

700

600

500

400

300

200

100

0

–50 –25

0

25 50 75 100 125 150

TEMPERATURE (°C)

3689 G05

INCREASED SUPPLY

CURRENT DUE TO

CATCH DIODE LEAKAGE

AT HIGH TEMPERATURE

LOAD CURRENT (A)

CATCH DIODE: DIODES INC. B140HB

V

IN

= 12V

V

OUT

= 3.3V

1.4

1.2

1.0

0.8

0.6

0.4

0.2

0

Maximum Load Current, 5V

OUT

TYPICAL

T

A

= 25°C

MINIMUM

T

A

= 25°C

L = 12μH

f = 1MHz

6

12

18

24

V

IN

(V)

30

36

3689 G06

Maximum V

IN

for Full Frequency

40

35

30

25

V

IN

(V)

V

IN

(V)

20

15

10

5

0

0.1

10

V

O

= 5V

L = 6.8μH

f = 2MHz

SYNC = 5V

0.2 0.3 0.4 0.5

LOAD CURRENT (A)

0.6

0.7

5

0

T

A

= 25°C

T

A

= 85°C

35

30

25

20

15

Maximum V

IN

for Full Frequency

1.2

T

A

= 25°C

LOAD CURRENT (A)

T

A

= 85°C

1.0

0.8

0.6

0.4

0.2

0

Maximum Output Current, 3.3V

OUT

TYPICAL

MINIMUM

V

O

= 3.3V

L = 4.7μH

f = 2MHz

SYNC = 3.3V

0

0.1

0.2 0.3 0.4 0.5

LOAD CURRENT (A)

0.6

0.7

T

A

= 25°C

L = 12μH

f = 700kHz

4

8

12

16

20 24

V

IN

(V)

28

32

36

3689 G07

3689 G08

3689 G09

Switch Current Limit

vs Duty Cycle

2.0

T

A

= 25°C

SYNC = LOW

TYPICAL

CURRENT LIMIT (A)

MINIMUM

2.0

Switch Current Limit

vs Temperature

600

500

VOLTAGE DROP (mV)

400

300

Switch Voltage Drop

1.6

CURRENT LIMIT (A)

SYNC = LOW

1.5

SYNC = HIGH

1.0

1.2

T

A

= 25°C

0.8

T

A

= –50°C

200

T

A

= 125°C

100

0.4

0.5

0

20

40

60

DUTY CYCLE (%)

80

100

3689 G10

0

–50 –25

0

25 50 75 100 125 150

TEMPERATURE (°C)

3689 G11

0

200

400

600

800

1000

3689 G12

SWITCH CURRENT (mA)

3689fa

5

LT3689EUD-PBF

LT3689EUD-PBF概述

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