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FEATURES

n

LT3694/LT3694-1

36V, 2.6A Monolithic Buck

Regulator With Dual LDO

DESCRIPTION

The LT

®

3694/LT3694-1 are monolithic, current mode

DC/DC converters with dual, low dropout regulator con-

trollers. The switching converter is a step-down converter

capable of generating up to 2.6A at its output. Each regula-

tor has independent track/soft-start circuits simplifying

power supply sequencing and interfacing with micro-

controllers and DSPs.

The switching frequency is set with a single resistor with a

range of 250kHz to 2.5MHz. The high switching frequency

permits the use of small inductors and ceramic capacitors

leading to very small triple output solutions. The constant-

switching frequency, combined with low impedance ce-

ramic capacitors, results in low, predictable output ripple.

Protection circuitry senses the current in the power switch

and external Schottky catch diode to protect the LT3694

against short-circuit conditions. Frequency foldback and

thermal shutdown provide additional protection.

With its wide input voltage range of 4V to 36V, the LT3694

regulates a broad array of power sources from 4-cell batteries

and 5V logic rails to unregulated wall transformers, lead acid

batteries and distributed power supplies. The LT3694 can be

synchronized to an external clock with the SYNC pin while

the LT3694-1 offers a CLKOUT pin allowing other DC/DC

converters to synchronize to the LT3694-1 clock.

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: 4V to 36V

Overvoltage Shutdown Protects Circuit Through

70V Transients

2.6A Output Switching Regulator with Internal

Power Switch

Dual, Low Dropout, Linear Regulator Controllers

with Programmable Current Limit

Tracking/Soft-Start Inputs and Power Good Output

Simplify Soft-Start and Supply Sequencing

Uses Small Inductors and Ceramic Capacitors

V

OUT(MIN)

= 0.75V (Buck and LDOs)

Adjustable 250kHz to 2.5MHz Switching Frequency

Accurate Enable Threshold Allows User

Programmable Undervoltage Lockout

Options for Clock Synchronization (LT3694) or Clock

Output to Enable Synchronization to Other Switching

Regulators (LT3694-1)

Thermally Enhanced 28-Lead 4mm

×

5mm QFN and

20-Lead TSSOP Packages

APPLICATIONS

n

Automotive

Industrial

DSL and Cable Modems

Distributed Power Regulation

Wall Transformer Regulation

TYPICAL APPLICATION

V

IN

4.5V TO 36V

4.7μF

EN/UVLO

TRK/SS1

TRK/SS2

TRK/SS3

0.1Ω

OUT1

OUT2

2.5V

450mA

2.2μF

LIM2

DRV2

LT3694

V

IN

BIAS

BST

0.22μF

SW

B340A

DA

FB1

V

C1

LIM3

24.9k

FB2

SYNC

10.7k

51.1k

PGOOD

RT

f

SW

= 800kHz

GND

FB3

11k

36941 TA01a

Efficiency at V

OUT

= 3.3V

100

4.7μH

34k

1nF

OUT1

3.3V

1.7A

47μF

EFFICIENCY (%)

f

SW

= 800kHz

V

IN

= 4.5V

V

IN

= 12V

90

41.2k

330pF

10k

OUT1

0.1Ω

80

V

IN

= 36V

70

DRV3

15.4k

2.2μF

OUT3

1.8V

450mA

60

50

0

1

I

OUT

(A)

2

3

36941 TA01b

36941fa

1

LT3694/LT3694-1

ABSOLUTE MAXIMUM RATINGS

(Note 1)

V

IN

, EN/UVLO (Note 6) ............................... –0.3V to 70V

BST ...........................................................................55V

BST Above SW ..........................................................25V

PGOOD......................................................................16V

TRK/SS, V

C

, FB, RT, SYNC Pins ...................................6V

BIAS, LIM2, LIM3 Pins ...............................................7V

Operating Junction Temperature Range (Notes 2 and 5)

LT3694E ............................................. –40°C to 125°C

LT3694I .............................................. –40°C to 125°C

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

Lead Temperature (Soldering, 10 Sec)

(TSSOP Only) ................................................... 300°C

PIN CONFIGURATION

TOP VIEW

GND

SW

V

IN

V

IN

TOP VIEW

V

IN

22 DA

21 BST

20 BIAS

29

GND

19 V

C1

18 FB1

17 TRK/SS3

16 FB3

15 DRV3

9 10 11 12 13 14

LIM2

GND

LIM3

EN/UVLO

SYNC(CLKOUT)

PGOOD

RT

TRK/SS1

TRK/SS2

FB2

DRV2

1

2

3

4

5

6

7

8

9

21

GND

20 SW

19 DA

18 BST

17 BIAS

16 V

C1

15 FB1

14 TRK/SS3

13 FB3

12 DRV3

11 LIM3

28 27 26 25 24 23

EN/UVLO 1

SYNC (CLKOUT) 2

PGOOD 3

RT 4

TRK/SS1 5

TRK/SS2 6

FB2 7

DRV2 8

LIM2 10

UFD PACKAGE

28-LEAD (4mm 5mm) PLASTIC QFN

θ

JA

= 34°C/W

EXPOSED PAD (PIN 29) IS GND, MUST BE SOLDERED TO PCB

LT3694-1 PINOUT IS SHOWN IN PARENTHESIS

FE PACKAGE

20-LEAD PLASTIC TSSOP

θ

JA

= 38°C/W

EXPOSED PAD (PIN 21) IS GND, MUST BE SOLDERED TO PCB

LT3694-1 PINOUT IS SHOWN IN PARENTHESIS

ORDER INFORMATION

LEAD FREE FINISH

LT3694EUFD#PBF

LT3694IUFD#PBF

LT3694EFE#PBF

LT3694IFE#PBF

LT3694-1EUFD#PBF

LT3694-1IUFD#PBF

LT3694-1EFE#PBF

LT3694-1IFE#PBF

TAPE AND REEL

LT3694EUFD#TRPBF

LT3694IUFD#TRPBF

LT3694EFE#TRPBF

LT3694IFE#TRPBF

LT3694-1EUFD#TRPBF

LT3694-1IUFD#TRPBF

LT3694-1EFE#TRPBF

LT3694-1IFE#TRPBF

PART MARKING*

3694

LT3694FE

36941

LT3694FE-1

PACKAGE DESCRIPTION

28-Lead (4mm

×

5mm) Plastic QFN

28-Lead (4mm

×

5mm) Plastic QFN

20-Lead Plastic TSSOP

28-Lead (4mm

×

5mm) Plastic QFN

28-Lead (4mm

×

5mm) Plastic QFN

20-Lead Plastic TSSOP

TEMPERATURE RANGE

–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/

36941fa

2

LT3694/LT3694-1

ELECTRICAL CHARACTERISTICS

PARAMETER

V

IN

Internal Undervoltage Lockout

Overvoltage Shutdown Threshold

Input Quiescent Current

Bias Quiescent Current

Shutdown Current

EN/UVLO Threshold, Bias On

EN/UVLO Threshold, Switching On

Reference Voltage Line Regulation

Switching Frequency

V

IH

, SYNC

V

IL

, SYNC

V

OH

, CLKOUT

V

OL

, CLKOUT

PGOOD Output Voltage Low

PGOOD Leakage

PGOOD Threshold (Relative to V

FB

)

Switching Regulator

Feedback Pin Voltage

Feedback Pin Bias Current

Error Amplifier Transconductance

Error Amplifier Voltage Gain

TRK/SS Pull-Up Current

TRK/SS Threshold to Start Switching

V

C1

Source Current

V

C1

Sink Current

V

C1

Clamp Voltage

V

C1

Switching Threshold

V

C1

to Switch Current Gain

Switch Leakage Current

Minimum Boost Voltage Above Switch

Switch Current Limit (Note 3)

Switch V

CESAT

BST Operating Current

V

F

, BST Diode

I

L

BST Diode

DA Current Limit

Minimum Switch Off-Time

V

IN

= 36V

(Note 4)

(Note 3) 10% Duty Cycle

I

SW1

= 3A

I

SW1

= 3A

I

BST

= 100mA

V

BST

– V

BIAS

= 36V

l

The

l

denotes the specifications which apply over the full operating

temperature range, otherwise specifications are at T

A

= 25°C, V

IN

= 12V, V

BIAS

= 3V, unless otherwise noted. (Notes 2, 9)

CONDITIONS

l

MIN

3.5

36

TYP

3.8

38

1

2

0.1

MAX

4

40

2

3.5

2

1.23

1.1

0.35

UNITS

V

mA

μA

mV

V

%/V

MHz

V

nA

%

mV

nA

μS

V/V

Not Switching

V

EN/UVLO

= 0.1V

350

1.16

0.9

1.5

1.6

5V < V

IN

< 36V

R

T

= 40.2K

l

500

1.2

0.01

1.0

I

CLKOUT

= –50μA

I

CLKOUT

= 50μA

I

PGOOD

= 250μA

V

PGOOD

= 2V

(Note 8)

2.6

0.3

0.2

10

0.4

1000

94

765

–500

86

735

90

750

–50

350

600

–2

35

V

C

= 0.6V

V

C

= 0.6V

–3

50

–20

28

2

0.75

3.6

0.01

1.8

–4

70

μA

mV

μA

V

A/V

10

2.5

6

μA

V

A

mV

mA

V

μA

3.5

4.9

600

60

0.8

1

2.6

3.6

4.5

140

A

ns

36941fa

3

LT3694/LT3694-1

ELECTRICAL CHARACTERISTICS

PARAMETER

LDO Regulator

Feedback Pin Voltage

Feedback Pin Bias Current

Error Amplifier Voltage Gain

TRK/SS Pull-Up Current

TRK/SS Threshold to Shut Down LDO

Line Regulation

Load Regulation

Base Drive

Current Limit Threshold

Short-Circuit Current Limit Threshold

Minimum BIAS to DRV Voltage (Note 7)

Minimum V

IN

to DRV Voltage

V

FB

= 0

I

DRV

= 10mA

I

DRV

= 10mA

l

The

l

denotes the specifications which apply over the full operating

temperature range, otherwise specifications are at T

A

= 25°C, V

IN

= 12V, V

BIAS

= 3V, unless otherwise noted. (Notes 2, 9)

CONDITIONS

MIN

735

TYP

750

–50

2800

–2

35

5V < V

IN

< 36V

I

DRV

From 0.1mA to 10mA

l

MAX

765

–500

–4

70

UNITS

mV

nA

μA

mV

%/V

mV/mA

–3

50

0.025

0.5

10

47

22

15

60

26

0.3

2.0

20

70

30

0.9

2.3

mA

mV

V

Note 1:

Stresses beyond those listed under Absolute Maximum Ratings

may cause permanent damage to the device. Exposure to any Absolute

Maximum Rating condition for extended periods may affect device

reliability and lifetime.

Note 2:

The LT3694E 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

LT3694I is guaranteed to meet performance specifications from –40°C to

125°C junction temperature.

Note 3:

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

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

Note 4:

This is the minimum voltage across the boost capacitor needed to

guarantee full saturation of the internal power switch.

Note 5:

This IC includes overtemperature protection that is intended

to protect the device during momentary overload conditions.

Junction temperature will exceed the maximum operating range when

overtemperature protection is active. Continuous operation above the

specified maximum operating junction temperature may impair

device reliability.

Note 6:

Absolute Maximum Voltage at V

IN

and EN/UVLO pins is 70V for

non-repetitive, 1 second transients and 36V for continuous operation.

Note 7:

The LDO will function if the BIAS to DRV differential is not met,

but the base drive current will be drawn from V

IN

instead of BIAS.

Note 8:

The PGOOD pin will pull low when the voltage on any of the three

FB pins is lower than the PGOOD threshold value.

Note 9:

Positive currents flow into pins, negative currents flow out of pins.

Minimum and maximum values refer to absolute values.

36941fa

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LT3694/LT3694-1

TYPICAL PERFORMANCE CHARACTERISTICS

Efficiency at V

OUT

= 5V

100

V

IN

= 6.3V

90

V

IN

= 12V

f

SW

= 800kHz

V

IN

= 12V, T

A

= 25°C, unless otherwise noted.

BST Pin Current

vs Switch Current

70

60

BOOST PIN CURRENT (mA)

50

40

30

20

10

0

Switch V

CESAT

vs Switch Current

0.8

0.7

0.6

SWITCH V

CESAT

(V)

0.5

0.4

0.3

0.2

0.1

EFFICIENCY (%)

80

V

IN

= 36V

70

60

50

0

1

I

OUT

(A)

2

3

36941 G01

0

2

1

SWITCH CURRENT (A)

3

36941 G02

0

1

2

SWITCH CURRENT (A)

36941 G03

3

Switch Current Limit

vs Temperature

5.0

Switch Current Limit

vs Duty Cycle

5.0

–45°C

25°C

160

140

120

TIME (ns)

Switch Minimum On-Time and

Off-Time vs Temperature

I

SW

= 1A

MINIMUM

ON-TIME

4.5

SWITCH I

LIM

(A)

4.5

SWITCH I

LIM

(A)

150°C

4.0

100

80

60

MINIMUM

OFF-TIME

4.0

3.5

40

20

3.0

–50

3.0

0

100

50

TEMPERATURE (°C)

150

36941 G04

0

80

20

40

60

SWITCH DUTY CYCLE (%)

100

36941 G05

0

–50

0

100

50

TEMPERATURE (°C)

150

36941 G06

V

FB

vs Temperature

760

758

756

FREQUENCY (MHz)

2.5

2.0

1.5

1.0

0.5

0

3.0

Frequency vs R

T

5

NORMALIZED FREQUENCY SHIFT (%)

4

3

2

1

0

–1

–2

–3

–4

Frequency Shift vs Temperature

R

T

= 200k

R

T

=

40.2k

R

T

= 10.7k

754

V

FB

(mV)

752

750

748

746

744

742

740

–50

0

100

50

TEMPERATURE (°C)

150

36941 G07

0

50

100

R

T

(k)

150

200

36941 G08

–5

–50

0

100

50

TEMPERATURE (°C)

150

36941 G09

36941fa

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LT3694-1EUFDPBF

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