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Electrical Specifications Subject to Change

FEATURES

n

LT3640

Dual Monolithic Buck

Regulator with Power-On

Reset and Watchdog Timer

DESCRIPTION

The LT

®

3640 is a dual channel, current mode monolithic

buck switching regulator with a power-on reset and a

watchdog timer. Both regulators are synchronized to a

single oscillator with an adjustable frequency (350kHz to

2.5MHz). At light loads, both regulators operate in low

ripple Burst Mode

®

to maintain high efﬁciency and low

output ripple.

The high voltage channel is a nonsynchronous buck with

an internal 2.4A top switch that operates from an input

of 4V to 36V; a 38V OVLO protects the device to 55V. The

low voltage channel operates from an input of 2.5V to

5.5V. Internal synchronous power switches provide high

efﬁciency without the need of external Schottky diode.

Both channels have cycle-by-cycle current limit, providing

protection against shorted outputs.

The power-on reset and watchdog timeout periods are

both adjustable using external capacitors. The window

mode watchdog timer ﬂags when the μP pulses group

too close together or too far apart.

The LT3640 is available in a 28-pin 4mm

×

5mm QFN

package and 28-pin TSSOP package. Both packages have

an exposed pad for low thermal resistance.

n

High Voltage Buck Regulator:

4V to 36V Operating Range

1.3A Output Current

OVLO Protects Input to 55V

Low Voltage Synchronous Buck Regulator:

2.5V to 5.5V Input Voltage Range

1A Output Current

Synchronizable, Adjustable 350kHz to 2.5MHz

Switching Frequency

Programmable Power-On Reset Timer

Programmable Window Mode Watchdog Timer

Quiescent Current: 275μA

Short-Circuit Robust

Programmable Soft-Start

Low Shutdown Current: I

Q

< 1μA

Available in Thermally Enhanced 28-Lead

(4mm

×

5mm) QFN and 28-Lead TSSOP Packages

APPLICATIONS

n

Industrial Power Supplies

Automotive Electronic Control Units

L,

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

and ThinSOT is a trademark of Linear Technology Corporation. All other trademarks are the

property of their respective owners.

TYPICAL APPLICATION

2MHz 3.3V/0.8A and 1.8V/0.8A Step Down Regulators

0.22μF

V

IN

5V TO 34V

10μF

EN/UVLO V

IN

SW

SYNC

PGOOD

WDE

V

OUT1

100k

RST1

RST2

WDO

WDI

CWDT

CPOR

1.5nF

LT3640

V

IN2

EN2

SW2

100k

RT GND SS2

FB2

SS1

1nF

3640 TA01a

HV Channel Efﬁciency,

2MHz, V

OUT1

= 3.3V

90

LV Channel Efﬁciency,

2MHz, V

OUT2

= 1.8V

V

IN2

= 3.3V

3.3μH

BST SW1

DA

FB1

80.6k

V

OUT1

3.3V/0.8A

22μF

EFFICIENCY (%)

85

V

IN

= 12V

EFFICIENCY (%)

0.2

0.4

0.6

0.8

1.0

V

OUT1

CURRENT (A)

85

49.9k

80

10μF

1μH

V

OUT2

1.8V/0.8A

47μF

μP

75

49.9k

70

0

1.2

3640 TA01b

70

0

0.2

0.4

0.6

V

OUT2

CURRENT (A)

0.8

3640 TA01c

1.5nF

32.4k

1

3640p

1

LT3640

ABSOLUTE MAXIMUM RATINGS

(Note 1)

V

IN

, EN/UVLO Voltage (Note 7) .................................55V

WDE

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

BST Above SW, SW1 Voltage ....................... –0.3V to 6V

SW1 Above SW Voltage ............................... –0.3V to 6V

V

IN2

, SYNC, EN2,

PGOOD,

WDI,

WDO, RST1, RST2,

Voltages ....................... –0.3V to 6V

SS1, SS2, FB1, FB2, RT, CWDT,

CPOR Voltages………... ........................... –0.3V to 2.5V

SW2 Voltage ................................ –0.3V to (V

IN2

+ 0.3V)

DA Current ..................................................................2A

Operating Junction Temperature Range (Note 2)

LT3640E ................................................. –40°C to 125°C

LT3640I .................................................. –40°C to 125°C

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

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

PIN CONFIGURATION

TOP VIEW

EN/UVLO

PGOOD

FB2

PGOOD

EN/UVLO

SYNC

SS1

FB1

RT

RST2

RST1

1

2

3

4

5

6

7

8

9

29

28 SS2

27 EN2

26 GND

25 SW2

24 V

IN2

23 GND

22 V

IN

21 BST

20 SW

19 SW1

18 DA

17 NC

16 GND

15 GND

SYNC 1

SS1 2

FB1 3

RT 4

RST2

5

RST1

6

WDO

7

CWDT 8

9 10 11 12 13 14

WDE

WDI

GND

CPOR

NC

29

GND

TOP VIEW

GND

22 SW2

21 V

IN2

20 GND

19 V

IN

18 BST

17 SW

16 SW1

15 DA

SS2

EN2

FB2

28 27 26 25 24 23

WDO

10

CWDT 11

CPOR 12

WDE

13

WDI 14

FE PACKAGE

28-LEAD PLASTIC TSSOP

θ

JA

= 30°C/W

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

UFD PACKAGE

28-LEAD (4mm 5mm) PLASTIC QFN

θ

JA

= 34°C/W

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

ORDER INFORMATION

LEAD FREE FINISH

LT3640EFE#PBF

LT3640IFE#PBF

LT3640EUFD#PBF

LT3640IUFD#PBF

TAPE AND REEL

LT3640EFE#TRPBF

LT3640IFE#TRPBF

LT3640EUFD#TRPBF

LT3640IUFD#TRPBF

PART MARKING*

LT3640FE

3640

PACKAGE DESCRIPTION

28-Lead Plastic TSSOP

28-Lead (4mm × 5mm) Plastic QFN

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/

3640p

2

LT3640

ELECTRICAL CHARACTERISTICS

PARAMETER

V

IN

Undervoltage Lockout Threshold

V

IN

Undervoltage Release Threshold

V

IN

Overvoltage Lockout Threshold

V

IN

Overvoltage Release Threshold

Quiescent Current from V

IN

EN/UVLO Threshold Voltage

EN/UVLO High Bias Current

EN/UVLO Low Bias Current

SYNC Input Frequency

SYNC Threshold Voltage

Switching Frequency

FB1 Voltage

FB1 Bias Current

FB1 Line Regulation

SW1 Minimum Off-Time

SW1 V

CESAT

SW1 Leakage Current

SW1 Current Limit

DA Current limit

BST Pin Current

Minimum BST-SW Voltage

ΔFB1 to Start LV Channel

ΔFB1 Hysteresis to Stop LV Channel

V

IN2

Minimum Operating Voltage

V

IN2

Maximum Operating Voltage

EN2 Threshold Voltage

FB2 Voltage

FB2 Bias Current

FB2 Line Regulation

SW2 Minimum Off-Time

SW2 PMOS Current Limit

SW2 NMOS Current Limit

SW2 PMOS R

DS(ON)

SW2 NMOS R

DS(ON)

ΔFB2 to Enable

PGOOD

ΔFB2 Hysteresis to Disable

PGOOD

Voltage

FB2 = 0.6V, I

PGOOD

= 1mA

(Note 5)

I

SW2

= 0.5A (Note 6)

I

SW2

= 0.5A (Note 6)

20

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

IN2

= 3.3V, EN/UVLO = 12V, EN2 = 3.3V, unless otherwise noted.

CONDITIONS

l

MIN

TYP

3.6

3.8

MAX

4

4.2

38

37

1

375

1.3

UNITS

V

μA

V

μA

35

34

36.5

35.5

0.1

275

EN/UVLO = 0.3V

Not Switching

1.2

EN/UVLO = Threshold + 60mV

EN/UVLO = Threshold – 60mV

0.35

0.4

RT = 32.4k

RT = 182k

FB1 = 1.265V

5V < V

IN

< 30V

I

SW1

= 800mA

FB1 = 1V (Note 3)

FB1 = 0.1V

FB1 = 1V (Note 4)

FB1 = 0.1V

I

SW1

= 800mA

80

30

l

1.26

2

0.1

2.5

0.8

2

500

1.265

30

0.001

70

400

0.1

1

3.3

1.7

50

2.7

130

90

2.5

5.5

1.5

612

100

1

2.35

550

1.29

100

MHz

V

MHz

kHz

V

nA

%/V

ns

mV

μA

A

mA

V

mV

V

mV

nA

%/V

ns

1.75

450

1.24

2.2

1.1

2.8

1.8

1.35

0.8

30

2

100

50

2.3

0.3

588

FB2 = 0.6V

2.5V < V

IN2

< 5.5V

1.5

0.9

1

600

0

0.01

70

1.9

1.1

275

200

40

200

2.2

1.3

A

mΩ

80

320

mV

3640p

3

LT3640

ELECTRICAL CHARACTERISTICS

PARAMETER

SS1, SS2 Charge Current

SS1 to FB1 Offset Voltage

SS2 to FB2 Offset Voltage

RST1

Threshold as Percentage of V

FB

RST2

Threshold as Percentage of V

FB

Undervoltage to

RST

Assert Time

RST1, RST2, WDO

Pull-Up Current

RST1, RST2, WDO

Output Voltage

RST1, RST2

Timeout Period (t

RST

)

Watchdog Start Delay Time (t

DLY

)

Watchdog Upper Boundary (t

WDU

)

Watchdog Lower Boundary (t

WDL

)

WDI Pull-Up Current

WDI Voltage Threshold

WDI Low Minimum Pulse Width

WDI High Minimum Pulse Width

WDE

Pull-Down Current

WDE

Threshold

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 LT3640E 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

LT3640I is guaranteed and tested over the full –40°C to 125°C operating

junction temperature range.

WDE

= 2V

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

IN2

= 3.3V, EN/UVLO = 12V, EN2 = 3.3V, unless otherwise noted.

CONDITIONS

SS1 = 0.5V, SS2 = 0.5V

SS1 = 0.6V

SS2 = 0.3V

l

MIN

1.4

TYP

1.9

5

MAX

2.5

30

94

30

250

11

18

35

2.2

1.15

UNITS

μA

mV

%

μs

μA

mV

ms

μA

V

ns

90

89

5

92

20

15

150

9.5

16

32

2

0.85

RST1, RST2, WDO

= 0V

I

RST1

, I

RST2

, I

WDO

= 2mA

CPOR = 220pF

CWDT = 820pF

WDI = 1.2V

l

8

14

27

1.68

0.55

300

1

0.5

0.7

0.9

μA

V

Note 3:

SW1, SW2 current limit is guaranteed by design and/or correlation

to static test. Slope compensation reduces current limit at higher duty

cycle.

Note 4:

The oscillator cycle is extended when DA current exceeds its limit.

DA current limit is ﬂat over duty cycle.

Note 5:

If the SW2 NMOS current exceeds its limit at the start of an

oscillator cycle, the PMOS will not be turned on in the cycle.

Note 6:

The QFN switch R

DS(ON)

is guaranteed by correlation to wafer level

measurement.

Note 7:

Absolute maximum voltage at V

IN

and RUN/SS pin is 55V for

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

3640p

4

LT3640

TYPICAL PERFORMANCE CHARACTERISTICS

HV Channel Efﬁciency

(2MHz, V

OUT1

= 3.3V)

90

T

A

= 25°C, unless otherwise noted.

LV Channel Efﬁciency

(2MHz, V

OUT2

= 1.2V)

HV Channel Efﬁciency

(2MHz, V

OUT1

= 5V)

V

IN

= 12V

90

85

EFFICIENCY (%)

V

IN

= 16V

80

V

IN

= 24V

75

V

IN

= 12V

EFFICIENCY (%)

85

V

IN

= 16V

EFFICIENCY (%)

V

IN

= 24V

80

85

V

IN2

= 3.3V

80

V

IN2

= 5V

75

70

0

0.2

0.4

0.6

0.8

1.0

V

OUT1

CURRENT (A)

1.2

3640 G01

70

0

0.2

0.4

0.6

0.8

1.0

V

OUT1

CURRENT (A)

1.2

3640 G02

70

0

0.2

0.4

0.6

V

OUT2

CURRENT (A)

0.8

1.0

3640 G03

LV Channel Efﬁciency

(2MHz, V

OUT2

= 1.8V)

90

V

IN

QUIESCENT CURRENT (mA)

V

IN2

= 3.3V

85

EFFICIENCY (%)

V

IN2

= 5V

0.35

0.30

0.25

0.20

0.15

0.10

0.05

0.00

0

0.2

0.4

0.6

V

OUT2

CURRENT (A)

0.8

1.0

3640 G04

Quiescent Current vs V

IN

350

V

IN

QUIESCENT CURRENT (μA)

20

V

IN

VOLTAGE (V)

3640 G05

Quiescent Current vs Temperature

300

250

200

150

100

50

0

–50

80

75

70

0

10

30

40

0

50

100

150

3640 G06

TEMPERATURE (°C)

FB1 Voltage vs Temperature

1.40

1.35

1.30

FB1 VOLTAGE (V)

1.25

1.20

1.15

1.10

1.05

1.00

–50

0

50

100

150

3640 G07

FB1 Voltage vs SS1

1.4

1.2

0.65

FB1 VOLTAGE (V)

FB2 VOLTAGE (V)

0.70

FB2 Voltage vs Temperature

REGULATION

1.0

0.8

0.6

0.4

0.60

REGULATION

RST1 THRESHOLD

0.55

RST2 THRESHOLD

0.50

0.2

0.0

0

0.5

1.0

SS1 VOLTAGE (V)

3640 G08

1.5

2.0

0.45

–50

0

50

100

150

3640 G09

TEMPERATURE (°C)

3640p

5

LT3640IFEPBF

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