LINEAR TECHNOLOGY LTC3421 Technical data

FEATURES

■

Synchronous Rectification: Up to 96% Efficiency

■

True Output Disconnect

■

Inrush Current Limiting

■

Very Low Quiescent Current: 12µA

■

Up to 1.5A Continuous Output Current

■

Fixed Frequency Operation Up to 3MHz

■

0.5V to 4.5V Input Range

■

2.4V to 5.25V Adjustable Output Voltage

■

Guaranteed 1V Start-Up

■

Programmable Current Limit

■

Programmable Soft-Start

■

Synchronizable Oscillator

■

Manual or Automatic Burst Mode® Operation

■

Low-Battery Comparator

■

<1µA Shutdown Current

■

1.22V Reference Output Voltage

■

Small (4mm × 4mm) Thermally Enhanced QFN
Package

U

APPLICATIO S

■

Handheld Computers

■

Cordless Phones

■

GPS Receivers

■

Battery Backup Supplies

LTC3421

3A, 3MHz Micropower

Synchronous Boost Converter

with Output Disconnect

U

DESCRIPTIO

The LTC®3421 is a high efficiency, current mode, fixed
frequency, step-up DC/DC converter with true output dis­connect and inrush current limiting. The device includes a

0.10Ω N-channel MOSFET switch and a 0.14Ω P-channel
synchronous rectifier. This product has the ability to sim­ply program the output voltage, switching frequency, cur­rent limit, soft-start, Burst Mode threshold and loop
compensation with external passive components.

Quiescent current is only 12µA during Burst Mode opera-
tion, maximizing battery life in portable applications. The
oscillator frequency can be programmed up to 3MHz and
can be synchronized to an external clock applied to the
SYNC pin. An open-drain uncommitted low-battery com­parator is included. The part maintains operation in
applications with a secondary cell powering the output
voltage during shutdown.

Other features include: 1µA shutdown, antiringing control,
thermal limit and reference output.

The LTC3421 is available in a small 4mm × 4mm QFN
package.

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

Burst Mode is a registered trademark of Linear Technology Corporation.

TYPICAL APPLICATIO

V

IN

1.8V TO 3V

*LOCATE COMPONENTS CLOSE TO PINS
C1: TAIYO YUDEN JMK212BJ106MM

2
CELLS

C1*

4.7µF

45.3k

0.1µF

221141516

V

SHDN

REF

LIM

SS R

IN

GND

T

28k

4

ENB

3

V

23

LBI

22

LBO

7

SYNC

8

I

U

L1

4.7µH

SW SW

SW

18

V

OUTS

17

V

OUT

19

V

OUT

20

LTC3421

PGND PGNDPGND

C5: TAIYO YUDEN JMK325BJ226MM
L1: TDK RLF7030T-4R7M3R4

V

OUT

BURST

FB
V

C

1312111056

0.1µF

1
24
9

470pF

20k

100k

340k

200k

C5*
22µF

3421 TA01

V

3.3V

1.2A

OUT

2-Cell to 3.3V Efficiency

100

Burst Mode OPERATION

90
80
70
60
50
40

EFFICIENCY (%)

30
20

V

10

f

0

0.1 10 100 1000

= 3.3V

OUT

= 1MHz

OSC

1
OUTPUT CURRENT (mA)

VIN = 3V

VIN = 2.4V

VIN = 2V

3421 G02

3421f

1

LTC3421

WW

W

U

ABSOLUTE AXI U RATI GS

(Note 1)

VIN, V
BURST, SHDN, SS, ENB, SW,

LBO, LBI, SYNC Voltages .......................... –0.3V to 6V

Operating Temperature Range

(Notes 2, 5)............................................. –40°C to 85°C

Storage Temperature Range ................. –65°C to 125°C

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

OUT

, V

Voltage ............................ –0.3V to 6V

OUTS

UUW

PACKAGE/ORDER I FOR ATIO

TOP VIEW

LBO

VINV

25

10 11 12

GND

BURST

OUTVOUT

18

V

OUTS

V

17

OUT

SW

16

SW

15

SW

14
13

PGND

PGND

PGND

LTC3421EUF

VCLBI

24 23 22 21 20 19

FB

1

SHDN

2

V

3

REF

ENB

4

R

5

T

SS

6

7 8 9

LIM

I

SYNC

24-LEAD (4mm × 4mm) PLASTIC QFN

T

JMAX

= 35°C/W 4 LAYER BOARD, θJC = 2.6°C/W

θ

JA

EXPOSED PAD IS GND (PIN 25) MUST BE SOLDERED TO PCB

UF PACKAGE

= 125°C, θJA = 40°C/W 1 LAYER BOARD,

ORDER PART NUMBER

UF PART MARKING 3421

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

ELECTRICAL CHARACTERISTICS

temperature range, otherwise specifications are at TA = 25°C. VIN = 1.2V, V

PARAMETER CONDITIONS MIN TYP MAX UNITS

Minimum VIN Start-Up Voltage I
Minimum VIN Operating Voltage (Note 4) ● 0.5 V
Output Voltage Adjust Range 2.25 5.25 V

Feedback Voltage ● 1.196 1.220 1.244 V
Feedback Input Current VFB = 1.22V 1 50 nA
Quiescent Current—Burst Mode Operation VC = 0V, ENB = 0V (Note 3) 12 20 µA

Quiescent Current—Shutdown SHDN = 0V, ENB = 0V 0.1 1 µA

Quiescent Current—Active (Note 3) 0.6 1.1 mA
NMOS Switch Leakage 0.1 5 µA
PMOS Switch Leakage 0.1 10 µA
NMOS Switch On Resistance 0.1 Ω
PMOS Switch On Resistance 0.14 Ω
NMOS Current Limit I

Max Duty Cycle ● 84 91 %
Min Duty Cycle ● 0%

LOAD

V

= 0V, ENB = 2V (Note 3) 23 50 µA

C

SHDN = 0V, ENB > 1.4V 0.2 2 µA

Resistor = 105k ● 1 1.5 A

LIM

I

Resistor = 36.5k ● 3 4.2 A

LIM

The ● denotes the specifications which apply over the full operating

= 3.3V, RT = 28k, unless otherwise noted.

OUT

< 1mA 0.88 1 V

● 2.40 5.25 V

2

3421f

LTC3421

ELECTRICAL CHARACTERISTICS

temperature range, otherwise specifications are at TA = 25°C. VIN = 1.2V, V

PARAMETER CONDITIONS MIN TYP MAX UNITS

Frequency Accuracy ● 0.85 1 1.15 MHz
SYNC Input High ● 2.2 V
SYNC Input Low ● 0.8 V
SYNC Input Current ● 0.01 1 µA
ENB Input High ● 1.2 V
ENB Input Low ● 0.4 V
ENB Input Current ● 1 µA
SHDN Input High V

SHDN Input Low 0.25 V
SHDN Input Current ● 0.01 1 µA
REF Output Voltage ● 1.183 1.22 1.257 V
REF Output Current Range –100 8 µA
Error Amp Transconductance 45 µs
LBI Threshold Falling Edge ● 0.58 0.6 0.62 V
LBI Input Current ● 0.01 1 µA
LBO Low Voltage VIN = 0V, I

LBO Leakage V
SS Current Source VSS = 1V 1.2 2.4 5 µA
BURST Threshold Voltage Falling Edge 0.87 0.97 1.07 V

OUT

V

OUT

V

IN

PGOOD

= 0V, I

The ● denotes the specifications which apply over the full operating

= 3.3V, RT = 28k, unless otherwise noted.

OUT

= 0V (Initial Start-Up) 1.00 V
> 2.4V 0.65 V

= 1mA 12.0 50 mV

SINK

= 20mA 0.25 0.5 V

SINK

= 5.5V 0.01 1 µA

Note 1: Absolute Maximum Ratings are those values beyond which the life
of a device may be impaired.

Note 2: The LTC3421E is guaranteed to meet performance specifications
from 0°C to 70°C. Specifications over the 40°C to 85°C operating
temperature range are assured by design, characterization and correlation
with statistical process controls.

Note 3: Current is measured into the V
bootstrapped to the output. The current will reflect to the input supply by
(V

) • Efficiency. The outputs are not switching.

OUT/VIN

pin since the supply current is

OUTS

Note 4: Once V
supply.

Note 5: This IC includes overtemperature protection that is intended to
protect the device during momentary overload conditions. Junction
temperature will exceed 125°C when overtemperature protection is active.
Continuous operation above the specified maximum operating junction
temperature may impair device reliability.

is greater than 2.4V, the IC is not dependent on the V

OUT

IN

3421f

3

LTC3421

OUTPUT CURRENT (mA)

0

START VOLTAGE (V)

1.00

1.05

1.10

200

3421 G09

0.95

0.90

0.80
50

100

150

0.85

1.20

1.15

UW

TYPICAL PERFOR A CE CHARACTERISTICS

(TA = 25°C, unless otherwise specified)

Single Cell to 3.3V Efficiency

100

Burst Mode OPERATION

90
80
70
60
50
40

EFFICIENCY (%)

30
20

V

= 3.3V

10

OUT

= 1MHz

f

OSC

0

0.1 10 100 1000

1
OUTPUT CURRENT (mA)

VIN = 1.2V

Burst Mode Operation Load Transient Response Inrush Current Control

V

OUT

50mV/DIV

AC COUPLED

SW

INDUCTOR

CURRENT

0.5A/DIV

VIN = 1.5V

VIN = 1V

3421 G01

2-Cell to 3.3V Efficiency Li-Ion to 5V Efficiency

100

Burst Mode OPERATION

90
80
70
60
50
40

EFFICIENCY (%)

30
20

V

10

OUT

f

OSC

0

0.1 10 100 1000

V

OUT

100mV/DIV

AC COUPLED

600mA

I

OUT

50mA

VIN = 3V

= 3.3V

= 1MHz

1
OUTPUT CURRENT (mA)

VIN = 2.4V

VIN = 2V

3421 G02

100

90

Burst Mode

80

OPERATION

70
60
50
40

EFFICIENCY (%)

30
20

V

= 5V

10

OUT

= 1MHz

f

OSC

0

0.1 10 100 1000

V

OUT

1V/DIV

INDUCTOR

CURRENT

0.5A/DIV

1
OUTPUT CURRENT (mA)

VIN = 4.2V

= 3.6V

V

IN

= 2.7V

V

IN

3421 G03

100

90
80
70
60

50
40

EFFICIENCY (%)

30
20
10

0

4

2.5µs/DIV

3421 G04

= 2.4V 2.5ms/DIV 3421 G05

V

IN

V

= 3.3V

OUT

= 44µF

C

OUT

Efficiency vs Frequency Efficiency vs V

100

f = 300kHz

f = 3MHz

f = 1MHz

1

10 100 1000

OUTPUT CURRENT (mA)

VIN = 2.4V
V

OUT

= 3.3V

3421 G07

90
80
70
60
50
40

EFFICIENCY (%)

30
20

10

0

1

1.5 2.5

2

INPUT VOLTAGE (V)

IN

PMOS LDO MODE

3

3.5

V

OUT

I

OUT

VIN > V

4

= 3.3V

= 200mA

OUT

4.5

3421 G08

= 0V TO 2.4V 500µs/DIV 3421 G06

V

IN

C

= 44µF

OUT

Start-Up Voltage vs Output Current

5

3421f

UW

TYPICAL PERFOR A CE CHARACTERISTICS

LTC3421

(TA = 25°C, unless otherwise specified)

Burst Mode Threshold vs R

180

160

140

120

100

80

60

OUTPUT CURRENT (mA)

40

20

INTO BURST

0

20

OUT OF BURST

70

R

BURST

BURST

120

(kΩ)

3421 G10

FB Voltage Frequency Accuracy

1.24

1.23

1.22

VOLTAGE (V)

1.21

1.20
–45

–30 –15 0 15

TEMPERATURE (°C)

30 45 60 75 90

3421 G11

Burst Mode Quiescent Current Current Limit Accuracy R

20

15

10

CURRENT (µA)

5

0

–45

–30 –15 0 15

TEMPERATURE (°C)

30 45 60 75 90

3421 G13

1.70

R

LIM

1.65

1.60

1.55

1.50

1.45

1.40

CURRENT (A)

1.35

1.30

1.25

1.20

–30 0

–45

= 105k

–15

TEMPERATURE (°C)

30 90

45

15

60

75

3421 G14

1.03

1.01

0.99

FREQUENCY (MHz)

0.97

0.95
–45

–30 –15 0 15

DS(ON)

0.20

0.18

0.16

0.14

0.12

0.10

0.08

RESISTANCE (Ω)

0.06

0.04

0.02

0

–30 0

–15

–45

TEMPERATURE (°C)

TEMPERATURE (°C)

30 45 60 75 90

PMOS

NMOS

30 90

45

15

3421 G12

60

75

3421 G15

UUU

PI FU CTIO S

FB (Pin 1): Feedback Pin. Connect resistor divider tap
here. The output voltage can be adjusted from 2.4V to

5.25V. The feedback reference voltage is typically 1.220V.
SHDN (Pin 2): Shutdown Pin. Less than 0.25V on this pin

shuts down the IC. The IC is enabled when the SHDN
voltage is greater than 1V. Once V
hysteresis is applied to the pin (–500nA out of the pin)
allowing it to operate at a logic high while the battery can
drop to 0.5V.

V

(Pin 3): Buffered 1.22V Reference Output. This pin

REF

can source up to 100µA and sink up to 8µA. This pin must
be decoupled with a 0.1µF capacitor for stability.

is above 2.2V,

OUT

ENB (Pin 4): Reference Output (V
Comparator Enable. When ENB = Low, the V

) and Low-Battery

REF

output and

REF

low-battery comparator are disabled, which lowers the
quiescent current by 5µA. When ENB = High, the V

REF

output and the low-battery comparator are enabled. Dur­ing shutdown, if the ENB = High and the output voltage is
pulled up to greater than 2.5V from a secondary source
such as a coin cell through a Schottky diode, the V

REF

output and low-battery comparator becomes powered
from the output voltage and enabled.

3421f

5