LTC3421
3A, 3MHz Micropower Synchronous
Boost Converter with Output Disconnect
FEATURES
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DESCRIPTION
The
LTC
®
3421
is a high efficiency, current mode, fixed
frequency, step-up DC/DC converter with true output dis-
connect 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 sim-
ply 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.
All registered trademarks and trademarks are the property of their respective owners.
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
APPLICATIONS
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Handheld Computers
Cordless Phones
GPS Receivers
Battery Backup Supplies
TYPICAL APPLICATION
V
IN
1.8V TO 3V
4.7µF
2
SHDN
4
ENB
3
V
REF
23
LBI
22
LBO
7
SYNC
8
I
LIM
SS
6
0.1µF
R
T
5
28k
21
V
IN
4.7µH
100
2-Cell to 3.3V Efficiency
15
16
V
OUTS
V
OUT
V
OUT
18
17
19
20
1
24
9
470pF
20k
100k
200k
22µF
340k
V
OUT
3.3V
1A
90
80
EFFICIENCY (%)
70
60
50
40
30
20
10
V
OUT
= 3.3V
f
OSC
= 1MHz
10
100
1
OUTPUT CURRENT (mA)
1000
3421 G02
14
Burst Mode OPERATION
V
IN
= 3V
V
IN
= 2.4V
V
IN
= 2V
SW SW SW
2
CELLS
LTC3421
V
OUT
FB
V
C
BURST
45.3k
GND PGND PGND PGND
10
11
12
13
0.1µF
0
0.1
3421 TA01
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1
Rev. A
LTC3421
TABLE OF CONTENTS
Features ............................................................................................................................ 1
Applications ....................................................................................................................... 1
Typical Application ............................................................................................................... 1
Description......................................................................................................................... 1
Absolute Maximum Ratings ..................................................................................................... 4
Order Information ................................................................................................................. 4
Electrical Characteristics ........................................................................................................ 4
Package/Order Information...................................................................................................... 4
Electrical Characteristics ........................................................................................................ 5
Typical Performance Characteristics .......................................................................................... 6
Pin Functions ...................................................................................................................... 7
Block Diagram..................................................................................................................... 9
Operation..........................................................................................................................10
Low Voltage Start-Up............................................................................................................................................ 10
Low Noise Fixed Frequency Operation .................................................................................................................. 10
Burst Mode Operation ........................................................................................................................................... 11
Output Disconnect and Inrush Limiting ................................................................................................................ 12
Applications Information .......................................................................................................13
Component Selection............................................................................................................................................ 13
Typical Application ..............................................................................................................16
Package Description ............................................................................................................17
Revision History .................................................................................................................18
Typical Application ..............................................................................................................19
Related Parts .....................................................................................................................19
2
Rev. A
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LTC3421
ABSOLUTE MAXIMUM RATINGS
(Note 1)
PACKAGE/ORDER INFORMATION
TOP VIEW
V
OUT
PGND
V
OUT
18 V
OUTS
17 V
OUT
25
GND
16 SW
15 SW
14 SW
13 PGND
7
SYNC
8
I
LIM
9 10 11 12
BURST
PGND
GND
LBO
LBI
V
IN
V
C
FB 1
SHDN
2
V
REF
3
ENB 4
R
T
5
SS 6
V
IN
, V
OUT
, V
OUTS
Voltage ............................. –0.3V to 6V
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
24 23 22 21 20 19
UF PACKAGE
24-LEAD (4mm
×
4mm) PLASTIC QFN
T
JMAX
= 125°C,
θ
JA
= 40°C/W 1 LAYER BOARD
θ
JA
= 35°C/W 4 LAYER BOARD,
θ
JC
= 2.6°C/W
EXPOSED PAD IS GND (PIN 25) MUST BE SOLDERED TO PCB
ORDER INFORMATION
LEAD FREE FINISH
LTC3421EUF#PBF
TAPE AND REEL
LTC3421EUF#TRPBF
PART MARKING
3421
PACKAGE DESCRIPTION
24-Lead Plastic QFN
TEMPERATURE RANGE
–40°C to 85°C
Contact the factory for parts specified with wider operating temperature ranges.
Tape and reel specifications.
Some packages are available in 500 unit reels through designated sales channels with #TRMPBF suffix.
ELECTRICAL CHARACTERISTICS
PARAMETER
Minimum V
IN
Start-Up Voltage
Minimum V
IN
Operating Voltage
Output Voltage Adjust Range
(Note 4)
The
l
denotes the specifications which apply over the full operating
junction temperature range, otherwise specifications are at T
A
= 25°C. V
IN
= 1.2V, V
OUT
= 3.3V, R
T
= 28k, unless otherwise noted.
CONDITIONS
I
LOAD
< 1mA
l
l
MIN
TYP
0.88
MAX
1
0.5
5.25
5.25
UNITS
V
V
V
V
V
nA
µA
µA
2.25
2.40
1.196
1.220
1
12
23
Feedback Voltage
Feedback Input Current
Quiescent Current—Burst Mode Operation
V
FB
= 1.22V
V
C
= 0V, ENB = 0V (Note 3)
V
C
= 0V, ENB = 2V (Note 3)
l
1.244
50
20
50
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3
Rev. A
LTC3421
ELECTRICAL CHARACTERISTICS
PARAMETER
Quiescent Current—Shutdown
Quiescent Current—Active
NMOS Switch Leakage
PMOS Switch Leakage
NMOS Switch On Resistance
PMOS Switch On Resistance
NMOS Current Limit
Max Duty Cycle
Min Duty Cycle
Frequency Accuracy
SYNC Input High
SYNC Input Low
SYNC Input Current
ENB Input High
ENB Input Low
ENB Input Current
SHDN
Input High
SHDN
Input Low
SHDN
Input Current
REF Output Voltage
REF Output Current Range
Error Amp Transconductance
LBI Threshold
LBI Input Current
LBO
Low Voltage
LBO
Leakage
SS Current Source
BURST Threshold Voltage
V
IN
= 0V, I
SINK
= 1mA
V
IN
= 0V, I
SINK
= 20mA
V
LBO
= 5.5V
V
SS
= 1V
Falling Edge
1.2
0.87
Falling Edge
l
l
l
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
= 1.2V, V
OUT
= 3.3V, R
T
= 28k, unless otherwise noted.
CONDITIONS
SHDN
= 0V, ENB = 0V
SHDN
= 0V, ENB > 1.4V
(Note 3)
MIN
TYP
0.1
0.2
0.6
0.1
0.1
0.1
0.14
I
LIM
Resistor = 105k
I
LIM
Resistor = 36.5k
l
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l
l
l
l
l
l
l
l
l
MAX
1
2
1.1
5
10
UNITS
µA
µA
mA
µA
µA
Ω
Ω
A
A
%
1
3
84
0.85
2.2
1.5
4.2
91
0
1
1.15
0.8
0.01
1
0.4
1
%
MHz
V
V
µA
V
V
µA
V
V
1.2
V
OUT
= 0V (Initial Start-Up), ENB = 0V
V
OUT
> 2.4V, ENB = 0V
1.00
0.65
0.25
0.01
1.183
–100
45
0.58
0.6
0.01
12.0
0.25
0.01
2.4
0.97
0.62
1
50
0.5
1
5
1.07
1.22
1
1.257
8
V
µA
V
µA
µs
V
µA
mV
V
µA
µA
V
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
OUTS
pin since the supply current is
bootstrapped to the output. The current will reflect to the input supply by
(V
OUT
/V
IN
) • Efficiency. The outputs are not switching.
Note 4:
Once V
OUT
is greater than 2.4V, the IC is not dependent on the V
IN
supply.
Note 5:
This IC includes overtemperature protection that is intended
to protect the device during momentary overload conditions. Junction
temperature will exceed 85°C when overtemperature protection is active.
Continuous operation above the specified maximum operating junction
temperature may impair device reliability.
4
Rev. A
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LTC3421
TYPICAL PERFORMANCE CHARACTERISTICS
Single Cell to 3.3V Efficiency
100
90
80
EFFICIENCY (%)
70
60
50
40
30
20
10
V
OUT
= 3.3V
f
OSC
= 1MHz
1
10
100
OUTPUT CURRENT (mA)
1000
3421 G01
T
A
= 25°C, unless otherwise noted.
Li-Ion to 5V Efficiency
100
90
80
EFFICIENCY (%)
70
60
50
40
30
20
V
OUT
= 5V
f
OSC
= 1MHz
1
10
100
OUTPUT CURRENT (mA)
1000
3421 G03
2-Cell to 3.3V Efficiency
100
90
80
EFFICIENCY (%)
70
60
50
40
30
20
10
V
OUT
= 3.3V
f
OSC
= 1MHz
1
10
100
OUTPUT CURRENT (mA)
1000
3421 G02
Burst Mode OPERATION
V
IN
= 1.5V
V
IN
= 1.2V
V
IN
= 1V
Burst Mode OPERATION
V
IN
= 3V
V
IN
= 2.4V
V
IN
= 2V
Burst Mode
OPERATION
V
IN
= 4.2V
V
IN
= 3.6V
V
IN
= 2.7V
10
0
0.1
0
0.1
0
0.1
Burst Mode Operation
V
OUT
50mV/DIV
AC COUPLED
SW
INDUCTOR
CURRENT
0.5A/DIV
2.5µs/DIV
600mA
I
OUT
50mA
3421 G04
Load Transient Response
V
OUT
100mV/DIV
AC COUPLED
V
OUT
1V/DIV
Inrush Current Control
INDUCTOR
CURRENT
0.5A/DIV
V
IN
= 2.4V
V
OUT
= 3.3V
C
OUT
= 44µF
2.5ms/DIV
V
IN
= 0V TO 2.4V 500µs/DIV
C
OUT
= 44µF
3421 G06
3421 G05
Efficiency vs Frequency
100
90
80
EFFICIENCY (%)
EFFICIENCY (%)
70
60
50
40
30
20
10
0
1
V
IN
= 2.4V
V
OUT
= 3.3V
10
100
OUTPUT CURRENT (mA)
1000
3421 G07
100
f = 300kHz
90
80
f = 3MHz
f = 1MHz
Efficiency vs V
IN
V
OUT
= 3.3V
I
OUT
= 200mA
START VOLTAGE (V)
1.20
1.15
1.10
1.05
1.00
0.95
0.90
0.85
1
1.5
2
2.5 3
3.5 4
INPUT VOLTAGE (V)
4.5
5
0.80
Start-Up Voltage vs Output
Current
70
60
50
40
30
20
10
0
V
IN
> V
OUT
PMOS LDO MODE
0
100
50
150
OUTPUT CURRENT (mA)
200
3421 G09
3421 G08
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5
Rev. A
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