LTC3421
3A, 3MHz Micropower
Synchronous Boost Converter
with Output Disconnect
FEATURES
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DESCRIPTIO
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
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, cur-
rent 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 com-
parator 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.
APPLICATIO S
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Handheld Computers
Cordless Phones
GPS Receivers
Battery Backup Supplies
TYPICAL APPLICATIO
V
IN
1.8V TO 3V
C1*
4.7µF
4
3
23
2
CELLS
22
7
8
45.3k
SS
6
0.1µF
R
T
5
28k
2
SHDN
ENB
V
REF
LBI
LBO
SYNC
I
LIM
21
V
IN
L1
4.7µH
14
15
16
V
OUT
3.3V
1.2A
340k
EFFICIENCY (%)
SW SW SW
18
V
OUTS
17
V
OUT
19
V
OUT
20
V
OUT
LTC3421
1
FB
24
V
C
9
BURST
10
11
12
13
0.1µF
C5*
22µF
470pF
20k
100k
200k
GND PGND PGND PGND
*LOCATE COMPONENTS CLOSE TO PINS
C1: TAIYO YUDEN JMK212BJ106MM
C5: TAIYO YUDEN JMK325BJ226MM
L1: TDK RLF7030T-4R7M3R4
3421 TA01
U
2-Cell to 3.3V Efficiency
100
90
80
70
60
50
40
30
20
10
V
OUT
= 3.3V
f
OSC
= 1MHz
10
100
1
OUTPUT CURRENT (mA)
1000
3421 G02
U
U
Burst Mode OPERATION
V
IN
= 3V
V
IN
= 2.4V
V
IN
= 2V
0
0.1
3421f
1
LTC3421
ABSOLUTE
(Note 1)
AXI U RATI GS
PACKAGE/ORDER I FOR ATIO
TOP VIEW
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
V
OUT
24 23 22 21 20 19
FB 1
SHDN 2
V
REF
3
ENB 4
R
T
5
SS 6
7
8
9 10 11 12
25
18 V
OUTS
17 V
OUT
16 SW
15 SW
14 SW
13 PGND
GND
PGND
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 PART NUMBER
UF PART MARKING
BURST
PGND
SYNC
I
LIM
V
OUT
LBO
LBI
V
IN
V
C
LTC3421EUF
3421
Consult LTC Marketing for parts specified with wider operating temperature ranges.
The
q
denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at T
A
= 25°C. V
IN
= 1.2V, V
OUT
= 3.3V, R
T
= 28k, unless otherwise noted.
PARAMETER
Minimum V
IN
Start-Up Voltage
Minimum V
IN
Operating Voltage
Output Voltage Adjust Range
q
ELECTRICAL CHARACTERISTICS
CONDITIONS
I
LOAD
< 1mA
(Note 4)
q
MIN
TYP
0.88
MAX
1
0.5
5.25
5.25
UNITS
V
V
V
V
V
nA
µA
µA
µA
µA
mA
µA
µA
Ω
Ω
A
A
%
2.25
2.40
1.196
1.220
1
12
23
0.1
0.2
0.6
0.1
0.1
0.1
0.14
Feedback Voltage
Feedback Input Current
Quiescent Current—Burst Mode Operation
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
I
LIM
Resistor = 105k
I
LIM
Resistor = 36.5k
V
FB
= 1.22V
V
C
= 0V, ENB = 0V (Note 3)
V
C
= 0V, ENB = 2V (Note 3)
SHDN = 0V, ENB = 0V
SHDN = 0V, ENB > 1.4V
(Note 3)
q
1.244
50
20
50
1
2
1.1
5
10
q
q
q
q
1
3
84
1.5
4.2
91
0
2
U
%
3421f
W
U
U
W W
W
LTC3421
The
q
denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at T
A
= 25°C. V
IN
= 1.2V, V
OUT
= 3.3V, R
T
= 28k, unless otherwise noted.
PARAMETER
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
PGOOD
= 5.5V
V
SS
= 1V
Falling Edge
1.2
0.87
Falling Edge
q
q
q
q
ELECTRICAL CHARACTERISTICS
CONDITIONS
q
q
q
q
q
q
q
MIN
0.85
2.2
TYP
1
MAX
1.15
0.8
UNITS
MHz
V
V
µA
V
V
µA
V
V
0.01
1.2
1
0.4
1
V
OUT
= 0V (Initial Start-Up)
V
OUT
> 2.4V
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 125°C when overtemperature protection is active.
Continuous operation above the specified maximum operating junction
temperature may impair device reliability.
3421f
3
LTC3421
TYPICAL PERFOR A CE CHARACTERISTICS
(T
A
= 25°C, unless otherwise specified)
Single Cell to 3.3V Efficiency
100
90
80
70
EFFICIENCY (%)
60
50
40
30
20
10
V
OUT
= 3.3V
f
OSC
= 1MHz
1
10
100
OUTPUT CURRENT (mA)
1000
3421 G01
Burst Mode OPERATION
V
IN
= 1.2V
EFFICIENCY (%)
V
IN
= 1V
60
50
40
30
20
10
V
OUT
= 3.3V
f
OSC
= 1MHz
1
10
100
OUTPUT CURRENT (mA)
1000
3421 G02
EFFICIENCY (%)
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
Efficiency vs Frequency
100
90
80
EFFICIENCY (%)
f = 3MHz
f = 1MHz
f = 300kHz
100
90
80
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
70
60
50
40
30
20
10
0
1
1.5
2
2.5 3
3.5 4
INPUT VOLTAGE (V)
4.5
5
V
IN
> V
OUT
PMOS LDO MODE
START VOLTAGE (V)
4
U W
V
IN
= 1.5V
2-Cell to 3.3V Efficiency
100
90
80
70
Burst Mode OPERATION
V
IN
= 3V
V
IN
= 2.4V
V
IN
= 2V
100
90
80
70
60
50
40
30
20
10
Li-Ion to 5V Efficiency
V
IN
= 4.2V
V
IN
= 3.6V
V
IN
= 2.7V
Burst Mode
OPERATION
0
0.1
0
0.1
V
OUT
= 5V
f
OSC
= 1MHz
1
10
100
OUTPUT CURRENT (mA)
1000
3421 G03
Load Transient Response
V
OUT
100mV/DIV
AC COUPLED
Inrush Current Control
V
OUT
1V/DIV
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
C
OUT
= 44µF
500µs/DIV
3421 G05
3421 G06
Efficiency vs V
IN
V
OUT
= 3.3V
I
OUT
= 200mA
1.20
1.15
1.10
1.05
1.00
0.95
0.90
0.85
0.80
Start-Up Voltage vs Output Current
0
100
50
150
OUTPUT CURRENT (mA)
200
3421 G09
3421 G08
3421f
LTC3421
TYPICAL PERFOR A CE CHARACTERISTICS
(T
A
= 25°C, unless otherwise specified)
Burst Mode Threshold vs R
BURST
180
160
OUTPUT CURRENT (mA)
1.24
140
VOLTAGE (V)
120
100
80
60
OUT OF BURST
40
20
INTO BURST
0
20
70
R
BURST
(kΩ)
120
3421 G10
1.22
FREQUENCY (MHz)
Burst Mode Quiescent Current
20
15
1.50
1.45
1.40
1.35
1.30
1.25
RESISTANCE (Ω)
CURRENT (µA)
CURRENT (A)
10
5
0
–45 –30 –15
0 15 30 45 60
TEMPERATURE (°C)
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
OUT
is above 2.2V,
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
REF
(Pin 3):
Buffered 1.22V Reference Output. This pin
can source up to 100µA and sink up to 8µA. This pin must
be decoupled with a 0.1µF capacitor for stability.
3421f
U W
75
3421 G13
FB Voltage
1.03
Frequency Accuracy
1.23
1.01
0.99
1.21
0.97
1.20
–45 –30 –15
0 15 30 45 60
TEMPERATURE (°C)
75
90
0.95
–45 –30 –15
0 15 30 45 60
TEMPERATURE (°C)
75
90
3421 G11
3421 G12
Current Limit Accuracy
1.70
1.65
1.60
1.55
R
LIM
= 105k
0.20
0.18
0.16
0.14
0.12
0.10
0.08
0.06
0.04
0.02
0 15 30 45 60
TEMPERATURE (°C)
75
90
R
DS(ON)
PMOS
NMOS
90
1.20
–45 –30 –15
0
–45 –30 –15
0 15 30 45 60
TEMPERATURE (°C)
75
90
3421 G14
3421 G15
U
U
U
ENB (Pin 4):
Reference Output (V
REF
) and Low-Battery
Comparator Enable. When ENB = Low, the V
REF
output and
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. Dur-
ing 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.
5
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