LTC3125
1.2A Synchronous Step-Up
DC/DC Converter with
Input Current Limit
FEATURES
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DESCRIPTION
The LTC
®
3125 is a high efficiency, synchronous step-up
DC/DC converter with an accurate programmable average
input current limit. The resistor programmable average
input current limit is 5% accurate at 500mA and is suitable
for a wide variety of applications. In mobile computing,
GSM and GPRS cards demand high current pulses well
beyond the capability of the PC Card and CompactFlash
slots. The LTC3125 in concert with a reservoir capacitor,
keeps the slot power safely within its capabilities providing
a high performance and simple solution.
Synchronous rectification produces high efficiency while
the 1.6MHz switching frequency minimizes the solution
footprint. The current mode PWM design is internally com-
pensated. Output disconnect allows the load to discharge in
shutdown, while also providing inrush current limiting.
Other features include a <1μA shutdown current, short-
circuit and thermal overload protection. The LTC3125 is
offered in a low profile 0.75mm
×
2mm
×
3mm package.
L,
LT, LTC, LTM and Burst Mode are registered trademarks of Linear Technology Corporation.
All other trademarks are the property of their respective owners.
Programmable Average Input Current Limit
5% Input Current Accuracy
200mA to 1000mA Program Range
V
IN
: 1.8V to 5.5V, V
OUT
: 2V to 5.25V
Supports High Current GSM/GPRS Load Burst
V
IN
> V
OUT
Operation
1.6MHz Fixed Frequency Operation
Internal Current Sense Resistor
1.2A Peak Current Limit
Up to 93% Efficiency
Output Disconnect in Shutdown
Soft-Start
Low Quiescent Current Burst Mode
®
Operation
Available in 2mm
×
3mm
×
0.75mm DFN Package
APPLICATIONS
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GSM/GPRS PCMCIA/CompactFlash PC Card Modems
Wireless Emergency Locators
Portable Radios
Supercap Chargers
TYPICAL APPLICATION
PCMCIA/CompactFlash (3.3V/500mA Max),
4V GSM Pulsed Load
2.2μH
V
IN
3.3V
500mA
CS
V
IN
10μF
CER
OFF ON
SHDN
SW
V
OUT
LTC3125
1.24M
V
OUT
4V
2A PULSED LOAD
2200μF
2
TANT
3125 TA01a
Efficiency vs Load Current
100
90
80
EFFICIENCY (%)
70
60
50
40
30
20
10
0
0.001
0.01
0.1
LOAD CURRENT (A)
V
OUT
= 4V
V
IN
= 3.3V
V
IN
= 2.4V
1
3125 TA01b
1
POWER LOSS (W)
0.1
PROG
44.2k
GND
FB
536k
0.01
0.001
3125fa
1
LTC3125
ABSOLUTE MAXIMUM RATINGS
(Note 1)
PIN CONFIGURATION
TOP VIEW
GND 1
FB 2
PROG 3
V
IN
4
9
8 SW
7 V
OUT
6
SHDN
5 CS
V
IN
, V
OUT
Voltage ......................................... –0.3V to 6V
SW Voltage .................................................. –0.3V to 6V
SW Voltage < 100ns .................................... –0.3V to 7V
All Other Pins ............................................... –0.3V to 6V
Operating Junction Temperature Range
(Notes 2, 5) ............................................ –40°C to 125°C
Junction Temperature ........................................... 125°C
Storage Temperature Range................... –65°C to 125°C
DCB PACKAGE
8-LEAD (2mm 3mm) PLASTIC DFN
T
JMAX
= 125°C,
θ
JA
= 64°C/W (NOTE 6)
EXPOSED PAD (PIN 9) IS GND, MUST BE SOLDERED TO PCB
ORDER INFORMATION
LEAD FREE FINISH
LTC3125EDCB#PBF
TAPE AND REEL
LTC3125EDCB#TRPBF
PART MARKING
LDGY
PACKAGE DESCRIPTION
8-Lead (2mm
×
3mm) Plastic DFN
TEMPERATURE RANGE
–40°C to 125°C
Consult LTC Marketing for parts specified with wider operating temperature ranges.
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/
ELECTRICAL CHARACTERISTICS
PARAMETER
Input Voltage Range
Minimum Start-Up Voltage
Output Voltage Adjust Range
Feedback Voltage
Feedback Input Current
Quiescent Current—Shutdown
Quiescent Current—Active
Quiescent Current—Burst
N-Channel MOSFET Switch Leakage
P-Channel MOSFET Switch Leakage
N-Channel MOSFET Switch On-Resistance
P-Channel MOSFET Switch On-Resistance
N-Channel MOSFET Current Limit
Current Limit Delay to Output
Average Input Current Limit
(Note 3)
CONDITIONS
The
l
denotes the specifications which apply over the full operating
junction temperature range, otherwise specifications are at T
A
= 25°C. V
IN
= 3.3V, V
OUT
= 4.5V unless otherwise noted (Note 2).
MIN
1.8
l
l
l
TYP
1.6
MAX
5.5
1.8
5.25
1.229
50
1
500
25
10
20
UNITS
V
V
V
V
nA
μA
μA
μA
μA
μA
Ω
Ω
A
ns
2
1.176
1.200
1
0.01
300
15
0.1
0.1
0.125
0.200
V
SHDN
= 0V, Not Including Switch Leakage, V
OUT
= 0V
Measured on V
OUT
, Nonswitching
V
IN
= V
OUT
= 3.3V, Measured on V
IN
, FB ≥ 1.230V,
Nonswitching
V
SW
= 5V, V
IN
= 5V
V
SW
= 5V, V
OUT
= 0V, V
IN
= 5V
V
OUT
= 3.3V
V
OUT
= 3.3V
l
1.2
475
465
1.8
60
500
500
525
535
R
PROG
= 44.2k
R
PROG
= 44.2k, (Note 4)
l
mA
mA
3125fa
2
LTC3125
ELECTRICAL CHARACTERISTICS
PARAMETER
PROG Current Gain
Maximum Duty Cycle
Minimum Duty Cycle
Frequency
SHDN
Input High
SHDN
Input Low
SHDN
Input Current
V
SHDN
= 1.2V
0.3
CONDITIONS
(Note 3)
V
FB
= 1.15V
V
FB
= 1.3V
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
= 3.3V, V
OUT
= 4.5V unless otherwise noted (Note 2).
MIN
85
1.3
1
0.35
1
TYP
22.1
92
0
1.6
1.9
MAX
UNITS
kΩ-A/A
%
%
MHz
V
V
μA
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 LTC3125 is tested under pulsed load conditions such that
T
J
≈ T
A
. The LTC3125E (E Grade) is guaranteed to meet specifications
from 0°C to 85°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
junction temperature (T
J
) is calculated from the ambient temperature
(T
A
) and power dissipation (P
D
) according to the formula: T
J
= T
A
+ (P
D
)
(θ
JA
°C/W), where
θ
JA
is the package thermal impedance. The maximum
ambient temperature consistent with these specifications is determined by
specific operating conditions in conjunction with board layout, the rated
package thermal resistance and other environmental factors.
Note 3:
Specification is guaranteed by design and not 100% tested in
production.
Note 4:
Current measurements are made when the output is not switching.
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 result in device degradation or failure.
Note 6:
Failure to solder the exposed backside of the package to the PC
board ground plane will result in a thermal resistance much higher than
60°C/W.
TYPICAL PERFORMANCE CHARACTERISTICS
(T
A
= 25°C unless otherwise noted)
Efficiency vs Load Current,
V
OUT
= 2.5V
100
90
80
EFFICIENCY (%)
70
60
50
40
30
20
10
0
0.001
0.01
0.1
LOAD CURRENT (A)
V
IN
= 2.1V
V
IN
= 1.8V
1
3125 G01
Efficiency vs Load Current,
V
OUT
= 3.3V
1
100
90
0.1
POWER LOSS (W)
EFFICIENCY (%)
80
70
60
50
40
30
20
10
0.0001
0
0.001
0.01
0.1
LOAD CURRENT (A)
V
IN
= 2.8V
V
IN
= 2.4V
V
IN
= 2V
1
3125 G02
1
POWER LOSS (W)
0.1
0.01
0.01
0.001
0.001
3125fa
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LTC3125
TYPICAL PERFORMANCE CHARACTERISTICS
(T
A
= 25°C unless otherwise noted)
Efficiency vs Load Current,
V
OUT
= 5V
100
90
80
EFFICIENCY (%)
70
60
50
40
30
20
10
0
0.001
0.01
0.1
LOAD CURRENT (A)
V
IN
= 4V
V
IN
= 3.3V
1
3125 G03
No-Load Input Current vs V
IN
AVERAGE INPUT CURRENT LIMIT CHANGE (%)
1
4.0
3.5
3.0
POWER LOSS (W)
0.1
2.5
I
IN
(mA)
V
OUT
= 4V
2.0
V
OUT
= 3.8V
1.5
V
OUT
= 3.3V
1.0
0.5
0.001
0
1.5
V
OUT
= 2.5V
2
2.5
V
IN
(V)
3125 G04
Average Input Current Limit vs V
IN
2.0
1.5
1.0
0.5
0
–0.5
–1.0
–1.5
–2.0
1.5
2.0
2.5
3.0
V
IN
(V)
3.5
4.0
4.5
3125 G05
NORMALIZED TO 25°C
0.01
3
3.5
4
Average Input Current Limit
vs Temperature
AVERAGE INPUT CURRENT LIMIT CHANGE (%)
1.50
1.00
0.50
0
–0.50
–1.00
–1.50
–50
NORMALIZED TO 25°C
AVERAGE INPUT CURRENT LIMIT (A)
1.25
Average Input Current vs R
PROG
2.55
2.50
1.00
INPUT CURRENT (A)
2.45
2.40
2.35
2.30
2.25
2.20
2.15
2.10
2.05
0
10 20 30 40 50 60 70 80 90 100 110
R
PROG
(kΩ)
3125 G07
Peak Current Limit vs V
IN
V
OUT
= 3.8V
R
PROG
= 0Ω
0.75
0.50
0.25
2.00
1.5
2
2.5
3
3.5 4
V
IN
(V)
4.5
5
5.5
–25
0
25
50
TEMPERATURE (°C)
75
100
3125 G06
3125 G08
Burst Mode Threshold Current
vs V
IN
50
V
OUT
= 3.3V
C
OUT
= 1500μF
L = 2.2μH
LOAD CURRENT (mA)
50
45
40
35
30
25
10
1.8
20
2.0
2.4
2.2
V
IN
(V)
2.6
2.8
3125 G09
Burst Mode Threshold Current
vs V
IN
2
V
OUT
= 2.5V
C
OUT
= 1500μF
L = 2.2μH
FREQUENCY CHANGE (%)
1
0
–1
–2
–3
–4
–5
–6
–7
1.8
1.9
2.0
V
IN
(V)
2.1
2.2
3125 G10
Oscillator Frequency vs V
OUT
NORMALIZED TO V
OUT
= 3.3V
LOAD CURRENT (mA)
40
30
20
–8
2.0
2.5
3.0
3.5
4.0
V
OUT
(V)
4.5
5.0
3125 G11
3125fa
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LTC3125
TYPICAL PERFORMANCE CHARACTERISTICS
(T
A
= 25°C unless otherwise noted)
R
DS(ON)
vs V
OUT
0.450
0.400
0.350
0.300
PMOS
0.250
0.200
0.150
0.100
0.050
1.5
2
2.5
3.5
3
V
OUT
(V)
4
4.5
5
NMOS
R
DS(ON)
(mΩ)
RDS(ON) (Ω)
240
220
200
180
160
140
120
100
–50 –30
NMOS
FREQUENCY CHANGE (%)
PMOS
R
DS(ON)
vs Temperature
V
OUT
= 4V
10
8
6
4
2
0
–2
–4
–6
–8
30
50
–10 10
TEMPERATURE (°C)
70
90
3125 G13
Oscillator Frequency
vs Temperature
NORMALIZED TO 25°C
–10
–50
–25
25
50
0
TEMPERATURE (°C)
75
100
3125 G14
3125 G12
Feedback vs Temperature
0.50
0.25
CHANGE IN V
FB
(%)
CHANGE IN V
FB
(%)
0
–0.25
–0.50
–0.75
–1.00
–50 –30
NORMALIZED TO 25°C
0.50
0.25
0
Current Sense Voltage (V
RPROG
)
vs Temperature
NORMALIZED TO 25°C
16.0
15.5
15.0
I
Q
(μA)
14.5
14.0
13.5
Burst Mode Current vs V
OUT
–0.25
–0.50
–0.75
–1.00
–50 –30
30
50
–10 10
TEMPERATURE (°C)
70
90
3125 G15
30
50
–10 10
TEMPERATURE (°C)
70
90
3125 G15
13.0
1.5
2
2.5
3.5
3
V
OUT
(V)
4
4.5
5
3125 G17
V
OUT
and I
IN
During Soft-Start
V
OUT
2V/DIV
SHDN
5V/DIV
V
OUT
2V/DIV
SHDN
5V/DIV
V
OUT
and I
IN
During Soft-Start
INPUT
CURRENT
200mA/DIV
V
IN
= 3.3V
V
OUT
= 4.5V
C
OUT
= 4.4mF
L = 2.7μH
20ms/DIV
BURST
CURRENT
3125 G18
INPUT
CURRENT
200mA/DIV
V
IN
= 3.3V
V
OUT
= 4.5V
C
OUT
= 0.47F
L = 2.7μH
1s/DIV
3125 G19
3125fa
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