LTC3125EDCB-PBF [Linear]
1.2A Synchronous Step-Up DC/DC Converter with Input Current Limit; 1.2A同步升压型DC / DC转换器的输入电流限制
| 型号： | LTC3125EDCB-PBF |
| 厂家： | 
Linear |
| 描述： | 1.2A Synchronous Step-Up DC/DC Converter with Input Current Limit |
| 文件： | 总16页 (文件大小：292K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
LTC3125
1.2A Synchronous Step-Up
DC/DC Converter with
Input Current Limit
FEATURES
DESCRIPTION
The LTC®3125 is a high efficiency, synchronous step-up
DC/DC converter with an accurate programmable average
inputcurrentlimit.The 5ꢀaccurateaverageinputcurrent
is resistor programmable and 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.
n
Programmable Average Input Current Limit
n
5% Input Current Accuracy
n
200mA to 1000mA Program Range
n
V : 1.8V to 5.5V, V : 2V to 5.25V
IN
OUT
n
n
n
n
n
n
n
n
n
n
Supports High Current GSM/GPRS Load Burst
V > V
Operation
IN
OUT
1.6MHz Fixed Frequency Operation
Internal Current Sense Resistor
1.2A Peak Current Limit
Up to 93ꢀ Efficiency
Output Disconnect in Shutdown
Soft-Start
Synchronous rectification produces high efficiency while
the 1.6MHz switching frequency minimizes the solution
footprint.ThecurrentmodePWMdesignisinternallycom-
pensated.Outputdisconnectallowstheloadtodischargein
shutdown, while also providing inrush current limiting.
Low Quiescent Current Burst Mode® Operation (15μA)
Available in 2mm × 3mm × 0.75mm DFN Package
APPLICATIONS
Other features include a <1μA shutdown current, short-
circuitandthermaloverloadprotection.TheLTC3125isof-
fered in a low profile 0.75mm × 2mm × 3mm package.
n
GSM/GPRS PCMCIA/CompactFlash PC Card Modems
n
Wireless Emergency Locators
n
Portable Radios
Supercap Chargers
L, LT, LTC, LTM and Burst Mode are registered trademarks of Linear Technology Corporation.
All other trademarks are the property of their respective owners.
n
TYPICAL APPLICATION
PCMCIA/CompactFlash (3.3V/500mA Max),
4V GSM Pulsed Load
Efficiency vs Load Current
100
90
1
2.2μH
80
CS
SW
V
V
IN
OUT
4V
70
0.1
3.3V
V
V
OUT
IN
500mA
2A PULSED LOAD
60
50
10μF
CER
LTC3125
1.24M
536k
OFF ON
SHDN
2200μF
s2
40
30
20
10
0
TANT
0.01
0.001
PROG
FB
GND
44.2k
3125 TA01a
V
V
V
= 4V
= 3.3V
= 2.4V
OUT
IN
IN
0.001
0.01
0.1
1
LOAD CURRENT (A)
3125 TA01b
3125f
1
LTC3125
ABSOLUTE MAXIMUM RATINGS
PIN CONFIGURATION
(Note 1)
TOP VIEW
V , V
IN 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 Temperature Range
8
7
6
5
GND
FB
1
2
3
4
SW
V
OUT
9
PROG
SHDN
V
IN
CS
(Notes 2, 5).............................................. –40°C to 85°C
Junction Temperature ........................................... 125°C
Storage Temperature Range................... –65°C to 125°C
DCB PACKAGE
8-LEAD (2mm s 3mm) PLASTIC DFN
T
= 125°C, θ = 64°C/W (NOTE 6)
JA
EXPOSED PAD (PIN 9) IS GND, MUST BE SOLDERED TO PCB
JMAX
ORDER INFORMATION
LEAD FREE FINISH
TAPE AND REEL
PART MARKING
PACKAGE DESCRIPTION
8-Lead (2mm × 3mm) Plastic DFN
TEMPERATURE RANGE
–40°C to 85°C
LTC3125EDCB#PBF
LTC3125EDCB#TRPBF
LDGY
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 The l denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. VIN = 3.3V, VOUT = 4.5V unless otherwise noted.
PARAMETER
CONDITIONS
MIN
TYP
MAX
5.5
1.8
5.25
1.229
50
UNITS
V
Input Voltage Range
1.8
l
l
l
Minimum Start-Up Voltage
Output Voltage Adjust Range
Feedback Voltage
1.6
V
2
V
1.176
1.200
1
V
Feedback Input Current
nA
μA
μA
μA
μA
μA
Ω
Quiescent Current—Shutdown
Quiescent Current—Active
Quiescent Current—Burst
V
SHDN
= 0V, Not Including Switch Leakage, V
= 0V
0.01
300
15
1
OUT
Measured on V , Nonswitching
500
25
OUT
Measured on V , FB = 1.230V
OUT
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
V
V
V
V
= 5V, V = 5V
0.1
10
SW
IN
= 5V, V
= 0V, V = 5V
0.1
20
SW
OUT
IN
= 3.3V
= 3.3V
0.125
0.200
1.8
OUT
OUT
Ω
l
l
1.2
A
(Note 3)
60
ns
R
PROG
R
PROG
= 44.2k
= 44.2k, –40°C to 85°C
475
465
500
500
525
535
mA
mA
3125f
2
LTC3125
ELECTRICAL CHARACTERISTICS The l denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. VIN = 3.3V, VOUT = 4.5V unless otherwise noted.
PARAMETER
CONDITIONS
MIN
TYP
22.1
92
MAX
UNITS
PROG Current Gain
Maximum Duty Cycle
Minimum Duty Cycle
Frequency
(Note 3)
kΩ-A/A
l
l
l
V
V
= 1.15V
= 1.3V
85
ꢀ
ꢀ
FB
FB
0
1.3
1
1.6
1.9
MHz
V
SHDN Input High
SHDN Input Low
SHDN Input Current
0.35
1
V
V
= 1.2V
0.3
μA
SHDN
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 guaranteed to meet performance specifications
from 0°C to 85°C. Specifications over –40°C to 85°C operating
temperature range are assured by design, characterization and correlation
with statistical process controls.
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.
Note 3: Specification is guaranteed by design and not 100ꢀ tested in
production.
TYPICAL PERFORMANCE CHARACTERISTICS (TA = 25°C unless otherwise noted)
Efficiency vs Load Current,
VOUT = 2.5V
Efficiency vs Load Current,
VOUT = 3.3V
100
90
1
100
90
1
80
80
0.1
70
70
0.1
60
50
60
50
0.01
0.001
0.0001
40
30
20
10
0
40
30
20
10
0
0.01
0.001
V
V
V
= 2.8V
= 2.4V
= 2V
IN
IN
IN
V
= 2.1V
= 1.8V
IN
IN
V
0.001
0.01
0.1
1
0.001
0.01
0.1
1
LOAD CURRENT (A)
LOAD CURRENT (A)
3125 G01
3125 G02
3125f
3
LTC3125
TYPICAL PERFORMANCE CHARACTERISTICS (TA = 25°C unless otherwise noted)
Efficiency vs Load Current,
VOUT = 5V
No-Load Input Current vs VIN
Average Input Current Limit vs VIN
100
90
1
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
NORMALIZED TO 25°C
80
70
0.1
60
50
V
= 4V
OUT
2.0
1.5
0
V
= 3.8V
OUT
40
30
20
10
0
–0.5
0.01
0.001
V
= 3.3V
OUT
1.0
0.5
0
–1.0
–1.5
–2.0
V
V
= 4V
= 3.3V
IN
IN
V
= 2.5V
OUT
2
2.5
3.5
1.5
4
3
2.0
2.5
3.5
0.001
0.01
0.1
1
1.5
4.0
4.5
3.0
(V)
V
(V)
LOAD CURRENT (A)
V
IN
IN
3125 G03
3125 G04
3125 G05
Average Input Current Limit vs
Temperature
Peak Current Limit vs VIN
Average Input Current vs RPROG
1.50
1.00
0.50
0
2.55
2.50
2.45
2.40
2.35
2.30
2.25
2.20
2.15
2.10
2.05
2.00
1.25
1.00
0.75
0.50
0.25
0
NORMALIZED TO 25°C
V
= 3.8V
PROG
OUT
R
= 0Ω
–0.50
–1.00
–1.50
–50
0
25
50
75
100
–25
1.5
3
3.5
(V)
4
4.5
5
5.5
2
2.5
10 20 30 40 50 60 70 80 90 100 110
(kΩ)
TEMPERATURE (°C)
R
PROG
V
IN
3125 G07
3125 G06
3125 G08
Burst Mode Threshold Current
vs VIN
Burst Mode Threshold Current
vs VIN
Oscillator Frequency vs VOUT
2
1
50
45
50
40
30
20
10
NORMALIZED TO V = 3.3V
OUT
V
C
= 3.3V
= 1500μF
V
C
= 2.5V
= 1500μF
OUT
OUT
OUT
OUT
L = 2.2μH
L = 2.2μH
0
–1
–2
–3
–4
–5
–6
–7
–8
40
35
30
25
20
1.8
1.9
2.0
(V)
2.1
2.2
3.5
(V)
5.0
1.8
2.0
2.2
V
2.4
(V)
2.6
2.8
2.0
2.5
3.0
4.0
4.5
V
V
IN
OUT
IN
3125 G10
3125 G09
3125 G11
3125f
4
LTC3125
TYPICAL PERFORMANCE CHARACTERISTICS (TA = 25°C unless otherwise noted)
Oscillator Frequency
vs Temperature
RDS(ON) vs VOUT
RDS(ON) vs Temperature
240
220
0.450
10
8
NORMALIZED TO 25°C
V
OUT
= 4V
0.400
0.350
6
PMOS
200
180
160
140
120
4
0.300
0.250
0.200
0.150
0.100
2
PMOS
0
–2
–4
–6
–8
–10
NMOS
NMOS
100
0.050
30
TEMPERATURE (°C)
70
90
–50 –30 –10 10
50
–50
–25
25
50
75
100
2
2.5
3.5
(V)
4
4.5
5
0
1.5
3
TEMPERATURE (°C)
V
OUT
3125 G13
3125 G14
3125 G12
Current Sense Voltage (VRPROG
)
Feedback vs Temperature
vs Temperature
Burst Mode Current vs VOUT
16.0
15.5
15.0
14.5
0.50
0.25
0
0.50
0.25
0
NORMALIZED TO 25°C
NORMALIZED TO 25°C
–0.25
–0.25
14.0
13.5
13.0
–0.50
–0.75
–1.00
–0.50
–0.75
–1.00
3.5
(V)
4.5
5
30
TEMPERATURE (°C)
70
90
1.5
2
2.5
3
4
–50 –30 –10 10
50
30
TEMPERATURE (°C)
70
90
–50 –30 –10 10
50
V
OUT
3125 G17
3125 G15
3125 G15
VOUT and IIN During Soft-Start
V
OUT and IIN During Soft-Start
V
V
OUT
2V/DIV
OUT
2V/DIV
SHDN
5V/DIV
SHDN
5V/DIV
INPUT
CURRENT
200mA/DIV
INPUT
BURST
CURRENT
CURRENT
200mA/DIV
3125 G18
3125 G19
V
V
C
= 3.3V
20ms/DIV
V
V
C
= 3.3V
IN
1s/DIV
IN
= 4.5V
= 4.5V
OUT
OUT
OUT
OUT
= 4.4mF
= 0.47F
L = 2.7μH
L = 2.7μH
3125f
5
LTC3125
TYPICAL PERFORMANCE CHARACTERISTICS (TA = 25°C unless otherwise noted)
VOUT and IIN During Soft-Start
Efficiency vs VIN
100
95
I
= 200mA
= 3.8V
OUT
LOAD
V
V
OUT
2V/DIV
90
85
80
SHDN
5V/DIV
75
70
INPUT
CURRENT
200mA/DIV
65
60
55
50
3125 G20
V
V
C
= 3.3V
2s/DIV
IN
= 4.5V
= 1F
OUT
OUT
L = 2.7μH
2
4
5
3
V
IN
(V)
3125 G21
PIN FUNCTIONS
GND (Pin 1): Signal Ground.
CS (Pin 5): Current Sense Resistor Connection Point.
Connect the inductor directly to CS. An internal 60mΩ
FB (Pin 2): Feedback Input to the Error Amplifier. Connect
the resistor divider tap to this pin. The top of the divider
sense resistor is connected between CS and V .
IN
connects to V
and the bottom of the divider connects
SHDN (Pin 6): Logic Controlled Shutdown Input. Bringing
this pin above 1V enables the part, forcing this pin below
0.35V disables the part.
OUT
to GND. The output voltage can be adjusted from 1.8V
to 5.25V.
PROG (Pin 3): Programming Input for Average Input
V
(Pin 7): Output Voltage Sense and the Output of the
OUT
Current. This pin should be connected to ground through
Synchronous Rectifier. Connect the output filter capaci-
tor from V to GND, close to the IC. A minimum value
an external resistor (R
) to set input average current
PROG
OUT
limit threshold. PROG pin gain = 22.1kꢁ-A/A. The average
of 10μF ceramic is recommended. Due to the output
disconnect feature, V
SHDN is low.
input current limit threshold is set by R
to the following:
according
is disconnected from V when
OUT IN
PROG
SW (Pin 8): Switch Pin. Connect an inductor from this
pin to CS. An internal anti-ringing resistor is connected
across SW and CS after the inductor current has dropped
near zero.
22.1kΩ-A
Desired IAVG (A)
RPROG
=
V (Pin 4): Input Voltage. The device is powered from V
IN
IN
until V
exceeds V . Once V
is greater than (V +
OUT
IN
OUT IN
Exposed Pad (Pin 9): Power Ground. This pin must be
soldered to the PCB ground plane.
0.25V), the device is powered from V . Place a ceramic
OUT
bypass capacitor from V to GND. A minimum value of
IN
1μF is recommended. Also connects to CS through 60mΩ
internal sense resistor.
3125f
6
LTC3125
BLOCK DIAGRAM
L1
V
IN
C
IN
4
5
8
V
CS
SW
IN
R
SENSE
V
SEL
V
BEST
–
+
g
m
V
B
PROG
SHDN
3
6
WELL-SWITCH
ANTI-RING
R
PROG
INPUT CURRENT
SENSE AMP
V
OUT
V
7
OUT
V
SEL
+
–
I
ZERO
COMP
GATE DRIVE
AND
SD
SHUTDOWN
R2
ANTI-CROSS
CONDUCTION
4M
C
OUT
SLOPE COMP
I
PK
COMP
V
V
REF
I
+
–
PK
V
+
–
REF
V
BG
FB
GOOD
REF
I
CLMP
COMP
2
+
–
I
I
CLMP
ZERO
R1
LOGIC
CLK
TSD
OSC
CLK
MODE
CONTROL
WAKE
THERMAL
SHUTDOWN
SOFT START
+
–
g
m
AVERAGING
CIRCUIT
V
REF
I
AVG
V
CLAMP
ERROR
AMP
EXPOSED
PAD
GND
1
9
3125 BD
3125f
7
LTC3125
OPERATION
The LTC3125 provides high efficiency, low noise power
for applications in portable instrumentation and those
with pulsed-load, power-limited requirements such as
GSM modems.
with the internal slope compensation. The summed signal
is compared to the error amplifier output to provide a peak
current control command for the PWM.
A second current limit comparator shuts off the N-chan-
nel MOSFET switch once the peak current signal clamp
threshold is reached. The current limit comparator delay
to output is typically 60ns. Peak switch current is limited
to approximately 1.8A, independent of input or output
The LTC3125 directly and accurately controls the average
input current. The high efficiency of the LTC3125 provides
the maximum possible output current to the load without
impacting the host. Together with an external bulk capaci-
tor the LTC3125 with average input current limit allows a
GSM/GPRS modem to be interfaced directly to a PCMCIA
or CompactFlash power bus without overloading it.
voltage, unless V
falls below 0.8V, in which case the
OUT
current limit is cut in half.
AVERAGE INPUT CURRENT LIMIT
The current mode architecture with adaptive slope com-
pensation provides excellent transient load response,
requiring minimal output filtering. Internal soft-start and
loop compensation simplifies the design process while
minimizing the number of external components.
Input current is sensed with an internal 60mꢁ resistor
and converted to a voltage. A transconductance amplifier
creates a proportional current which is then forced into an
externalresistor.Thevoltageacrosstheresistorisaveraged
and compared to a temperature stable internal reference.
The result of this comparison is used to actively control
the current limit comparator’s clamp threshold. The high
gain of this loop forces the average input current to the
With its low R
and low gate charge internal N-chan-
DS(ON)
nel MOSFET switch and P-channel MOSFET synchronous
rectifier, the LTC3125 achieves high efficiency over a wide
range of load currents. Automatic Burst Mode operation
maintains high efficiency at very light loads, reducing the
quiescent current to just 15μA.
limit set by the external resistor, R
.
PROG
22.1kΩ-A
Desired IAVG (A)
RPROG
=
ERROR AMPLIFIER
NotethattheLTC3125’sinputcurrentaveragingcircuitmay
introduce a slightly higher inductor ripple than expected.
This is normal and has no affect on the average input
current seen by the power source.
The noninverting input of the transconductance error
amplifier is internally connected to the 1.2V reference
and the inverting input is connected to FB. Clamps limit
the minimum and maximum error amp output voltage for
improvedlarge-signaltransientresponse.Powerconverter
controlloopcompensationisprovidedinternally.Anexter-
ZERO CURRENT COMPARATOR
nalresistivevoltagedividerfromV togroundprograms
OUT
The zero current comparator monitors the inductor cur-
rent to the output and shuts off the synchronous rectifier
when this current reduces to approximately 30mA. This
prevents the inductor current from reversing in polarity,
improving efficiency at light loads.
the output voltage via FB from 2V to 5.25V.
R2
R1
⎛
⎝
⎞
VOUT = 1.2V 1+
⎜
⎟
⎠
INTERNAL CURRENT LIMIT
OSCILLATOR
Losslesscurrentsensingconvertsthepeakcurrentsignalof
theN-channelMOSFETswitchintoavoltagethatissummed
An internal oscillator sets the switching frequency to
1.6MHz.
3125f
8
LTC3125
OPERATION
SHUTDOWN
SOFT-START
Shutdown of the boost converter is accomplished by
The LTC3125 contains internal circuitry to provide soft-
start operation. The soft-start circuitry slowly ramps the
peak inductor current from zero to its peak value of 1.8A
(typical) in approximately 0.5ms, allowing start-up into
heavy loads. The soft-start circuitry is reset in the event
of a shutdown command or a thermal shutdown.
pulling SHDN below 0.35V and enabled by pulling SHDN
above 1V. Note that SHDN can be driven above V or
IN
V
, as long as it is limited to less than the absolute
OUT
maximum rating.
OUTPUT DISCONNECT
Burst Mode OPERATION
The LTC3125 is designed to allow true output disconnect
by eliminating body diode conduction of the internal
P-channel MOSFET rectifier. This allows for V
The LTC3125 will automatically enter Burst Mode opera-
tion at light load and return to fixed frequency PWM mode
when the load increases. Refer to the Typical Performance
Characteristics to see the output load Burst Mode thresh-
to go
OUT
to zero volts during shutdown, drawing no current from
the input source. It also limits inrush current at turn-on,
minimizingsurgecurrentsseenbytheinputsupply. Note
that to obtain the advantages of output disconnect, there
cannotbeanexternalSchottkydiodeconnectedbetween
old current vs V . The load current at which Burst Mode
IN
operation is entered can be changed by adjusting the
inductor value. Raising the inductor value will lower the
load current at which Burst Mode operation is entered.
the SW pin and V . The output disconnect feature also
OUT
allowsV tobepulledhigh,withoutanyreversecurrent
OUT
In Burst Mode operation, the LTC3125 still switches at a
fixed frequency of 1.6MHz, using the same error amplifier
and loop compensation for peak current mode control.
This control method eliminates any output transient
when switching between modes. In Burst Mode opera-
tion, energy is delivered to the output until it reaches the
nominal regulation value, then the LTC3125 transitions to
sleep mode where the outputs are off and the LTC3125
into the power source connected to V .
IN
THERMAL SHUTDOWN
If the die temperature exceeds 160°C typical, the LTC3125
will go into thermal shutdown. All switches will be off and
the soft-start capacitor will be discharged. The device
will be enabled again when the die temperature drops by
approximately 15°C.
consumesonly15μAofquiescentcurrentfromV .When
OUT
the output voltage droops slightly, switching resumes.
Thismaximizesefficiencyatverylightloadsbyminimizing
switching and quiescent losses.
SYNCHRONOUS RECTIFIER
To control inrush current and to prevent the inductor
As the load current increases, the LTC3125 will automati-
callyleaveBurstModeoperation.OncetheLTC3125hasleft
Burst Mode operation and returned to normal operation,
it will remain there until the output load is reduced below
the burst threshold.
current from running away when V
is close to V , the
OUT
IN
P-channel MOSFET synchronous rectifier is only enabled
when V > (V + 0.38V).
OUT
IN
ANTI-RINGING CONTROL
BurstModeoperationisinhibitedduringstart-upandsoft-
The anti-ringing control connects a resistor across the
inductor to prevent high frequency ringing on the SW pin
during discontinuous current mode operation. Although
start and until V
is at least 0.38V greater than V .
OUT
IN
the ringing of the resonant circuit formed by L and C
SW
(capacitance on SW pin) is low energy, it can cause EMI
radiation.
3125f
9
LTC3125
APPLICATIONS INFORMATION
GSM and GPRS modems have become a popular wireless
data transfer solution for use in notebook PCs and other
mobile systems. GSM transmission requires large bursts
ofcurrentthatexceedthemaximumpeakcurrentspecifica-
tions for CompactFlash and PCMCIA bus power.
SCHOTTKY DIODE
Althoughitisnotnecessary, addingaSchottkydiodefrom
SW to V
will improve efficiency by about 4ꢀ. Note that
OUT
thisdefeatstheoutputdisconnect,short-circuitprotection
and average input limiting during start-up.
The GSM standard specifies a 577μs, 2A (typical) trans-
mission burst within a 4.6ms period (12.5ꢀ duty cycle).
During the receive and standby periods the current con-
sumption drops to 70mA (typical), yielding an average
current requirement of 320mA.
PCB LAYOUT GUIDELINES
The high speed operation of the LTC3125 demands care-
ful attention to board layout. A careless layout will result
in reduced performance. A large ground pin copper area
will help to lower the die temperature. A multilayer board
with a separate ground plane is ideal, but not absolutely
necessary.
Other standards (such as GPRS, Class 10) define a higher
datarate.Onepopularrequirementtransmitstwo2Abursts
(3Aworstcase)withina4.6msframeperiod(70mAstandby
current) demanding an 800mA average input current. The
LTC3125 external current limit programming resistor can
be easily adjusted for this requirement.
COMPONENT SELECTION
Inductor Selection
Further, the GSM module is typically specified to operate
over an input power range that is outside that allowed in
the PCMCIA or CompactFlash bus power specification.
The LTC3125 can utilize small surface mount chip induc-
tors due to its fast 1.6MHz switching frequency. Inductor
values between 2.2μH and 4.7μH are suitable for most
applications.Largervaluesofinductancewillallowslightly
greateroutputcurrentcapability(andlowertheBurstMode
threshold)byreducingtheinductorripplecurrent.Increas-
ing the inductance above 10μH will increase size while
providing little improvement in output current capability.
The minimum inductance value is given by:
The LTC3125 is a high efficiency boost converter with
programmable input average current limit that provides
the needed flexibility when designing a GSM/GPRS power
supply solution. The high efficiency of the converter maxi-
mizes the average output power without overloading the
bus. A bulk output capacitor is used to supply the energy
and maintain the output voltage during the high current
pulses.
V
• VOUT(MAX) – V
(
)
IN(MIN)
IN(MIN)
L >
Ripple • VOUT(MAX) • fSW
V > V
OPERATION
IN
OUT
The LTC3125 will maintain voltage regulation even when
the input voltage is above the desired output voltage.
Note that the efficiency and the maximum output current
capability are reduced. Refer to the Typical Performance
Characteristics for details.
where:
Ripple = Allowable inductor current ripple
(amps peak-peak)
V
V
= Minimum input voltage
IN(MIN)
= Maximum output voltage
OUT(MAX)
SHORT-CIRCUIT PROTECTION
The inductor current ripple is typically set for 20ꢀ to
40ꢀ of the maximum inductor current. High frequency
ferritecoreinductormaterialsreducefrequencydependent
power losses compared to cheaper powdered iron types,
improving efficiency. The inductor should have low DCR
The LTC3125 output disconnect feature enables output
shortcircuitprotection.Toreducepowerdissipationunder
short-circuit conditions; the peak switch current limit is
reduced to 800mA (typical).
3125f
10
LTC3125
APPLICATIONS INFORMATION
2
(DC resistance of the windings) to reduce the I R power
standby mode. The above is a worst-case approximation
assuming all the pulsing energy comes from the output
capacitor.
losses, and must be able to support the peak inductor
current without saturating. Molded chokes and some chip
inductorsusuallydonothaveenoughcoreareatosupport
the peak inductor currents of 1.8A seen on the LTC3125.
To minimize radiated noise, use a shielded inductor. See
Table 1 for suggested components and suppliers.
The ripple due to the capacitor ESR is:
V
= (I
– I
) • ESR
RIPPLE_ESR
PULSE
STANDBY
Low ESR and high capacitance are critical to maintain low
output voltage ripple. Typically, two low profile 2200μF
parallel Vishay TANTAMOUNT® tantalum, low ESR capaci-
tors are used. The capacitor has less than 40mΩ ESR.
These capacitors can be used in parallel for even larger
capacitance values. For applications requiring very high
capacitance, the GS, GS2 and GW series from Cap-XX,
the BestCapTM series from AVX and PowerStor® Aerogel
CapacitorsfromCooperallofferveryhighcapacitanceand
low ESR in various package options. Table 2 shows a list
of several reservoir capacitor manufacturers.
Table 1. Recommended Inductors
VENDOR
PART/STYLE
Coilcraft
(847) 639-6400
www.coilcraft.com
LPO2506
LPS4012, LPS4018
MSS6122
MSS4020
MOS6020
DS1605, DO1608
Coiltronics
SD52, SD53, SD3114, SD3118
LQH55D
www.cooperet.com
Murata
(714) 852-2001
www.murata.com
High capacitance values and low ESR can lead to instabil-
ity in some internally compensated boost converters. The
internalloopcompensationoftheLTC3125isoptimizedto
be stable with output capacitor values greater than 500μF
with very low ESR.
Sumida
(847) 956-0666
www.sumida
CDH40D11
Taiyo-Yuden
www.t-yuden.com
NP04SB
NR3015
NR4018
Multilayer ceramic capacitors are an excellent choice for
input decoupling of the step-up converter as they have
extremely low ESR and are available in small footprints.
Input capacitors should be located as close as possible
to the device. While a 10μF input capacitor is sufficient
for most applications, larger values may be used to re-
duce input current ripple without limitations. Consult the
manufacturers directly for detailed information on their
selection of ceramic capacitors. Although ceramic capaci-
tors are recommended, low ESR tantalum capacitors may
be used as well.
TDK
VLP, LTF
(847) 803-6100
www.component.tdk.com
VLF, VLCF
Wurth
(201) 785-8800
www.we-online.com
WE-TPC Type S, M, MH, MS
Output and Input Capacitor Selection
When selecting output capacitors for large pulsed loads,
themagnitudeanddurationofthepulsingcurrent,together
with the ripple voltage specification, determine the choice
of the output capacitor. Both the ESR of the capacitor and
the charge stored in the capacitor each cycle contribute
to the output voltage ripple. The ripple due to the charge
is approximately:
Table 2. Capacitor Vendor Information
SUPPLIER
Vishay
PHONE
WEBSITE
(402) 563-6866
(803) 448-9411
(516) 998-4100
(843) 267-0720
(800) 394-2112
www.vishay.com
www.avxcorp.com
www.cooperbussman.com
www.cap-xx-com
www.panasonic.com
AVX
I
PULSE –ISTANDBY • t
(
)
ON
VRIPPLE (mV) =
Cooper Bussman
Cap-XX
COUT
Panasonic
where I
and t are the peak current and on time
PULSE
ON
during transmission burst and I
is the current in
STANDBY
3125f
11
LTC3125
TYPICAL APPLICATIONS
PC Card or CompactFlash (3.3V/500mA Maximum) 4.5V Output, GSM Pulsed Load
2.2μH*
V
CC
IN
CS
SW
V
OUT
PC CARD V
4.5V, 2A PULSED LOAD
V
IN
V
OUT
3.3V 10ꢀ
(577μs PW, 4.6ms PERIOD)
10μF
CER
500mA MAX
LTC3125
2200μF**
s2
55mΩ
TANT
2.74M
1M
OFF ON
SHDN
PROG
FB
GND
44.2k
3125 TA02a
*COILTRONICS SD3118-2R2-R
**VISHAY 592D228X6R3X220H
Waveforms of Input Current, VOUT for Pulsed Load Current
V
OUT
100mV/DIV
INPUT CURRENT
200mA/DIV
LOAD CURRENT
2A/DIV
3125 TA02b
V
V
C
= 3.3V
1ms/DIV
IN
= 4.5V
OUT
OUT
= 4.4mF
L = 2.2μH
R
= 44.2k
PROG
PC Card (3.3V/1000mA Maximum) 4.5V Output, GSM Pulsed Load
2.7μH*
V
CC
IN
CS
SW
V
OUT
PC CARD V
4.5V, 2A PULSED LOAD
V
IN
V
OUT
3.3V 10ꢀ
(577μs PW, 4.6ms PERIOD)
10μF
CER
1000mA MAX
LTC3125
2200μF**
s2
55mΩ
TANT
2.74M
1M
OFF ON
SHDN
PROG
FB
GND
22.6k
3125 TA03a
*WURTH 7440420027
**VISHAY 592D228X6R3X220H
Waveforms of Input Current, VOUT for Pulsed Load Current
V
OUT
100mV/DIV
INPUT CURRENT
500mA/DIV
LOAD CURRENT
2A/DIV
3125 TA03b
V
V
C
= 3.3V
1ms/DIV
IN
= 4.5V
OUT
OUT
= 4.4mF
L = 2.7μH
R
= 22.6k
PROG
3125f
12
LTC3125
TYPICAL APPLICATIONS
PC Card (3.3V/1000mA Maximum) 4.5V Output, GPRS, Class 10 Pulsed Load
2.7μH*
V
CC
IN
CS
SW
V
OUT
PC CARD V
4.5V, 2A PULSED LOAD
V
IN
V
OUT
3.3V 10ꢀ
(1154μs PW, 4.6ms PERIOD)
10μF
CER
1000mA MAX
LTC3125
2200μF**
s3
55mΩ
TANT
2.74M
1M
OFF ON
SHDN
PROG
FB
GND
22.6k
3125 TA04a
*WURTH 7440420027
**VISHAY 592D228X6R3X220H
Waveforms of Input Current, VOUT for Pulsed Load Current
V
OUT
100mV/DIV
INPUT CURRENT
500mA/DIV
LOAD CURRENT
2A/DIV
3125 TA04b
V
V
C
= 3.3V
1ms/DIV
IN
= 4.5V
OUT
OUT
= 6.6mF
L = 2.7μH
R
= 22.6k
PROG
Single Supercap Charger
2.2μH*
CS
SW
V
IN
V
OUT
2.5V
V
IN
V
OUT
3.3V 10ꢀ
1000mA MAX
10μF
CER
LTC3125
SC**
1.07M
OFF ON
SHDN
10F
60mΩ
PROG
FB
GND
1M
22.6k
3125 TA05a
*COILTRONICS SD3118-2R2-R
**COOPER B1325-2R5106-R
Waveforms of Input Current, VOUT for Pulsed Load Current
V
OUT
500mV/DIV
INPUT CURRENT
500mA/DIV
LOAD CURRENT
1A/DIV
3125 TA05b
V
V
C
= 3.3V
200ms/DIV
IN
= 2.5V
= 10F
OUT
OUT
L = 2.2μH
R
= 22.6k
PROG
3125f
13
LTC3125
TYPICAL APPLICATIONS
Stacked Supercap Charger
2.2μH*
CS
SW
V
IN
V
OUT
V
V
2.5V TO 5V
IN
OUT
4.5V
500mA MAX
10μF
CER
LTC3125
+
+
30F**
2.3V
2.74M
1M
100k
100k
OFF ON
SHDN
30F**
2.3V
PROG
FB
GND
44.2k
3125 TA06a
*TDK VLF4014ST-2R2M1R9
**PANASONIC EECHWOD306
Waveforms of Input Current, VOUT During Charging
V
OUT
2V/DIV
SHDN
5V/DIV
LOAD CURRENT
200mA/DIV
3125 TA06b
V
V
C
= 4.5V
20s/DIV
IN
OUT
OUT_SERIES
= 4.5V
= 15F
L = 2.2μH
= 44.2k
R
PROG
3.3V to 5V with Selectable Input Current Limit
2.2μH*
CS
SW
V
V
IN
OUT
5V
V
IN
V
OUT
3.3V 10ꢀ
10μF
CER
LTC3125
C
OUT
3.2M
1M
OFF ON
SHDN
PROG
FB
GND
44.2k
300mA 500mA
28.7k
M1
3125 TA07a
*TDK VLF4014ST-2R2M1R9
Waveforms of Input Current, VOUT for Pulsed Input Current Limit
INPUT CURRENT
200mA/DIV
M1 GATE DRIVE
5V/DIV
3125 TA07b
V
V
C
= 3.3V
2ms/DIV
IN
= 5V
OUT
OUT
= 4.4mF
L = 2.2μH
I
= 500mA
LOAD
3125f
14
LTC3125
PACKAGE DESCRIPTION
DCB Package
8-Lead Plastic DFN (2mm × 3mm)
(Reference LTC DWG # 05-08-1718 Rev A)
0.70 0.05
1.35 0.05
1.65 0.05
3.50 0.05
2.10 0.05
PACKAGE
OUTLINE
0.25 0.05
0.45 BSC
1.35 REF
RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS
APPLY SOLDER MASK TO AREAS THAT ARE NOT SOLDERED
R = 0.115
2.00 0.10
(2 SIDES)
0.40 0.10
TYP
5
R = 0.05
TYP
8
1.35 0.10
1.65 0.10
3.00 0.10
(2 SIDES)
PIN 1 NOTCH
PIN 1 BAR
TOP MARK
(SEE NOTE 6)
R = 0.20 OR 0.25
s 45° CHAMFER
(DCB8) DFN 0106 REV A
4
1
0.23 0.05
0.45 BSC
0.75 0.05
0.200 REF
1.35 REF
BOTTOM VIEW—EXPOSED PAD
0.00 – 0.05
NOTE:
1. DRAWING IS NOT A JEDEC PACKAGE OUTLINE
2. DRAWING NOT TO SCALE
3. ALL DIMENSIONS ARE IN MILLIMETERS
4. DIMENSIONS OF EXPOSED PAD ON BOTTOM OF PACKAGE DO NOT INCLUDE
MOLD FLASH. MOLD FLASH, IF PRESENT, SHALL NOT EXCEED 0.15mm ON ANY SIDE
5. EXPOSED PAD SHALL BE SOLDER PLATED
6. SHADED AREA IS ONLY A REFERENCE FOR PIN 1 LOCATION ON THE
TOP AND BOTTOM OF PACKAGE
3125f
Information furnished by Linear Technology Corporation is believed to be accurate and reliable.
However, no responsibility is assumed for its use. Linear Technology Corporation makes no representa-
tion that the interconnection of its circuits as described herein will not infringe on existing patent rights.
15
LTC3125
RELATED PARTS
PART NUMBER
DESCRIPTION
COMMENTS
92ꢀ Efficiency, V : 0.85V to 5V, V = 5V,
OUT(MAX)
LTC3400/LTC3400B 600mA (I ), 1.2MHz, Synchronous Step-Up DC/DC
SW
Converters
IN
™
I = 19ꢂA/300ꢂA, I < 1ꢂA, ThinSOT Package
Q SD
LTC3421
LTC3422
LTC3426
LTC3427
3A (I ), 3MHz, Synchronous Step-Up DC/DC Converter with
94ꢀ Efficiency, V : 0.85V to 4.5V, V
= 5.25V,
SW
IN
OUT(MAX)
Output Disconnect
I = 12ꢂA, I < 1ꢂA, 4mm × 4mm QFN24 Package
Q SD
1.5A (I ), 3MHz Synchronous Step-Up DC/DC Converter with 94ꢀ Efficiency, V : 0.85V to 4.5V, V = 5.25V,
OUT(MAX)
SW
IN
Output Disconnect
I = 25ꢂA, I < 1ꢂA, 3mm × 3mm DFN10 Package
Q SD
2A (I ), 1.5MHz Step-Up DC/DC Converter
92ꢀ Efficiency, V : 1.6V to 5.5V, V
SD
= 5V, I = 600ꢂA,
SW
IN
OUT(MAX) Q
I
< 1ꢂA, ThinSOT Package
500mA (I ), 1.25MHz Synchronous Step-Up DC/DC Converter 94ꢀ Efficiency, V : 1.8V to 5V, V
= 5.25V, I = 350ꢂA,
Q
SW
IN
OUT(MAX)
with Output Disconnect
I
< 1ꢂA, 2mm × 2mm DFN6 Package
SD
LTC3429/LTC3429B 600mA (I ), 500kHz, Synchronous Step-Up DC/DC Converter 96ꢀ Efficiency, V : 0.85V to 4.3V, V
= 5V, I = 20ꢂA,
Q
SW
IN
OUT(MAX)
with Output Disconnect and Soft-Start
I
< 1ꢂA, ThinSOT Package
SD
LTC3458/LTC3458L 1.4A/1.7A (I ), 1.5MHz Synchronous Step-Up DC/DC
94ꢀ Efficiency V : 0.85V to 6V, V
= 7.5V/6V, I = 15μA,
OUT(MAX) Q
SW
IN
Converter
I
< 1μA, 3mm × 4mm DFN12 Package
SD
LTC3459
80mA (I ), Synchronous Step-Up DC/DC Converter
92ꢀ Efficiency, V : 1.5V to 5.5V, V
SD
= 10V, I = 10ꢂA,
Q
SW
IN
OUT(MAX)
I
< 1ꢂA, ThinSOT Package
LT3494/LT3494A
180mA/350mA (I ), High Efficiency Step-Up DC/DC
85ꢀ Efficiency, V : 2.3V to 16V, V
SD
= 38V, I = 65ꢂA,
Q
SW
IN
OUT(MAX)
Converters with Output Disconnect
I
< 1ꢂA, 2mm × 3mm DFN6, ThinSOT Packages
LTC3523/LTC3523-2 600mA (I ), Step-Up and 400MHz Synchronous Step-Down
94ꢀ Efficiency V : 1.8V to 5.5V, V
SD
= 5.25V, I = 45μA,
Q
SW
IN
OUT(MAX)
1.2MHz/2.4MHz DC/DC Converter with Output Disconnect
I
< 1μA, 3mm × 3mm QFN16 Package
LTC3525-3/
LTC3525-3.3/
LTC3525-5
400mA (I ), Micropower Synchronous Step-Up DC/DC
94ꢀ Efficiency, V : 0.85V to 4V, V
SD
= 5V, I = 7μA,
Q
SW
IN
OUT(MAX)
Converter with Output Disconnect
I
< 1μA, SC-70 Package
LTC3526/LTC3526L 500mA (I ), 1MHz Synchronous Step-Up DC/DC Converter
94ꢀ Efficiency V : 0.85V to 5V, V
= 5.25V, I = 9μA,
OUT(MAX) Q
SW
IN
LTC3526B
with Output Disconnect
I
< 1μA, 2mm × 2mm DFN6 Package
SD
LTC3527/LTC3527-1 Dual 800mA/400mA (I ), 2.2MHz Synchronous Step-Up
94ꢀ Efficiency V : 0.7V to 5V, V
= 5.25V, I = 12μA,
OUT(MAX) Q
SW
IN
DC/DC Converter with Output Disconnect
I
< 1μA, 3mm × 3mm QFN16 Package
SD
LTC3528/LTC3528B 1A (I ), 1MHz Synchronous Step-Up DC/DC Converter with
94ꢀ Efficiency V : 0.7V to 5.5V, V
SD
= 5.25V, I = 12μA,
OUT(MAX) Q
SW
IN
Output Disconnect
I
< 1μA, 2mm × 3mm DFN8 Package
LTC3537
600mA (I ), 2.2MHz Synchronous Step-Up DC/DC Converter
94ꢀ Efficiency V : 0.7V to 5V, V
= 5.25V, I = 30μA,
OUT(MAX) Q
SW
IN
with Output Disconnect and 100mA LDO
I
< 1μA, 3mm × 3mm QFN16 Package
SD
LTC3539/LTC3539-2 2A (I ), 1MHz, 2.2MHz Synchronous Step-Up DC/DC
94ꢀ Efficiency, VIN: 0.7V to 5V, VOUT(MAX) = 5.25V,
IQ = 10μA, ISD < 1μA, 2mm 3mm DFN Package
SW
Converter with Output Disconnect
×
3125f
LT 1108 • PRINTED IN USA
LinearTechnology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
16
●
●
© LINEAR TECHNOLOGY CORPORATION 2008
(408) 432-1900 FAX: (408) 434-0507 www.linear.com
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