LINER LTC3125

LTC3125
1.2A Synchronous Step-Up
DC/DC Converter with
Input Current Limit
FEATURES
DESCRIPTION
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The LTC®3125 is a high efficiency, synchronous step-up
DC/DC converter with an accurate programmable average
input current limit. The ±5% accurate average input current
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.
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Programmable Average Input Current Limit
5% Input Current Accuracy
200mA to 1000mA Program Range
VIN: 1.8V to 5.5V, VOUT: 2V to 5.25V
Supports High Current GSM/GPRS Load Burst
VIN > VOUT 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 (15μA)
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
Synchronous rectification produces high efficiency while
the 1.6MHz switching frequency minimizes the solution
footprint. The current mode PWM design is internally compensated. Output disconnect allows the load to discharge in
shutdown, while also providing inrush current limiting.
Other features include a <1μA shutdown current, shortcircuit 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.
TYPICAL APPLICATION
PCMCIA/CompactFlash (3.3V/500mA Max),
4V GSM Pulsed Load
Efficiency vs Load Current
1
100
2.2μH
90
10μF
CER
OFF ON
LTC3125
SHDN
1.24M
PROG
44.2k
VOUT
4V
2A PULSED LOAD
2200μF
s2
TANT
FB
GND
536k
3125 TA01a
70
0.1
60
50
40
0.01
30
20
VOUT = 4V
VIN = 3.3V
VIN = 2.4V
10
0
0.001
0.01
0.1
LOAD CURRENT (A)
POWER LOSS (W)
SW
VOUT
CS
VIN
EFFICIENCY (%)
80
VIN
3.3V
500mA
0.001
1
3125 TA01b
3125f
1
LTC3125
ABSOLUTE MAXIMUM RATINGS
PIN CONFIGURATION
(Note 1)
VIN, VOUT 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
(Notes 2, 5) .............................................. –40°C to 85°C
Junction Temperature ........................................... 125°C
Storage Temperature Range................... –65°C to 125°C
TOP VIEW
8 SW
GND 1
FB 2
7 VOUT
9
6 SHDN
PROG 3
5 CS
VIN 4
DCB PACKAGE
8-LEAD (2mm s 3mm) PLASTIC DFN
TJMAX = 125°C, θJA = 64°C/W (NOTE 6)
EXPOSED PAD (PIN 9) IS GND, MUST BE SOLDERED TO PCB
ORDER INFORMATION
LEAD FREE FINISH
TAPE AND REEL
PART MARKING
PACKAGE DESCRIPTION
TEMPERATURE RANGE
LTC3125EDCB#PBF
LTC3125EDCB#TRPBF
LDGY
8-Lead (2mm × 3mm) Plastic DFN
–40°C to 85°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
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
Input Voltage Range
1.8
Minimum Start-Up Voltage
l
Output Voltage Adjust Range
l
2
Feedback Voltage
l
1.176
MAX
5.5
1.6
UNITS
V
1.8
V
5.25
V
1.200
1.229
V
1
50
nA
VSHDN = 0V, Not Including Switch Leakage, VOUT = 0V
0.01
1
μA
Feedback Input Current
Quiescent Current—Shutdown
TYP
Quiescent Current—Active
Measured on VOUT, Nonswitching
300
500
μA
Quiescent Current—Burst
Measured on VOUT, FB = 1.230V
15
25
μA
N-Channel MOSFET Switch Leakage
VSW = 5V, VIN = 5V
0.1
10
μA
P-Channel MOSFET Switch Leakage
VSW = 5V, VOUT = 0V, VIN = 5V
0.1
20
μA
N-Channel MOSFET Switch On-Resistance
VOUT = 3.3V
P-Channel MOSFET Switch On-Resistance
VOUT = 3.3V
l
N-Channel MOSFET Current Limit
Current Limit Delay to Output
(Note 3)
Average Input Current Limit
RPROG = 44.2k
RPROG = 44.2k, –40°C to 85°C
l
1.2
475
465
0.125
Ω
0.200
Ω
1.8
A
60
ns
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
PROG Current Gain
(Note 3)
Maximum Duty Cycle
VFB = 1.15V
l
Minimum Duty Cycle
VFB = 1.3V
l
TYP
MAX
22.1
l
Frequency
SHDN Input High
85
kΩ-A/A
92
%
0
1.3
1.6
1.9
1
VSHDN = 1.2V
0.3
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 3: Specification is guaranteed by design and not 100% tested in
production.
%
MHz
V
SHDN Input Low
SHDN Input Current
UNITS
0.35
V
1
μA
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 (TA = 25°C unless otherwise noted)
Efficiency vs Load Current,
VOUT = 2.5V
Efficiency vs Load Current,
VOUT = 3.3V
100
1
100
90
80
50
0.01
40
30
0.001
20
70
0.1
60
50
40
0.01
30
20
VIN = 2.1V
VIN = 1.8V
10
0.01
0.1
LOAD CURRENT (A)
VIN = 2.8V
VIN = 2.4V
VIN = 2V
10
0.0001
1
3125 G01
0
0.001
0.01
0.1
LOAD CURRENT (A)
POWER LOSS (W)
60
EFFICIENCY (%)
0.1
70
POWER LOSS (W)
EFFICIENCY (%)
80
0
0.001
1
90
0.001
1
3125 G02
3125f
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LTC3125
TYPICAL PERFORMANCE CHARACTERISTICS (TA = 25°C unless otherwise noted)
Efficiency vs Load Current,
VOUT = 5V
No-Load Input Current vs VIN
1
4.0
90
3.5
80
3.0
60
50
40
0.01
30
2.5
IIN (mA)
EFFICIENCY (%)
0.1
POWER LOSS (W)
70
VOUT = 4V
2.0
VOUT = 3.8V
1.5
VOUT = 3.3V
1.0
20
0.5
VIN = 4V
VIN = 3.3V
10
0
0.001
0.01
0.1
LOAD CURRENT (A)
0.001
0
1.5
1
VOUT = 2.5V
2
2.5
3.5
3
0
–0.50
–1.00
–0.5
–1.0
–1.5
–2.0
1.5
2.0
2.5
75
0.75
0.50
0.25
VOUT = 3.8V
RPROG = 0Ω
2.35
2.30
2.25
2.20
2.15
2.10
2.00
1.5
10 20 30 40 50 60 70 80 90 100 110
RPROG (kΩ)
2
2.5
3
3.5 4
VIN (V)
4.5
5
3125 G07
Oscillator Frequency vs VOUT
2
LOAD CURRENT (mA)
VOUT = 2.5V
COUT = 1500μF
L = 2.2μH
1
FREQUENCY CHANGE (%)
45
5.5
3125 G08
50
VOUT = 3.3V
COUT = 1500μF
L = 2.2μH
4.5
2.40
Burst Mode Threshold Current
vs VIN
20
4.0
2.05
Burst Mode Threshold Current
vs VIN
30
3.5
2.45
1.00
0
100
40
3.0
VIN (V)
Peak Current Limit vs VIN
2.50
3125 G06
LOAD CURRENT (mA)
0
2.55
INPUT CURRENT (A)
AVERAGE INPUT CURRENT LIMIT (A)
AVERAGE INPUT CURRENT LIMIT CHANGE (%)
0.50
50
0.5
Average Input Current vs RPROG
1.00
0
25
50
TEMPERATURE (°C)
1.0
3125 G05
1.25
NORMALIZED TO 25°C
–25
NORMALIZED TO 25°C
1.5
3125 G04
Average Input Current Limit vs
Temperature
–1.50
–50
4
2.0
VIN (V)
3125 G03
1.50
Average Input Current Limit vs VIN
AVERAGE INPUT CURRENT LIMIT CHANGE (%)
100
40
35
30
NORMALIZED TO VOUT = 3.3V
0
–1
–2
–3
–4
–5
–6
25
–7
10
1.8
20
2.0
2.4
2.2
VIN (V)
2.6
2.8
3125 G09
1.8
1.9
2.0
VIN (V)
2.1
2.2
3125 G10
–8
2.0
2.5
3.0
3.5
4.0
VOUT (V)
4.5
5.0
3125 G11
3125f
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LTC3125
TYPICAL PERFORMANCE CHARACTERISTICS (TA = 25°C unless otherwise noted)
0.450
240
0.400
220
PMOS
0.250
0.200
NMOS
0.150
180
160
140
NMOS
4
2
0
–2
–4
–6
120
0.100
0.050
1.5
2
2.5
3.5
3
VOUT (V)
4
4.5
–8
100
–50 –30
5
30
50
–10 10
TEMPERATURE (°C)
0.50
CHANGE IN VFB (%)
0.25
0
–0.25
–0.50
–0.75
70
90
0.25
15.5
0
15.0
–0.25
14.0
–0.75
13.5
30
50
–10 10
TEMPERATURE (°C)
70
90
13.0
1.5
2
2.5
3.5
3
VOUT (V)
4
4.5
5
3125 G17
3125 G15
VOUT and IIN During Soft-Start
VOUT
2V/DIV
VOUT
2V/DIV
SHDN
5V/DIV
SHDN
5V/DIV
INPUT
CURRENT
200mA/DIV
BURST
CURRENT
20ms/DIV
100
14.5
–0.50
–1.00
–50 –30
75
16.0
VOUT and IIN During Soft-Start
VIN = 3.3V
VOUT = 4.5V
COUT = 4.4mF
L = 2.7μH
25
50
0
TEMPERATURE (°C)
Burst Mode Current vs VOUT
NORMALIZED TO 25°C
3125 G15
INPUT
CURRENT
200mA/DIV
–25
3125 G14
IQ (μA)
NORMALIZED TO 25°C
30
50
–10 10
TEMPERATURE (°C)
–10
–50
90
Current Sense Voltage (VRPROG)
vs Temperature
Feedback vs Temperature
–1.00
–50 –30
70
3125 G13
3125 G12
CHANGE IN VFB (%)
6
FREQUENCY CHANGE (%)
0.300
NORMALIZED TO 25°C
8
200
RDS(ON) (mΩ)
RDS(ON) (Ω)
10
VOUT = 4V
PMOS
0.350
0.50
Oscillator Frequency
vs Temperature
RDS(ON) vs Temperature
RDS(ON) vs VOUT
3125 G18
VIN = 3.3V
VOUT = 4.5V
COUT = 0.47F
L = 2.7μH
1s/DIV
3125 G19
3125f
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LTC3125
TYPICAL PERFORMANCE CHARACTERISTICS (TA = 25°C unless otherwise noted)
VOUT and IIN During Soft-Start
Efficiency vs VIN
100
ILOAD = 200mA
95 VOUT = 3.8V
VOUT
2V/DIV
EFFICIENCY (%)
90
SHDN
5V/DIV
INPUT
CURRENT
200mA/DIV
VIN = 3.3V
VOUT = 4.5V
COUT = 1F
L = 2.7μH
2s/DIV
3125 G20
85
80
75
70
65
60
55
50
2
4
3
5
VIN (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
sense resistor is connected between CS and VIN.
FB (Pin 2): Feedback Input to the Error Amplifier. Connect
the resistor divider tap to this pin. The top of the divider
connects to VOUT and the bottom of the divider connects
to GND. The output voltage can be adjusted from 1.8V
to 5.25V.
SHDN (Pin 6): Logic Controlled Shutdown Input. Bringing
this pin above 1V enables the part, forcing this pin below
0.35V disables the part.
PROG (Pin 3): Programming Input for Average Input
Current. This pin should be connected to ground through
an external resistor (RPROG) to set input average current
limit threshold. PROG pin gain = 22.1kΩ-A/A. The average
input current limit threshold is set by RPROG according
to the following:
VOUT (Pin 7): Output Voltage Sense and the Output of the
Synchronous Rectifier. Connect the output filter capacitor from VOUT to GND, close to the IC. A minimum value
of 10μF ceramic is recommended. Due to the output
disconnect feature, VOUT is disconnected from VIN when
SHDN is low.
RPROG =
22.1kΩ-A
Desired IAVG ( A)
VIN (Pin 4): Input Voltage. The device is powered from VIN
until VOUT exceeds VIN. Once VOUT is greater than (VIN +
0.25V), the device is powered from VOUT. Place a ceramic
bypass capacitor from VIN to GND. A minimum value of
1μF is recommended. Also connects to CS through 60mΩ
internal sense resistor.
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.
Exposed Pad (Pin 9): Power Ground. This pin must be
soldered to the PCB ground plane.
3125f
6
LTC3125
BLOCK DIAGRAM
L1
VIN
CIN
4
5
VIN
8
CS
SW
RSENSE
VSEL
VBEST
+ –
gm
3
VB
PROG
WELL-SWITCH
ANTI-RING
RPROG
INPUT CURRENT
SENSE AMP
VOUT
VOUT
7
VSEL
– +
6
SHDN
SHUTDOWN
SD
4M
VREF
VBG
IZERO
COMP
GATE DRIVE
AND
ANTI-CROSS
CONDUCTION
COUT
IPK
COMP
IPK
VREF GOOD
ICLMP
LOGIC
OSC
THERMAL
SHUTDOWN
R2
ICLMP
COMP
IZERO
SLOPE COMP
+
–
+
–
VREF
FB
2
+
–
R1
CLK
CLK
TSD
MODE
CONTROL
WAKE
AVERAGING
CIRCUIT
VREF
IAVG
ERROR
AMP
EXPOSED
PAD
GND
9
1
SOFT START
+
gm
–
VCLAMP
3125 BD
3125f
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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.
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 capacitor 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.
The current mode architecture with adaptive slope compensation 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.
With its low RDS(ON) and low gate charge internal N-channel 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.
ERROR AMPLIFIER
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
improved large-signal transient response. Power converter
control loop compensation is provided internally. An external resistive voltage divider from VOUT to ground programs
the output voltage via FB from 2V to 5.25V.
⎛ R2 ⎞
VOUT = 1.2V ⎜ 1+ ⎟
⎝ R1⎠
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-channel 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
voltage, unless VOUT falls below 0.8V, in which case the
current limit is cut in half.
AVERAGE INPUT CURRENT LIMIT
Input current is sensed with an internal 60mΩ resistor
and converted to a voltage. A transconductance amplifier
creates a proportional current which is then forced into an
external resistor. The voltage across the resistor is averaged
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
limit set by the external resistor, RPROG.
RPROG =
22.1kΩ-A
Desired IAVG ( A)
Note that the LTC3125’s input current averaging circuit may
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.
ZERO CURRENT COMPARATOR
The zero current comparator monitors the inductor current 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.
INTERNAL CURRENT LIMIT
OSCILLATOR
Lossless current sensing converts the peak current signal of
the N-channel MOSFET switch into a voltage that is summed
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
pulling SHDN below 0.35V and enabled by pulling SHDN
above 1V. Note that SHDN can be driven above VIN or
VOUT, as long as it is limited to less than the absolute
maximum rating.
The LTC3125 contains internal circuitry to provide softstart 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.
OUTPUT DISCONNECT
The LTC3125 is designed to allow true output disconnect
by eliminating body diode conduction of the internal
P-channel MOSFET rectifier. This allows for VOUT to go
to zero volts during shutdown, drawing no current from
the input source. It also limits inrush current at turn-on,
minimizing surge currents seen by the input supply. Note
that to obtain the advantages of output disconnect, there
cannot be an external Schottky diode connected between
the SW pin and VOUT. The output disconnect feature also
allows VOUT to be pulled high, without any reverse current
into the power source connected to VIN.
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.
SYNCHRONOUS RECTIFIER
To control inrush current and to prevent the inductor
current from running away when VOUT is close to VIN, the
P-channel MOSFET synchronous rectifier is only enabled
when VOUT > (VIN + 0.38V).
ANTI-RINGING CONTROL
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
the ringing of the resonant circuit formed by L and CSW
(capacitance on SW pin) is low energy, it can cause EMI
radiation.
Burst Mode OPERATION
The LTC3125 will automatically enter Burst Mode operation 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 threshold current vs VIN. The load current at which Burst Mode
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.
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 operation, 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
consumes only 15μA of quiescent current from VOUT. When
the output voltage droops slightly, switching resumes.
This maximizes efficiency at very light loads by minimizing
switching and quiescent losses.
As the load current increases, the LTC3125 will automatically leave Burst Mode operation. Once the LTC3125 has left
Burst Mode operation and returned to normal operation,
it will remain there until the output load is reduced below
the burst threshold.
Burst Mode operation is inhibited during start-up and softstart and until VOUT is at least 0.38V greater than VIN.
3125f
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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
of current that exceed the maximum peak current specifications for CompactFlash and PCMCIA bus power.
The GSM standard specifies a 577μs, 2A (typical) transmission burst within a 4.6ms period (12.5% duty cycle).
During the receive and standby periods the current consumption drops to 70mA (typical), yielding an average
current requirement of 320mA.
Other standards (such as GPRS, Class 10) define a higher
data rate. One popular requirement transmits two 2A bursts
(3A worst case) within a 4.6ms frame period (70mA standby
current) demanding an 800mA average input current. The
LTC3125 external current limit programming resistor can
be easily adjusted for this requirement.
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 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 maximizes 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.
VIN > VOUT OPERATION
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.
SHORT-CIRCUIT PROTECTION
The LTC3125 output disconnect feature enables output
short circuit protection. To reduce power dissipation under
short-circuit conditions; the peak switch current limit is
reduced to 800mA (typical).
SCHOTTKY DIODE
Although it is not necessary, adding a Schottky diode from
SW to VOUT will improve efficiency by about 4%. Note that
this defeats the output disconnect, short-circuit protection
and average input limiting during start-up.
PCB LAYOUT GUIDELINES
The high speed operation of the LTC3125 demands careful 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.
COMPONENT SELECTION
Inductor Selection
The LTC3125 can utilize small surface mount chip inductors due to its fast 1.6MHz switching frequency. Inductor
values between 2.2μH and 4.7μH are suitable for most
applications. Larger values of inductance will allow slightly
greater output current capability (and lower the Burst Mode
threshold) by reducing the inductor ripple current. Increasing the inductance above 10μH will increase size while
providing little improvement in output current capability.
The minimum inductance value is given by:
L>
(
VIN(MIN) • VOUT(MAX ) – VIN(MIN)
)
Ripple • VOUT(MAX ) • fSW
where:
Ripple = Allowable inductor current ripple
(amps peak-peak)
VIN(MIN) = Minimum input voltage
VOUT(MAX) = Maximum output voltage
The inductor current ripple is typically set for 20% to
40% of the maximum inductor current. High frequency
ferrite core inductor materials reduce frequency dependent
power losses compared to cheaper powdered iron types,
improving efficiency. The inductor should have low DCR
3125f
10
LTC3125
APPLICATIONS INFORMATION
(DC resistance of the windings) to reduce the I2R power
losses, and must be able to support the peak inductor
current without saturating. Molded chokes and some chip
inductors usually do not have enough core area to support
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.
Table 1. Recommended Inductors
VENDOR
PART/STYLE
Coilcraft
(847) 639-6400
www.coilcraft.com
LPO2506
LPS4012, LPS4018
MSS6122
MSS4020
MOS6020
DS1605, DO1608
Coiltronics
www.cooperet.com
SD52, SD53, SD3114, SD3118
Murata
(714) 852-2001
www.murata.com
LQH55D
Sumida
(847) 956-0666
www.sumida
CDH40D11
Taiyo-Yuden
www.t-yuden.com
NP04SB
NR3015
NR4018
TDK
(847) 803-6100
www.component.tdk.com
VLP, LTF
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,
the magnitude and duration of the pulsing current, 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:
VRIPPLE (mV) =
(IPULSE – ISTANDBY ) • tON
COUT
where IPULSE and tON are the peak current and on time
during transmission burst and ISTANDBY is the current in
standby mode. The above is a worst-case approximation
assuming all the pulsing energy comes from the output
capacitor.
The ripple due to the capacitor ESR is:
VRIPPLE_ESR = (IPULSE – ISTANDBY) • ESR
Low ESR and high capacitance are critical to maintain low
output voltage ripple. Typically, two low profile 2200μF
parallel Vishay TANTAMOUNT® tantalum, low ESR capacitors 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
Capacitors from Cooper all offer very high capacitance and
low ESR in various package options. Table 2 shows a list
of several reservoir capacitor manufacturers.
High capacitance values and low ESR can lead to instability in some internally compensated boost converters. The
internal loop compensation of the LTC3125 is optimized to
be stable with output capacitor values greater than 500μF
with very low ESR.
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 reduce input current ripple without limitations. Consult the
manufacturers directly for detailed information on their
selection of ceramic capacitors. Although ceramic capacitors are recommended, low ESR tantalum capacitors may
be used as well.
Table 2. Capacitor Vendor Information
SUPPLIER
PHONE
WEBSITE
Vishay
(402) 563-6866
www.vishay.com
AVX
(803) 448-9411
www.avxcorp.com
Cooper Bussman
(516) 998-4100
www.cooperbussman.com
Cap-XX
(843) 267-0720
www.cap-xx-com
Panasonic
(800) 394-2112
www.panasonic.com
3125f
11
LTC3125
TYPICAL APPLICATIONS
PC Card or CompactFlash (3.3V/500mA Maximum) 4.5V Output, GSM Pulsed Load
2.2μH*
VIN
PC CARD VCC
3.3V ±10%
500mA MAX
SW
VOUT
CS
VIN
10μF
CER
OFF ON
VOUT
4.5V, 2A PULSED LOAD
(577μs PW, 4.6ms PERIOD)
LTC3125
SHDN
PROG
FB
GND
44.2k
2200μF**
s2
55mΩ
TANT
2.74M
1M
3125 TA02a
*COILTRONICS SD3118-2R2-R
**VISHAY 592D228X6R3X220H
Waveforms of Input Current, VOUT for Pulsed Load Current
VOUT
100mV/DIV
INPUT CURRENT
200mA/DIV
LOAD CURRENT
2A/DIV
VIN = 3.3V
VOUT = 4.5V
COUT = 4.4mF
L = 2.2μH
RPROG = 44.2k
3125 TA02b
1ms/DIV
PC Card (3.3V/1000mA Maximum) 4.5V Output, GSM Pulsed Load
2.7μH*
VIN
PC CARD VCC
3.3V ±10%
1000mA MAX
SW
VOUT
CS
VIN
10μF
CER
OFF ON
VOUT
4.5V, 2A PULSED LOAD
(577μs PW, 4.6ms PERIOD)
LTC3125
SHDN
PROG
22.6k
2200μF**
s2
55mΩ
TANT
2.74M
FB
GND
1M
3125 TA03a
*WURTH 7440420027
**VISHAY 592D228X6R3X220H
Waveforms of Input Current, VOUT for Pulsed Load Current
VOUT
100mV/DIV
INPUT CURRENT
500mA/DIV
LOAD CURRENT
2A/DIV
VIN = 3.3V
VOUT = 4.5V
COUT = 4.4mF
L = 2.7μH
RPROG = 22.6k
1ms/DIV
3125 TA03b
3125f
12
LTC3125
TYPICAL APPLICATIONS
PC Card (3.3V/1000mA Maximum) 4.5V Output, GPRS, Class 10 Pulsed Load
2.7μH*
VIN
PC CARD VCC
3.3V ±10%
1000mA MAX
SW
VOUT
CS
VIN
10μF
CER
OFF ON
VOUT
4.5V, 2A PULSED LOAD
(1154μs PW, 4.6ms PERIOD)
LTC3125
SHDN
FB
PROG
GND
22.6k
2200μF**
s3
55mΩ
TANT
2.74M
1M
3125 TA04a
*WURTH 7440420027
**VISHAY 592D228X6R3X220H
Waveforms of Input Current, VOUT for Pulsed Load Current
VOUT
100mV/DIV
INPUT CURRENT
500mA/DIV
LOAD CURRENT
2A/DIV
VIN = 3.3V
VOUT = 4.5V
COUT = 6.6mF
L = 2.7μH
RPROG = 22.6k
3125 TA04b
1ms/DIV
Single Supercap Charger
2.2μH*
VIN
3.3V ±10%
1000mA MAX
SW
VOUT
CS
VIN
10μF
CER
OFF ON
LTC3125
SHDN
1.07M
SC**
10F
60mΩ
FB
PROG
22.6k
VOUT
2.5V
GND
1M
3125 TA05a
*COILTRONICS SD3118-2R2-R
**COOPER B1325-2R5106-R
Waveforms of Input Current, VOUT for Pulsed Load Current
VOUT
500mV/DIV
INPUT CURRENT
500mA/DIV
LOAD CURRENT
1A/DIV
VIN = 3.3V
VOUT = 2.5V
COUT = 10F
L = 2.2μH
RPROG = 22.6k
200ms/DIV
3125 TA05b
3125f
13
LTC3125
TYPICAL APPLICATIONS
Stacked Supercap Charger
2.2μH*
SW
VOUT
CS
VIN
VIN
2.5V TO 5V
500mA MAX
10μF
CER
OFF ON
VOUT
4.5V
LTC3125
SHDN
2.74M
GND
44.2k
30F**
2.3V
+
100k
FB
PROG
+
100k
30F**
2.3V
1M
3125 TA06a
*TDK VLF4014ST-2R2M1R9
**PANASONIC EECHWOD306
Waveforms of Input Current, VOUT During Charging
VOUT
2V/DIV
SHDN
5V/DIV
LOAD CURRENT
200mA/DIV
VIN = 4.5V
VOUT = 4.5V
COUT_SERIES = 15F
L = 2.2μH
RPROG = 44.2k
3125 TA06b
20s/DIV
3.3V to 5V with Selectable Input Current Limit
2.2μH*
SW
VOUT
CS
VIN
VIN
3.3V ±10%
10μF
CER
OFF ON
LTC3125
300mA 500mA
M1
COUT
SHDN
3.2M
PROG
44.2k
VOUT
5V
FB
GND
1M
28.7k
3125 TA07a
*TDK VLF4014ST-2R2M1R9
Waveforms of Input Current, VOUT for Pulsed Input Current Limit
INPUT CURRENT
200mA/DIV
M1 GATE DRIVE
5V/DIV
VIN = 3.3V
VOUT = 5V
COUT = 4.4mF
L = 2.2μH
ILOAD = 500mA
2ms/DIV
3125 TA07b
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
3.50 ±0.05
1.65 ± 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
TYP
R = 0.05
5
TYP
2.00 ±0.10
(2 SIDES)
0.40 ± 0.10
8
1.35 ±0.10
1.65 ± 0.10
3.00 ±0.10
(2 SIDES)
PIN 1 NOTCH
R = 0.20 OR 0.25
s45° CHAMFER
PIN 1 BAR
TOP MARK
(SEE NOTE 6)
(DCB8) DFN 0106 REV A
4
0.200 REF
1
0.23 ± 0.05
0.45 BSC
0.75 ±0.05
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 representation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
15
LTC3125
RELATED PARTS
PART NUMBER
DESCRIPTION
COMMENTS
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LTC3426
2A (ISW), 1.5MHz Step-Up DC/DC Converter
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LTC3427
500mA (ISW), 1.25MHz Synchronous Step-Up DC/DC Converter 94% Efficiency, VIN: 1.8V to 5V, VOUT(MAX) = 5.25V, IQ = 350μA,
with Output Disconnect
ISD < 1μA, 2mm × 2mm DFN6 Package
LTC3429/LTC3429B
600mA (ISW), 500kHz, Synchronous Step-Up DC/DC Converter
with Output Disconnect and Soft-Start
96% Efficiency, VIN: 0.85V to 4.3V, VOUT(MAX) = 5V, IQ = 20μA,
ISD < 1μA, ThinSOT Package
LTC3458/LTC3458L
1.4A/1.7A (ISW), 1.5MHz Synchronous Step-Up DC/DC
Converter
94% Efficiency VIN: 0.85V to 6V, VOUT(MAX) = 7.5V/6V, IQ = 15μA,
ISD < 1μA, 3mm × 4mm DFN12 Package
LTC3459
80mA (ISW), Synchronous Step-Up DC/DC Converter
92% Efficiency, VIN: 1.5V to 5.5V, VOUT(MAX) = 10V, IQ = 10μA,
ISD < 1μA, ThinSOT Package
LT3494/LT3494A
180mA/350mA (ISW), High Efficiency Step-Up DC/DC
Converters with Output Disconnect
85% Efficiency, VIN: 2.3V to 16V, VOUT(MAX) = 38V, IQ = 65μA,
ISD < 1μA, 2mm × 3mm DFN6, ThinSOT Packages
LTC3523/LTC3523-2
600mA (ISW), Step-Up and 400MHz Synchronous Step-Down
1.2MHz/2.4MHz DC/DC Converter with Output Disconnect
94% Efficiency VIN: 1.8V to 5.5V, VOUT(MAX) = 5.25V, IQ = 45μA,
ISD < 1μA, 3mm × 3mm QFN16 Package
LTC3525-3/
LTC3525-3.3/
LTC3525-5
400mA (ISW), Micropower Synchronous Step-Up DC/DC
Converter with Output Disconnect
94% Efficiency, VIN: 0.85V to 4V, VOUT(MAX) = 5V, IQ = 7μA,
ISD < 1μA, SC-70 Package
LTC3526/LTC3526L
LTC3526B
500mA (ISW), 1MHz Synchronous Step-Up DC/DC Converter
with Output Disconnect
94% Efficiency VIN: 0.85V to 5V, VOUT(MAX) = 5.25V, IQ = 9μA,
ISD < 1μA, 2mm × 2mm DFN6 Package
LTC3527/LTC3527-1
Dual 800mA/400mA (ISW), 2.2MHz Synchronous Step-Up
DC/DC Converter with Output Disconnect
94% Efficiency VIN: 0.7V to 5V, VOUT(MAX) = 5.25V, IQ = 12μA,
ISD < 1μA, 3mm × 3mm QFN16 Package
LTC3528/LTC3528B
1A (ISW), 1MHz Synchronous Step-Up DC/DC Converter with
Output Disconnect
94% Efficiency VIN: 0.7V to 5.5V, VOUT(MAX) = 5.25V, IQ = 12μA,
ISD < 1μA, 2mm × 3mm DFN8 Package
LTC3537
600mA (ISW), 2.2MHz Synchronous Step-Up DC/DC Converter
with Output Disconnect and 100mA LDO
94% Efficiency VIN: 0.7V to 5V, VOUT(MAX) = 5.25V, IQ = 30μA,
ISD < 1μA, 3mm × 3mm QFN16 Package
LTC3539/LTC3539-2
2A (ISW), 1MHz, 2.2MHz Synchronous Step-Up DC/DC
Converter with Output Disconnect
94% Efficiency, VIN: 0.7V to 5V, VOUT(MAX) = 5.25V,
IQ = 10μA, ISD < 1μA, 2mm × 3mm DFN Package
3125f
16 Linear Technology Corporation
LT 1108 • PRINTED IN USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900 ● FAX: (408) 434-0507
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