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LTC3125 1.2A Synchronous Step-Up DC/DC Converter with Input Current Limit FEATURES
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DESCRIPTION
The LTC(R)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. 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 <1A shutdown current, shortcircuit and thermal overload protection. The LTC3125 is offered in a low profile 0.75mm x 2mm x 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 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(R) Operation (15A) Available in 2mm x 3mm x 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
2.2H VIN 3.3V 500mA CS VIN 10F CER OFF ON SHDN SW VOUT LTC3125 1.24M VOUT 4V 2A PULSED LOAD 2200F 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) VOUT = 4V VIN = 3.3V VIN = 2.4V 1
3125 TA01b
1
POWER LOSS (W)
0.1
PROG 44.2k GND
FB 536k
0.01
0.001
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LTC3125 ABSOLUTE MAXIMUM RATINGS
(Note 1)
PIN CONFIGURATION
TOP VIEW GND 1 FB 2 PROG 3 VIN 4 9 8 SW 7 VOUT 6 SHDN 5 CS
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) .............................................. -40C to 85C Junction Temperature ........................................... 125C Storage Temperature Range................... -65C to 125C
DCB PACKAGE 8-LEAD (2mm 3mm) PLASTIC DFN TJMAX = 125C, JA = 64C/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 x 3mm) Plastic DFN TEMPERATURE RANGE -40C to 85C 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 temperature range, otherwise specifications are at TA = 25C. VIN = 3.3V, VOUT = 4.5V unless otherwise noted.
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 A A A A A A ns
2 1.176 1.200 1 0.01 300 15 0.1 0.1 0.125 0.200
VSHDN = 0V, Not Including Switch Leakage, VOUT = 0V Measured on VOUT, Nonswitching Measured on VOUT, FB = 1.230V VSW = 5V, VIN = 5V VSW = 5V, VOUT = 0V, VIN = 5V VOUT = 3.3V VOUT = 3.3V
l
1.2 475 465
1.8 60 500 500 525 535
RPROG = 44.2k RPROG = 44.2k, -40C to 85C
l
mA mA
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LTC3125 ELECTRICAL CHARACTERISTICS
PARAMETER PROG Current Gain Maximum Duty Cycle Minimum Duty Cycle Frequency SHDN Input High SHDN Input Low SHDN Input Current VSHDN = 1.2V 0.3 CONDITIONS (Note 3) VFB = 1.15V VFB = 1.3V
l l l
The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25C. VIN = 3.3V, VOUT = 4.5V unless otherwise noted.
MIN 85 1.3 1 0.35 1 TYP 22.1 92 0 1.6 1.9 MAX UNITS k-A/A % % MHz V V A
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 0C to 85C. Specifications over -40C to 85C 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.
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 125C 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 60C/W.
TYPICAL PERFORMANCE CHARACTERISTICS (TA = 25C unless otherwise noted)
Efficiency vs Load Current, VOUT = 2.5V
100 90 80 EFFICIENCY (%) 70 60 50 40 30 20 10 0 0.001 0.01 0.1 LOAD CURRENT (A) VIN = 2.1V VIN = 1.8V 1
3125 G01
Efficiency vs Load Current, VOUT = 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) VIN = 2.8V VIN = 2.4V VIN = 2V 1
3125 G02
1
POWER LOSS (W)
0.1
0.01
0.01
0.001
0.001
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LTC3125 TYPICAL PERFORMANCE CHARACTERISTICS (TA = 25C unless otherwise noted)
Efficiency vs Load Current, VOUT = 5V
100 90 80 EFFICIENCY (%) 70 60 50 40 30 20 10 0 0.001 0.01 0.1 LOAD CURRENT (A) VIN = 4V VIN = 3.3V 1
3125 G03
No-Load Input Current vs VIN
AVERAGE INPUT CURRENT LIMIT CHANGE (%) 1 4.0 3.5 3.0 POWER LOSS (W) 0.1 2.5 IIN (mA) VOUT = 4V 2.0 VOUT = 3.8V 1.5 VOUT = 3.3V 1.0 0.5 0.001 0 1.5 VOUT = 2.5V 2 2.5 VIN (V)
3125 G04
Average Input Current Limit vs VIN
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 VIN (V) 3.5 4.0 4.5
3125 G05
NORMALIZED TO 25C
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 25C AVERAGE INPUT CURRENT LIMIT (A) 1.25
Average Input Current vs RPROG
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 RPROG (k)
3125 G07
Peak Current Limit vs VIN
VOUT = 3.8V RPROG = 0
0.75
0.50
0.25
2.00 1.5 2 2.5 3 3.5 4 VIN (V) 4.5 5 5.5
-25
0 25 50 TEMPERATURE (C)
75
100
3125 G06
3125 G08
Burst Mode Threshold Current vs VIN
50 VOUT = 3.3V COUT = 1500F L = 2.2H LOAD CURRENT (mA) 50 45 40 35 30 25 10 1.8 20 2.0 2.4 2.2 VIN (V) 2.6 2.8
3125 G09
Burst Mode Threshold Current vs VIN
2 VOUT = 2.5V COUT = 1500F L = 2.2H FREQUENCY CHANGE (%) 1 0 -1 -2 -3 -4 -5 -6 -7 1.8 1.9 2.0 VIN (V) 2.1 2.2
3125 G10
Oscillator Frequency vs VOUT
NORMALIZED TO VOUT = 3.3V
LOAD CURRENT (mA)
40
30
20
-8 2.0 2.5 3.0
3.5 4.0 VOUT (V)
4.5
5.0
3125 G11
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LTC3125 TYPICAL PERFORMANCE CHARACTERISTICS (TA = 25C unless otherwise noted)
RDS(ON) vs VOUT
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 VOUT (V) 4 4.5 5 NMOS RDS(ON) (m) RDS(ON) () 240 220 200 180 160 140 120 100 -50 -30 NMOS FREQUENCY CHANGE (%) PMOS
RDS(ON) vs Temperature
VOUT = 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 25C
-10 -50
-25
25 50 0 TEMPERATURE (C)
75
100
3125 G14
3125 G12
Feedback vs Temperature
0.50 0.25 CHANGE IN VFB (%) CHANGE IN VFB (%) 0 -0.25 -0.50 -0.75 -1.00 -50 -30 NORMALIZED TO 25C 0.50 0.25 0
Current Sense Voltage (VRPROG) vs Temperature
NORMALIZED TO 25C 16.0 15.5 15.0 IQ (A) 14.5 14.0 13.5
Burst Mode Current vs VOUT
-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 VOUT (V)
4
4.5
5
3125 G17
VOUT and IIN During Soft-Start
VOUT 2V/DIV SHDN 5V/DIV VOUT 2V/DIV SHDN 5V/DIV
VOUT and IIN During Soft-Start
INPUT CURRENT 200mA/DIV VIN = 3.3V VOUT = 4.5V COUT = 4.4mF L = 2.7H 20ms/DIV
BURST CURRENT
3125 G18
INPUT CURRENT 200mA/DIV VIN = 3.3V VOUT = 4.5V COUT = 0.47F L = 2.7H 1s/DIV
3125 G19
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LTC3125 TYPICAL PERFORMANCE CHARACTERISTICS (TA = 25C unless otherwise noted)
VOUT and IIN During Soft-Start
100 VOUT 2V/DIV SHDN 5V/DIV INPUT CURRENT 200mA/DIV VIN = 3.3V VOUT = 4.5V COUT = 1F L = 2.7H 2s/DIV
3125 G20
Efficiency vs VIN
ILOAD = 200mA 95 VOUT = 3.8V
90 EFFICIENCY (%) 85 80 75 70 65 60 55 50 2 3 VIN (V)
3125 G21
4
5
PIN FUNCTIONS
GND (Pin 1): Signal Ground. 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. 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 = 22.1k-A/A. The average input current limit threshold is set by RPROG according to the following: RPROG = 22.1k-A Desired IAVG ( A) CS (Pin 5): Current Sense Resistor Connection Point. Connect the inductor directly to CS. An internal 60m sense resistor is connected between CS and VIN. SHDN (Pin 6): Logic Controlled Shutdown Input. Bringing this pin above 1V enables the part, forcing this pin below 0.35V disables the part. 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 10F ceramic is recommended. Due to the output disconnect feature, VOUT is disconnected from VIN when SHDN is low. 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.
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 1F is recommended. Also connects to CS through 60m internal sense resistor.
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LTC3125 BLOCK DIAGRAM
L1 VIN CIN 4 VIN RSENSE VBEST VSEL 5 CS 8 SW
3 RPROG
PROG ANTI-RING INPUT CURRENT SENSE AMP VSEL
6
SHDN 4M
SHUTDOWN
SD
GATE DRIVE AND ANTI-CROSS CONDUCTION IPK COMP ICLMP COMP SLOPE COMP
VREF VBG VREF GOOD
OSC
CLK CLK TSD WAKE MODE CONTROL
THERMAL SHUTDOWN
AVERAGING CIRCUIT EXPOSED PAD 9
+-
gm VB WELL-SWITCH VOUT 7 VOUT
-+
IZERO COMP R2 COUT IPK
+ - + -
+ -
VREF FB 2 R1
ICLMP LOGIC IZERO
VREF
+ gm -
IAVG ERROR AMP VCLAMP
SOFT START
GND 1
3125 BD
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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 15A. 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 INTERNAL CURRENT LIMIT Lossless current sensing converts the peak current signal of the N-channel MOSFET switch into a voltage that is summed 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 60m 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 = 22.1k-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. OSCILLATOR An internal oscillator sets the switching frequency to 1.6MHz.
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LTC3125 OPERATION
SHUTDOWN 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. 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 160C 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 15C. 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 15A 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. SOFT-START 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.
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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 577s, 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 2.2H and 4.7H 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 10H will increase size while providing little improvement in output current capability. The minimum inductance value is given by: L> 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
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VIN(MIN) * VOUT(MAX ) - VIN(MIN) Ripple * VOUT(MAX ) * fSW
(
)
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 Coilcraft (847) 639-6400 www.coilcraft.com PART/STYLE LPO2506 LPS4012, LPS4018 MSS6122 MSS4020 MOS6020 DS1605, DO1608 SD52, SD53, SD3114, SD3118 LQH55D
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 2200F parallel Vishay TANTAMOUNT(R) tantalum, low ESR capacitors are used. The capacitor has less than 40m 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(R) 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 500F 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 10F 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 Vishay AVX Cooper Bussman Cap-XX Panasonic PHONE (402) 563-6866 (803) 448-9411 (516) 998-4100 (843) 267-0720 (800) 394-2112 WEBSITE www.vishay.com www.avxcorp.com www.cooperbussman.com www.cap-xx-com www.panasonic.com
Coiltronics www.cooperet.com Murata (714) 852-2001 www.murata.com Sumida (847) 956-0666 www.sumida Taiyo-Yuden www.t-yuden.com TDK (847) 803-6100 www.component.tdk.com Wurth (201) 785-8800 www.we-online.com
CDH40D11
NP04SB NR3015 NR4018 VLP LTF , VLF VLCF , 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
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LTC3125 TYPICAL APPLICATIONS
PC Card or CompactFlash (3.3V/500mA Maximum) 4.5V Output, GSM Pulsed Load
2.2H* VIN PC CARD VCC 3.3V 10% 500mA MAX CS VIN 10F CER OFF ON SHDN SW VOUT LTC3125 2.74M
VOUT 4.5V, 2A PULSED LOAD (577s PW, 4.6ms PERIOD) 2200F** 2 55m TANT
3125 TA02a
PROG 44.2k GND
FB 1M
*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 = 2.2H RPROG = 44.2k 1ms/DIV
3125 TA02b
PC Card (3.3V/1000mA Maximum) 4.5V Output, GSM Pulsed Load
2.7H* VIN PC CARD VCC 3.3V 10% 1000mA MAX CS VIN 10F CER OFF ON SHDN SW VOUT LTC3125 2.74M
VOUT 4.5V, 2A PULSED LOAD (577s PW, 4.6ms PERIOD) 2200F** 2 55m TANT
3125 TA03a
PROG 22.6k GND
FB 1M
*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 = 2.7H RPROG = 22.6k 1ms/DIV
3125 TA03b
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LTC3125 TYPICAL APPLICATIONS
PC Card (3.3V/1000mA Maximum) 4.5V Output, GPRS, Class 10 Pulsed Load
2.7H* VIN PC CARD VCC 3.3V 10% 1000mA MAX CS VIN 10F CER OFF ON SHDN SW VOUT LTC3125 2.74M
VOUT 4.5V, 2A PULSED LOAD (1154s PW, 4.6ms PERIOD) 2200F** 3 55m TANT
3125 TA04a
PROG 22.6k GND
FB 1M
*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 = 2.7H RPROG = 22.6k 1ms/DIV
3125 TA04b
Single Supercap Charger
2.2H* CS VIN 10F CER OFF ON SHDN SW VOUT LTC3125 1.07M
VIN 3.3V 10% 1000mA MAX
VOUT 2.5V SC** 10F 60m
PROG 22.6k GND
FB 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 = 2.2H RPROG = 22.6k 200ms/DIV
3125 TA05b
3125f
13
LTC3125 TYPICAL APPLICATIONS
Stacked Supercap Charger
2.2H* CS VIN 10F CER OFF ON SHDN SW VOUT LTC3125 2.74M 100k 100k 1M
3125 TA06a
VIN 2.5V TO 5V 500mA MAX
VOUT 4.5V
+ +
30F** 2.3V 30F** 2.3V
PROG 44.2k GND
FB
*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 = 2.2H RPROG = 44.2k 20s/DIV
3125 TA06b
3.3V to 5V with Selectable Input Current Limit
2.2H* CS VIN 10F CER OFF ON SHDN SW VOUT LTC3125 3.2M
VIN 3.3V 10%
VOUT 5V COUT
PROG 44.2k GND
FB 1M
300mA 500mA
M1
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 = 2.2H ILOAD = 500mA 2ms/DIV
3125 TA07b
3125f
14
LTC3125 PACKAGE DESCRIPTION
DCB Package 8-Lead Plastic DFN (2mm x 3mm)
(Reference LTC DWG # 05-08-1718 Rev A)
0.70 0.05
3.50 0.05 2.10 0.05
1.35 0.05 1.65 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 3.00 0.10 (2 SIDES) PIN 1 BAR TOP MARK (SEE NOTE 6) 4 0.200 REF 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 1 0.23 0.05 0.45 BSC 1.65 0.10 PIN 1 NOTCH R = 0.20 OR 0.25 45 CHAMFER
(DCB8) DFN 0106 REV A
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 LTC3400/LTC3400B LTC3421 LTC3422 LTC3426 LTC3427 LTC3429/LTC3429B LTC3458/LTC3458L LTC3459 LT3494/LT3494A LTC3523/LTC3523-2 LTC3525-3/ LTC3525-3.3/ LTC3525-5 LTC3526/LTC3526L LTC3526B LTC3527/LTC3527-1 LTC3528/LTC3528B LTC3537 LTC3539/LTC3539-2 DESCRIPTION 600mA (ISW), 1.2MHz, Synchronous Step-Up DC/DC Converters 3A (ISW), 3MHz, Synchronous Step-Up DC/DC Converter with Output Disconnect 1.5A (ISW), 3MHz Synchronous Step-Up DC/DC Converter with Output Disconnect 2A (ISW), 1.5MHz Step-Up DC/DC Converter COMMENTS 92% Efficiency, VIN: 0.85V to 5V, VOUT(MAX) = 5V, IQ = 19A/300A, ISD < 1A, ThinSOTTM Package 94% Efficiency, VIN: 0.85V to 4.5V, VOUT(MAX) = 5.25V, IQ = 12A, ISD < 1A, 4mm x 4mm QFN24 Package 94% Efficiency, VIN: 0.85V to 4.5V, VOUT(MAX) = 5.25V, IQ = 25A, ISD < 1A, 3mm x 3mm DFN10 Package 92% Efficiency, VIN: 1.6V to 5.5V, VOUT(MAX) = 5V, IQ = 600A, ISD < 1A, ThinSOT Package
500mA (ISW), 1.25MHz Synchronous Step-Up DC/DC Converter 94% Efficiency, VIN: 1.8V to 5V, VOUT(MAX) = 5.25V, IQ = 350A, with Output Disconnect ISD < 1A, 2mm x 2mm DFN6 Package 600mA (ISW), 500kHz, Synchronous Step-Up DC/DC Converter with Output Disconnect and Soft-Start 1.4A/1.7A (ISW), 1.5MHz Synchronous Step-Up DC/DC Converter 80mA (ISW), Synchronous Step-Up DC/DC Converter 180mA/350mA (ISW), High Efficiency Step-Up DC/DC Converters with Output Disconnect 600mA (ISW), Step-Up and 400MHz Synchronous Step-Down 1.2MHz/2.4MHz DC/DC Converter with Output Disconnect 400mA (ISW), Micropower Synchronous Step-Up DC/DC Converter with Output Disconnect 500mA (ISW), 1MHz Synchronous Step-Up DC/DC Converter with Output Disconnect Dual 800mA/400mA (ISW), 2.2MHz Synchronous Step-Up DC/DC Converter with Output Disconnect 1A (ISW), 1MHz Synchronous Step-Up DC/DC Converter with Output Disconnect 600mA (ISW), 2.2MHz Synchronous Step-Up DC/DC Converter with Output Disconnect and 100mA LDO 2A (ISW), 1MHz, 2.2MHz Synchronous Step-Up DC/DC Converter with Output Disconnect 96% Efficiency, VIN: 0.85V to 4.3V, VOUT(MAX) = 5V, IQ = 20A, ISD < 1A, ThinSOT Package 94% Efficiency VIN: 0.85V to 6V, VOUT(MAX) = 7.5V/6V, IQ = 15A, ISD < 1A, 3mm x 4mm DFN12 Package 92% Efficiency, VIN: 1.5V to 5.5V, VOUT(MAX) = 10V, IQ = 10A, ISD < 1A, ThinSOT Package 85% Efficiency, VIN: 2.3V to 16V, VOUT(MAX) = 38V, IQ = 65A, ISD < 1A, 2mm x 3mm DFN6, ThinSOT Packages 94% Efficiency VIN: 1.8V to 5.5V, VOUT(MAX) = 5.25V, IQ = 45A, ISD < 1A, 3mm x 3mm QFN16 Package 94% Efficiency, VIN: 0.85V to 4V, VOUT(MAX) = 5V, IQ = 7A, ISD < 1A, SC-70 Package 94% Efficiency VIN: 0.85V to 5V, VOUT(MAX) = 5.25V, IQ = 9A, ISD < 1A, 2mm x 2mm DFN6 Package 94% Efficiency VIN: 0.7V to 5V, VOUT(MAX) = 5.25V, IQ = 12A, ISD < 1A, 3mm x 3mm QFN16 Package 94% Efficiency VIN: 0.7V to 5.5V, VOUT(MAX) = 5.25V, IQ = 12A, ISD < 1A, 2mm x 3mm DFN8 Package 94% Efficiency VIN: 0.7V to 5V, VOUT(MAX) = 5.25V, IQ = 30A, ISD < 1A, 3mm x 3mm QFN16 Package 94% Efficiency, VIN: 0.7V to 5V, VOUT(MAX) = 5.25V, IQ = 10A, ISD < 1A, 2mm x 3mm DFN Package
3125f
16 Linear Technology Corporation
(408) 432-1900 FAX: (408) 434-0507
LT 1108 * PRINTED IN USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
www.linear.com
(c) LINEAR TECHNOLOGY CORPORATION 2008

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