LTC3125 1.2A Synchronous Step-Up DC/DC Converter with Input Current Limit FEATURES DESCRIPTION n The LTC®3125 is a high efficiency, synchronous step-up DC/DC converter with an accurate programmable average input current limit. The resistor programmable average input current limit is 5% accurate at 500mA and is suitable for a wide variety of applications. In mobile computing, GSM and GPRS cards demand high current pulses well beyond the capability of the PC Card and CompactFlash slots. The LTC3125 in concert with a reservoir capacitor, keeps the slot power safely within its capabilities providing a high performance and simple solution. n n n n n n n n n n n n n 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 Available in 2mm × 3mm × 0.75mm DFN Package APPLICATIONS n n n n 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 100 2.2μH 1 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 3125fa 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 Junction Temperature Range (Notes 2, 5) ............................................ –40°C to 125°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 125°C Consult LTC Marketing for parts specified with wider operating temperature ranges. Consult LTC Marketing for information on non-standard lead based finish parts. For more information on lead free part marking, go to: http://www.linear.com/leadfree/ For more information on tape and reel specifications, go to: http://www.linear.com/tapeandreel/ ELECTRICAL CHARACTERISTICS The l denotes the specifications which apply over the full operating junction temperature range, otherwise specifications are at TA = 25°C. VIN = 3.3V, VOUT = 4.5V unless otherwise noted (Note 2). 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 VIN = VOUT = 3.3V, Measured on VIN , FB ≥ 1.230V, Nonswitching 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 0.125 Ω P-Channel MOSFET Switch On-Resistance VOUT = 3.3V 0.200 Ω A N-Channel MOSFET Current Limit Current Limit Delay to Output (Note 3) Average Input Current Limit RPROG = 44.2k RPROG = 44.2k, (Note 4) l 1.2 1.8 l 475 465 500 500 60 ns 525 535 mA mA 3125fa 2 LTC3125 ELECTRICAL CHARACTERISTICS The l denotes the specifications which apply over the full operating junction temperature range, otherwise specifications are at TA = 25°C. VIN = 3.3V, VOUT = 4.5V unless otherwise noted (Note 2). 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 tested under pulsed load conditions such that TJ ≈ TA. The LTC3125E (E Grade) is guaranteed to meet specifications from 0°C to 85°C junction temperature. Specifications over the –40°C to 125°C operating junction temperature range are assured by design, characterization and correlation with statistical process controls. The junction temperature (TJ) is calculated from the ambient temperature (TA) and power dissipation (PD) according to the formula: TJ = TA + (PD) (θJA °C/W), where θJA is the package thermal impedance. The maximum ambient temperature consistent with these specifications is determined by % MHz V SHDN Input Low SHDN Input Current UNITS 0.35 V 1 μA specific operating conditions in conjunction with board layout, the rated package thermal resistance and other environmental factors. Note 3: Specification is guaranteed by design and not 100% tested in production. Note 4: Current measurements are made when the output is not switching. Note 5: This IC includes overtemperature protection that is intended to protect the device during momentary overload conditions. Junction temperature will exceed 125°C when overtemperature protection is active. Continuous operation above the specified maximum operating junction temperature may result in device degradation or failure. Note 6: Failure to solder the exposed backside of the package to the PC board ground plane will result in a thermal resistance much higher than 60°C/W. TYPICAL PERFORMANCE CHARACTERISTICS (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 3125fa 3 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 –0.5 –1.0 –1.5 –2.0 1.5 2.0 2.5 0 –0.50 –1.00 0 25 50 TEMPERATURE (°C) 75 2.50 0.75 0.50 0.25 VOUT = 3.8V RPROG = 0Ω 2.40 2.35 2.30 2.25 2.20 2.15 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 50 LOAD CURRENT (mA) VOUT = 2.5V COUT = 1500μF L = 2.2μH 1 FREQUENCY CHANGE (%) 45 5.5 3125 G08 Burst Mode Threshold Current vs VIN VOUT = 3.3V COUT = 1500μF L = 2.2μH 4.5 2.05 Burst Mode Threshold Current vs VIN 20 4.0 2.10 3125 G06 30 3.5 2.45 1.00 0 100 40 3.0 VIN (V) Peak Current Limit vs VIN 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 –25 1.0 3125 G05 1.25 NORMALIZED TO 25°C –1.50 –50 NORMALIZED TO 25°C 1.5 3125 G04 Average Input Current Limit vs Temperature 1.50 4 2.0 VIN (V) 3125 G03 LOAD CURRENT (mA) 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 3125fa 4 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 0.100 120 0.050 1.5 100 –50 –30 2 2.5 3.5 3 VOUT (V) 4 4.5 5 0.50 –4 –8 30 50 –10 10 TEMPERATURE (°C) 70 –10 –50 90 0 –0.25 –0.50 –0.75 70 90 0.25 15.5 0 15.0 –0.25 14.5 –0.50 14.0 –0.75 13.5 –1.00 –50 –30 100 75 16.0 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 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 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) CHANGE IN VFB (%) 0.25 CHANGE IN VFB (%) 0 –2 Current Sense Voltage (VRPROG) vs Temperature NORMALIZED TO 25°C VIN = 3.3V VOUT = 4.5V COUT = 4.4mF L = 2.7μH 2 3125 G13 Feedback vs Temperature 30 50 –10 10 TEMPERATURE (°C) 4 –6 3125 G12 –1.00 –50 –30 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 3125fa 5 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, Exposed Pad Pin 9): Ground. The exposed pad must be soldered to the PCB ground plane for electrical connection and for rated thermal performance. 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. Refer to the Component Selection section in Applications Information for details on selecting RPROG. 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. 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 150μF 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. 3125fa 6 LTC3125 BLOCK DIAGRAM L1 VIN CIN 4 5 VIN 8 SW CS 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 3125fa 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. 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. 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 A current proportional to the internally sensed input current is sourced out of the PROG pin. The voltage across the external resistor on the PROG pin is averaged and compared to a temperature stable internal reference, providing a signal 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 value of the external resistor, RPROG. ERROR AMPLIFIER The LTC3125 is trimmed and tested at 500mA to obtain a ±5% initial accuracy. At other current limit settings, nonidealities such as random offsets in the input current limit loop will degrade the accuracy in the application. RPROG tolerance must also be considered when setting the input current limit as the accuracies listed in the Electrical Characteristics section do not include external resistor variation. 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. Traditional, internally compensated, current mode controlled boost converters can be unstable with the high capacitance and low ESR values used in supercapacitor chargers and pulsed load applications. The internal loop compensation of the LTC3125 is optimized to be stable with output capacitor values greater than 150μF with very low ESR. Output capacitor values below 150μF will degrade transient response and can lead to instability. ⎛ 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. Note that the LTC3125’s input current averaging circuit may introduce a slightly higher inductor current 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 3125fa 8 LTC3125 OPERATION prevents the inductor current from reversing in polarity, improving efficiency at light loads. OSCILLATOR An internal oscillator sets the switching frequency to 1.6MHz. 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 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. 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. 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. 3125fa 9 LTC3125 APPLICATIONS INFORMATION Burst Mode operation is inhibited during start-up and soft-start and until VOUT is at least 0.38V greater than VIN. 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 although input current limit functionality is maintained. 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 3125fa 10 LTC3125 APPLICATIONS INFORMATION power losses compared to cheaper powdered iron types, improving efficiency. The inductor should have low DCR (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)= 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. 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 (IPULSE –ISTANDBY ) •tON COUT 3125fa 11 LTC3125 APPLICATIONS INFORMATION AVERAGE INPUT ILIMIT PROGRAMMING RESISTOR SELECTION The input current limit is user programmable by selection of an external resistor, RPROG. It is important to locate the resistor as close to the pin as possible to minimize capacitance and noise pick-up. Resistor tolerance directly affects the current limit accuracy so it must be factored in to the application requirements. Table 3 shows standard resistors for typical current limit values. Also refer to the graph, “Average Input Current vs RPROG”, in the Typical Performance Characteristics section of this datasheet. Table 3. STANDARD 1% RESISTOR VALUE (K) TYPICAL APPLICATION INPUT LIMIT (A) 22.1 1.001 24.9 0.890 28.0 0.791 29.4 0.750 31.6 0.699 37.4 0.588 54.9 0.393 71.5 0.295 82.5 0.252 For most applications the loss in accuracy from standard 1% resistors is tolerated but for critical applications the use of 0.1% resistors is recommended. TYPICAL APPLICATIONS Waveforms of Input Current, VOUT for Pulsed Load Current 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 LTC3125 SHDN 2200μF** s2 55mΩ TANT 2.74M PROG 22.6k VOUT 4.5V, 2A PULSED LOAD (577μs PW, 4.6ms PERIOD) FB GND 1M 3125 TA03a *WURTH 7440420027 **VISHAY 592D228X6R3X220H 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 3125fa 12 LTC3125 TYPICAL APPLICATIONS PC Card (3.3V/1000mA Maximum) 4.5V Output, GPRS, Class 10 Pulsed Load Waveforms of Input Current, VOUT for Pulsed Load Current 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 PROG FB GND 22.6k 2200μF** s3 55mΩ TANT 2.74M 1M VOUT 100mV/DIV INPUT CURRENT 500mA/DIV 3125 TA04a LOAD CURRENT 2A/DIV *WURTH 7440420027 **VISHAY 592D228X6R3X220H VIN = 3.3V VOUT = 4.5V COUT = 6.6mF L = 2.7μH RPROG = 22.6k Single Supercap Charger SW VOUT CS VIN 10μF CER OFF ON 1.07M PROG 22.6k VOUT 2.5V LTC3125 SHDN 3125 TA04b Waveforms of Input Current, VOUT for Pulsed Load Current 2.2μH* VIN 3.3V ±10% 1000mA MAX 1ms/DIV SC** 10F 60mΩ VOUT 500mV/DIV INPUT CURRENT 500mA/DIV FB GND 1M 3125 TA05a *COILTRONICS SD3118-2R2-R **COOPER B1325-2R5106-R LOAD CURRENT 1A/DIV VIN = 3.3V VOUT = 2.5V COUT = 10F L = 2.2μH RPROG = 22.6k 200ms/DIV 3125 TA05b 3125fa 13 LTC3125 TYPICAL APPLICATIONS Stacked Supercap Charger 2.2μH* VIN 2.5V TO 5V 500mA MAX SW VOUT CS VIN 10μF CER OFF ON LTC3125 SHDN 2.74M PROG 44.2k VOUT 4.5V + 100k FB GND + 100k 1M 30F** 2.3V 30F** 2.3V 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 20s/DIV 3125 TA06b 3125fa 14 LTC3125 TYPICAL APPLICATIONS 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 3125fa 15 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 3125fa 16 LTC3125 REVISION HISTORY REV DATE DESCRIPTION PAGE NUMBER A 12/10 Text change to Description 1 Change to Electrical Characteristics Quiescent Current-Burst 2 Modification of Note 2 3 Pin Functions; change to GND (Pin 1), PROG (Pin 3) and VOUT (Pin 7) 6 Replaced Average Input Current Limit section 8 Added Average Input Limit Programming Resistor Selection section 12 Updated Related Parts table 18 3125fa 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. 17 LTC3125 TYPICAL APPLICATION PC Card or CompactFlash (3.3V/500mA Maximum) 4.5V Output, GSM Pulsed Load Waveforms of Input Current, VOUT for Pulsed Load Current VOUT 100mV/DIV 2.2μH* VIN PC CARD VCC 3.3V ±10% 500mA MAX SW VOUT CS VIN 10μF CER OFF ON 44.2k VOUT 4.5V, 2A PULSED LOAD (577μs PW, 4.6ms PERIOD) LTC3125 2200μF** ×2 55mΩ TANT 2.74M SHDN PROG FB GND 1M INPUT CURRENT 200mA/DIV LOAD CURRENT 2A/DIV 3125 TA02a VIN = 3.3V VOUT = 4.5V COUT = 4.4mF L = 2.2μH RPROG = 44.2k *COILTRONICS SD3118-2R2-R **VISHAY 592D228X6R3X220H 1ms/DIV 3125 TA02b RELATED PARTS PART NUMBER DESCRIPTION COMMENTS LTC3127 1A Buck-Boost Converter with Programmable Input Current Limit 96% Efficiency, ±4% Accurate Average Input Current Limit, VIN: 1.8V to 5.5V, VOUT = 1.8V to 5.25V, IQ = 35μA, DFN Package LTC3421 3A (ISW), 3MHz, Synchronous Step-Up DC/DC Converter with Output Disconnect 94% Efficiency, VIN: 0.85V to 4.5V, VOUT(MAX) = 5.25V, IQ = 12μA, ISD < 1μA, 4mm × 4mm QFN24 Package LTC3422 1.5A (ISW), 3MHz Synchronous Step-Up DC/DC Converter with Output Disconnect 94% Efficiency, VIN: 0.85V to 4.5V, VOUT(MAX) = 5.25V, IQ = 25μA, ISD < 1μA, 3mm × 3mm DFN10 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 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 3125fa 18 Linear Technology Corporation LT 1210 REV A • PRINTED IN USA 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com © LINEAR TECHNOLOGY CORPORATION 2008