LTM4605

LTM4605 High Efficiency Buck-Boost DC/DC µModule FEATURES n n n n n n n n n n n n DESCRIPTION The LTM®4605 is a high efficiency switching mode buckboost power supply. Included in the package are the switching controller, power FETs, and support components. Operating over an input voltage range of 4.5V to 20V, the LTM4605 supports an output voltage range of 0.8V to 16V, set by a resistor. This high efficiency design delivers up to 5A continuous current in boost mode (12A in buck mode). Only the inductor, sense resistor, bulk input and output capacitors are needed to finish the design. The low profile package enables utilization of unused space on the bottom of PC boards for high density point of load regulation. The high switching frequency and current mode architecture enable a very fast transient response to line and load changes. The LTM4605 can be frequency synchronized with an external clock to reduce undesirable frequency harmonics. Fault protection features include overvoltage and foldback current protection. The DC/DC μModule™ is offered in a small and thermally enhanced 15mm × 15mm × 2.8mm LGA package. The LTM4605 is Pb-free and RoHS compliant. L, LT, LTC and LTM are registered trademarks of Linear Technology Corporation. Burst Mode is a registered trademark of Linear Technology Corporation. μModule is a trademark of Linear Technology Corporation. All other trademarks are the property of their respective owners. Single Inductor Architecture Allows VIN Above, Below or Equal to VOUT Wide VIN Range: 4.5V to 20V Wide VOUT Range: 0.8V to 16V 5A DC Typical (12A DC Typical at Buck Mode) High Efficiency Up to 98% Current Mode Control Power Good Output Signal Phase-Lockable Fixed Frequency: 200kHz to 400kHz Ultra-Fast Transient Response Current Foldback Protection Output Overvoltage Protection Small, Low Profile Surface Mount LGA Package (15mm × 15mm × 2.8mm) APPLICATIONS n n n Telecom, Servers and Networking Equipment Industrial and Automotive Equipment High Power Battery-Operated Devices TYPICAL APPLICATION 12V/5A Buck-Boost DC/DC μModule with 4.5V to 20V Input VIN 4.5V TO 20V CLOCK SYNC 10μF 35V ON/OFF VIN RUN PLLIN V OUT FCB LTM4605 SW1 SW2 RSENSE SENSE+ 0.1μF SS SGND PGND SENSE– VFB 7.15k 4605 TA01 Efficiency and Power Loss vs Input Voltage 99 VOUT 12V 5A EFFICIENCY (%) 98 97 96 95 4 94 93 92 91 90 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 VIN (V) 4605 TA01b 10μF 35V 4.7μH + VOUT = 12V ILOAD = 5A f = 200kHz 8 7 6 POWER LOSS (W) 5 330μF 25V 3 2 1 0 6mΩ 4605fa 1 LTM4605 ABSOLUTE MAXIMUM RATINGS VIN ............................................................. –0.3V to 20V VOUT ............................................................. 0.8V to 16V INTVCC, EXTVCC, RUN, SS, PGOOD .............. –0.3V to 7V SW1, SW2 .................................................... –5V to 20V VFB, COMP ................................................ –0.3V to 2.4V FCB, STBYMD ....................................... –0.3V to INTVCC PLLIN ........................................................ –0.3V to 5.5V PLLFLTR.................................................... –0.3V to 2.7V Operating Temperature Range (Note 2) ...............................................–40°C to 85°C Junction Temperature ........................................... 125°C Storage Temperature Range...................–55°C to 125°C PIN CONFIGURATION (See Table 6. Pin Assignment) TOP VIEW BANK 2 M L BANK 4 K J H G BANK 1 BANK 3 BANK 5 F E D C BANK 6 B A 1 2 3 4 5 6 7 8 9 10 11 12 LGA PACKAGE 141-LEAD (15mm 15mm 2.8mm) TJMAX = 125°C, θJP = 4°C/W WEIGHT = 1.5g ORDER INFORMATION LEAD FREE FINISH LTM4605EV#PBF LTM4605IV#PBF PART MARKING* LTM4605V LTM4605V PACKAGE DESCRIPTION 141-Lead (15mm × 15mm × 2.8mm) LGA 141-Lead (15mm × 15mm × 2.8mm) LGA TEMPERATURE RANGE –40°C to 85°C –40°C to 85°C Consult LTC Marketing for parts specified with wider operating temperature ranges. *The temperature grade is identified by a label on the shipping container. For more information on lead free part marking, go to: http://www.linear.com/leadfree/ This product is only offered in trays. For more information go to: http://www.linear.com/packaging/ ELECTRICAL CHARACTERISTICS SYMBOL Input Specifications VIN(DC) VIN(UVLO) IQ(VIN) Input DC Voltage Undervoltage Lockout Threshold Input Supply Bias Current Normal Standby Shutdown Supply Current PARAMETER The l denotes the specifications which apply over the –40°C to 85°C temperature range, otherwise specifications are at TA = 25°C, VIN = 12V. Per typical application (front page) configuration. CONDITIONS l MIN 4.5 TYP MAX 20 UNITS V V mA mA μA VIN Falling l 3.4 2.8 1.6 35 4 VRUN = 0V, VSTBYMD > 2V VRUN = 0V, VSTBYMD = Open 60 4605fa 2 LTM4605 ELECTRICAL CHARACTERISTICS SYMBOL IOUTDC PARAMETER Output Specifications Output Continuous Current Range VIN = 12V, VOUT = 5V (See Output Current Derating Curves VIN = 6V, VOUT = 12V for Different VIN, VOUT and TA) Reference Voltage Line Regulation Accuracy Load Regulation Accuracy VIN = 4.5V to 20V, VCOMP = 1.2V (Note 3) VCOMP = 1.2V to 0.7V VCOMP = 1.2V to 1.8V (Note 3) Drain to Source Voltage VDS = 12V, Bias Current ISW = 10mA Drain to Source Voltage VDS = 12V, Bias Current ISW = 10mA Drain to Source Voltage VDS = 12V, Bias Current ISW = 10mA Drain to Source Voltage VDS = 12V, Bias Current ISW = 10mA Drain to Source Voltage VDS = 12V, Bias Current ISW = 10mA Drain to Source Voltage VDS = 12V, Bias Current ISW = 10mA Drain to Source Voltage VDS = 12V, Bias Current ISW = 10mA Drain to Source Voltage VDS = 12V, Bias Current ISW = 10mA Drain to Source Voltage VDS = 12V, Bias Current ISW = 10mA Drain to Source Voltage VDS = 12V, Bias Current ISW = 10mA Drain to Source Voltage VDS = 12V, Bias Current ISW = 10mA Drain to Source Voltage VDS = 12V, Bias Current ISW = 10mA Bias Current ISW = 3A Bias Current ISW = 3A Bias Current ISW = 3A Bias Current ISW = 3A l l The l denotes the specifications which apply over the –40°C to 85°C temperature range, otherwise specifications are at TA = 25°C, VIN = 12V. Per typical application (front page) configuration. CONDITIONS MIN TYP 12 5 0.002 0.15 –0.15 50 40 25 20 20 20 50 50 50 50 220 220 6.5 8 8 8 12 12 12 0.02 0.5 –0.5 MAX UNITS A A % % % ns ns ns ns ns ns ns ns ns ns ns ns mΩ mΩ mΩ mΩ ΔVFB/VFB(NOM) ΔVFB/VFB(LOAD) Switch Section M1 tr M1 tf M3 tr M3 tf M2, M4 tr M2, M4 tf t1d t2d t3d t4d Mode Transition 1 Mode Transition 2 M1 RDS(ON) M2 RDS(ON) M3 RDS(ON) M4 RDS(ON) Turn-On Time (Note 4) Turn-Off Time Turn-On Time Turn-Off Time Turn-On Time Turn-Off Time M1 Off to M2 On Delay (Note 4) M2 Off to M1 On Delay M3 Off to M4 On Delay M4 Off to M3 On Delay M2 Off to M4 On Delay M4 Off to M2 On Delay Static Drain-to-Source OnResistance Static Drain-to-Source OnResistance Static Drain-to-Source OnResistance Static Drain-to-Source OnResistance Nominal Frequency Lowest Frequency Highest Frequency PLLIN Input Resistance Phase Detector Output Current Oscillator and Phase-Locked Loop fNOM fLOW fHIGH RPLLIN IPLLFLTR VPLLFLTR = 1.2V VPLLFLTR = 0V VPLLFLTR = 2.4V fPLLIN < fOSC fPLLIN > fOSC 260 170 340 300 200 400 50 –15 15 330 220 440 kHz kHz kHz kΩ μA μA 4605fa 3 LTM4605 ELECTRICAL CHARACTERISTICS SYMBOL Control Section VFB VRUN ISS VSTBYMD(START) VSTBYMD(KA) VFCB IFCB VBURST DF(BOOST, MAX) DF(BUCK, MAX) tON(MIN, BUCK) RFBHI INTVCC ΔVLDO/VLDO VEXTVCC ΔVEXTVCC(HYS) ΔVEXTVCC VSENSE(MAX) VSENSE(MIN, BUCK) ISENSE PGOOD ΔVFBH ΔVFBL ΔVFB(HYS) VPGL IPGOOD PGOOD Upper Threshold PGOOD Lower Threshold PGOOD Hysteresis PGOOD Low Voltage PGOOD Leakage Current VFB Rising VFB Falling VFB Returning IPGOOD = 2mA VPGOOD = 5V 5.5 –5.5 7.5 –7.5 2.5 0.2 0.3 1 10 –10 % % % V μA Feedback Reference Voltage RUN Pin ON/OFF Threshold Soft-Start Charging Current Start-Up Threshold Keep-Active Power On Threshold Forced Continuous Threshold Forced Continuous Pin Current Burst Inhibit (Constant Frequency) Threshold Maximum Duty Factor Maximum Duty Factor VFCB = 0.85V Measured at FCB Pin % Switch M4 On % Switch M1 On VRUN = 2.2V VSTBYMD Rising VSTBYMD Rising, VRUN = 0V 0.76 –0.3 VCOMP = 1.2V l The l denotes the specifications which apply over the –40°C to 85°C temperature range, otherwise specifications are at TA = 25°C, VIN = 12V. Per typical application (front page) configuration. PARAMETER CONDITIONS MIN 0.792 1 1 0.4 TYP 0.8 1.6 1.7 0.7 1.25 0.8 –0.2 5.3 99 99 200 99.5 VIN > 7V, VEXTVCC = 5V ICC = 0mA to 20mA, VEXTVCC = 5V ICC = 20mA, VEXTVCC Rising ICC = 20mA, VEXTVCC = 6V Boost Mode Buck Mode Discontinuous Mode VSENSE– = VSENSE+ = 0V l l l l MAX 0.808 2.2 UNITS V V μA V V 0.84 –0.1 5.5 V μA V % % Minimum On-Time for Synchronous Switch M1 (Note 5) Switch in Buck Operation Resistor Between VOUT and VFB pins Internal VCC Voltage Internal VCC Load Regulation EXTVCC Switchover Voltage EXTVCC Switchover Hysteresis EXTVCC Switch Drop Voltage Maximum Current Sense Threshold Minimum Current Sense Threshold Sense Pins Total Source Current 250 100.5 6.3 2 ns kΩ V % V mV 100 6 0.3 5.6 300 60 160 –130 –6 –380 Internal VCC Regulator 5.7 5.4 150 190 –150 mV mV mV mV μA Current Sensing Section –95 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 LTM4605E is guaranteed to meet performance specifications from 0°C to 85°C. Specifications over the –40°C to 85°C operating temperature range are assured by design, characterization and correlation with statistical process controls. The LTM4605I is guaranteed over the –40°C to 85°C temperature range. Note 3: The LTM4605 is tested in a feedback loop that servos VCOMP to a specified voltage and measures the resultant VFB. Note 4: Turn-on and turn-off time are measured using 10% and 90% levels. Transition delay time is measured using 50% levels. Note 5: 100% tested at wafer level only. 4605fa 4 LTM4605 TYPICAL PERFORMANCE CHARACTERISTICS Efficiency vs Load Current 6VIN to 12VOUT 100 90 80 EFFICIENCY (%) EFFICIENCY (%) 70 60 50 40 30 20 10 0 0.01 0.1 1 LOAD CURRENT (A) CCM DCM BURST 10 4605 G01 (Refer to Figure 16) Efficiency vs Load Current 18VIN to 12VOUT 95 85 75 EFFICIENCY (%) 65 55 45 35 CCM DCM BURST 25 15 0.01 CCM DCM SKIP CYCLE 0.1 1 10 LOAD CURRENT (A) 100 4605 G03 Efficiency vs Load Current 12VIN to 12VOUT 100 90 80 70 60 50 40 30 20 10 0 0.01 0.1 1 LOAD CURRENT (A) 10 4605 G02 Efficiency vs Load Current 3.3μH Inductor (CCM) 100 95 90 EFFICIENCY (%) EFFICIENCY (%) 85 80 75 70 65 60 0 3 18VIN TO 5VOUT 12VIN TO 5VOUT 5VIN TO 5VOUT 6 9 LOAD CURRENT (A) 12 4605 G04 Efficiency vs Load Current 1.5μH Inductor (CCM) 100 95 90 EFFICIENCY (%) 18VIN TO 3.3VOUT 12VIN TO 3.3VOUT 5VIN TO 3.3VOUT 0 3 6 9 LOAD CURRENT (A) 12 4605 G05 Efficiency vs Load Current 1.5μH Inductor (CCM) 100 95 90 85 80 75 70 65 60 55 50 0 3 18VIN TO 2.5VOUT 12VIN TO 2.5VOUT 5VIN TO 2.5VOUT 6 9 LOAD CURRENT (A) 12 4605 G06 85 80 75 70 65 60 55 50 Transient Response from 6VIN to 12VOUT IOUT 2A/DIV IOUT 2A/DIV Transient Response from 12VIN to 12VOUT IOUT 2A/DIV Transient Response from 18VIN to 12VOUT VOUT 200mV/DIV 200μs/DIV 4605 G07 VOUT 200mV/DIV 200μs/DIV 4605 G08 VOUT 100mV/DIV 200μs/DIV 4605 G09 LOAD STEP: 0A TO 3A AT CCM OUTPUT CAPS: 4x 22μF CERAMIC CAPS AND 2x 180μF ELECTROLYTIC CAPS 2x 15mΩ SENSING RESISTORS LOAD STEP: 0A TO 3A AT CCM OUTPUT CAPS: 4x 22μF CERAMIC CAPS AND 2x 180μF ELECTROLYTIC CAPS 2x 15mΩ SENSING RESISTORS LOAD STEP: 0A TO 4A AT CCM OUTPUT CAPS: 4x 22μF CERAMIC CAPS AND 2x 180μF ELECTROLYTIC CAPS 2x 15mΩ SENSING RESISTORS 4605fa 5 LTM4605 TYPICAL PERFORMANCE CHARACTERISTICS Start-Up with 6VIN to 12VOUT at IOUT = 5A VOUT 5V/DIV IIN 5A/DIV IL 5A/DIV 50ms/DIV 4605 G10 Start-Up with 18VIN to 12VOUT at IOUT = 5A VOUT 5V/DIV IIN 2A/DIV IL 5A/DIV 50ms/DIV 4605 G11 0.22μF SOFT-START CAP OUTPUT CAPS: 4x 22μF CERAMIC CAPS AND 2x 180μF ELECTROLYTIC CAPS 2x 15mΩ SENSING RESISTORS 0.22μF SOFT-START CAP OUTPUT CAPS: 4x 22μF CERAMIC CAPS AND 2x 180μF ELECTROLYTIC CAPS 2x 15mΩ SENSING RESISTORS Short Circuit with 6VIN to 12VOUT at IOUT = 5A VOUT 5V/DIV VOUT 10V/DIV IIN 5A/DIV Short Circuit with 18VIN to 12VOUT at IOUT = 5A IIN 10A/DIV 20μs/DIV 4605 G12 100μs/DIV 4605 G13 OUTPUT CAPS: 4x 22μF CERAMIC CAPS AND 2x 180μF ELECTROLYTIC CAPS 2x 15mΩ SENSING RESISTORS OUTPUT CAPS: 4x 22μF CERAMIC CAPS AND 2x 180μF ELECTROLYTIC CAPS 2x 15mΩ SENSING RESISTORS 4605fa 6 LTM4605 PIN FUNCTIONS VIN (Bank 1): Power Input Pins. Apply input voltage between these pins and PGND pins. Recommend placing input decoupling capacitance directly between VIN pins and PGND pins. VOUT (Bank 5): Power Output Pins. Apply output load between these pins and PGND pins. Recommend placing output decoupling capacitance directly between these pins and PGND pins. PGND (Bank 6): Power Ground Pins for Both Input and Output Returns. SW1, SW2 (Bank 4, Bank 2): Switch Nodes. The power inductor is connected between SW1 and SW2. RSENSE (Bank 3): Sensing Resistor Pin. The sensing resistor is connected from this pin to PGND. SENSE+ (Pin A4): Positive Input to the Current Sense and Reverse Current Detect Comparators. SENSE– (Pin A5): Negative Input to the Current Sense and Reverse Current Detect Comparators. EXTVCC (Pin F6): External VCC Input. When EXTVCC exceeds 5.7V, an internal switch connects this pin to INTVCC and shuts down the internal regulator so that the controller and gate drive power is drawn from EXTVCC. Do not exceed 7V at this pin and ensure that EXTVCC < VIN. INTVCC (Pin F5): Internal 6V Regulator Output. This pin is for additional decoupling of the 6V internal regulator. PLLIN (Pin B9): External Clock Synchronization Input to the Phase Detector. This pin is internally terminated to SGND with a 50k resistor. The phase-locked loop will force the rising bottom gate signal of the controller to be synchronized with the rising edge of PLLIN signal. PLLFLTR (Pin B8): The lowpass filter of the phase-locked loop is tied to this pin. This pin can also be used to set the frequency of the internal oscillator with an AC or DC voltage. See the Applications Information section for details. SS (Pin A6): Soft-Start Pin. Soft-start reduces the input power sources’ surge currents by gradually increasing the controller’s current limit. STBYMD (Pin A10): LDO Control Pin. Determine whether the internal LDO remains active when the controller is shut down. See Operations section for details. If the STBYMD pin is pulled to ground, the SS pin is internally pulled to ground to disable start-up and thereby providing a single control pin for turning off the controller. An internal decoupling capacitor is tied to this pin. VFB (Pin B6): The Negative Input of the Error Amplifier. Internally, this pin is connected to VOUT with a 100k precision resistor. Different output voltages can be programmed with an additional resistor between VFB and SGND pins. See the Applications Information section. FCB (Pin A9): Forced Continuous Control Input. The voltage applied to this pin sets the operating mode of the module. When the applied voltage is less than 0.8V, the forced continuous current mode is active in boost operation and the skip cycle mode is active in buck operation. When the pin is tied to INTVCC, the constant frequency discontinuous current mode is active in buck or boost operation. See the Applications Information section. SGND (Pin A7): Signal Ground Pin. This pin connects to PGND at output capacitor point. COMP (Pin B7): Current Control Threshold and Error Amplifier Compensation Point. The current comparator threshold increases with this control voltage. The voltage ranges from 0V to 2.4V. PGOOD (Pin B5): Output Voltage Power Good Indicator. Open drain logic output that is pulled to ground when the output voltage is not within ±10% of the regulation point, after a 25μs power bad mask timer expires. RUN (Pin A8): Run Control Pin. A voltage below 1.6V will turn off the module. There is a 100k resistor between the RUN pin and SGND in the module. Do not apply more than 6V to this pin. See Applications Information section. 4605fa 7 LTM4605 SIMPLIFIED BLOCK DIAGRAM VIN 4.5V TO 20V EXTVCC M1 INTVCC PGOOD M2 L SW1 VOUT 12V 5A CO1 M3 0.1μF COMP M4 INT COMP SS SS 0.1μF PLLIN INT FILTER PLLFLTR SENSE– RSENSE CONTROLLER RSENSE SENSE+ 100k VFB COUT RFB 7.15k SW2 C1 CIN RUN ON/OFF 100k STBYMD INT FILTER FCB SGND TO PGND PLANE AS SHOWN IN FIGURE 13 1000pF PGND 4605 BD Figure 1. Simplified LTM4605 Block Diagram DECOUPLING REQUIREMENTS TA = 25°C. Use Figure 1 configuration. SYMBOL CIN COUT PARAMETER External Input Capacitor Requirement (VIN = 4.5V to 20V, VOUT = 12V) External Output Capacitor Requirement (VIN = 4.5V to 20V, VOUT = 12V) CONDITIONS IOUT = 5A IOUT = 5A MIN 10 200 300 TYP MAX UNITS μF μF 4605fa 8 LTM4605 OPERATION Power Module Description The LTM4605 is a non-isolated buck-boost DC/DC power supply. It can deliver a wide range output voltage from 0.8V to 16V over a wide input range from 4.5V to 20V, by only adding the sensing resistor, inductor and some external input and output capacitors. It provides precisely regulated output voltage programmable via one external resistor. The typical application schematic is shown in Figure 16. The LTM4605 has an integrated current mode buck-boost control, ultralow RDS(ON) FETs with fast switching speed and integrated Schottky diodes. With current mode control and internal feedback loop compensation, the LTM4605 module has sufficient stability margins and good transient performance under a wide range of operating conditions and with a wide range of output capacitors. The frequency of LTM4605 can be operated from 200kHz to 400kHz by setting the voltage on the PLLFLTR pin. Alternatively, its frequency can be synchronized by the input clock signal from the PLLIN pin. The typical switching frequency is 400kHz. The Burst Mode and skip-cycle mode operations can be enabled at light loads in the LTM4605 to improve its efficiency, while the forced continuous mode and discontinuous mode operations are used for constant frequency applications. Foldback current limiting is activated in an overcurrent condition as VFB drops. Internal overvoltage and undervoltage comparators pull the open-drain PGOOD output low if the output feedback voltage exits the ±10% window around the regulation point. Pulling the RUN pin below 1.6V forces the controller into its shutdown state. If an external bias supply is applied on the EXTVCC pin, then an efficiency improvement will occur due to the reduced power loss in the internal linear regulator. This is especially true at the higher input voltage range. APPLICATIONS INFORMATION The typical LTM4605 application circuit is shown in Figure 16. External component selection is primarily determined by the maximum load current and output voltage. Refer to Table 3 for specific external capacitor requirements for a particular application. Output Voltage Programming The PWM controller has an internal 0.8V±1% reference voltage. As shown in the Block Diagram, a 100k, 0.5% internal feedback resistor connects VOUT and VFB pins together. Adding a resistor RFB from the VFB pin to the SGND pin programs the output voltage: VOUT = 0.8 V • 100k + RFB RFB Operation Frequency Selection The LTM4605 uses current mode control architecture at constant switching frequency, which is determined by the internal oscillator’s capacitor. This internal capacitor is charged by a fixed current plus an additional current that is proportional to the voltage applied to the PLLFLTR pin. Table 1. RFB Resistor (0.5%) vs Various Output Voltages VOUT RFB VOUT RFB 0.8V Open 8V 11k 1.5V 115k 9V 9.76k 2.5V 47.5k 10V 8.66k 3.3V 32.4k 12V 7.15k 5V 19k 15V 5.62k 6V 15.4k 16V 5.23k 4605fa 9 LTM4605 APPLICATIONS INFORMATION The PLLFLTR pin can be grounded to lower the frequency to 200kHz or tied to 2.4V to yield approximately 400kHz. When PLLIN is left open, the PLLFLTR pin goes low, forcing the oscillator to its minimum frequency. A graph for the voltage applied to the PLLFLTR pin vs frequency is given in Figure 2. As the operating frequency increases, the gate charge losses will be higher, thus the efficiency is low. The maximum switching frequency is approximately 400kHz. 450 400 OPERATING FREQUENCY (kHz) 350 300 250 200 150 100 50 0 0 1.0 1.5 2.0 0.5 PLLFLTR PIN VOLTAGE (V) 2.5 4605 F02 by accepting a logic input on the FCB pin. Table 2 shows the different operation modes. Table 2. Different Operating Modes FCB PIN 0V to 0.75V 0.85V to 5V >5.3V BUCK Force Continuous Mode Skip-Cycle Mode DCM with Constant Freq BOOST Force Continuous Mode Burst Mode Operation DCM with Constant Freq Figure 2. Frequency vs PLLFLTR Pin Voltage When the FCB pin voltage is lower than 0.8V, the controller behaves as a continuous, PWM current mode synchronous switching regulator. When the FCB pin voltage is below VINTVCC – 1V, but greater than 0.8V, the controller enters Burst Mode operation in boost operation or enters skipcycle mode in buck operation. During boost operation, Burst Mode operation is activated if the load current is lower than the preset minimum output current level. The MOSFETs will turn on for several cycles, followed by a variable “sleep” interval depending upon the load current. During buck operation, skip-cycle mode sets a minimum positive inductor current level. In this mode, some cycles will be skipped when the output load current drops below 1% of the maximum designed load in order to maintain the output voltage. When the FCB pin is tied to the INTVCC pin, the controller enters constant frequency discontinuous current mode (DCM). For boost operation, if the output voltage is high enough, the controller can enter the continuous current buck mode for one cycle to discharge inductor current. In the following cycle, the controller will resume DCM boost operation. For buck operation, constant frequency discontinuous current mode is turned on if the preset minimum negative inductor current level is reached. At very light loads, this constant frequency operation is not as efficient as Burst Mode operation or skip-cycle, but does provide low noise, constant frequency operation. Input Capacitors In boost mode, since the input current is continuous, only minimum input capacitors are required. However, the input current is discontinuous in buck mode, so the selection of input capacitor CIN is driven by the need of filtering the input square wave current. 4605fa FREQUENCY SYNCHRONIZATION The LTM4605 can also be synchronized to an external source via the PLLIN pin instead of adjusting the voltage on the PLLFLTR pin directly. The power module has a phaselocked loop comprised of an internal voltage controlled oscillator and a phase detector. This allows turning on the internal top MOSFET for locking to the rising edge of the external clock. A pulse detection circuit is used to detect a clock on the PLLIN pin to turn on the phase lock loop. The input pulse width of the clock has to be at least 400ns, and 2V in amplitude. The synchronized frequency ranges from 200kHz to 400kHz, corresponding to a DC voltage input from 0V to 2.4V at PLLFLTR. During the start up of the regulator, the phase-lock loop function is disabled. Low Current Operation To improve the efficiency at low current operation, LTM4605 provides three modes for both buck and boost operations 10 LTM4605 APPLICATIONS INFORMATION For a buck converter, the switching duty-cycle can be estimated as: D= VOUT VIN The LTM4605 is designed for low output voltage ripple. The bulk output capacitors defined as COUT are chosen with low enough ESR to meet the output voltage ripple and transient requirements. COUT can be a low ESR tantalum capacitor, a low ESR polymer capacitor or a ceramic capacitor. Multiple capacitors can be placed in parallel to meet the ESR and RMS current handling requirements. The typical capacitance is 300μF Additional output filtering may . be required by the system designer, if further reduction of output ripple or dynamic transient spike is required. Table 3 shows a matrix of different output voltages and output capacitors to minimize the voltage droop and overshoot at a current transient. Inductor Selection The inductor is chiefly decided by the required ripple current and the operating frequency. The inductor current ripple ΔIL is typically set to 20% to 40% of the maximum inductor current. In the inductor design, the worst cases in continuous mode are considered as follows: LBOOST ≥ VIN • VOUT(MAX ) − VIN Without considering the inductor current ripple, the RMS current of the input capacitor can be estimated as: ICIN(RMS) = IOUT(MAX ) η • D • (1− D) In the above equation, η is the estimated efficiency of the power module. CIN can be a switcher-rated electrolytic aluminum capacitor, OS-CON capacitor or high volume ceramic capacitors. Note the capacitor ripple current ratings are often based on temperature and hours of life. This makes it advisable to properly derate the input capacitor, or choose a capacitor rated at a higher temperature than required. Always contact the capacitor manufacturer for derating requirements. Output Capacitors In boost mode, the discontinuous current shifts from the input to the output, so the output capacitor COUT must be capable of reducing the output voltage ripple. For boost and buck modes, the steady ripple due to charging and discharging the bulk capacitance is given by: VRIPPLE,BOOST = IOUT(MAX ) • VOUT − VIN(MIN) COUT • VOUT • f VOUT • VIN(MAX ) − VOUT ( ) VOUT(MAX ) • f • IOUT(MAX ) • Ripple% VOUT • VIN(MAX ) − VOUT LBUCK ≥ where: ( ) VIN(MAX ) • f • IOUT(MAX ) • Ripple% ( ) f is operating frequency, Hz Ripple% is allowable inductor current ripple, % VOUT(MAX) is maximum output voltage, V VIN(MAX) is maximum input voltage, V VOUT is output voltage, V IOUT(MAX) is maximum output load current, A The inductor should have low DC resistance to reduce the I2R losses, and must be able to handle the peak inductor current without saturation. To minimize radiated noise, use a toroid, pot core or shielded bobbin inductor. Please refer to Table 3 for the recommended inductors for different cases. 4605fa VRIPPLE,BUCK = 8 • L • COUT • VIN(MAX ) • f 2 ( ) The steady ripple due to the voltage drop across the ESR (effective series resistance) is given by: VESR,BUCK = ΔIL(MAX ) • ESR VESR,BOOST = IL(MAX ) • ESR 11 LTM4605 APPLICATIONS INFORMATION RSENSE Selection and Maximum Output Current RSENSE is chosen based on the required inductor current. Since the maximum inductor valley current at buck mode is much lower than the inductor peak current at boost mode, different sensing resistors are suggested to use in buck and boost modes. The current comparator threshold sets the peak of the inductor current in boost mode and the maximum inductor valley current in buck mode. In boost mode, the allowed maximum average load current is: ⎛ 160mV ΔIL ⎞ VIN IOUT(MAX,BOOST) = ⎜ − • 2 ⎟ VOUT ⎝ RSENSE ⎠ where ΔIL is peak-to-peak inductor ripple current. In buck mode, the allowed maximum average load current is: IOUT(MAX,BUCK ) = 130mV ΔIL + RSENSE 2 the internal reference and the output voltage. The total soft-start time can be calculated as: t SOFTSTART = 2.4V • CSS 1.7µA When the RUN pin falls below 1.6V, then soft-start pin is reset to allow for proper soft-start control when the regulator is enabled again. Current foldback and force continuous mode are disabled during the soft-start process. The softstart function can also be used to control the output ramp up time, so that another regulator can be easily tracked. Do not apply more than 6V to the SS pin. Run Enable The RUN pin is used to enable the power module. The pin can be driven with a logic input, and not exceed 6V. The RUN pin can also be used as an undervoltage lockout (UVLO) function by connecting a resistor from the input supply to the RUN pin. The equation: V _ UVLO = Power Good The PGOOD pin is an open drain pin that can be used to monitor valid output voltage regulation. This pin monitors a ±7.5% window around the regulation point, and tracks with margining. COMP Pin This pin is the external compensation pin. The module has already been internally compensated for most output voltages. A spice model will be provided for other control loop optimization. Fault Conditions: Current Limit and Overcurrent Foldback LTM4605 has a current mode controller, which inherently limits the cycle-by-cycle inductor current not only in steady state operation, but also in transient. Refer to Table 3. To further limit current in the event of an overload condition, the LTM4605 provides foldback current limiting. If the 4605fa The maximum current sensing RSENSE value for the boost mode is: RSENSE(MAX,BOOST) = 2 • 160mV • VIN 2 • IOUT(MAX,BOOST) • VOUT + ΔIL • VIN The maximum current sensing RSENSE value for the buck mode is: RSENSE(MAX,BUCK ) = 2 • 130mV 2 • IOUT(MAX,BUCK ) – ΔIL R + 100k • 1.6 V 100k A 20% to 30% margin on the calculated sensing resistor is usually recommended. Please refer to Table 3 for the recommended sensing resistors for different applications. Soft-Start The SS pin provides a means to soft-start the regulator. A capacitor on this pin will program the ramp rate of the output voltage. A 1.7μA current source will charge up the external soft-start capacitor. This will control the ramp of 12 LTM4605 APPLICATIONS INFORMATION output voltage falls by more than 70%, then the maximum output current is progressively lowered to about 30% of its full current limit value for boost mode and about 40% for buck mode. Standby Mode (STBYMD) The standby mode (STBYMD) pin provides several choices for start-up and standby operational modes. If the pin is pulled to ground, the SS pin is internally pulled to ground, preventing start-up and thereby providing a single control pin for turning off the controller. If the pin is left open or decoupled with a capacitor to ground, the SS pin is internally provided with a starting current, permitting external control for turning on the controller. If the pin is connected to a voltage greater than 1.25V, the internal regulator (INTVCC) will be on even when the controller is shut down (RUN pin voltage