LTC3521EUF-PBF

LTC3521 Wide VIN, 1A Buck-Boost DC/DC and Dual 600mA Buck DC/DC Converters FeaTures n DescripTion The LTC®3521 combines a 1A buck-boost DC/DC converter and dual 600mA synchronous buck DC/DC converters. The 1.1MHz switching frequency minimizes the solution footprint while maintaining high efficiency. All three converters feature soft-start and internal compensation to minimize the solution footprint and simplify the design process. The buck converters are current mode controlled and utilize an internal synchronous rectifier to improve efficiency. The buck converters support 100% duty cycle operation to extend battery life. If the PWM pin is held low, the buck converters automatically transition from Burst Mode operation to PWM mode at high loads. With the PWM pin held high, the buck converters remain in low noise, 1.1MHz PWM mode. The buck-boost converter features continuous conduction operation to maximize efficiency and minimize noise. At light loads, the buck-boost converter can be operated in Burst Mode operation to improve efficiency and reduce no-load standby current. The LTC3521 provides a < 2μA shutdown mode, overtemperature shutdown and current limit protection on all converters. The LTC3521 is available in a 24-pin 0.75mm × 4mm × 4mm QFN package, and a 20-pin thermally enhanced TSSOP package. n n n n n n n n Three High Efficiency DC/DC Converters: Buck-Boost (VOUT: 1.8V to 5.25V, IOUT: 1A) Dual Buck (VOUT: 0.6V to VIN, IOUT: 600mA) 1.8V to 5.5V Input Voltage Range Pin-Selectable Burst Mode® Operation 30µA Total Quiescent Current in Burst Mode Operation Independent Power Good Indicator Outputs Integrated Soft-Start Thermal and Overcurrent Protection 3.0V) VOUT2 1.8V 10µF 600mA 22µF 1.0M 221k ON LTC3521 FB1 SHDN1 SHDN2 SHDN3 PWM SW3 FB3 PGOOD1 PGOOD2 PGOOD3 PGND1 GND PGND2 3521 TA01a OFF VOUT3 1.2V 10µF 600mA BURST PWM 100 98 96 94 92 90 88 86 84 82 80 78 76 74 72 70 2.4 Efficiency vs VIN VOUT1 = 3.3V IOUT = 500mA VOUT2 = 1.8V IOUT = 200mA VOUT3 = 1.2V IOUT = 200mA 3.4 VIN (V) 4.4 5.4 3521 TA01b 3521f  LTC3521 absoluTe MaxiMuM raTinGs (Note 1) PVIN1, PVIN2 Voltage .................................... –0.3V to 6V SW1A, SW1B, SW2, SW3 Voltage DC............................................................ –0.3V to 6V Pulsed < 100ns ........................................... –1V to 7V Voltage, All Other Pins ................................. –0.3V to 6V Operating Junction Temperature Range (Note 2).................................................... –40°C to 85°C Maximum Junction Temperature (Note 5)............. 125°C Storage Temperature Range................... –65°C to 150°C pin conFiGuraTion PGND1A TOP VIEW FB2 FB3 FB2 SHDN2 PGOOD3 PGOOD2 PGOOD1 VIN GND PWM 1 2 3 4 5 6 7 8 9 21 GND 20 PVIN2 19 SW2 18 PGND2 17 SW3 16 VOUT1 15 SW1A 14 SW1B 13 PVIN1 12 SHDN1 11 SHDN3 SHDN2 1 PGOOD3 2 PGOOD2 3 PGOOD1 4 VIN 5 GND 6 7 PWM 8 FB1 9 10 11 12 PGND1B SHDN3 SHDN1 PVIN1 25 GND TOP VIEW PVIN2 SW2 FB3 NC 18 PGND2 17 SW3 16 VOUT1 15 SW1A 14 SW1B 13 NC 24 23 22 21 20 19 FB1 10 FE PACKAGE 20-LEAD PLASTIC TSSOP TJMAX = 150°C, θJA = 40°C/W (NOTE 4) UNDERSIDE METAL INTERNALLY CONNECTED TO V – (PCB CONNECTION OPTIONAL) EXPOSED PAD (PIN 21) IS GND AND MUST BE SOLDERED TO PCB GROUND UF PACKAGE 24-LEAD (4mm 4mm) PLASTIC QFN TJMAX = 125°C, θJA = 37°C/W EXPOSED PAD (PIN 25) IS GND AND MUST BE SOLDERED TO PCB GROUND orDer inForMaTion LEAD FREE FINISH LTC3521EUF#PBF LTC3521EFE#PBF TAPE AND REEL LTC3521EUF#TRPBF LTC3521EFE#TRPBF PART MARKING 3521 LTC3521FE PACKAGE DESCRIPTION 24-Lead (4mm × 4mm) Plastic QFN 20-Lead Plastic TSSOP TEMPERATURE RANGE –40°C to 85°C –40°C to 85°C Consult LTC Marketing for parts specified with wider operating temperature ranges. Consult LTC Marketing for information on non-standard lead based finish parts. For more information on lead free part marking, go to: http://www.linear.com/leadfree/ For more information on tape and reel specifications, go to: http://www.linear.com/tapeandreel/ elecTrical characTerisTics PARAMETER Input Voltage Quiescent Current—Shutdown Burst Mode Quiescent Current The l denotes the specifications which apply over the full operating junction temperature range, otherwise specifications are at TJ = 25°C. VIN = 3.6V, VOUT1 = 3.3V, unless otherwise noted. CONDITIONS l MIN 1.8 l TYP 0.01 30 MAX 5.5 2 UNITS V µA µA 3521f VSHDN1 = VSHDN2 = VSHDN3 = 0V VFB1 = 0.66V, VFB2 = 0.66V, VFB3 = 0.66V, VPWM = 0V  LTC3521 elecTrical characTerisTics PARAMETER Oscillator Frequency SHDN1, SHDN2, SHDN3, PWM Input High Voltage SHDN1, SHDN2, SHDN3, PWM Input Low Voltage Power Good Outputs Low Voltage Power Good Outputs Leakage Current Buck Converters PMOS Switch Resistance NMOS Switch Resistance NMOS Switch Leakage Current PMOS Switch Leakage Current Feedback Voltage Feedback Input Current PMOS Current Limit Maximum Duty Cycle Minimum Duty Cycle PGOOD Threshold Power Good Hysteresis Buck-Boost Converter Output Voltage PMOS Switch Resistance NMOS Switch Resistance NMOS Switch Leakage Current PMOS Switch Leakage Current Feedback Voltage Feedback Input Current Average Current Limit Reverse Current Limit Maximum Duty Cycle Minimum Duty Cycle PGOOD Threshold Power Good Hysteresis VSW1A = VSW1B = 5.5V, VIN = 5.5V VSW1A = VSW1B = 0V, VIN = 5.5V (Note 4) VFB1 = 0.6V (Note 3) (Note 3) VFB1 = 0.55V VFB1 = 0.66V VFB1 Falling VFB1 Returning Good l l l l l The l denotes the specifications which apply over the full operating junction temperature range, otherwise specifications are at TJ = 25°C. VIN = 3.6V, VOUT1 = 3.3V, unless otherwise noted. CONDITIONS l l l MIN 0.85 1.4 TYP 1.1 MAX 1.35 0.4 UNITS MHz V V V µA Ω Ω IPGOOD1 = IPGOOD2 = IPGOOD3 = 1mA VPGOOD1 = VPGOOD2 = VPGOOD3 = 5.5V 0.1 0.1 0.205 0.170 0.2 10 VSW2 = VSW3 = 5.5V, VIN = 5.5V VSW2 = VSW3 = 0V, VIN = 5.5V (Note 4) VFB2 = VFB3 = 0.6V (Note 3) VFB2 = VFB3 = 0.55V VFB2 = VFB3 = 0.66V VFB2,3 Falling VFB2,3 Returning Good 1.8 l l l l 0.1 0.1 0.585 750 100 0.6 1 1050 5 10 0.612 50 µA µA V nA mA % 0 –12 –9 2 5.25 0.110 0.085 0.1 0.1 0.585 1.65 85 –12 0.6 1 2.1 375 94 0 –9 3 –6 5 10 0.612 50 –6 % % % V Ω Ω µA µA V nA A mA % % % % 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 LTC3521 is guaranteed to meet performance specifications from 0°C to 85°C. Specifications over the –40°C to 85°C operating junction temperature range are assured by design, characterization and correlation with statistical process controls. Note 3: Current measurements are performed when the LTC3521 is not switching. The current limit values in operation will be somewhat higher due to the propagation delay of the comparators. Note 4: The LTC3521 is tested in a proprietary test mode that connects each FB pin to the output of the respective error amplifier. 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 impair device reliability. 3521f  LTC3521 Typical perForMance characTerisTics Buck-Boost Efficiency, Li-Ion to 3.3V 100 90 80 EFFICIENCY (%) 60 Burst Mode 40 OPERATION 30 20 10 0 0.1 1 Burst Mode POWER LOSS 100 10 LOAD CURRENT (mA) 50 EFFICIENCY (%) 70 100 80 60 40 20 0 1000 3521 G01 TA = 25°C, unless otherwise noted. Buck Efficiency, Li-Ion to 2.5V 140 120 POWER LOSS (mW) 100 90 80 70 60 50 40 30 20 10 0 0.1 1 100 10 LOAD CURRENT (mA) Burst Mode POWER LOSS VIN = 3.6V VIN = 4.2V Burst Mode OPERATION 140 PWM MODE 120 POWER LOSS (mW) 100 80 60 40 20 0 1000 3521 G02 VIN = 2.7V VIN = 4.2V PWM MODE Buck Efficiency, Li-Ion to 1.8V 100 90 80 EFFICIENCY (%) 70 60 50 40 30 20 10 0 0.1 1 100 10 LOAD CURRENT (mA) Burst Mode POWER LOSS VIN = 2.7V VIN = 4.2V Burst Mode OPERATION PWM MODE 140 120 100 80 60 40 20 0 1000 3521 G03 Buck Burst Mode Current Threshold 60 50 LOAD CURRENT (mA) 40 30 20 10 0 1.5 VOUT = 1.2V POWER LOSS (mW) VOUT = 1.8V VOUT = 2.5V 2 2.5 3 3.5 VIN (V) 4 4.5 5 5.5 3521 G04 160 Buck-Boost Switches RDS(ON) 350 300 250 RDS(ON) (m ) 200 150 100 50 0 20 40 60 80 100 120 3521 G05 Buck Switches RDS(ON) VIN = 3.6V PMOS NMOS Switching Frequency vs Temperature 1.0 0.8 0.6 CHANGE FROM 25°C (%) 0.4 0.2 0 –0.2 –0.4 –0.6 –0.8 VIN = 3.6V 140 VOUT1 = 3.3V PMOS (SWITCHES A AND D) NMOS (SWITCHES B AND C) 120 RDS(ON) (m ) 100 80 60 40 20 0 –40 –20 0 –40 –20 0 20 40 60 80 100 120 3521 G06 TEMPERATURE (°C) TEMPERATURE (°C) –1.0 –50 –30 –10 10 30 50 70 TEMPERATURE (°C) 90 110 3521 G07 3521f  LTC3521 Typical perForMance characTerisTics CHANGE IN FEEDBACK VOLTAGE FROM 25°C (%) 2.0 1.5 CHANGE FROM VIN = 3.6V (%) 1.0 0.5 0 –0.5 –1.0 –1.5 –2.0 1.8 2.3 2.8 3.3 3.8 4.3 4.8 5.3 3521 G08 TA = 25°C, unless otherwise noted. Buck Feedback Voltage vs Temperature CHANGE IN FEEDBACK VOLTAGE FROM 25°C (%) 0.2 0.1 0 – 0.1 – 0.2 – 0.3 – 0.4 – 0.5 –50 –25 0 25 50 75 100 125 3521 G10 Switching Frequency vs VIN Buck-Boost Feedback Voltage vs Temperature 0.2 0.1 0 –0.1 –0.2 –0.3 –0.4 –0.5 –40 –20 0 20 40 60 80 100 120 3521 G09 VIN (V) TEMPERATURE (°C) TEMPERATURE (°C) 33 Burst Mode Quiescent Current vs VIN ALL THREE CONVERTERS ENABLED MAXIMUM LOAD CURRENT (mA) Buck-Boost Maximum Load Current, Burst Mode Operation 90 80 70 LOAD CURRENT (mA) 60 50 40 30 20 10 VOUT = 5V VOUT = 3V 1500 1300 1100 900 700 500 300 2.3 2.8 3.3 3.8 4.3 4.8 5.3 3521 G12 Buck-Boost Maximum Load Current, PWM Mode L = 4.7µH QUIESCENT CURRENT (µA) 31 VOUT = 3.3V 29 VOUT = 5V 27 25 1.8 2.3 2.8 3.3 3.8 4.3 4.8 5.3 3521 G11 0 1.8 100 1.8 2.3 2.8 VIN (V) VIN (V) 3.3 3.8 VIN (V) 4.3 4.8 5.3 3521 G13 60 No Load Quiescent Current vs VIN Buck-Boost Load Step, 0mA to 750mA VIN = 3.6V, VOUT = 3.3V L = 4.7µH COUT = 22µF QUIESCENT CURRENT (µA) 55 VOUT 100mV/DIV 50 INDUCTOR CURRENT 500mA/DIV 100µs/DIV 40 1.8 2.3 2.8 3.3 3.8 4.3 4.8 5.3 3521 G14 3521 G15 45 VIN (V) 3521f  LTC3521 Typical perForMance characTerisTics Buck-Boost Burst to PWM Transition VOUT 100mV/DIV TA = 25°C, unless otherwise noted. Buck Load Step, Burst Mode Operation, 10mA to 400mA VOUT 100mV/DIV Buck Load Step, PWM Mode, 10mA to 400mA INDUCTOR CURRENT 200mA/DIV VOUT 20mV/DIV INDUCTOR CURRENT 200mA/DIV 50µs/DIV L = 4.7µH COUT = 22µF 3521 G16 INDUCTOR CURRENT 200mA/DIV 100µs/DIV L = 4.7µH COUT = 10µF 3521 G17 VIN = 3.6V VOUT = 3.3V VIN = 3.6V VOUT = 1.8V VIN = 3.6V VOUT = 1.8V 100µs/DIV L = 4.7µH COUT = 10µF 3521 G18 Buck-Boost Current Limit vs Temperature 2150 3350 Buck-Boost Peak Current Limit vs Temperature 1150 Buck Current Limit vs Temperature CURRENT LIMIT (mA) CURRENT LIMIT (mA) 3300 CURRENT LIMIT (mA) –25 0 25 50 75 100 125 3521 G20 2100 1100 1050 2050 3250 1000 2000 950 3200 –50 900 –50 1950 –50 –25 0 25 50 75 100 125 3521 G19 –25 0 25 50 75 100 125 3521 G21 TEMPERATURE (°C) TEMPERATURE (°C) TEMPERATURE (°C) pin FuncTions (FE/UF Packages) FB3 (Pin 1/Pin 23): Feedback Voltage for the Buck Converter Derived from a Resistor Divider Connected to the Buck VOUT3 Output Voltage. The buck output voltage is given by the following equation, where R1 is a resistor between FB3 and ground, and R2 is a resistor between FB3 and the buck output voltage:  R2 VOUT 3 = 0.6 V 1 +   R1 given by the following equation, where R1 is a resistor between FB2 and ground, and R2 is a resistor between FB2 and the buck output voltage:  R2 VOUT 2 = 0.6 V 1 +   R1 SHDN2 (Pin 3/Pin 1): Forcing this pin above 1.4V enables the buck converter output at SW2. Forcing this pin below 0.4V disables the buck converter. This pin cannot be left floating. PGOOD3 (Pin 4/Pin 2): This pin is an open-drain output which pulls low under any of the following conditions: VOUT3 buck output voltage is out of regulation, the part is 3521f FB2 (Pin 2/Pin 24): Feedback Voltage for the Buck Converter Derived from a Resistor Divider Connected to the Buck VOUT2 Output Voltage. The buck output voltage is  LTC3521 pin FuncTions in overtemperature shutdown, the part is in undervoltage lockout, or if the SHDN3 pin is pulled low. PGOOD2 (Pin 5/Pin 3): This pin is an open-drain output which pulls low under any of the following conditions: VOUT2 buck output voltage is out of regulation, the part is in overtemperature shutdown, the part is in undervoltage lockout, or if the SHDN2 pin is pulled low. PGOOD1 (Pin 6/Pin 4): This pin is an open-drain output which pulls low under any of the following conditions: VOUT1 buck-boost output voltage is out of regulation, the part is in overtemperature shutdown, the part is in undervoltage lockout, the buck-boost converter is in current limit, or if the SHDN1 pin is pulled low. See the Operation section of this data sheet for details on the functionality of this pin in PWM mode. VIN (Pin 7/Pin 5): Low Current Power Supply Connection Used to Power the Internal Circuitry of the LTC3521. This pin should be bypassed by a 4.7µF or larger, ceramic , capacitor. The bypass capacitor should be placed as close to the pin as possible and should have a short return path to ground. Pins VIN , PVIN1, and PVIN2 must be connected together in the application circuit. GND (Pin 8/Pin 6): Small Signal Ground. This pin is used as a ground reference for the internal circuitry of the LTC3521. PWM (Pin 9/Pin 7): Logic Input Used to Choose Between Burst Mode Operation and PWM Mode for All Three Converters. This pin cannot be left floating. PWM = Low: Burst Mode operation is enabled on all three converters. The buck converters will operate in Burst Mode operation at light current but will automatically transition to PWM operation at high currents. The buck converters can supply maximum output current (600mA) in this mode. The buck-boost converter will operate in variable frequency mode and can only supply a reduced load current (typically 50mA). PWM = High: All three converters are forced into PWM mode operation. The buck converters will remain at constant-frequency operation until their minimum ontime is reached. The buck-boost converter will remain in PWM mode at all load currents. FB1 (Pin 10/Pin 8): Feedback Voltage for the Buck-Boost Converter Derived from a Resistor Divider on the BuckBoost Output Voltage. The buck-boost output voltage is given by the following equation, where R1 is a resistor between FB1 and ground, and R2 is a resistor between FB1 and the buck output voltage:  R2 VOUT1 = 0.6 V 1 +   R1 SHDN3 (Pin 11/Pin 9): Forcing this pin above 1.4V enables the buck converter output at SW3. Forcing this pin below 0.4V disables the buck converter. This pin cannot be left floating. SHDN1 (Pin 12/Pin 10): Forcing this pin above 1.4V enables the buck-boost converter. Forcing this pin below 0.4V disables the buck-boost converter. This pin cannot be left floating. PVIN1 (Pin 13/Pin 11): High current power supply connection used to supply switch A of the buck-boost converter. This pin should be bypassed by a 4.7µF or larger, ceramic , cap. The bypass capacitor should be placed as close to the pin as possible and should have a short return path to ground. Pins VIN, PVIN1, and PVIN2 must be connected together in the application circuit. NC (Pin 13, UF Package Only): No Internal Connection. SW1B (Pin 14/Pin 14): Buck-Boost Switch Node. This pin must be connected to one side of the buck-boost inductor. SW1A (Pin 15/Pin 15): Buck-Boost Switch Node. This pin must be connected to one side of the buck-boost inductor. VOUT1 (Pin 16/Pin 16): Buck-Boost Output Voltage Node. This pin should be connected to a low ESR ceramic capacitor. The capacitor should be placed as close to the IC as possible and should have a short return to ground. SW3 (Pin 17/Pin 17): Buck converter Switch Node. This pin must be connected to the opposite side of the inductor connected to VOUT3. PGND2 (Pin 18/Pin 18): High Current Ground Connection for Both Buck Converters. The PCB trace connecting this pin to ground should be made as short and wide as possible. SW2 (Pin 19/Pin 20): Buck Converter Switch Node. This pin must be connected to the opposite side of the inductor connected to VOUT2. 3521f  LTC3521 pin FuncTions NC (Pin 19, UF Package Only): No Internal Connection. PVIN2 (Pin 20/Pin 22): High Current Power Supply Connection Used to Supply the Buck Converter Power Switches. This pin should be bypassed by a 10µF or larger ceramic cap. The bypass capacitor should be placed as close to the pin as possible and should have a short return path to ground. Pins VIN, PVIN1, and PVIN2 must be connected together in the application circuit. Exposed Pad (Pin 21/Pin 25): The Exposed Pad must be electrically connected to ground. Pins GND, PGND1A, PGND1B, PGND2 and the Exposed Pad must be connected together in the application circuit. PGND1A (Pin 21, UF Package Only): High Current Ground Connection for the Buck-Boost Switch B. The PCB trace connecting this pin to ground should be made as short and wide as possible. block DiaGraM 11 PVIN1 (UF Package) 15 SW1A 14 SW1B 16 PVOUT FORWARD + ILIMIT 2.1A FILTER 0.55V PGOOD1 3 A INTERNAL VCC 4 VIN D – REVERSE ILIMIT + – B C 0.375A + – + – IZERO PVOUT PGND1A PGND1B GATE DRIVES PWM SHDN2 PVIN2 PVIN2 OSCILLATOR UVLO SW2 0A 6 8 22 19 GATE DRIVES BUCK PWM LOGIC BUCK PWM LOGIC PGND2 0A + – 1.05A ZERO CROSSING ZERO CROSSING ILIMIT + – + ILIMIT SLOPE COMPENSATION 24 0.60V SLOPE COMPENSATION SOFT-START RAMP 2 PGOOD2 1V 0.6V 0.9V 0.55V 0.55V GND 5 0.55V PGND1A 21 PGND1B 12 PGND2 18  + – BANDGAP REFERENCE AND OT SHUTDOWN + + – + – + + – FB2 gm + – + – + + – + BUCK-BOOST PWM LOGIC FB1 0.6V SOFT-START RAMP SHDN1 SHDN3 PVIN2 10 9 7 GATE DRIVES SW3 17 + – + – 1.05A PGND2 0A gm FB3 0.60V SOFT-START RAMP PGOOD3 23 1 3521 BD 3521f LTC3521 operaTion The LTC3521 combines dual synchronous buck DC/DC converters and a 4-switch buck-boost DC/DC converter in a 4mm × 4mm QFN package and a 20-pin thermally enhanced TSSOP package. The buck-boost converter utilizes a proprietary switching algorithm which allows its output voltage to be regulated above, below or equal to the input voltage. The buck converters provide a high efficiency lower voltage output and support 100% duty cycle operation to extend battery life. In Burst Mode operation, the total quiescent current for the LTC3521 is reduced to 30μA. All three converters are synchronized to the same internal 1.1MHz oscillator. BUCK CONVERTER OPERATION PWM Mode Operation When the PWM pin is held high, the LTC3521 buck converters use a constant-frequency, current mode control architecture. Both the main (P-channel MOSFET) and synchronous rectifier (N-channel MOSFET) switches are internal. At the start of each oscillator cycle, the P-channel switch is turned on and remains on until the current waveform with superimposed slope compensation ramp exceeds the error amplifier output. At this point, the synchronous rectifier is turned on and remains on until the inductor current falls to zero or a new switching cycle is initiated. As a result, the buck converters operate with discontinuous inductor current at light loads, which improves efficiency. At extremely light loads, the minimum on-time of the main switch will be reached and the buck converters will begin turning off for multiple cycles in order to maintain regulation. Burst Mode Operation When the PWM pin is forced low, the buck converters will automatically transition between Burst Mode operation at sufficiently light loads (below approximately 15mA) and PWM mode at heavier loads. Burst Mode entry is determined by the peak inductor current. Therefore, the load current at which Burst Mode operation will be entered depends on the input voltage, the output voltage and the inductor value. Typical curves for Burst Mode entry threshold are provided in the Typical Performance Characteristics section of this data sheet. In dropout and near dropout conditions, Burst Mode operation is disabled. Dropout Operation As the input voltage decreases to a value approaching the output regulation voltage, the duty cycle increases toward the maximum on-time. Further reduction of the supply voltage will force the main switch to remain on for more than one cycle until 100% duty cycle operation is reached where the main switch remains on continuously. In this dropout state, the output will be determined by the input voltage less the resistive voltage drop across the main switch and series resistance of the inductor. Slope Compensation Current mode control requires the use of slope compensation to prevent subharmonic oscillations in the inductor current at high duty cycle operation. This is accomplished internally on the LTC3521 through the addition of a compensating ramp to the current sense signal. In some current mode ICs, current limiting is performed by clamping the error amplifier voltage to a fixed maximum. This leads to a reduced output current capability at low step-down ratios. In contrast, the LTC3521 performs current limiting prior to addition of the slope compensation ramp and therefore achieves a peak inductor current limit that is independent of duty cycle. Short-Circuit Protection When the output is shorted to ground, the error amplifier will saturate high and the P-channel MOSFET switch will turn on at the start of each cycle and remain on until the current limit trips. During this minimum on-time, the inductor current will increase rapidly and will decrease very slowly during the remainder of the period due to the very small reverse voltage produced by a hard output short. To eliminate the possibility of inductor current runaway in this situation, the buck converter switching frequency is reduced to 250kHz when the voltage on the buck FB pin falls below 0.25V. The buck soft-start circuit is reset when the buck FB pin falls below 0.25V to provide a smooth restart once the short-circuit condition at the output voltage is no longer present. Additionally, the PMOS current limit is decreased from 1050mA to 700mA when the voltage on the buck FB pin falls below 0.25V. 3521f  LTC3521 operaTion Soft-Start The buck converters have an internal voltage mode soft-start circuit with a nominal duration of 500μs. The converters remain in regulation during soft-start and will therefore respond to output load transients which occur during this time. In addition, the output voltage rise time has minimal dependency on the size of the output capacitor or load current. Error Amplifier and Compensation The LTC3521 buck converters utilize an internal transconductance error amplifier. Compensation of the feedback loop is performed internally to reduce the size of the application circuit and simplify the design process. The compensation network has been designed to allow use of a wide range of output capacitors while simultaneously ensuring rapid response to load transients. PGOOD Comparators The PGOOD2 and PGOOD3 pins are open-drain outputs which indicate the status of the buck converters. If the buck output voltage falls 9% below the regulation voltage, the respective PGOOD open-drain output will pull low. The output voltage must rise 2% above the falling threshold before the pull-down will turn off. In addition, there is a 60μs typical deglitching delay in the flag in order to prevent false trips due to voltage transients on load steps. The respective PGOOD output will also pull low during overtemperature shutdown, undervoltage lockout or if the respective buck converter SHDN pin is pulled low to indicate these fault conditions. L PVIN1 A SW1A SW1B D VOUT1 BUCK-BOOST CONVERTER OPERATION PWM Mode Operation When the PWM pin is held high, the LTC3521 buck-boost converter operates in a constant-frequency PWM mode with voltage mode control. A proprietary switching algorithm allows the converter to switch between buck, buckboost and boost modes without discontinuity in inductor current or loop characteristics. The switch topology for the buck-boost converter is shown in Figure 1. When the input voltage is significantly greater than the output voltage, the buck-boost converter operates in buck mode. Switch D turns on continuously and switch C remains off. Switches A and B are pulse width modulated to produce the required duty cycle to support the output regulation voltage. As the input voltage decreases, switch A remains on for a larger portion of the switching cycle. When the duty cycle reaches approximately 85%, the switch pair AC begins turning on for a small fraction of the switching period. As the input voltage decreases further, the AC switch pair remains on for longer durations and the duration of the BD phase decreases proportionally. As the input voltage drops below the output voltage, the AC phase will eventually increase to the point that there is no longer any BD phase. At this point, switch A remains on continuously while switch pair CD is pulse width modulated to obtain the desired output voltage. At this point, the converter is operating solely in boost mode. This switching algorithm provides a seamless transition between operating modes and eliminates discontinuities in average inductor current, inductor current ripple and loop transfer function throughout all three operational B C LTC3521 PGND1A PGND1B 3521 F01 Figure 1. Buck-Boost Switch Topology 3521f 0 LTC3521 operaTion modes. These advantages result in increased efficiency and stability in comparison to the traditional 4-switch buck-boost converter. Error Amplifier and Compensation The buck-boost converter utilizes a voltage mode error amplifier with an internal compensation network as shown in Figure 2. LTC3521 PVOUT VOUT this case, the increased bandwidth created by decreasing R2 is used to counteract the reduced converter bandwidth caused by the large output capacitor. Current Limit Operation The buck-boost converter has two current limit circuits. The primary current limit is an average current limit circuit which injects an amount of current into the feedback node which is proportional to the extent that the switch A current exceeds the current limit value. Due to the high gain of this loop, the injected current forces the error amplifier output to decrease until the average current through switch A decreases approximately to the current limit value. The average current limit utilizes the error amplifier in an active state and thereby provides a smooth recovery with little overshoot once the current limit fault condition is removed. Since the current limit is based on the average current through switch A, the peak inductor current in current limit will have a dependency on the duty cycle (i.e., on the input and output voltages in the overcurrent condition). The speed of the average current limit circuit is limited by the dynamics of the error amplifier. On a hard output short, it would be possible for the inductor current to increase substantially beyond current limit before the average current limit circuit would react. For this reason, there is a second current limit circuit which turns off switch A if the current ever exceeds approximately 165% of the average current limit value. This provides additional protection in the case of an instantaneous hard output short. Reverse Current Limit The reverse current comparator on switch D monitors the inductor current entering PVOUT. When this current exceeds 375mA (typical), switch D will be turned off for the remainder of the switching cycle. + – 0.6V FB1 R2 R1 GND 3521 F02 Figure 2. Buck-Boost Error Amplifier and Compensation Notice that resistor R2 of the external resistor divider network plays an integral role in determining the frequency response of the compensation network. The ratio of R2 to R1 must be set to program the desired output voltage but this still allows the value of R2 to be adjusted to optimize the transient response of the converter. Increasing the value of R2 generally leads to greater stability at the expense of reduced transient response speed. Increasing the value of R2 can yield substantial transient response improvement in cases where the phase margin has been reduced due to the use of a small value output capacitor or a large inductance (particularly with large boost step-up ratios). Conversely, decreasing the value of R2 increases the loop bandwidth which can improve the speed of the converter’s transient response. This can be useful in improving the transient response if a large valued output capacitor is utilized. In 3521f  LTC3521 operaTion Burst Mode Operation With the PWM pin held low, the buck-boost converter operates utilizing a variable frequency switching algorithm designed to improve efficiency at light load and reduce the standby current at zero load. In Burst Mode operation, the inductor is charged with fixed peak amplitude current pulses. These current pulses are repeated as often as necessary to maintain the output regulation voltage. The maximum output current which can be supplied in Burst Mode operation is dependent upon the input and output voltage as given by the following formula: IOUT(MAX ),BURST = 0.1 • VIN A VIN + VOUT voltage through the action of the voltage mode error amplifier. Since the soft-start is voltage mode, the feedback voltage will track the output voltage correctly during soft-start, and the PGOOD1 output will correctly indicate the point at which the buck-boost attains regulation at the end of soft-start. Therefore, the PGOOD1 output can be utilized for sequencing purposes. Once in regulation, the feedback voltage will no longer track the output voltage, and the PGOOD1 pin will not directly respond to a loss of regulation in the output. However, the only means by which a loss of regulation can occur is if the current limit has been reached, thereby preventing the buck-boost converter from delivering the required output current. In such cases, the occurrence of current limit will cause the PGOOD1 flag to fall indicating a fault state. There can be cases, however, when the buck-boost converter is continuously in current limit, causing the PGOOD1 output to pull low, while the output voltage still remains slightly above the PGOOD1 comparator trip point. The PGOOD1 output also pulls low during overtemperature shutdown, undervoltage lockout or if the SHDN1 pin is pulled low. COMMON FUNCTIONS Thermal Shutdown If the die temperature exceeds 150°C, all three converters will be disabled. All power devices will be turned off and all switch nodes will be high impedance. The soft-start circuits for all three converters are reset during thermal shutdown to provide a smooth recovery once the overtemperature condition is eliminated. All three converters will restart (if enabled) when the die temperature drops to approximately 140°C. Undervoltage Lockout If the supply voltage decreases below 1.7V (typical) then all three converters will be disabled and all power devices will be turned off. The soft-start circuits for all three converters are reset during undervoltage lockout to provide a smooth restart once the input voltage rises above the undervoltage lockout threshold. 3521f () In Burst Mode operation, the error amplifier is not used but is instead placed in a low current standby mode to reduce supply current and improve light load efficiency. Soft-Start The buck-boost converter has an internal voltage mode soft-start circuit with a nominal duration of 500μs. The converter remains in regulation during soft-start and will therefore respond to output load transients that occur during this time. In addition, the output voltage rise time has minimal dependency on the size of the output capacitor or load. During soft-start, the buck-boost converter is forced into PWM operation regardless of the state of the PWM pin. PGOOD Comparator The PGOOD1 pin is an open-drain output which indicates the status of the buck-boost converter. In Burst Mode operation (PWM = Low), the PGOOD1 open-drain output will pull low when the feedback voltage falls 9% below the regulation voltage. There is approximately 3% hysteresis in this threshold when the output voltage is returning good. In addition, there is a 60μs typical deglitching delay to prevent false trips due to short duration voltage transients in response to load steps. In PWM mode, operation of the PGOOD1 comparator is complicated by the fact that the feedback pin voltage is driven to the reference voltage independent of the output  LTC3521 applicaTions inForMaTion The basic LTC3521 application circuit is shown as the Typical Application on the front page of this data sheet. The external component selection is determined by the desired output voltages, output currents and ripple voltage requirements of each particular application. Basic guidelines and considerations for the design process are provided in this section. Buck Inductor Selection The choice of buck inductor value influences both the efficiency and the magnitude of the output voltage ripple. Larger inductance values will reduce inductor current ripple and lead to lower output voltage ripple. For a fixed DC resistance, a larger value inductor will yield higher efficiency by lowering the peak current closer to the average. However, a larger inductor within the same family will generally have a greater series resistance, thereby offsetting this efficiency advantage. Given a desired peak-to-peak current ripple, ΔIL, the required inductance can be calculated via the following expression, where f represents the switching frequency in MHz: V 1 L= VOUT 1 – OUT  µH fΔIL VIN   Table 1 depicts the recommended inductance for several common output voltages. Table 1. Buck Recommended Inductance OUTPUT VOLTAGE 0.6V 1.2V 1.8V 2.5V MINIMUM INDUCTANCE 1.5μH 2.2μH 3.3μH 4.7μH MAXIMUM INDUCTANCE 2.2μH 4.7μH 6.8μH 8.2μH Buck Output Capacitor Selection A low ESR output capacitor should be utilized at the buck output in order to minimize voltage ripple. Multilayer ceramic capacitors are an excellent choice as they have low ESR and are available in small footprints. In addition to controlling the ripple magnitude, the value of the output capacitor also sets the loop crossover frequency and can, therefore, impact loop stability. There is both a minimum and maximum capacitance value required to ensure stability of the loop. If the output capacitance is too small, the loop crossover frequency will increase to the point where the switching delay and the high frequency parasitic poles of the error amplifier will degrade the phase margin. In addition, the wider bandwidth produced by a small output capacitor will make the loop more susceptible to switching noise. At the other extreme, if the output capacitor is too large, the crossover frequency can decrease too far below the compensation zero and lead to a degraded phase margin. Table 2 provides a guideline for the range of allowable values of low ESR output capacitors. Larger value output capacitors can be accommodated provided they have sufficient ESR to stabilize the loop. Table 2. Buck Output Capacitor Range VOUT 0.6V 0.8V 1.2V 1.8V 2.7V 3.3V CMIN 15μF 15μF 10μF 10μF 10μF 6.8μF CMAX 300μF 230μF 150μF 90μF 70μF 50μF () A reasonable choice for ripple current is ΔIL = 240mA which represents 40% of the maximum 600mA load current. The DC current rating of the inductor should be at least equal to the maximum load current, plus half the ripple current, in order to prevent core saturation and loss of efficiency during operation. To optimize efficiency, the inductor should have a low series resistance. In particularly space-restricted applications, it may be advantageous to use a much smaller value inductor at the expense of larger ripple current. In such cases, the converter will operate in discontinuous conduction for a wider range of output loads and efficiency will be reduced. In addition, there is a minimum inductor value required to maintain stability of the current loop (given the fixed internal slope compensation). Specifically, if the buck converter is going to be utilized at duty cycles over 40%, the inductance value must be at least LMIN , as given by the following equation: LMIN = 2.5 • VOUT (µH) 3521f  LTC3521 applicaTions inForMaTion Buck Input Capacitor Selection The PVIN2 pin provides current to the buck converter power switch and is the supply pin for the IC’s internal circuitry. It is recommended that a low ESR ceramic capacitor with a value of at least 4.7µF be used to bypass this pin. The capacitor should be placed as close to the pin as possible and have a short return to ground. Buck Output Voltage Programming The output voltage is set by a resistive divider, according to the following formula:  R2 VOUT 2,3 = 0.6 V 1 +   R1 The external divider is connected to the output, as shown in Figure 3. It is recommended that a feedforward capacitor, CFF , be placed in parallel with resistor R2 to improve the noise immunity of the feedback node. Table 3 provides the recommended resistor and feedforward capacitor combinations for common output voltage options. Table 3. Buck Resistor Divider Values VOUT 0.6V 0.8V 1.0V 1.2V 1.5V 1.8V 2.7V 3.3V R1 – 200k 118k 100k 78.7k 68.1k 63.4k 60.4k R2 0 69.8k 80.6k 102k 121k 137k 226k 274k CFF – 22pF 22pF 22pF 22pF 22pF 33pF 33pF Buck-Boost Output Voltage Programming The buck-boost output voltage is set by a resistive divider according to the following formula:  R2 VOUT1 = 0.6 V 1 +   R1 The external divider is connected to the output, as shown in Figure 4. The buck-boost converter utilizes voltage mode control and the value of R2 plays an integral role in the dynamics of the feedback loop. In general, a larger value for R2 will increase stability and reduce the speed of the transient response. A smaller value of R2 will reduce stability but increase the transient response speed. A good starting point is to choose R2 = 1MΩ, then calculate the required value of R1 to set the desired output voltage according to the above formula. If a large output capacitor is used, the bandwidth of the converter is reduced. In such cases R2 can be reduced to improve the transient response. If a large inductor or small output capacitor is utilized, the loop will be less stable and the phase margin can be improved by increasing the value of R2. Buck-Boost Inductor Selection To achieve high efficiency, a low ESR inductor should be utilized for the buck-boost converter. The inductor must have a saturation rating greater than the worst case average inductor current plus half the ripple current. The peak-topeak inductor current ripple will be larger in buck and boost mode than in the buck-boost region. The peak-to-peak inductor current ripple for each mode can be calculated 0.6V VOUT2 R2 5.25V 0.6V VOUT3 R2 5.25V 1.8V VOUT1 R2 5.25V FB2 R1 FB3 R1 FB1 LTC3521 GND 3521 F03 3521 F04 LTC3521 GND R1 Figure 3. Setting the Buck Output Voltage Figure 4. Setting the Buck-Boost Output Voltage 3521f  LTC3521 applicaTions inForMaTion from the following formulas, where f is the frequency in MHz and L is the inductance in μH: ΔIL,P-P,BUCK = 1 VOUT (VIN – VOUT ) • fL VIN 1 VIN (VOUT – VIN) • fL VOUT Since the output current is discontinuous in boost mode, the ripple in this mode will generally be much larger than the magnitude of the ripple in buck mode. In addition to controlling the ripple magnitude, the value of the output capacitor also affects the location of the resonant frequency in the open loop converter transfer function. If the output capacitor is too small, the bandwidth of the converter will extend high enough to degrade the phase margin. To prevent this from happening, it is recommended that a minimum value of 10μF be used for the buck-boost output capacitor. Buck-Boost Input Capacitor Selection The supply current to the buck-boost converter is provided by the PVIN1 pin. It is recommended that a low ESR ceramic capacitor with a value of at least 4.7μF be located as close to this pin as possible. Inductor Style and Core Material Different inductor core materials and styles have an impact on the size and price of an inductor at any given peak current rating. Toroid or shielded pot cores in ferrite or permalloy materials are small and reduce emissions, but generally cost more than powdered iron core inductors with similar electrical characteristics. The choice of inductor style depends upon the price, sizing, and EMI requirements of a particular application. Table 4 provides a sampling of inductors that are well suited to many LTC3521 application circuits. Table 4. Representative Surface Mount Inductors MANUFACTURER PART NUMBER NP03SB6R8M Coilcraft CooperBussmann Sumida MSS7341-502NL DT1608C-472ML SD7030-5R0-R SD20-6R2-R CDRH4D16FB/ND6R8N MAX VALUE CURRENT 4.7μH 6.8μH 5μH 4.7µH 5µH 6.2µH 6.8µH 1.2A 1A 2.3A 1.2A 2.4A 1.12A 2.6A 1A DCR HEIGHT ΔIL,P-P,BOOST = In addition to affecting output current ripple, the size of the inductor can also affect the stability of the feedback loop. In boost mode, the converter transfer function has a right half plane zero at a frequency that is inversely proportional to the value of the inductor. As a result, a large inductor can move this zero to a frequency that is low enough to degrade the phase margin of the feedback loop. It is recommended that the chosen inductor value be less than 10μH if the buck-boost converter is to be used in the boost region. Buck-Boost Output Capacitor Selection A low ESR output capacitor should be utilized at the buckboost converter output in order to minimize output voltage ripple. Multilayer ceramic capacitors are an excellent choice as they have low ESR and are available in small footprints. The capacitor should be chosen large enough to reduce the output voltage ripple to acceptable levels. Neglecting the capacitor ESR and ESL, the peak-to-peak output voltage ripple can be calculated by the following formulas, where f is the frequency in MHz, COUT is the capacitance in μF L is the inductance in μH and ILOAD is , the output current in amps: ΔVP-P,BOOST = ΔVP-P,BUCK = ILOAD (VOUT – VIN) COUT • VOUT • f 1 8 • L • COUT • f 2 Taiyo Yuden NP03SB4R7M 0.047Ω 1.8mm 0.084Ω 1.8mm 0.024Ω 4.1mm 0.085Ω 2.92mm 0.026Ω 0.072Ω 3mm 2mm (V – V ) V • IN OUT OUT VIN CDR6D23MNNP-4R2 4.2µH 0.052Ω 2.5mm 0.081Ω 1.8mm 3521f  LTC3521 applicaTions inForMaTion Capacitor Vendor Information Both the input and output capacitors used with the LTC3521 must be low ESR and designed to handle the large AC currents generated by switching converters. The vendors in Table 5 provide capacitors that are well suited to LTC3521 application circuits. Table 5. Capacitor Vendor Information MANUFACTURER Taiyo Yuden TDK Murata AVX WEB SITE www.t-yuden.com www.component. tdk.com www.murata.com www.avxcorp.com REPRESENTATIVE PART NUMBERS JMK212BJ106K 10μF 6.3V , JMK212BJ226K 22μF 6.3V , C2012X5R0J106K 10μF 6.3V , GRM21BR60J106K 10μF 6.3V , GRM32ER61C226K 22μF 16V , SM055C106KHN480 10μF PCB Layout Considerations The LTC3521 switches large currents at high frequencies. Special care should be given to the PCB layout to ensure stable, noise-free operation. Figure 5 depicts the recommended PCB layout to be utilized for the LTC3521. A few key guidelines follow: 1. All circulating high current paths should be kept as short as possible. This can be accomplished by keeping the routes to all bold components in Figure 5 as short and as wide as possible. Capacitor ground connections should via down to the ground plane in the shortest route possible. The bypass capacitors on PVIN1 and PVIN2 should be placed as close to the IC as possible and should have the shortest possible paths to ground. 2. The small-signal ground pad (GND) should have a single point connection to the power ground. A convenient way to achieve this is to short the pin directly to the Exposed Pad as shown in Figure 5. 3. The components shown in bold, and their connections, should all be placed over a complete ground plane. 4. To prevent large circulating currents from disrupting the output voltage sensing, the ground for each resistor divider should be returned directly to the small signal ground pin (GND). 5. Use of vias in the die attach pad will enhance the thermal environment of the converter, especially if the vias extend to a ground plane region on the exposed bottom surface of the PCB. Minimizing solution size is usually a priority. Please be aware that ceramic capacitors can exhibit a significant reduction in effective capacitance when a bias is applied. The capacitors exhibiting the highest reduction are those packaged in the smallest case size. 3521f  LTC3521 applicaTions inForMaTion KELVIN TO VOUT PAD BUCK VOUT MINIMIZE TRACE LENGTH VIA TO GROUND PLANE MINIMIZE TRACE LENGTH PGND1A (9) PVIN2 (22) SW2 (20) FB2 (24) FB3 (23) NC (19) SHDN2 (1) PGOOD3 (2) PGOOD2 (3) PGOOD1 (4) VIN (5) GND (6) DIRECT TIE BACK TO GND PIN PGND1B (12) SHDN3 (9) SHDN1 (10) PWM (7) PVIN (11) FB1 (8) PGND2 (18) SW3 (17) VOUT1 (16) SW1A (15) SW1B (14) NC (13) KELVIN TO VOUT PAD BUCK VOUT BUCK-BOOST VOUT KELVIN TO VOUT PAD MINIMIZE TRACE LENGTH UNINTERRUPTED GROUND PLANE MUST EXIST UNDER ALL COMPONENTS SHOWN IN BOLD, AND UNDER TRACES CONNECTING TO THOSE COMPONENTS 3521 F05 Figure 5. LTC3521 Recommended PCB Layout 3521f  LTC3521 Typical applicaTion Dual Supercapacitor to 3.3V at 200mA, 1.8V at 50mA and 1.2V at 100mA Backup Power Supply VIN 1.8V TO 5.5V + + 1F 1F C4 L1 4.7µF 4.7µH PVIN1 SW1A SW1B PVIN2 SW2 FB2 L2 4.7µH R3 137k R4 68.1k C2 10µF VOUT2 1.8V 50mA VOUT1 3.3V 200mA C1 22µF R1 1.0M R2 221k ON VOUT1 LTC3521 FB1 SHDN1 SHDN2 SHDN3 SW3 L3 4.7µH OFF BURST PWM FB3 PGOOD1 PWM PGOOD2 PGOOD3 PGND1 GND PGND2 3521 TA02a R5 100k R6 100k C3 10µF VOUT3 1.2V 100mA Converter Output Voltages VIN 2V/DIV VOUT1 2V/DIV VOUT2 2V/DIV VOUT3 2V/DIV 50µs/DIV 3521 TA02b Efficiency vs VIN 100 96 92 EFFICIENCY (%) 88 84 80 76 72 1.8 2.8 3.8 VIN (V) 4.8 3521 TA02c VOUT1 = 3.3V IOUT = 200mA VOUT3 = 1.2V IOUT = 100mA VOUT2 = 1.8V IOUT = 50mA 3521f  LTC3521 packaGe DescripTion FE Package 20-Lead Plastic TSSOP (4.4mm) (Reference LTC DWG # 05-08-1663) Exposed Pad Variation CB 3.86 (.152) 6.40 – 6.60* (.252 – .260) 3.86 (.152) 20 19 18 17 16 15 14 13 12 11 6.60 ± 0.10 4.50 ± 0.10 SEE NOTE 4 2.74 (.108) 0.45 ± 0.05 1.05 ± 0.10 0.65 BSC 6.40 2.74 (.252) (.108) BSC RECOMMENDED SOLDER PAD LAYOUT 1 2 3 4 5 6 7 8 9 10 1.20 (.047) MAX 0° – 8 ° 4.30 – 4.50* (.169 – .177) 0.25 REF 0.09 – 0.20 (.0035 – .0079) 0.50 – 0.75 (.020 – .030) 0.65 (.0256) BSC NOTE: 1. CONTROLLING DIMENSION: MILLIMETERS MILLIMETERS 2. DIMENSIONS ARE IN (INCHES) 3. DRAWING NOT TO SCALE 0.195 – 0.30 (.0077 – .0118) TYP 0.05 – 0.15 (.002 – .006) FE20 (CB) TSSOP 0204 4. RECOMMENDED MINIMUM PCB METAL SIZE FOR EXPOSED PAD ATTACHMENT *DIMENSIONS DO NOT INCLUDE MOLD FLASH. MOLD FLASH SHALL NOT EXCEED 0.150mm (.006") PER SIDE UF Package 24-Lead Plastic QFN (4mm × 4mm) (Reference LTC DWG # 05-08-1697) 4.00 ± 0.10 (4 SIDES) 0.70 ± 0.05 PIN 1 TOP MARK (NOTE 6) BOTTOM VIEW—EXPOSED PAD PIN 1 NOTCH R = 0.20 TYP OR R = 0.115 0.35 × 45° CHAMFER TYP 23 24 0.40 ± 0.10 1 2 4.50 2.45 ± 0.05 ± 0.05 (4 SIDES) 3.10 ± 0.05 2.45 ± 0.10 (4-SIDES) 0.75 ± 0.05 PACKAGE OUTLINE 0.25 ± 0.05 0.50 BSC RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS NOTE: 1. DRAWING PROPOSED TO BE MADE A JEDEC PACKAGE OUTLINE MO-220 VARIATION (WGGD-X)—TO BE APPROVED 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, IF PRESENT 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 0.200 REF 0.00 – 0.05 (UF24) QFN 0105 0.25 ± 0.05 0.50 BSC 3521f 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.  LTC3521 Typical applicaTion Li-Ion to 3.3V at 800mA, 1.8V at 600mA and 1.2V at 600mA with Sequenced Start-Up VIN 2.4V TO 4.2V + Li-Ion 4.7µF L1 4.7µH L3 4.7µH C3 10µF 499k R5 100k R6 100k L2 4.7µH R3 137k R4 68.1k VOUT1 FB1 PGOOD3 SHDN1 SHDN2 PGOOD1 PGOOD2 PWM SHDN3 PGND1 GND PGND2 3521 TA03a Sequenced Start-Up Waveforms VOUT2 2V/DIV VOUT3 2V/DIV VOUT1 2V/DIV VOUT1 3.3V 800mA (1A, VIN > 3.0V) SHDN2, 5V/DIV PGOOD2, 5V/DIV PGOOD3, 5V/DIV 500µs/DIV 3521 TA03b PVIN1 SW1A SW1B SW3 PVIN2 SW2 FB2 VOUT3 1.2V 600mA C2 10µF VOUT2 1.8V 600mA LTC3521 FB3 R1 1.0M R2 221k R5 499k ON C1 22µF PGOOD1 BURST PWM OFF relaTeD parTs PART NUMBER LTC3100 LTC3101 DESCRIPTION COMMENTS 700mA ISW, 1.5MHz, Synchronous Step-Up, 250mA 94% Efficiency, VIN: 0.7V to 5V, VOUT(MAX) = 5.25V, IQ = 15µA, Synchronous Step-Down DC/DC Converter and 100mA LDO ISD < 1µA, 3mm × 3mm QFN-16 Package Wide VIN, Multioutput DC/DC Converter and PowerPath™ Controller, 800mA Buck-Boost, Dual 350mA Buck Converters, 50mA Always-On LDO 600mA IOUT, 1.7MHz/2.6MHz, Synchronous Step-Down DC/DC Converter 1.2A IOUT, 2MHz, Synchronous Buck-Boost DC/DC Converter 1A 2MHz, Synchronous Buck-Boost and 600mA Buck Converter 400mA 2MHz, Synchronous Buck-Boost and 200mA Buck Converter 95% Efficiency, VIN: 1.8V to 5.5V, IQ = 38µA, Standby IQ = 15µA, 4mm × 4mm QFN-24 Package 96% Efficiency, VIN: 1.6V to 5.5V, VOUT(MIN) = 0.6V, IQ = 65µA, ISD < 1µA, DFN Package 95% Efficiency, VIN: 2.4V to 5.5V, VOUT(MIN): 2.4V to 5.25V, IQ = 50µA, ISD < 1µA, DFN Package 95% Efficiency, VIN: 2.2V to 5.5V, VOUT(MAX) = 5.25V, IQ = 55µA, ISD < 1µA, 4mm × 4mm QFN-24 Package 95% Efficiency, VIN: 2.4V to 5.5V, VOUT(MAX) = 5.25V, IQ = 25µA, ISD < 1µA, 3mm × 3mm QFN-16 Package 95% Efficiency, VIN: 1.8V to 5.5V, VOUT(MIN): 2V to 5V, IQ = 16µA, ISD < 1µA, ThinSOT and DFN Packages 95% Efficiency, VIN: 2.4V to 5.5V, VOUT(MIN): 2.4V to 5.25V, IQ = 35µA, ISD < 1µA, MS10 and DFN Packages 95% Efficiency, VIN: 2.5V to 5.5V, VOUT(MIN) = 0.6V, IQ = 40µA, ISD < 1µA, DFN-8 Package LTC3409 LTC3441/LTC3442/ LTC3443 LTC3520 LTC3522 LTC3531/LTC3531-3/ 200mA IOUT, 1.5MHz, Synchronous Buck-Boost DC/DC LTC3531-3.3 Converter LTC3532 LTC3547 500mA IOUT, 2MHz, Synchronous Buck-Boost DC/DC Converter Dual 300mA IOUT, 2.25MHz, Synchronous Step-Down DC/DC Converter 3521f 0 Linear Technology Corporation 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 ● FAX: (408) 434-0507 ● LT 0310 • PRINTED IN USA www.linear.com  LINEAR TECHNOLOGY CORPORATION 2010