LT3514 Triple Step-Down Switching Regulator with 100% Duty Cycle Operation DESCRIPTION FEATURES Wide Input Voltage Range: 3.2V to 36V (40V Transient) n Three Outputs: 2A, 1A, 1A n 100% Duty Cycle Operation n Resistor-Programmed Constant Frequency n Short-Circuit Robust n Wide SYNC Range: 350kHz to 2.2MHz n Anti-Phase Switching Reduces Ripple n Feedback Voltage: 800mV n Independent Run/Soft-Start Pins n Shutdown with UVLO n Internal Compensation n Thermal Shutdown n Tiny 28-Lead (4mm × 5mm) Thermally Enhanced QFN Package n 24-Lead Exposed Pad TSSOP The LT®3514 consists of three buck regulators (2A, 1A, 1A output current). The device has a wide operating input range of 3.2V to 36V. An on-chip boost regulator allows each channel to operate up to 100% duty cycle. The LT3514 is designed to minimize external component count and results in a simple and small application circuit. APPLICATIONS The LT3514 has one fewer channel (CH2) than the LT3504, and has one channel (CH3) that outputs 2A instead of 1A. The LT3514 in QFN is pin compatible with the LT3504. The LT3504 provides four 1A outputs. n n n n n The LT3514 operates robustly in fault conditions. Cycleby-cycle peak current limit and catch diode current limit sensing protect the IC during overload conditions. Thermal shutdown protects the power switches at elevated temperatures. Soft-start helps control the peak inductor current during startup. The LT3514 also features output voltage tracking and sequencing, programmable frequency, programmable undervoltage lockout, and a power good pin to indicate when all outputs are in regulation. Automotive Battery Regulation Industrial Control Supplies Wall Transformer Regulation Distributed Supply Regulation L, LT, LTC, LTM, Linear Technology, the Linear logo and Burst Mode are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners. TYPICAL APPLICATION Triple Buck Regulator 3.3µH SW1 SKY SW5 1µF VIN 5.4V TO 20V TRANSIENT TO 40V 10µH 1µF ×2 PG LT3514 47nF 1.8V/1A 12.7k 22µF 10.2k 4.7µH SW3 PG DA3 FB3 RT/SYNC 18.2k 1MHz DA1 FB1 VIN VIN EN/UVLO RUN/SS1 RUN/SS3 RUN/SS4 100pF LT3514 Start-Up and Shutdown Waveform. VIN (Top Trace) Is Ramped from OV Up to 8V and Then Back Down to 0V. The Other Three Traces Are the Output Voltages of All Three Channels 47pF 3.3V/2A 31.6k 47µF 10.2k 5V CHANNEL BEGINS 100% DC OPERATION 3.3V CHANNEL BEGINS 100% DC OPERATION 8.2µH SW4 DA4 FB4 22pF UVLO = ~2.9V PARTS SHUTS OFF 5V/1A 53.6k 22µF 10.2k GND 100ms/DIV VIN 1V/DIV CH4 1V/DIV CH3 1V/DIV CH1 1V/DIV 3514 TA01b 3514 TA01a 3514fa For more information www.linear.com/LT3514 1 LT3514 ABSOLUTE MAXIMUM RATINGS (Note 1) EN/UVLO....................................................................40V EN/UVLO Pin Above VIN...............................................5V VIN.............................................................................40V SKY............................................................................46V SW5...........................................................................47V RUN/SS........................................................................6V FB................................................................................6V RT/SYNC......................................................................6V PG..............................................................................25V Operating Junction Temperature Range (Notes 2, 7) LT3514EUFD....................................... –40°C to 125°C LT3514IUFD........................................ –40°C to 125°C LT3514EFE.......................................... –40°C to 125°C LT3514IFE........................................... –40°C to 125°C LT3514HFE.......................................... –40°C to 150°C Storage Temperature Range................... –65°C to 150°C PIN CONFIGURATION PG TOP VIEW SW5 SKY VIN GND VIN TOP VIEW VIN 1 24 SKY NC 2 23 SW5 NC 1 22 NC DA3 3 22 GND NC 2 21 FB3 SW3 4 21 PG DA3 3 20 FB1 SW3 5 20 FB3 SW3 4 19 FB4 SW1 6 18 GND DA1 7 DA1 6 17 RT/SYNC 8 17 RT/SYNC SW4 7 16 EN/UVLO SW4 DA4 8 15 RUN/SS3 DA4 9 16 EN/UVLO 28 27 26 25 24 23 29 GND SW1 5 NC RUN/SS1 RUN/SS4 VIN VIN GND 9 10 11 12 13 14 UFD PACKAGE 28-LEAD (4mm × 5mm) PLASTIC QFN θJA = 43°C/W EXPOSED PAD (PIN 29) IS GND, MUST BE SOLDERED TO PCB 25 GND 19 FB1 18 FB4 NC 10 15 RUN/SS3 VIN 11 14 RUN/SS1 VIN 12 13 RUN/SS4 FE PACKAGE 24-LEAD PLASTIC TSSOP θJA = 33°C/W EXPOSED PAD (PIN 25) IS GND, MUST BE SOLDERED TO PCB ORDER INFORMATION LEAD FREE FINISH TAPE AND REEL PART MARKING* PACKAGE DESCRIPTION TEMPERATURE RANGE LT3514EUFD#PBF LT3514EUFD#TRPBF 3514 28-Lead (4mm × 5mm) Plastic QFN –40°C to 125°C LT3514IUFD#PBF LT3514IUFD#TRPBF 3514 28-Lead (4mm × 5mm) Plastic QFN –40°C to 125°C LT3514EFE#PBF LT3514EFE#TRPBF LT3514FE 24-Lead Plastic TSSOP –40°C to 125°C LT3514IFE#PBF LT3514IFE#TRPBF LT3514FE 24-Lead Plastic TSSOP –40°C to 125°C LT3514HFE#PBF LT3514HFE#TRPBF LT3514FE 24-Lead Plastic TSSOP –40°C to 150°C Consult LTC Marketing for parts specified with wider operating temperature ranges. *The temperature grade is identified by a label on the shipping container. 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/ 3514fa 2 For more information www.linear.com/LT3514 LT3514 ELECTRICAL CHARACTERISTICS The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. VIN = 12V unless otherwise noted. SYMBOL CONDITIONS EN/UVLO Threshold Voltage Rising l MIN TYP MAX 1.2 1.44 1.6 EN/UVLO Threshold Voltage Hysteresis EN/UVLO Threshold Current Hysteresis VEN/UVLO = Measured Rising Threshold – 50mV (Note 3) Internal VIN Undervoltage Lockout 2.4 UNITS V 110 mV 1.3 µA 2.9 3.2 V 0.01 2 µA 4 10 Quiescent Current (VIN) in Shutdown VEN/UVLO = 0V Quiescent Current (VIN) VEN/UVLO = 1V Quiescent Current (VIN) VEN/UVLO = 1.5V, VRUN/SS(1,3,4) = Open, VFB(1,3,4) = 0.9V, VSKY = 17V (Note 4) 2.7 mA Quiescent Current (SKY) VEN/UVLO = 1.5V, VRUN/SS(1,3,4) = Open, VFB(1,3,4) = 0.9V, VSKY = 17V (Note 4) 4.4 mA RUN/SS Pin Source Current VRUN/SS = 0V RUN/SS Pin Threshold for Switching VFB = 0V Feedback Voltage l FB Pin Current VFB = Measured VFB (Note 5) Reference Line Regulation VIN = 5V to 40V SKY Pin Current ISW1 = 1A or ISW4 = 1A 1.3 µA 50 100 mV 790 784 800 800 810 816 mV mV 15 150 nA l –0.015 40 mA 80 mA SKY Pin Current ISW3 = 2A 54 VSKY – VIN 4.85 Switching Frequency RT = 6.34k RT = 18.2k RT = 100k Switching Phase RT = 18.2k 1.8 0.85 220 150 SYNC Threshold Voltage %/V 27 SKY Voltage above VIN Voltage l l l V 2.1 1 270 2.4 1.15 320 MHz MHz kHz 180 210 Deg 1.25 SYNC Input Frequency 0.35 Switch Current Limit (SW1,4) (Note 6) Switch VCESAT (SW1,4) ISW1, SW4 = 1A µA 1.45 V 2.2 1.75 2.1 400 Switch Leakage Current (SW1,4) MHz A mV 0.1 2 µA Catch Diode Current Limit (SW1,4) FB = 0V FB = 0.7V 0.75 1.0 1.15 1.45 1.33 1.67 A A Switch Current Limit (SW3) (Note 6) 3 3.5 4.2 A Switch VCESAT (SW3) ISW3 = 2A 400 Switch Leakage Current (SW3) mV 0.1 4 µA Catch Diode Current Limit (SW3) FB = 0V FB = 0.7V 1.5 2.0 2 2.5 2.4 3.0 A A Switch Current Limit (SW5) (Note 6) 220 320 mA Switch VCESAT (SW5) ISW = 200mA 230 mV Switch Leakage Current (SW5) 0.1 Boost Diode Current Limit (SW5) VIN = 5V 350 450 PG Threshold Offset VFB Rising 65 90 PG Hysteresis VFB Rising – VFB Falling 35 2 µA mA 125 mV mV 3514fa For more information www.linear.com/LT3514 3 LT3514 The l denotes the specifications which apply over the full operating ELECTRICAL CHARACTERISTICS temperature range, otherwise specifications are at TA = 25°C. VIN = 12V unless otherwise noted. SYMBOL CONDITIONS PG Voltage Output Low PG Pin Leakage TYP MAX UNITS IPG = 250µA 180 300 mV VPG = 2V 0.01 1 µA 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 LT3514E is guaranteed to meet performance specifications from 0°C to 125°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 LT3514I is guaranteed over the full –40°C to 125°C operating junction temperature range. Note 3: Current flows into pin. Note 4: The VIN pin quiescent current and the SKY pin quiescent current are specified in the Electrical Characteristics table. However, the quiescent MIN current for an application circuit is higher than the sum of these two currents because the SKY voltage is higher than VIN, and there are power losses in the boost regulator. See the Typical Performance Characteristics section for a plot of input quiescent current vs input voltage for a typical application. Note 5: Current flows out of pin. Note 6: Current limit is guaranteed by design and/or correlation to static test. Slope compensation reduces current limit at higher duty cycles. Note 7: 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. TYPICAL PERFORMANCE CHARACTERISTICS Efficiency, Channel 4, f = 1MHz Efficiency, Channel 4, f = 1MHz 90 VOUT = 1.8V Efficiency, Channel 4, f = 1MHz 90 VOUT = 2.5V 60 70 70 60 60 40 30 20 VIN = 6V VIN = 12V VIN = 24V VIN = 36V 10 0 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 LOAD CURRENT (A) EFFICIENCY (%) 80 50 50 40 30 VIN = 6V VIN = 12V VIN = 24V VIN = 36V 20 10 1 314 G01 0 0 VOUT = 3.3V 80 70 EFFICIENCY (%) EFFICIENCY (%) 80 TA = 25°C, unless otherwise noted. 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 LOAD CURRENT (A) 50 40 30 VIN = 6V VIN = 12V VIN = 24V VIN = 36V 20 10 1 3514 G02 0 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 LOAD CURRENT (A) 1 3514 G03 3514fa 4 For more information www.linear.com/LT3514 LT3514 TYPICAL PERFORMANCE CHARACTERISTICS Efficiency, Channel 4, f = 1MHz 100 Efficiency, Channel 3, f = 1MHz 80 VOUT = 5V 90 60 50 40 30 70 10 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 LOAD CURRENT (A) 50 40 30 20 VIN = 6V VIN = 12V VIN = 24V VIN = 36V 20 VIN = 6V VIN = 12V VIN = 24V VIN = 36V 10 0 1 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 LOAD CURRENT (A) 0 3514 G04 50 40 30 60 50 40 2 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 LOAD CURRENT (A) –3.5 –2.0 –5.0 2 2 3514 G10 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 LOAD CURRENT (A) EN/UVLO Pin Current 1.55 1.8 1.6 1.4 RISING 1.45 1.40 FALLING 1.35 1.20 –50 1 3514 G09 2.0 –45°C 1.2 25°C 1.0 0.8 0.6 150°C 0.4 1.25 VIN = 12V VOUT = 1.8V VOUT = 2.5V VOUT = 3.3V VOUT = 5V –4.5 1.30 1.5 1 LOAD CURRENT (A) –2.5 –3.0 –4.0 IEN/UVLO (µA) –1.5 0.5 –2.0 1.60 1.50 –1.0 0 –1.5 EN/UVLO Threshold –0.5 VIN = 12V 3514 G08 THRESHOLD (V) PERCENT ERROR (%) 3514 G06 VIN = 6V VIN = 12V VIN = 24V VIN = 36V 10 0 2 –1.0 20 VOUT = 1.8V VOUT = 2.5V VOUT = 3.3V VOUT = 5V 0.0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 LOAD CURRENT (A) Load Regulation Channels 1 and 4 70 Load Regulation Channel 3 0.5 0 –0.5 30 VIN = 6V VIN = 12V VIN = 24V VIN = 36V 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 LOAD CURRENT (A) VIN = 6V VIN = 12V VIN = 24V VIN = 36V 0 3514 G07 –2.5 0 2 PERCENT ERROR (%) EFFICIENCY (%) EFFICIENCY (%) 60 0 30 10 80 10 40 20 VOUT = 5V 90 70 20 50 Efficiency, Channel 3, f = 1MHz 100 VOUT = 3.3V 80 60 3514 G05 Efficiency, Channel 3, f = 1MHz 90 VOUT = 2.5V 80 EFFICIENCY (%) EFFICIENCY (%) EFFICIENCY (%) VOUT = 1.8V 60 70 0 Efficiency, Channel 3, f = 1MHz 90 70 80 0 TA = 25°C, unless otherwise noted. 0.2 –25 25 50 0 75 TEMPERATURE (°C) 100 125 3514 G11 0 0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 VEN/UVLO (V) 3514 G12 3514fa For more information www.linear.com/LT3514 5 LT3514 TYPICAL PERFORMANCE CHARACTERISTICS Input Voltage Undervoltage Lockout 10 40 3.4 9 35 8 7 3.0 IVIN (µA) UVLO (V) INPUT QUIESCENT CURRENT (mA) 3.6 2.8 2.6 6 5 4 3 2.4 2 2.2 1 2.0 –50 –25 25 50 0 75 TEMPERATURE (°C) 100 0 125 0 30 25 20 15 10 0 1.20 800 –0.2 1.15 700 –0.4 FREQUENCY (MHz) IRUN/SS (µA) –0.8 –1.0 –1.2 –1.4 200 –1.6 100 –1.8 0 200 600 800 1000 400 RUN/SS VOLTAGE (mV) –25 25 50 0 75 TEMPERATURE (°C) CURRENT LIMIT (A) CHANNELS 1, 4 CHANNEL 3 200 100 0 0 100 0.95 0.80 –50 –25 125 500 1000 1500 SWITCH CURRENT (mA) 2000 3514 G19 0 25 50 75 100 125 150 TEMPERATURE (°C) 3514 G18 Switch and Diode Current Limit 4.0 2.0 3.8 1.9 3.6 1.8 3.4 1.7 CURRENT LIMIT (A) 600 300 1.00 3514 G17 700 400 1.05 Switch and Diode Current Limit, Channel 3 500 RT = 18.2k 0.85 3514 G16 Switch Voltage Drop 45 0.90 –2.0 –50 1200 40 1.10 –0.6 300 0 15 20 25 30 35 INPUT VOLTAGE (V) Switching Frequency vs Temperature 900 400 10 3514 G15 Soft Start Current 500 5 0 3514 G14 FB Voltage vs RUN/SS 600 ALL SS = 2V ALL SS = 0V 5 0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 VEN/UVLO (V) 3514 G13 FB VOLTAGE (mV) Input Quiescent Current vs Input Voltage VIN Pin Current 3.2 SWITCH VOLTAGE DROP (mV) TA = 25°C, unless otherwise noted. 3.2 3.0 2.8 2.6 1.6 1.5 1.4 1.3 2.4 1.2 2.2 1.1 2.0 –50 –25 25 50 0 75 TEMPERATURE (°C) 100 125 3514 G20 CHANNELS 1, 4 1.0 –50 –25 25 50 0 75 TEMPERATURE (°C) 100 125 3514 G21 3514fa 6 For more information www.linear.com/LT3514 LT3514 TYPICAL PERFORMANCE CHARACTERISTICS 3.6 56 3.4 54 3.2 52 3.0 2.0 1.9 1A 50 2A 2.8 48 2.6 46 2.4 44 2.2 42 2.0 40 –50 –25 60 40 DUTY CYCLE (%) 80 100 0 25 50 75 100 125 150 TEMPERATURE (°C) Switch Beta, Channels 1 and 4 1.6 1.5 1.4 1.3 1.2 1.0 0 20 40 60 DUTY CYCLE (%) 80 100 3514 G24 Minimum On-Time 120 65 110 0.5A ON-TIME (ns) 100 55 50 1.7 3514 G23 70 60 1.8 1.1 3514 G22 BETA 90 80 70 1A 45 60 40 –50 –25 25 50 0 75 TEMPERATURE (°C) 100 50 –50 125 –25 25 50 0 75 TEMPERATURE (°C) 3514 G25 Feedback Voltage 125 Power Good Threshold 805 740 804 RISING 720 803 802 801 800 799 798 797 700 680 FALLING 660 640 620 796 795 –50 100 3514 G26 THRESHOLD (mV) 20 SWITCH CURRENT LIMIT (A) 58 BETA 60 3.8 0 Switch Current Limit, Channels 1 and 4 Switch Beta, Channel 3 4.0 FEEDBACK VOLTAGE (mV) SWITCH CURRENT LIMIT (A) Switch Current Limit, Channel 3 TA = 25°C, unless otherwise noted. –25 25 50 0 75 TEMPERATURE (°C) 100 125 600 –50 3514 G27 –25 25 50 0 75 TEMPERATURE (°C) 100 125 3514 G28 3514fa For more information www.linear.com/LT3514 7 LT3514 TYPICAL PERFORMANCE CHARACTERISTICS Operating Waveforms, Discontinuous Mode Operating Waveforms, Continuous Mode SW1 10V/DIV SW1 10V/DIV SW3 10V/DIV SW3 10V/DIV SW4 10V/DIV SW4 10V/DIV 500ns/DIV 3514 G29 IOUT1,3,4 = 40mA VOUT1,3,4 = 5V PIN FUNCTIONS TA = 25°C, unless otherwise noted. 500ns/DIV 3514 G30 IOUT1,3,4 = 0.5A VOUT1,3,4 = 5V (QFN/TSSOP) NC (Pins 1, 2, 14, 22/Pins 2, 10): No Connection. These pins have no connection to internal circuitry. They can be grounded or left floating. DA (Pins 3, 6, 8/Pins 3, 7, 9): Return the Schottky catch diode anode to the diode anode (DA) pin. An internal comparator senses the diode current and prevents switching when the diode current is higher than the DA pin current limit. SW (Pins 4, 5, 7/Pins 4, 5, 6, 8): The SW pins are the output of the internal power switches. Connect each SW pin to an inductor and Schottky catch diode cathode. VIN (Pins 9, 11, 26, 28/Pins 1, 11, 12): The VIN pins supply current to the LT3514’s internal regulator and to the internal power switches. The VIN pins should be locally bypassed with a capacitor to ground, preferably to pins 27 and 10. They must be tied to the same input source. GND (Pins 10, 18, 27, Exposed Pad Pin 29/Pin 22, Exposed Pad Pin 25): Tie the GND pins to a local ground plane below the LT3514 and the circuit components. The exposed pad must be soldered to the PCB and electrically connected to ground. Use a large ground plane and thermal vias to optimize thermal performance. RUN/SS (Pins 12, 13, 15/Pins 13, 14, 15): The RUN/SS pins are used to soft start each channel and to allow each channel to track other outputs. Output tracking is implemented by connecting a resistor divider to this pin from the tracked output. For soft start, tie a capacitor from this pin to ground. An internal 1.3µA soft-start current charges the capacitor to create a voltage ramp at the pin. Each channel can be individually shut down by pulling RUN/SS below 0.1V. EN/UVLO (Pin 16/Pin 16): The EN/UVLO pin is used to start up the internal regulator to power the reference and oscillator. It also starts up the internal boost regulator. Pull the EN/UVLO pin below 1.44V to shut down the LT3514. The LT3514 will draw less than 10µA of current from the VIN pin when EN/UVLO is less than 1.44V. Pull EN/UVLO pin below 0.7V to put the LT3514 in a state where the part draws 0µA from the VIN pin. The threshold can function as an accurate undervoltage lockout (UVLO), preventing the regulator from operating until the input voltage has reached the programmed level. Do not drive the EN/UVLO pin more than 5V above VIN. 3514fa 8 For more information www.linear.com/LT3514 LT3514 PIN FUNCTIONS (QFN/TSSOP) RT/SYNC (Pin 17/Pin 17): Set the switching frequency of the LT3514 by tying an external resistor from this pin to ground. Select the value of the programming resistor (RT) according to Table 1 in the Applications Information section. The RT/SYNC pin is also used to synchronize the internal oscillator of the LT3514 to an external signal. The synchronization (sync) signal is directly logical compatible and can be driven by any signal with pulse width greater than 50ns. The synchronization range is from 350kHz to 2.2MHz. FB (Pins 19, 20, 21/Pins 18,19, 20): Each feedback pin is regulated to 800mV. Connect the feedback resistor divider to this pin. The output voltage is programmed according to the following equation: V R1= R2 • OUT − 1 0.8V PG (Pin 23/Pin 21): The Power Good pin is the open collector output of an internal comparator. PG remains low until all FB pins are greater than 710mV. If not in use, this pin can be left unconnected. The PG comparator is disabled in shutdown. SW5 (Pin 24/Pin 23): The SW5 pin is an open collector of an internal boost regulator power switch. This power switch generates the drive voltage 4.85V above the input voltage (VIN), to drive the internal buck regulator power switches. Connect an inductor from this pin to the VIN pin. SKY (Pin 25/Pin 24): The SKY pin is the output of an integrated power Schottky diode and is the source of drive voltage to the internal buck regulator power switches. Connect a 1µF capacitor from this pin to the VIN pin. Do not drive this pin with an external voltage source. Do not draw current from this pin with an external component. where R1 connects between OUT and FB, and R2 connects between FB and GND. A good value for R2 is 10.2kΩ. 3514fa For more information www.linear.com/LT3514 9 LT3514 BLOCK DIAGRAM PRECISION UVLO ON 1.44V VIN REF EN/UVLO SW5 BOOST ERROR AMP S SKY VIN 4.5V Q5 R NQ 5V VIN 1µA BOOST SWITCH AND DRIVE SKYBAD SKY 0.7V Σ 0.4V LOCK D5 BOOST REGULATOR 1SHOT STARTUP/SHUTDOWN THERMAL SHUTDOWN SLOPE CLK1 CLK2 TO CH3, CH4 SKY OSC 1SHOT SLOPE 0 SYNC DETECT 1 VIN FREQUENCY TO CURRENT 0.7V 2.2V 1µA S Σ 0.8V Q1 R NQ SW1 0.1V +– OUT1 SWITCH AND DRIVE SKYBAD DA1 SKYBAD FB1 ONE OF THREE BUCK REGULATORS SHOWN 0.8V CURRENT LIMIT FOLDBACK PG PGOOD 0.72V FB1 COMPARATORS FROM OTHER CHANNELS RT/SYNC RUN/SS1 POWER GOOD LOGIC GND 3514 BD 3514fa 10 For more information www.linear.com/LT3514 LT3514 OPERATION A comparator starts the reference when the EN/UVLO pin rises above the 1.44V rising threshold. Other comparators prevent switching when the input voltage is below 2.9V or the die temperature is above 175°C. When the EN/UVLO is above 1.44V, the input voltage is above 3.2V, and the temperature is below 175°C, the boost regulator begins switching and charges the SKY capacitor to 4.85V above VIN. When the SKY voltage is less than 4.5V above VIN, the RUN/SS pins and VC nodes are actively pulled low to prevent the buck regulators from switching. The boost regulator (Channel 5) consists of an internal 0.4A power switch (Q5), an internal power Schottky diode (D5), and the necessary logic and other control circuitry to drive the switch. The switch current is monitored to enforce cycle-by-cycle current limit. The diode current is monitored to prevent inductor current runaway during transient conditions. An error amplifier servos the SKY voltage to 4.85V above VIN. A comparator detects when the SKY voltage is 4.5V above VIN and allows the buck regulators to begin switching. The oscillator produces two antiphase clock signals running at 50% duty cycle. Channel 5 runs antiphase to Channels 3 and 4. The oscillator can be programmed by connecting a single resistor from RT/SYNC to ground, or by applying an external clock signal to RT/SYNC. A sync detect circuit distinguishes between the type of input. Tying a resistor to GND directly sets the bias current of the oscillator. The sync signal is converted to a current to set the bias current of the oscillator. The oscillator enables an RS flip-flop, turning on the power switch Q1. An amplifier and comparator monitor the current flowing between the VIN and SW pins, turning the switch off when this current reaches a level determined by the voltage at the VC node. A second comparator enforces a catch diode current limit to prevent inductor current runaway during transient conditions. An error amplifier measures the output voltage through an external resistor tied to the FB pin and servos the VC node. If the error amplifier’s output increases, more current is delivered to the output; if it decreases, less current is delivered. A clamp on the VC pin provides switch current limit. Each buck regulator switch driver operates by drawing current from the SKY pin. Regulating the SKY pin to 4.85V above the VIN pin voltage is necessary to fully saturate the bipolar power switch for efficient operation. Soft-start is implemented by generating a voltage ramp at the RUN/SS pin. An internal 1.3µA current source pulls the RUN/SS pin up to 2.1V. Connecting a capacitor from the RUN/SS pin to ground programs the rate of the voltage ramp on the RUN/SS pin. A voltage follower circuit with a 0.1V offset connected from the RUN/SS pin to the RAMP node prevents switching until the voltage at the RUN/SS pin increases above 0.1V. When the voltage at the RAMP node is less than 0.9V, the error amplifier servos the FB voltage to the RAMP node voltage. When the RAMP node voltage increases above 0.9V, then the error amplifier servos the FB voltage to 0.8V. Additionally, a current amplifier reduces the catch diode current limit when the FB voltage is below 0.8V to limit the inductor current during startup. Each channel can be placed in shutdown by pulling the respective RUN/SS pin below 0.1V. The EN/UVLO pin can be pulled low (below a VBE) to place the entire part in shutdown, disconnecting the outputs and reducing the input current to less than 2µA. 3514fa For more information www.linear.com/LT3514 11 LT3514 APPLICATIONS INFORMATION The three step-down converters in the LT3514 are referred to as channels 1, 3, and 4, while the boost converter is referred to as channel 5. There is no channel 2. This naming convention is intended to maintain consistency and limited pin compatibility with the LT3504, a four channel step-down converter. Essentially, two 1A converters (channels 2 and 3) of the LT3504 were combined to make the 2A converter (channel 3) of the LT3514. FB Resistor Network The output voltage is programmed with a resistor divider connected from the output and the FB pin. Choose the 1% resistor according to: V R1= R2 • OUT − 1 0.8V A good value for R2 is 10.2kΩ, R2 should not exceed 20kΩ to avoid bias current error. Input Voltage Range The input voltage range for LT3514 applications depends on the output voltage and on the absolute maximum rating of the VIN pin. The minimum input voltage to regulate the output generally has to be at least 400mV greater than the greatest programmed output voltage. The only exception is when the largest programmed output voltage is less than 2.8V. In this case the minimum input voltage is 3.2V. The absolute maximum input voltage of the LT3514 is 40V and the part will regulate output voltages as long as the input voltage remains less than or equal to 40V. However for constant-frequency operation (no pulseskipping) the maximum input voltage is determined by the minimum on-time of the LT3514 and the programmed switching frequency. The minimum on-time is the shortest period of time that it takes the switch to turn on and off. Therefore the maximum input voltage to operate without pulse-skipping is: VIN(PS) = [ (VOUT + VD)/(fSW • tON(MIN)) ] + VSW – VD where: •VIN(PS) is the maximum input voltage to operate in constant frequency operation without skipping pulses. •VOUT is the programmed output voltage •VSW is the switch voltage drop, at IOUT1,4 = 1A, VSW1,4 = 0.4V, at IOUT3 = 2A, VSW3 = 0.4V. •VD is the catch diode forward voltage drop, for an appropriately sized diode, VD = 0.4V •fSW is the programmed switching frequency •tON(MIN) is the minimum on-time, worst-case over temperature = 110ns (at T = 125°C) At input voltages that exceed VIN(PS) the part will continue to regulate the output voltage up to 40V. However the part will skip pulses (see Figure 1) resulting in unwanted harmonics, increased output voltage ripple, and increased IL 0.5A/DIV VSW 10V/DIV 2µs/DIV 3514 F01a Figure 1a. The LT3514 Operating in Constant-Frequency Operation (Below VIN(PS)), VIN = 26.5V, VOUT = 3.3V, fSW = 2MHz, tON(MIN) = 74ns at T = 25°C IL 0.5A/DIV VSW 10V/DIV 2µs/DIV 3514 F01b Figure 1b. The LT3514 Operating in Pulse-Skipping Mode (Above VIN(PS)), VIN = 27V, VOUT = 3.3V, fSW = 2MHz, tON(MIN) = 74ns at T = 25°C 3514fa 12 For more information www.linear.com/LT3514 LT3514 APPLICATIONS INFORMATION peak inductor current. Provided that the inductor does not saturate and that the switch current remains below 2A (SW1, SW4) or below 4A (SW3), operation above VIN(PS) is safe and will not damage the part. For a more detailed discussion on minimum on-time and pulse-skipping, refer to the Applications Information section of the LT3505 data sheet. Avoid starting up the LT3514 at input voltages greater than 36V, as the LT3514 must simultaneously conduct maximum currents at high VIN. The maximum operating junction temperature of 125°C may be exceeded due to the high instantaneous power dissipation. Frequency Selection The maximum frequency that the LT3514 can be programmed to is 2.5MHz. The minimum frequency is 250kHz. The switching frequency can be programmed in two ways. The first method is by tying a 1% resistor (RT) from the RT/SYNC pin to ground. Table 1 can be used to select the value of RT. The second method is to synchronize (sync) the internal oscillator to an external clock. The external clock must have a minimum amplitude from 0V to 1.5V and a minimum pulse-width of 50ns. Table 1. RT/SYNC Pin Resistance to Program Oscillator Frequency FREQUENCY (MHz) RT/SYNC PIN RESISTANCE (kΩ) 0.20 140 0.3 82.5 0.4 56.2 0.5 43.2 0.6 34.8 0.7 28.0 0.8 23.7 0.9 20.5 1.0 18.2 1.1 16.9 1.2 14.7 1.3 13.0 1.4 11.5 FREQUENCY (MHz) RT/SYNC PIN RESISTANCE (kΩ) 1.5 10.7 1.6 9.76 1.7 8.66 1.8 8.06 1.9 7.32 2.0 6.81 2.1 6.34 2.2 6.04 2.3 5.62 2.4 5.36 2.5 4.99 In certain applications, the LT3514 may be required to be alive and switching for a period of time before it begins to receive a sync signal. If the sync signal is in a high impedance state when it is inactive then the solution is to simply tie an RT resistor from the RT/SYNC pin to ground (Figure 2). The sync signal should be capable of driving the RT resistor. If the sync signal is in a low impedance state or an unknown state when it is inactive, then the solution is to tie the RT resistor from the RT/SYNC pin to ground and then to drive the RT/SYNC pin with the sync signal through a 1nF capacitor as shown in Figure 3. LT3514 PORT RT/SYNC GND RT 3514 F02 Figure 2. Driving the RT/SYNC Pin From a Port That Is in a High Impedance State When it Is Inactive LT3514 1nF PORT RT/SYNC RT GND 3514 F03 Figure 3. Driving the RT/SYNC Pin from a Port That Is in a Low Impedance State When it Is Inactive 3514fa For more information www.linear.com/LT3514 13 LT3514 APPLICATIONS INFORMATION BOOST Regulator and SKY Pin Considerations The on-chip boost regulator generates the SKY voltage to be 4.85V above VIN. The SKY voltage is the source of drive current for the buck regulators which is used to fully saturate the power switch. The boost regulator requires two external components: an inductor and a capacitor. A good first choice for an inductor is given by: 20.5µH L= f Thus, for a 250kHz programmed switching frequency, a good first choice for an inductor value is 82µH. For a 2.5MHz programmed switching frequency, a good first choice for an inductor value is 8.2µH. These values will ensure that each buck regulator will have sufficient drive current to saturate the power switch in all applications and under all operating conditions. A user desiring a lower inductor current value can calculate their optimum inductor size based on their output current requirements. Each buck regulator instantaneously requires 20mA from the SKY pin per 1A of switch current. The average current that each buck regulator draws from the SKY pin is 20mA per 1A of switch current multiplied by the duty cycle. So if all three buck regulators run at 100% duty cycle with channels 1 and 4 supplying 1A of output current and channel 3 supplying 2A of output current, then the SKY pin should be able to source 80mA. However if each channel runs at 50% duty cycle then the SKY pin only has to source 40mA. Alternatively if each channel runs at 100% duty cycle but the output current requirements are reduced by half, then again the SKY pin only has to source 40mA. To summarize, the SKY pin output current requirement is calculated from the following equation: (I Once the SKY pin output current requirement is determined, the inductor value can be calculated based on the maximum tolerable inductor current ripple from the following equation: L= where f is in MHz. ISKY = where IOUTX is the desired output current from Channel X, VOUTX is the programmed output voltage of Channel X, and VIN is input voltage. OUT1 • VOUT1 + IOUT3 • VOUT3 + IOUT4 • VOUT4 50 • VIN ) VIN • DC5 2 • fSW • 0.3 • (1− 0.25 • DC5) − ISKY where fSW is the programmed switching frequency and DC5 is the boost regulator duty cycle, given by: DC5 = 5V/(VIN + 5V). For a 1MHz application, with VIN = 12V, VOUT1 = 5V, VOUT3 = 2.5V, VOUT4 = 1.8V, IOUT1,4 = 1A, IOUT3 = 2A, and the required SKY pin current is 20mA and the inductor value is 6.8µH. Soft-Start/Tracking The RUN/SS pin can be used to soft-start the corresponding channel, reducing the maximum input current during start-up. The RUN/SS pin is pulled up through a 1µA current source to about 2.1V. A capacitor can be tied to the pin to create a voltage ramp at this pin. The buck regulator will not switch while the RUN/SS pin voltage is less than 0.1V. As the RUN/SS pin voltage increases above 0.1V, the channel will begin switching and the FB pin voltage will track the RUN/SS pin voltage (offset by 0.1V), until the RUN/SS pin voltage is greater than 0.8V + 0.1V. At this point the output voltage will be at 100% of it’s programmed value and the FB pin voltage will cease to track the RUN/SS pin voltage and remain at 0.8V (the RUN/SS pin will continue ramping up to about 2.1V with no effect on the output voltage). The ramp rate can be tailored so that the peak start up current can be reduced to the current that 3514fa 14 For more information www.linear.com/LT3514 LT3514 APPLICATIONS INFORMATION is required to regulate the output, with little overshoot. Figure 4 shows the start-up waveforms with and without a soft-start capacitor (CSS) on the RUN/SS pin. IL 0.5A/DIV VOUT 2V/DIV 3514 F04a 100µs/DIV Undervoltage Lockout The LT3514 prevents switching when the input voltage decreases below 3.2V. Alternatively, the EN/UVLO pin can be used to program an undervoltage lockout at input voltages exceeding 3.2V by tapping a resistor divider from VIN to EN/UVLO as shown in Figure 5. The rising threshold on the EN/UVLO pin is 1.44V. The falling threshold on the EN/UVLO pin is 1.33V. When EN/ UVLO is rising and less than 1.44V then the EN/UVLO pin sinks 1.3µA of current. This 1.3µA current can be used to program additional hysteresis on the EN/UVLO pin. For the circuit in Figure 5, R1 can be determined from: Figure 4a. Inductor Current Waveform During Start-Up without a Soft-Start Capacitor R1= ( 0.11 V 1.33 IN,FALLING 1.3µA VIN,HYSTERESIS − ) where VIN,HYSTERESIS is the desired amount of hysteresis on the input voltage and VIN,FALLING is the desired input voltage threshold at which the part will shut down. Notice that for a given falling threshold (VIN,FALLING), the amount of hysteresis (VIN,HYSTERESIS) must be at least: IL 0.5A/DIV VOUT 2V/DIV 3514 F04b 100µs/DIV VIN, HYSTERESIS > ( 0.11 • VIN,FALLING 1.33 ) Figure 4b. Inductor Current Waveform During Start-Up with a 1nF Soft-Start Capacitor (CSS) SWITCHING VIN VIN R1 133k LT3514 EN/UVLO R2 20.5k GND VIN, FALLING = 10V VIN, RISING = 11V NOT SWITCHING 9 10 11 VIN (V) 12 3514 F05 Figure 5. Circuit to Prevent Switching When VIN < 10V, with 700mV of Hysteresis 3514fa For more information www.linear.com/LT3514 15 LT3514 APPLICATIONS INFORMATION For a falling threshold of 10V, the minimum hysteresis is 0.827V. For a falling threshold of 30V, the minimum hysteresis is 2.48V. R2 can be calculated once R1 is known: R2 = R1• 1.33 VIN, FALLING − 1.33 The circuit shown in Figure 5 will start when the input voltage rises above 11V and will shutdown when the input voltage falls below 10V. Inductor Selection and Maximum Output Current A good first choice for the inductor value is: L = 2 • (VOUT + VD)/fSW for Channels 1, 4 L = (VOUT + VD)/fSW for Channel 3 CH4 or below 2A for CH3, then you can decrease the value of the inductor and operate with higher ripple current. This allows you to use a physically smaller inductor, or one with a lower DCR resulting in higher efficiency. Low inductance may result in discontinuous mode operation, which is okay, but further reduces maximum load current. For details on maximum output current and discontinuous mode operation, see Linear Technology Application Note 44. Catch Diode Use a 1A Schottky diode for channels 1 and 4 and a 2A Schottky diode for channel 3. The diode must have a reverse voltage rating equal to or greater than the maximum input voltage. Input Capacitor where VD is the voltage drop of the catch diode (~0.4V), L is in µH and fSW is in MHz. With this value there will be no subharmonic oscillation for applications with 50% or greater duty cycle. The inductor’s RMS current rating must be greater than your maximum load current and its saturation current should be about 30% higher. For robust operation in fault conditions, the saturation current should be above 2A for CH1, CH4 and above 4A for CH3. To keep efficiency high, the series resistance (DCR) should be less than 0.1 . Table 2 lists several vendors and types that are suitable. Of course, such a simple design guide will not always result in the optimum inductor for your application. A larger value provides a higher maximum load current and reduces output voltage ripple at the expense of slower transient response. If your load is lower than 1A for CH1, The input of the LT3514 circuit must be bypassed with a X7R or X5R type ceramic capacitor. Y5V types have poor performance over temperature and amplified voltage and should not be used. There are four VIN pins. Each VIN pin should be bypassed to the nearest ground pin. However it is not necessary to use a dedicated capacitor for each VIN pin. Pins 9 and 11 may be tied together on the board layout so that both pins can share a single bypass capacitor. Since the channels running on Pins 9 and 11 are 180 degrees out-of-phase, it is not necessary to double the capacitor value either. Similarly, Pins 26 and 28 may be tied together on the board layout to save a bypass capacitor. For switching frequencies greater than 750kHz, a 1µF capacitor or higher value ceramic capacitor should be used to bypass each group of two VIN pins. For switching frequencies less than 750kHz, a 2.2µF or higher value ceramic capacitor should be used to bypass each Table 2. Inductor Vendors VENDOR URL PART SERIES Sumida www.sumida.com CDRH4D28 CDRH5D28 CDRH5D28 INDUCTANCE (µH) 1.2 TO 4.7 2.5 TO 10 2.5 TO 33 SIZE (mm) 4.5 × 4.5 5.5 × 5.5 8.3 × 8.3 Toko www.toko.com A916CY D585LC 2 TO 12 1.1 TO 39 6.3 × 6.2 8.1 × 8 Würth Elektronik www.we-online.com WE-TPC(M) WE-PD2(M) WE-PD(S) 1 TO 10 2.2 TO 22 1 TO 27 4.8 × 4.8 5.2 × 5.8 7.3 × 7.3 3514fa 16 For more information www.linear.com/LT3514 LT3514 APPLICATIONS INFORMATION group of two VIN pins. The ceramic bypass capacitors should be located as close to the VIN pins as possible. See the sample layout shown in the PCB Layout section. All four VIN pins should be tied together on the board and bypassing with a low performance electrolytic capacitor is recommended especially if the input power source has high impedance, or there is significant inductance due to long wires or cables. Step-down regulators draw current from the input supply in pulses with very fast rise and fall times. The input capacitor is required to reduce the resulting voltage ripple at the LT3514 and to force this very high frequency switching current into a tight local loop, minimizing EMI. To accomplish this task, the input bypass capacitor must be placed close to the LT3514 and the catch diode; see the PCB Layout section. A second precaution regarding the ceramic input capacitor concerns the maximum input voltage rating of the LT3514. A ceramic input capacitor combined with trace or cable inductance forms a high quality (underdamped) tank circuit. If the LT3514 circuit is plugged into a live supply, the input voltage can ring to twice its nominal value, possibly exceeding the LT3514’s voltage rating. This situation can be easily avoided by adding an electrolytic capacitor in parallel with the ceramic input capacitors. See Application Note 88. Output Capacitor The output capacitor has two essential functions. Along with the inductor, it filters the square wave generated by the LT3514 to produce the DC output. In this role it deter- mines the output ripple so low impedance at the switching frequency is important. The second function is to store energy in order to satisfy transient loads and stabilize the LT3514’s control loop. Ceramic capacitors have very low equivalent series resistance (ESR) and provide the best ripple performance. A good value is: COUT = 33/(VOUT • fSW) for Channels 1, 4 COUT = 132/(VOUT • fSW) for Channel 3 where COUT is in µF and fSW is in MHz. Use X5R or X7R types and keep in mind that a ceramic capacitor biased with VOUT will have less than its nominal capacitance. This choice will provide low output ripple and good transient response. Transient performance can be improved with a high value capacitor, if the compensation network is also adjusted to maintain the loop bandwidth. A lower value of output capacitor can be used, but transient performance will suffer. Also, a lower value output capacitor may result in increased sensitivity to noise which can be alleviated by adding a 100pF phase lead capacitor from FB to VOUT. High performance electrolytic capacitors can be used for the output capacitor. Low ESR is important, so choose one that is intended for use in switching regulators. The ESR should be specified by the supplier and should be 0.1Ω or less. Such a capacitor will be larger than a ceramic capacitor and will have a larger capacitance, because the capacitor must be large to achieve low ESR. Table 3 lists several capacitor vendors. Table 3. Capacitor Vendors VENDOR PHONE URL PART SERIES COMMENTS Panasonic (714) 373-7366 www.panasonic.com Ceramic, Polymer, Tantalum EEF Series Kemet (864) 963-6300 www.kemet.com Ceramic, Tantalum T494, T495 Sanyo (408) 749-9714 www.sanyovideo.com Ceramic, Polymer, Tantalum POSCAP Murata (404) 436-1300 www.murata.com Ceramic www.avxcorp.com Ceramic, Tantalum www.taiyo-yuden.com Ceramic AVX Taiyo Yuden (864) 963-6300 TPS Series 3514fa For more information www.linear.com/LT3514 17 LT3514 APPLICATIONS INFORMATION Figure 6 shows the transient response of the LT3514 with several output capacitor choices. The output is 3.3V. The load current is stepped from 500mA to 1A and back to 500mA and the oscilloscope traces show the output voltage. The upper photo shows the recommended value. The second photo shows the improved response (less voltage drop) resulting from a larger output capacitor and a larger phase lead capacitor. The last photo shows the response to a high performance electrolytic capacitor. Transient performance is improved due to the large output capacitance. VOUT LT3514 31.6k FB Shorted and Reversed Input Protection If the inductor is chosen so that it won’t saturate excessively, an LT3514 buck regulator will tolerate a shorted output. There is another situation to consider in systems where the output will be held high when the input to the LT3514 is absent. This may occur in battery charging applications or in battery backup systems where a battery or some other supply is diode OR-ed with the LT3514’s output. If the VIN pin is allowed to float and the EN/UVLO pin is held high (either by a logic signal or because it is IOUT 1A/DIV 10µF 10k VOUT 20mV/DIV VOUT LT3514 31.6k 100pF 20µs/DIV 3514 F06a 20µs/DIV 3514 F06b 20µs/DIV 3514 F06c IOUT 1A/DIV 10µF ×2 FB 10k VOUT 20mV/DIV VOUT LT3514 31.6k FB + IOUT 1A/DIV 22µF 10k VOUT 20mV/DIV Figure 6. Transient Load Response of the LT3514 with Different Output Capacitors as the Load Current Is Stepped from 500mA to 1A. VIN = 12V, VOUT = 3.3V, L = 10µH, RT = 19.1k 3514fa 18 For more information www.linear.com/LT3514 LT3514 APPLICATIONS INFORMATION tied to VIN), then the LT3514’s internal circuitry will pull its quiescent current through its SW pin. This is fine if your system can tolerate a few mA in this state. If you ground the EN/UVLO pin, the SW pin current will drop to essentially zero. However, if the VIN pin is grounded while the output is held high, then parasitic diodes inside the LT3514 can pull large currents from the output through the SW pin and the VIN pin. Figure 7 shows a circuit that will run only when the input voltage is present and that protects against a shorted or reversed input. reduce the dependence of efficiency on input voltage. The die temperature is calculated by multiplying the LT3514 power dissipation by the thermal resistance from junction to ambient. Power dissipation within the LT3514 can be estimated by calculating the total power loss from an efficiency measurement and subtracting the catch diode losses. Thermal resistance depends on the layout of the circuit board, but 43°C/W is typical for the QFN package and 33°C/W is typical for the FE package. Thermal shutdown will turn off the buck regulators and the boost regulator when the die temperature exceeds 175°C, but this is not a warrant to allow operation at die temperatures exceeding 125°C. High Temperature Considerations While the LT3514 is capable of delivering total output current up to 4A, total power dissipation for an application circuit and the resulting temperature rise must be considered, especially if all three channels are operating at high duty cycle. Outputs Greater Than 9V For outputs greater than 9V, add a 1k resistor in series with a 1nF capacitor across the inductor to damp the discontinuous ringing of the SW node, preventing unintended SW current. The die temperature of the LT3514 must be lower than the maximum rating of 125°C. This is generally not a concern unless the ambient temperature is above 85°C. For higher temperatures, extra care should be taken in the layout of the circuit to ensure good heat sinking of the LT3514. The maximum load current should be derated as the ambient temperature approaches 125°C. Programming the LT3514 to a lower switching frequency will improve efficiency and VIN Other Linear Technology Publications Application Notes 19, 35, 44 contain more detailed descriptions and design information for step-down regulators and other switching regulators. Design Note 318 shows how to generate a bipolar output supply using a step-down regulator. EN/UVLO VOUT SW1 SKY SW5 LT3514 D4 DA1 VIN VIN RUN/SS1 BACKUP FB1 RT/SYNC GND 3514 F07 Figure 7. Diode D4 Prevents a Shorted Input from Discharging a Backup Battery Tied to the Output; It Also Protects the Circuit from a Reversed Input. The LT3514 Runs Only When the Input Is Present 3514fa For more information www.linear.com/LT3514 19 LT3514 APPLICATIONS INFORMATION PCB Layout For proper operation and minimum EMI, care must be taken during printed circuit board layout. Figure 8 shows the recommended component placement with trace, ground plane, and via locations for the QFN package. Note that large, switched currents flow in the LT3514’s VIN, SW and DA pins, the catch diodes (D1, D3, D4) and the input capacitors (C5, C6). The loop formed by these + C3 GND components should be as small as possible and tied to system ground in only one place. These components, along with the inductors (L1, L3, L4, L5) and output capacitors (C1, C3, C4, C7), should be placed on the same side of the circuit board, and their connections should be made on that layer. Place a local, unbroken ground plane below these components, and tie this ground plane to system ground at one location (ideally at the ground terminal of the output capacitors). For the QFN package ground C8 OUT3 L5 SW5 L3 GND VIN C7 SKY SW3 GND C5 D3 GND PG NC NC NC SW1 VIN SW4 R3 R1 R7 R9 GND L1 R6 RT/SYNC RUN/SS3 EN/UVLO GND C6 R5 NC R2 D4 RUN/SS4 RUN/SS1 D1 FB3 FB1 FB4 GND GND L4 C4 OUT1 C1 GND OUT4 GND VIA TO LOCAL GROUND PLANE OUTLINE OF LOCAL GROUND PLANE VIA TO VIN 3514 F08 Figure 8 3514fa 20 For more information www.linear.com/LT3514 LT3514 APPLICATIONS INFORMATION pins (Pins 10, 27) are provided near the VIN pins so that the VIN pins can be bypassed to these ground pins. The SW nodes should be kept as small as possible and kept far away from the RT/SYNC and FB nodes. Keep the RT/ SYNC node and FB nodes small so that the ground pin and ground traces will shield them from the SW nodes. If the user plans on using a SYNC signal to set the oscillator frequency then the RT/SYNC node should be kept away from the FB nodes. Include vias near the exposed pad of the LT3514 to help transfer heat from the LT3514 to the ground plane. Keep the SW5 pad/trace as far away from the FB pads as possible. Zener diode D1 clamps Q1’s gate voltage to 36V. The source follower configuration prevents VIN from rising any further than about 33V (a VGS below the Zener clamp voltage ) . Figure 10 shows the LT3514 regulating all three channels through a 180V surge event without interruption. VSUPPLY 50V/DIV VIN 50V/DIV VOUT1,3,4 2V/DIV Overvoltage Transient Protection Figure 9 shows the complete application circuit for a 3-output step-down regulator with 100% duty cycle operation that withstands 180V surges. Under normal operating conditions (VIN < 33V), the VSKY rail supplies gate drive to MOSFET Q1, providing the LT3514 with a low resistance path to VSUPPLY. In the event that a supply surge occurs, 3514 F10 100ms/DIV Figure 10. Overvoltage Protection Withstands 180V Surge VSUPPLY 3.2V TO 30V SURGE PROTECTION TO 180V R1 10Ω Q1 R2 100k D2 6.8V R3 1k C1 0.1µF D3 D1 36V VSKY CSKY 2.2µF L5 10µH VIN + 22µF 2.2µF ×2 PG Q1: FQB34N20L D1: BZT52C36-7-F D2: BZT52C6V8-7-F D3: BAT54-7-F L1: CDRH5D28-4R2 L3: CDRH8D28-4R7 L4: CDRH5D28-8R2 L5: CBC2016100M 4.2µH SW1 SKY SW5 LT3514 0.1µF 18.2k 2.5V/1A 21.5k 22µF 10.2k 4.7µH SW3 PG DA3 FB3 RT/SYNC 82pF DA1 FB1 VIN VIN EN/UVLO RUN/SS1 RUN/SS3 RUN/SS4 L1 L3 47pF 3.3V/2A 31.6k 47µF 10.2k 8.2µH SW4 L4 22pF DA4 FB4 5V/1A 53.6k 22µF 10.2k GND 3514 F09 Figure 9. Complete Triple Buck Regulator with 180V Surge Protection 3514fa For more information www.linear.com/LT3514 21 LT3514 APPLICATIONS INFORMATION Bear in mind that significant power dissipation occurs in Q1 during an overvoltage event. The MOSFET junction temperature must be kept below its absolute maximum rating. For the overvoltage transient shown in Figure 10, MOSFET Q1 conducts 0.55A (full load on all buck channels) while withstanding the voltage difference between VSUPPLY (180V peak) and VIN (33V). This results in a peak power of 81W. Since the overvoltage pulse in Figure 10 is roughly triangular, average power dissipation during the transient event (about 400ms) is approximately half the peak power. As such, the average power is given by: 1 • PPEAK (W) = 40.5W 2 In order to approximate the MOSFET junction temperature rise from an overvoltage transient, one must determine the MOSFET transient thermal response as well as the MOSFET power dissipation. Fortunately, most MOSFET transient thermal response curves are provided by the manufacturer (as shown in Figure 11). For a 400ms pulse duration, the FQB34N20L MOSFET thermal response ZθJC(t) is 0.65°C/W. The MOSFET junction temperature rise is given by: PAVG (W) = TRISE (°C) = ZθJC (t) • PAVG (W) = 26.3°C Note that, by properly selecting MOSFET Q1, it is possible to withstand even higher input voltage surges. Consult manufacturer data sheets to ensure that the MOSFET operates within its maximum safe operating area. The application circuit start-up behavior is shown in Figure 12. Resistor R2 pulls up on the gate of Q1, forcing source connected VIN to follow approximately 3V below VSUPPLY. Once VIN reaches the LT3514’s 3.2V minimum start-up voltage, the on-chip boost converter immediately regulates the VSKY rail 4.85V above VIN. Diode D3 and resistor R3 bootstrap Q1’s gate voltage to the VSKY, fully enhancing Q1. This connects VIN directly to VSUPPLY through Q1’s low resistance drain-source path. It should be noted that, prior to VSKY being present, the minimum input voltage is about 6.2V. However, with VSKY in regulation and Q1 enhanced, the minimum run voltage drops to 3.2V, permitting the LT3514 to maintain regulation through deep input voltage dips Figure 13 shows all channels operating down to the LT3514’s 3.2V minimum input voltage. SKY 2V/DIV VSUPPLY 2V/DIV VIN 2V/DIV 20ms/DIV 3514 F12 Figure 12. Figure 9’s Start-Up Behavior ZθJC(t), THERMAL RESPONSE (°C/W) 1 VIN 1V/DIV 0.1 VOUT4 1V/DIV 0.01 10–3 10–5 SINGLE PULSE VOUT3 1V/DIV VOUT1 1V/DIV D = 0.5 D = 0.2 D = 0.1 D = 0.05 D = 0.02 D = 0.01 0.1 1 10–4 10–3 0.01 10 t1, SQUARE WAVE PULSE DURATION (s) PDM t1 t2 ZθJC(t) = 0.7°C/W MAX DUTY FACTOR = D = t1/t2 TJM – TC = PDM • ZθJC(t) 100ms/DIV 3514 F13 Figure 13. Figure 9’s Dropout Performance 3514 F11 Figure 11. FQB34N20L Transient Thermal Response 3514fa 22 For more information www.linear.com/LT3514 LT3514 PACKAGE DESCRIPTION Please refer to http://www.linear.com/designtools/packaging/ for the most recent package drawings. FE Package 24-Lead Plastic TSSOP (4.4mm) (Reference LTC DWG # 05-08-1771 Rev B) Exposed Pad Variation AA 7.70 – 7.90* (.303 – .311) 3.25 (.128) 3.25 (.128) 24 23 22 21 20 19 18 17 16 15 14 13 6.60 ±0.10 2.74 (.108) 4.50 ±0.10 6.40 2.74 (.252) (.108) BSC SEE NOTE 4 0.45 ±0.05 1.05 ±0.10 0.65 BSC 1 2 3 4 5 6 7 8 9 10 11 12 RECOMMENDED SOLDER PAD LAYOUT 4.30 – 4.50* (.169 – .177) 0.09 – 0.20 (.0035 – .0079) 0.25 REF 0.50 – 0.75 (.020 – .030) NOTE: 1. CONTROLLING DIMENSION: MILLIMETERS MILLIMETERS 2. DIMENSIONS ARE IN (INCHES) 3. DRAWING NOT TO SCALE 1.20 (.047) MAX 0° – 8° 0.65 (.0256) BSC 0.195 – 0.30 (.0077 – .0118) TYP 0.05 – 0.15 (.002 – .006) FE24 (AA) TSSOP REV B 0910 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 3514fa For more information www.linear.com/LT3514 23 LT3514 PACKAGE DESCRIPTION Please refer to http://www.linear.com/designtools/packaging/ for the most recent package drawings. UFD Package UFDQFN Package 28-Lead Plastic (4mm × 5mm) 28-Lead Plastic QFN (4mm × 5mm) (Reference LTC DWG # 05-08-1712 Rev B) (Reference LTC DWG # 05-08-1712 Rev B) 0.70 ±0.05 4.50 ±0.05 3.10 ±0.05 2.50 REF 2.65 ±0.05 3.65 ±0.05 PACKAGE OUTLINE 0.25 ±0.05 0.50 BSC 3.50 REF 4.10 ±0.05 5.50 ±0.05 RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS APPLY SOLDER MASK TO AREAS THAT ARE NOT SOLDERED 4.00 ±0.10 (2 SIDES) 0.75 ±0.05 R = 0.05 TYP PIN 1 NOTCH R = 0.20 OR 0.35 × 45° CHAMFER 2.50 REF R = 0.115 TYP 27 28 0.40 ±0.10 PIN 1 TOP MARK (NOTE 6) 1 2 5.00 ±0.10 (2 SIDES) 3.50 REF 3.65 ±0.10 2.65 ±0.10 (UFD28) QFN 0506 REV B 0.200 REF 0.00 – 0.05 0.25 ±0.05 0.50 BSC BOTTOM VIEW—EXPOSED PAD NOTE: 1. DRAWING PROPOSED TO BE MADE A JEDEC PACKAGE OUTLINE MO-220 VARIATION (WXXX-X). 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 3514fa 24 For more information www.linear.com/LT3514 LT3514 REVISION HISTORY REV DATE DESCRIPTION PAGE NUMBER A 01/14 Added H-grade option 2 Clarified Switching Frequency parameters 3 Clarified resistor value for R2 11 3514fa 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 representaFor more information www.linear.com/LT3514 tion that the interconnection of its circuits as described herein will not infringe on existing patent rights. 25 LT3514 TYPICAL APPLICATION Complete Triple Buck Regulator with 180V Surge Protection VSUPPLY 3.2V TO 30V SURGE PROTECTION TO 180V R1 10Ω Q1 R2 100k D2 6.8V R3 1k C1 0.1µF D3 D1 36V VSKY CSKY 2.2µF L5 10µH VIN + 22µF 2.2µF ×2 PG Q1: FQB34N20L D1: BZT52C36-7-F D2: BZT52C6V8-7-F D3: BAT54-7-F L1: CDRH5D28-4R2 L3: CDRH8D28-4R7 L4: CDRH5D28-8R2 L5: CBC2016100M 4.2µH SW1 SKY SW5 LT3514 0.1µF 18.2k 2.5V/1A 21.5k 22µF 10.2k 4.7µH SW3 PG DA3 FB3 RT/SYNC 82pF DA1 FB1 VIN VIN EN/UVLO RUN/SS1 RUN/SS3 RUN/SS4 L1 L3 47pF 3.3V/2A 31.6k 47µF 10.2k 8.2µH SW4 DA4 FB4 L4 22pF 5V/1A 53.6k 22µF 10.2k GND 3514 TA02 RELATED PARTS PART DESCRIPTION COMMENTS LT3504 40V, Quad 1A Step-Down 2.5MHz DC/DC Converter with 100% Duty Cycle Operation VIN(MIN) = 3.2V, VIN(MAX) = 40V, IQ = 7.1mA, ISD < 1µA, 4mm x 5mm QFN-28 Package LT3507/ LT3507A 36V 2.5MHz, Triple [2.4A + 1.5A + 1.5A (IOUT)] with LDO Controller High Efficiency Step-Down DC/DC Converter VIN(MIN) = 4V, VIN(MAX) = 36V, VOUT(MIN) = 0.8V, IQ = 7mA, ISD = 1µA, 5mm x 7mm QFN-38 Package LT8610 42V 2.2MHz, Synchronous, Low IQ = 2.5µA, Step-Down DC/DC Converter VIN(MIN) = 3.4V, VIN(MAX) = 42V, VOUT(MIN) = 0.97V, IQ = 2.5µA, ISD = 1µA, MSOP-16E Package LT3988 60V with Transient Protection to 80V, 2.5MHz, Dual 1A High Efficiency Step-Down DC/DC Converter VIN(MIN) = 4.0V, VIN(MAX) = 60V, VOUT(MIN) = 0.75V, IQ = 2mA, ISD = 1µA, MSOP-16E Package LT3509 36V with Transient Protection to 60V, Dual 0.70(IOUT), 2.2MHz, High Efficiency Step-Down DC/DC Converter VIN(MIN) = 3.6V, VIN(MAX) = 36V, VOUT(MIN) = 0.8V, IQ = 1.9mA, ISD = 1µA, 3mm × 4mm DFN-14, MSOP-16E Packages LT3500 36V, 40VMAX, 2A, 2.5MHz High Efficiency Step-Down DC/DC Converter and LDO Controller VIN(MIN) = 3.6V, VIN(MAX) = 36V, VOUT(MIN) = 0.8V, IQ = 2.5mA, ISD <10µA, 3mm × 3mm DFN-10 Package LT3508 36V with Transient Protection to 40V, Dual 1.4A (IOUT), 3MHz, High Efficiency Step-Down DC/DC Converter VIN(MIN) = 3.7V, VIN(MAX) = 37V, VOUT(MIN) = 0.8V, IQ = 4.6mA, ISD = 1µA, 4mm × 4mm QFN-24, TSSOP-16E Packages LT3980 58V with Transient Protection to 80V, 2A (IOUT), 2.4MHz, High Efficiency Step-Down DC/DC Converter with Burst Mode® Operation VIN(MIN) = 3.6V, VIN(MAX) = 58V, Transient to 80V, VOUT(MIN) = 0.8V, IQ = 85µA, ISD <1µA, 3mm × 4mm DFN-16 and MSOP-16E Packages LT3480 36V with Transient Protection to 60V, 2A (IOUT), 2.4MHz, High Efficiency Step-Down DC/DC Converter with Burst Mode Operation VIN(MIN) = 3.6V, VIN(MAX) = 38V, VOUT(MIN) = 0.78V, IQ = 70µA, ISD <1µA, 3mm × 3mm DFN-10, MSOP-10E Packages LT3689 36V, 60V Transient Protection, 800mA, 2.2MHz High Efficiency Micropower Step-Down DC/DC Converter with POR Reset and Watchdog Timer VIN(MIN) = 3.6V, VIN(MAX) = 36V, Transient to 60V, VOUT(MIN) = 0.8V, IQ = 75µA, ISD <1µA. 3mm × 3mm QFN-16 Package LT3970 40V, 350mA, 2MHz High Efficiency Micropower Step-Down DC/DC Converter VIN(MIN) = 4V, VIN(MAX) = 40V, Transient to 60V, VOUT(MIN) = 1.21V, IQ = 2µA, ISD <1µA, 3mm × 2mm DFN-10 and MSOP-10 Packages LT3682 36V, 60VMAX, 1A, 2.2MHz High Efficiency Micropower Step-Down DC/DC Converter VIN(MIN) = 3.6V, VIN(MAX) = 36V, VOUT(MIN) = 0.8V, IQ = 75µA, ISD < 1µA, 3mm × 3mm DFN-12 Package 3514fa 26 Linear Technology Corporation 1630 McCarthy Blvd., Milpitas, CA 95035-7417 For more information www.linear.com/LT3514 (408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com/LT3514 LT 0114 REV A • PRINTED IN USA LINEAR TECHNOLOGY CORPORATION 2013