LT8614IUDC#PBF

LT8614 42V, 4A Synchronous Step-Down Silent Switcher with 2.5µA Quiescent Current Description Features Silent Switcher® Architecture nn Ultralow EMI/EMC Emissions nn High Efficiency at High Frequency nn Up to 96% Efficiency at 1MHz, 12V to 5V IN OUT nn Up to 94% Efficiency at 2MHz, 12V to 5V IN OUT nn Wide Input Voltage Range: 3.4V to 42V nn Ultralow Quiescent Current Burst Mode® Operation: nn 2.5μA I Regulating 12V to 3.3V Q IN OUT nn Output Ripple < 10mV P-P nn Fast Minimum Switch On-Time: 30ns nn Low Dropout Under All Conditions: 125mV at 1A nn Safely Tolerates Inductor Saturation in Overload nn Adjustable and Synchronizable: 200kHz to 3MHz nn Peak Current Mode Operation nn Accurate 1V Enable Pin Threshold nn Internal Compensation nn Output Soft-Start and Tracking nn Small 18-Lead 3mm × 4mm QFN nn Applications Automotive and Industrial Supplies General Purpose Step-Down nn GSM Power Supplies nn nn The LT®8614 step-down regulator features Silent Switcher architecture designed to minimize EMI/EMC emissions while delivering high efficiency at frequencies up to 3MHz. Assembled in a 3mm × 4mm QFN, the monolithic construction with integrated power switches and inclusion of all necessary circuitry yields a solution with a minimal PCB footprint. An ultralow 2.5µA quiescent current—with the output in full regulation— enables applications requiring highest efficiency at very small load currents. Transient response remains excellent and output voltage ripple is below 10mVP-P at any load, from zero to full current. The LT8614 allows high VIN to low VOUT conversion at high frequency with a fast minimum top switch on-time of 30ns. Operation is safe in overload even with a saturated inductor. Essential features are included and easy to use: An opendrain PG pin signals when the output is in regulation. The SYNC pin allows clock synchronization and choice of Burst Mode operation or pulse-skipping mode. Soft-start and tracking functionality is accessed via the TR/SS pin. An accurate enable threshold can be set using the EN/UV pin and a resistor at the RT pin programs switch frequency. L, LT, LTC, LTM, Linear Technology, the Linear logo, Silent Switcher and Burst Mode are registered trademarks and Silent Switcher is a trademark of Analog Devices, Inc. All other trademarks are the property of their respective owners. Protected by U.S. patents, including 8823345. Typical Application 12VIN to 5VOUT Efficiency 100 5V 4A Step-Down Converter 4.7µF VIN1 1µF EN/UV GND1 PG 10nF VIN2 GND2 LT8614 SYNC/MODE TR/SS BST fSW = 1MHz 1µF 0.1µF 4.7µH 4.7pF INTVCC RT VOUT 5V 4A SW BIAS 1µF 41.2k 2MHz 90 1M 47µF FB GND EFFICIENCY (%) VIN 5.8V TO 42V 1MHz 95 243k 85 80 75 70 65 60 0 0.5 1.0 1.5 2.0 2.5 3.0 LOAD CURRENT (A) 3.5 4.0 8614 TA01b 8614 TA01a 8614fd For more information www.linear.com/LT8614 1 LT8614 Pin Configuration VIN, EN/UV, PG...........................................................42V BIAS...........................................................................30V BST Pin Above SW Pin................................................4V FB, TR/SS, RT, INTVCC ................................................4V SYNC Voltage ..............................................................6V Operating Junction Temperature Range (Note 2) LT8614E.............................................. –40°C to 125°C LT8614I............................................... –40°C to 125°C LT8614H............................................. –40°C to 150°C LT8614MP.......................................... –55°C to 150°C Storage Temperature Range................... –65°C to 150°C SYNC/MODE 17 PG 20 19 18 FB GND TOP VIEW 16 TR/SS BIAS 1 INTVCC 2 15 RT BST 3 21 SW 13 VIN2 11 GND2 7 8 9 10 GND2 GND1 6 14 EN/UV SW VIN1 4 22 SW SW (Note 1) GND1 Absolute Maximum Ratings UDC PACKAGE 18-LEAD (3mm × 4mm) PLASTIC QFN θJA = 40°C/W, θJC(PAD) = 12°C/W (Note 3) EXPOSED PAD (PINS 21, 22) ARE SW, SHOULD BE SOLDERED TO PCB NOTE: PINS 5, 12 ARE REMOVED. CONFIGURATION DOES NOT MATCH JEDEC 20-PIN PACKAGE OUTLINE Order Information http://www.linear.com/product/LT8614#orderinfo LEAD FREE FINISH TAPE AND REEL PART MARKING* PACKAGE DESCRIPTION TEMPERATURE RANGE LT8614EUDC#PBF LT8614EUDC#TRPBF LGGQ 18-Lead (3mm × 4mm) Plastic QFN –40°C to 125°C LT8614IUDC#PBF LT8614IUDC#TRPBF LGGQ 18-Lead (3mm × 4mm) Plastic QFN –40°C to 125°C LT8614HUDC#PBF LT8614HUDC#TRPBF LGGQ 18-Lead (3mm × 4mm) Plastic QFN –40°C to 150°C LT8614MPUDC#PBF LT8614MPUDC#TRPBF LGGQ 18-Lead (3mm × 4mm) Plastic QFN –55°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. 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/. Some packages are available in 500 unit reels through designated sales channels with #TRMPBF suffix. Electrical Characteristics The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. PARAMETER CONDITIONS MIN Minimum Input Voltage VIN Quiescent Current TYP MAX l 2.9 3.4 V l 1.0 1.0 3 8 µA µA l 1.7 1.7 4 10 µA µA 0.26 0.5 mA 21 210 50 350 µA µA VEN/UV = 0V VEN/UV = 2V, Not Switching, VSYNC = 0V VEN/UV = 2V, Not Switching, VSYNC = 2V VIN Current in Regulation 2 VOUT = 0.97V, VIN = 6V, Output Load = 100µA VOUT = 0.97V, VIN = 6V, Output Load = 1mA l l UNITS 8614fd For more information www.linear.com/LT8614 LT8614 Electrical Characteristics The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. PARAMETER CONDITIONS Feedback Reference Voltage VIN = 6V, ILOAD = 0.5A VIN = 6V, ILOAD = 0.5A l Feedback Voltage Line Regulation VIN = 4.0V to 42V, ILOAD = 0.5A l Feedback Pin Input Current VFB = 1V –20 INTVCC Voltage ILOAD = 0mA, VBIAS = 0V ILOAD = 0mA, VBIAS = 3.3V INTVCC Undervoltage Lockout BIAS Pin Current Consumption VBIAS = 3.3V, ILOAD = 1A, 2MHz Minimum On-Time MIN TYP MAX UNITS 0.964 0.958 0.970 0.970 0.976 0.982 V V 0.004 0.02 %/V 20 nA 3.23 3.25 3.4 3.29 3.57 3.35 V V 2.5 2.6 2.7 V 9 mA ILOAD = 1.5A, SYNC = 0V ILOAD = 1.5A, SYNC = 3.3V l l 15 15 30 30 45 45 80 110 ns Oscillator Frequency RT = 221k, ILOAD = 1A RT = 60.4k, ILOAD = 1A RT = 18.2k, ILOAD = 1A l l l 180 665 1.85 210 700 2.00 240 735 2.15 kHz kHz MHz Top Power NMOS On-Resistance ISW = 1A Minimum Off-Time 85 Top Power NMOS Current Limit l Bottom Power NMOS On-Resistance VINTVCC = 3.4V, ISW = 1A Bottom Power NMOS Current Limit VINTVCC = 3.4V SW Leakage Current VIN = 42V, VSW = 0V, 42V EN/UV Pin Threshold EN/UV Rising 5.7 mΩ 10 40 l 4.5 6.9 –1.5 l 0.94 EN/UV Pin Hysteresis 1.0 VEN/UV = 2V –20 PG Upper Threshold Offset from VFB VFB Falling l 6 PG Lower Threshold Offset from VFB VFB Rising l –6 PG Hysteresis 8.5 A 1.5 µA 1.06 V mV 20 nA 9.0 12 % –9.0 –12 % 1.2 PG Leakage VPG = 3.3V PG Pull-Down Resistance VPG = 0.1V –40 SYNC Threshold SYNC Falling SYNC Rising 0.8 1.6 SYNC Pin Current VSYNC = 6V –40 l TR/SS Source Current l Fault Condition, TR/SS = 0.1V 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 LT8614E 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 LT8614I is guaranteed over the full –40°C to 125°C operating junction temperature range. The LT8614H is guaranteed to meet performance specifications from –40°C to 150°C operating junction temperature range. 1.5 A mΩ 40 EN/UV Pin Current TR/SS Pull-Down Resistance 8.5 ns ns % 40 nA 650 2000 Ω 1.1 2.0 1.4 2.4 V V 40 nA 2.9 µA 2.2 200 Ω High junction temperatures degrade operating lifetimes. The LT8614MP is 100% tested and guaranteed over the full –55°C to 150°C operating junction temperature range. Operating lifetime is derated at junction temperatures greater than 125°C. Note 3: θ values determined per JEDEC 51-7, 51-12. See the Applications Information section for information on improving the thermal resistance. Note 4: This IC includes overtemperature protection that is intended to protect the device during overload conditions. Junction temperature will exceed 150°C when overtemperature protection is active. Continuous operation above the specified maximum operating junction temperature will reduce lifetime. 8614fd For more information www.linear.com/LT8614 3 LT8614 Typical Performance Characteristics Efficiency at 3.3VOUT Efficiency at 5VOUT 95 95 90 90 90 80 85 70 EFFICIENCY (%) 75 70 fSW = 1MHz L = 4.7µH VIN = 12V VIN = 24V VIN = 36V 55 50 0 0.5 1 1.5 2 2.5 3 LOAD CURRENT (A) 3.5 75 70 fSW = 1MHz L = 2.2µH VIN = 12V VIN = 24V VIN = 36V 65 60 55 50 4 0 0.5 1 1.5 2 2.5 3 LOAD CURRENT (A) 8614 G01 98 90 94 60 50 40 fSW = 1MHz L = 4.7µH VIN = 12V VIN = 24V VIN = 36V 0 0.01 1 10 100 LOAD CURRENT (mA) 0.1 EFFICIENCY (%) EFFICIENCY (%) 80 10 95 85 75 65 2.25 1.75 2.75 0.75 1.25 SWITCHING FREQUENCY (MHz) EN THRESHOLD (V) 0.975 0.98 0.96 0.95 –50 –25 VOUT = 5V VIN = 12V 0.10 EN RISING 0.99 0.963 10 4 6 8 INDUCTOR VALUE (µH) Load Regulation 1.00 0.965 8614 G07 2 8614 G06 0.15 1.01 0.97 25 50 75 100 125 150 TEMPERATURE (°C) 0 8614 G05 1.02 0.967 VIN = 24V 70 84 0.25 0.977 0.969 VIN = 12V 80 EN Pin Thresholds 0.971 VOUT = 5V ILOAD = 10mA 90 86 1.03 1000 8614 G03 88 1000 0.973 1 10 100 LOAD CURRENT (mA) 0.1 Burst Mode Efficiency vs Inductor Value VIN = 24V 90 Reference Voltage 4 0 0.01 4 92 0.979 0 10 VIN = 12V 8614 G04 0.961 –50 –25 20 VOUT = 5V ILOAD = 1A L = 8.6µH 96 70 fSW = 1MHz L = 4.7µH VIN = 12V VIN = 24V VIN = 36V 30 Efficiency vs Frequency 100 20 50 40 8614 G02 Efficiency at 3.3VOUT 30 3.5 60 EFFICIENCY (%) 65 80 CHANGE IN VOUT (%) EFFICIENCY (%) 85 80 100 EFFICIENCY (%) 100 60 REFERENCE VOLAGE (V) Efficiency at 5VOUT 100 0.05 0 –0.05 EN FALLING 0 –0.10 25 50 75 100 125 150 TEMPERATURE (°C) 8614 G08 –0.15 0 0.5 1 2.5 3 1.5 2 LOAD CURRENT (A) 3.5 4 8614 G09 8614fd For more information www.linear.com/LT8614 LT8614 Typical Performance Characteristics No-Load Supply Current Line Regulation 0.10 5.0 VOUT = 5V ILOAD = 1A 0.08 0.02 0 –0.02 –0.04 3.5 3.0 2.5 2.0 1.5 –0.06 1.0 –0.08 0.5 5 10 15 20 25 30 35 INPUT VOLTAGE (V) 40 0 45 20 INPUT CURRENT (µA) INPUT CURRENT (µA) 0 5 10 15 20 25 30 35 INPUT VOLTAGE (V) 8614 G10 40 0 –55 45 Top FET Current Limit 8.5 9.0 7.0 6.5 6.0 CURRENT LIMIT (A) 7.0 8.5 5% DC 8.0 7.5 7.0 0.2 0.4 0.6 DUTY CYCLE 6.0 –50 –25 1.0 0.8 0 200 SWITCH DROP (mV) 150 TOP SWITCH 100 75 BOTTOM SWITCH 25 0 –50 –25 5.0 25 50 75 100 125 150 TEMPERATURE (°C) 8614 G16 0 25 50 75 100 125 150 TEMPERATURE (°C) 8614 G15 Minimum On-Time 500 40 450 38 400 36 350 300 TOP SWITCH 250 200 150 100 0 0 32 30 28 26 22 4 2 1 3 SWITCH CURRENT (A) 34 ILOAD = 1A, VSYNC = 0V ILOAD = 1A, VSYNC = 3V ILOAD = 2A ILOAD = 4A 24 BOTTOM SWITCH 50 0 5.5 Switch Drop SWITCH CURRENT = 1A 50 6.0 8614 G14 175 125 6.5 4.0 –50 –25 25 50 75 100 125 150 TEMPERATURE (°C) 8614 G13 Switch Drop 155 4.5 6.5 0 125 Bottom FET Current Limit 7.5 7.5 65 5 95 35 TEMPERATURE (°C) –25 8614 G12 9.5 CURRENT LIMIT (A) CURRENT LIMIT (A) 10 8614 G11 Top FET Current Limit vs Duty Cycle SWITCH DROP (mV) 15 9.0 8.0 VOUT = 3.3V VIN = 12V IN REGULATION 5 MINIMUM ON-TIME (ns) CHANGE IN VOUT (%) 4.0 0.04 –0.10 VOUT = 3.3V IN REGULATION 4.5 0.06 No-Load Supply Current 25 8614 G17 20 –50 –25 50 25 0 75 TEMPERATURE (°C) 100 125 8614 G18 8614fd For more information www.linear.com/LT8614 5 LT8614 Typical Performance Characteristics Switching Frequency 700 730 600 500 400 300 200 RT = 60.4k FRONT PAGE APPLICATION VIN = 12V 1000 VOUT = 5V 720 710 700 690 680 670 100 0 Burst Frequency 1200 SWITCHING FREQUENCY (kHz) 740 SWITCHING FREQUENCY (kHz) DROPOUT VOLTAGE (mV) Dropout Voltage 800 0 0.5 1 1.5 2 2.5 3 LOAD CURRENT (A) 3.5 660 –50 –25 4 0 SWITCHING FREQUENCY (kHz) 20 600 1.0 500 400 300 15 20 25 30 35 INPUT VOLTAGE (V) 40 0 45 0 0.2 0.4 0.6 FB VOLTAGE (V) SS PIN CURRENT (µA) 0 1 11.5 –7.5 10.5 2.3 FB RISING 10.0 2.2 2.1 2.0 25 50 75 100 125 150 TEMPERATURE (°C) 8614 G25 0.2 1.0 0.4 0.6 0.8 TR/SS VOLTAGE (V) 9.5 9.0 1.4 –8.0 –8.5 FB RISING –9.0 –9.5 –10.0 FB FALLING FB FALLING –10.5 8.5 –11.0 8.0 –11.5 7.5 7.0 –50 –25 1.2 PG Low Thresholds –7.0 11.0 2.4 0 8614 G24 12.0 PG THRESHOLD OFFSET FROM VREF (%) VSS = 0.5V 1.9 6 0.8 PG High Thresholds 2.5 0 0.4 8614 G23 Soft-Start Current 1.8 –50 –25 0.6 0.2 8614 G22 2.6 0.8 200 PG THRESHOLD OFFSET FROM VREF (%) 10 200 Soft-Start Tracking 100 5 50 100 150 LOAD CURRENT (mA) 1.2 FB VOLTAGE (V) LOAD CURRENT (mA) 40 0 8614 G21 VOUT = 3.3V VIN = 12V VSYNC = 0V RT = 60.4k 700 60 0 200 Frequency Foldback 800 FRONT PAGE APPLICATION VOUT = 5V fSW = 1MHz 80 400 8614 G20 Minimum Load to Full Frequency (SYNC DC High) 100 600 0 25 50 75 100 125 150 TEMPERATURE (°C) 8614 G19 800 0 25 50 75 100 125 150 TEMPERATURE (°C) 8614 G26 –12.0 –50 –25 0 25 50 75 100 125 150 TEMPERATURE (°C) 8614 G27 8614fd For more information www.linear.com/LT8614 LT8614 Typical Performance Characteristics RT Programmed Switching Frequency VIN UVLO 250 225 6.0 3.4 5.5 150 125 100 75 BIAS PIN CURRENT (mA) 3.2 175 INPUT VOLTAGE (V) RT PIN RESISTOR (kΩ) 200 Bias Pin Current 3.6 3.0 2.8 2.6 2.4 50 0 0.2 0.6 1.4 1.8 2.2 2.6 1 SWITCHING FREQUENCY (MHz) 3 2.0 –55 –25 95 65 35 TEMPERATURE (°C) 5 8614 G27 BIAS PIN CURRENT (mA) 12 10 4.0 3.5 125 155 2.5 5 8 6 15 20 25 30 35 INPUT VOLTAGE (V) VSW 5V/DIV IL 500mA/DIV IL 500mA/DIV VOUT 5mV/DIV VOUT 10mV/DIV 500ns/DIV FRONT PAGE APPLICATION 12VIN TO 5VOUT AT 1A 2 40 45 Switching Waveforms, Burst Mode Operation VSW 5V/DIV 4 2.6 1.4 1.8 2.2 0.6 1 SWITCHING FREQUENCY (MHz) 10 8614 G30 Switching Waveforms, Full Frequency Continuous Operation VBIAS = 5V VOUT = 5V VIN = 12V ILOAD = 1A 0 0.2 4.5 8614 G29 Bias Pin Current 14 5.0 3.0 2.2 25 VBIAS = 5V VOUT = 5V ILOAD = 1A fSW = 700kHz 5µs/DIV FRONT PAGE APPLICATION 12VIN TO 5VOUT AT 10mA VSYNC = 0V 8614 G32 8614 G33 3 8614 G31 Switching Waveforms IL 500mA/DIV VSW 5V/DIV 200ns/DIV FRONT PAGE APPLICATION 36VIN TO 5VOUT AT 1A Transient Response; Load Current Stepped from 100mA (Burst Mode Operation) to 1.1A Transient Response; Load Current Stepped from 1A to 2A 8614 G34 ILOAD 1A/DIV ILOAD 1A/DIV VOUT 100mV/DIV VOUT 200mV/DIV 50µs/DIV FRONT PAGE APPLICATION 1A TO 2A TRANSIENT 12VIN, 5VOUT COUT = 47µF 8614 G35 50µs/DIV FRONT PAGE APPLICATION 100mA (Burst Mode OPERATION) TO 1.1A TRANSIENT 12VIN, 5VOUT COUT = 47µF 8614 G36 8614fd For more information www.linear.com/LT8614 7 LT8614 Typical Performance Characteristics Start-Up Dropout Performance Start-Up Dropout Performance VIN VIN 2V/DIV VOUT VOUT 2V/DIV VIN VIN 2V/DIV VOUT VOUT 2V/DIV 100ms/DIV 2.5Ω LOAD (2A IN REGULATION) 100ms/DIV 20Ω LOAD (250mA IN REGULATION) 8614 G37 8614 G38 Radiated EMI Performance (CISPR25 Radiated Emission Test with Class 5 Peak Limits) 50 VERTICAL POLARIZATION AMPLITUDE (dBµV/m) 40 30 20 10 0 LT8614 CLASS 5 PEAK 0 50 100 200 300 600 400 500 FREQUENCY (MHz) 700 800 900 1000 HORIZONTIAL POLARIZATION AMPLITUDE (dBµV/m) 40 30 20 10 0 8 LT8614 CLASS 5 PEAK 600 400 500 FREQUENCY (MHz) DC2019A DEMO BOARD (WITH EMI FILTER INSTALLED) 14VIN TO 5VOUT AT 4A, fSW = 2MHz 0 100 200 300 700 800 900 1000 8614 G39 8614fd For more information www.linear.com/LT8614 LT8614 Pin Functions BIAS (Pin 1): The internal regulator will draw current from BIAS instead of VIN when BIAS is tied to a voltage higher than 3.1V. For output voltages of 3.3V to 30V this pin should be tied to VOUT. If this pin is tied to a supply other than VOUT use a 1µF local bypass capacitor on this pin. If no supply is available, tie to GND. GND2 (10, 11): Power Switch Ground. These pins are the return path of the internal bottom side power switch and must be tied together. Place the negative terminal of the input capacitor as close to the GND2 pins as possible. Also be sure to tie GND2 to the ground plane. See the Applications Information section for sample layout. INTVCC (Pin 2): Internal 3.4V Regulator Bypass Pin. The internal power drivers and control circuits are powered from this voltage. INTVCC maximum output current is 20mA. Do not load the INTVCC pin with external circuitry. INTVCC current will be supplied from BIAS if BIAS > 3.1V, otherwise current will be drawn from VIN. Voltage on INTVCC will vary between 2.8V and 3.4V when BIAS is between 3.0V and 3.6V. Decouple this pin to power ground with at least a 1μF low ESR ceramic capacitor placed close to the IC. VIN2 (Pin 13): The LT8614 requires two 1µF small input bypass capacitors. One 1µF capacitor should be placed between VIN1 and GND1. A second 1µF capacitor should be placed between VIN2 and GND2. These capacitors must be placed as close as possible to the LT8614. A third larger capacitor of 2.2µF or more should be placed close to the LT8614 with the positive terminal connected to VIN1 and VIN2, and the negative terminal connected to ground. See the Applications Information section for sample layout. BST (Pin 3): This pin is used to provide a drive voltage, higher than the input voltage, to the topside power switch. Place a 0.1µF boost capacitor as close as possible to the IC. EN/UV (Pin 14): The LT8614 is shut down when this pin is low and active when this pin is high. The hysteretic threshold voltage is 1.00V going up and 0.96V going down. Tie to VIN if the shutdown feature is not used. An external resistor divider from VIN can be used to program a VIN threshold below which the LT8614 will shut down. VIN1 (Pin 4): The LT8614 requires two 1µF small input bypass capacitors. One 1µF capacitor should be placed between VIN1 and GND1. A second 1µF capacitor should be placed between VIN2 and GND2. These capacitors must be placed as close as possible to the LT8614. A third larger capacitor of 2.2µF or more should be placed close to the LT8614 with the positive terminal connected to VIN1 and VIN2, and the negative terminal connected to ground. See applications section for sample layout. GND1 (6, 7): Power Switch Ground. These pins are the return path of the internal bottom side power switch and must be tied together. Place the negative terminal of the input capacitor as close to the GND1 pins as possible. Also be sure to tie GND1 to the ground plane. See the Applications Information section for sample layout. SW (Pins 8, 9): The SW pins are the outputs of the internal power switches. Tie these pins together and connect them to the inductor and boost capacitor. This node should be kept small on the PCB for good performance and low EMI. RT (Pin 15): A resistor is tied between RT and ground to set the switching frequency. TR/SS (Pin 16): Output Tracking and Soft-Start Pin. This pin allows user control of output voltage ramp rate during start-up. A TR/SS voltage below 0.97V forces the LT8614 to regulate the FB pin to equal the TR/SS pin voltage. When TR/SS is above 0.97V, the tracking function is disabled and the internal reference resumes control of the error amplifier. An internal 2.2μA pull-up current from INTVCC on this pin allows a capacitor to program output voltage slew rate. This pin is pulled to ground with an internal 230Ω MOSFET during shutdown and fault conditions; use a series resistor if driving from a low impedance output. This pin may be left floating if the tracking function is not needed. 8614fd For more information www.linear.com/LT8614 9 LT8614 Pin Functions FB (Pin 20): The LT8614 regulates the FB pin to 0.970V. Connect the feedback resistor divider tap to this pin. Also, connect a phase lead capacitor between FB and VOUT. Typically, this capacitor is 4.7pF to 22pF. SYNC/MODE (Pin 17): External Clock Synchronization Input. Ground this pin for low ripple Burst Mode operation at low output loads. Tie to a clock source for synchronization to an external frequency. Apply a DC voltage of 3V or higher or tie to INTVCC for pulse-skipping mode. When in pulse-skipping mode, the IQ will increase to several hundred µA. Do not float this pin. SW (Exposed Pad Pins 21, 22): The exposed pads should to connected and soldered to the SW trace for good thermal performance. If necessary due to manufacturing limitations Pins 21 and 22 may be left disconnected, however thermal performance will be degraded. GND (Pins 18): LT8614 Ground Pin. Connect this pin to system ground and to the ground plane. PG (Pin 19): The PG pin is the open-drain output of an internal comparator. PG remains low until the FB pin is within ±9% of the final regulation voltage, and there are no fault conditions. PG is valid when VIN is above 3.4V, regardless of EN/UV pin state. Block Diagram VIN 4 CIN3 VIN2 VIN1 CIN1 R3 OPT 14 R4 OPT 19 EN/UV 1V PG + – SHDN ±9% C1 R2 R1 20 CSS OPT 16 RT 15 17 FB TR/SS INTVCC OSCILLATOR 200kHz TO 3MHz ERROR AMP VC BST BURST DETECT SHDN TSD INTVCC UVLO VIN UVLO 2.2µA BIAS 3.4V REG SLOPE COMP + + – VOUT CIN2 – + INTERNAL 0.97V REF 13 SWITCH LOGIC AND ANTISHOOT THROUGH 1 2 CVCC 3 CBST M1 L SW 8, 9, 21, 22 VOUT COUT M2 GND1 SHDN TSD VIN UVLO 6, 7 GND2 10, 11 RT SYNC/MODE GND 18 10 8614 BD 8614fd For more information www.linear.com/LT8614 LT8614 Operation The LT8614 is a monolithic, constant frequency, current mode step-down DC/DC converter. An oscillator, with frequency set using a resistor on the RT pin, turns on the internal top power switch at the beginning of each clock cycle. Current in the inductor then increases until the top switch current comparator trips and turns off the top power switch. The peak inductor current at which the top switch turns off is controlled by the voltage on the internal VC node. The error amplifier servos the VC node by comparing the voltage on the VFB pin with an internal 0.97V reference. When the load current increases it causes a reduction in the feedback voltage relative to the reference leading the error amplifier to raise the VC voltage until the average inductor current matches the new load current. When the top power switch turns off, the synchronous power switch turns on until the next clock cycle begins or inductor current falls to zero. If overload conditions result in more than 6.9A flowing through the bottom switch, the next clock cycle will be delayed until switch current returns to a safe level. from the input supply when regulating with no load. The SYNC pin is tied low to use Burst Mode operation and can be tied to a logic high to use pulse-skipping mode. If a clock is applied to the SYNC pin the part will synchronize to an external clock frequency and operate in pulse-skipping mode. While in pulse-skipping mode the oscillator operates continuously and positive SW transitions are aligned to the clock. During light loads, switch pulses are skipped to regulate the output and the quiescent current will be several hundred µA. If the EN/UV pin is low, the LT8614 is shut down and draws 1µA from the input. When the EN/UV pin is above 1V, the switching regulator will become active. The oscillator reduces the LT8614’s operating frequency when the voltage at the FB pin is low. This frequency foldback helps to control the inductor current when the output voltage is lower than the programmed value which occurs during start-up or overcurrent conditions. When a clock is applied to the SYNC pin or the SYNC pin is held DC high, the frequency foldback is disabled and the switching frequency will slow down only during overcurrent conditions. To optimize efficiency at light loads, the LT8614 operates in Burst Mode operation in light load situations. Between bursts, all circuitry associated with controlling the output switch is shut down, reducing the input supply current to 1.7μA. In a typical application, 2.5μA will be consumed To improve efficiency across all loads, supply current to internal circuitry can be sourced from the BIAS pin when biased at 3.3V or above. Else, the internal circuitry will draw current from VIN. The BIAS pin should be connected to VOUT if the LT8614 output is programmed at 3.3V or above. Comparators monitoring the FB pin voltage will pull the PG pin low if the output voltage varies more than ±9% (typical) from the set point, or if a fault condition is present. 8614fd For more information www.linear.com/LT8614 11 LT8614 Applications Information Low EMI PCB Layout Note that large, switched currents flow in the LT8614 VIN1, VIN2, GND1, and GND2 pins and the input capacitors (CIN1, CIN2). The loops formed by the input capacitors should be as small as possible by placing the capacitors adjacent to the VIN1/2 and GND1/2 pins. Capacitors with small case size such as 0603 are optimal due to lowest parasitic inductance. The LT8614 is specifically designed to minimize EMI/EMC emissions and also to maximize efficiency when switching at high frequencies. For optimal performance the LT8614 requires the use of multiple VIN bypass capacitors. Two small 1µF capacitors should be placed as close as possible to the LT8614: One capacitor should be tied to VIN1/GND1; a second capacitor should be tied to VIN2/ GND2. A third capacitor with a larger value, 2.2µF or higher, should be placed near VIN1 or VIN2. The input capacitors, along with the inductor and output capacitors, 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 under the application circuit on the layer closest to the surface layer. The SW and BOOST nodes should be as small as possible. Finally, keep the FB and RT nodes small so that the ground See Figure 1 for a recommended PCB layout. For more detail and PCB design files refer to the Demo Board guide for the LT8614. GROUND PLANE ON LAYER 2 C1 R1 RPG V R2 CVCC CSS V V 1 V 16 17 20 V V 22 CIN1 6 21 7 RT 10 11 CIN2 CIN3 CBST L GROUND VIA VIN VIA VOUT VIA COUT V OTHER SIGNAL VIAS 9614 F01 Figure 1. Recommended PCB Layout for the LT8614 12 8614fd For more information www.linear.com/LT8614 LT8614 Applications Information To enhance efficiency at light loads, the LT8614 operates in low ripple Burst Mode operation, which keeps the output capacitor charged to the desired output voltage while minimizing the input quiescent current and minimizing output voltage ripple. In Burst Mode operation the LT8614 delivers single small pulses of current to the output capacitor followed by sleep periods where the output power is supplied by the output capacitor. While in sleep mode the LT8614 consumes 1.7μA. As the output load decreases, the frequency of single current pulses decreases (see Figure 2a) and the percentage of time the LT8614 is in sleep mode increases, resulting in much higher light load efficiency than for typical converters. By maximizing the time between pulses, the converter quiescent current approaches 2.5µA for a typical application when there is no output load. Therefore, to optimize the quiescent current performance at light loads, the current in the feedback resistor divider must be minimized as it appears to the output as load current. In order to achieve higher light load efficiency, more energy must be delivered to the output during the single small pulses in Burst Mode operation such that the LT8614 can stay in sleep mode longer between each pulse. This can be achieved by using a larger value inductor (i.e., 4.7µH), and should be considered independent of switching frequency when choosing an inductor. For example, while a lower inductor value would typically be used for a high switching frequency application, if high light load efficiency is desired, a higher inductor value should be chosen. See curve in Typical Performance Characteristics. Burst Frequency 1200 FRONT PAGE APPLICATION VIN = 12V 1000 VOUT = 5V SWITCHING FREQUENCY (kHz) Achieving Ultralow Quiescent Current While in Burst Mode operation the current limit of the top switch is approximately 600mA resulting in output voltage ripple shown in Figure 3. Increasing the output capacitance will decrease the output ripple proportionally. As load ramps upward from zero the switching frequency will increase but only up to the switching frequency programmed by the resistor at the RT pin as shown in Figure 2a. The output load at which the LT8614 reaches the programmed frequency varies based on input voltage, output voltage, and inductor choice. 800 600 400 200 0 0 50 100 150 LOAD CURRENT (mA) (2a) 200 8614 F02a Minimum Load to Full Frequency (SYNC DC High) 100 FRONT PAGE APPLICATION VOUT = 5V fSW = 1MHz 80 LOAD CURRENT (mA) traces will shield them from the SW and BOOST nodes. The exposed pad on the bottom of the package should be soldered to SW to reduce thermal resistance to ambient. To keep thermal resistance low, extend the ground plane from GND1 and GND2 as much as possible, and add thermal vias to additional ground planes within the circuit board and on the bottom side. 60 40 20 0 5 10 15 20 25 30 35 INPUT VOLTAGE (V) (2b) 40 45 8614 F02b Figure 2. SW Frequency vs Load Information in Burst Mode Operation (2a) and Pulse-Skipping Mode (2b) 8614fd For more information www.linear.com/LT8614 13 LT8614 Applications Information If low input quiescent current and good light-load efficiency are desired, use large resistor values for the FB resistor divider. The current flowing in the divider acts as a load current, and will increase the no-load input current to the converter, which is approximately: VSW 5V/DIV IL 500mA/DIV VOUT 10mV/DIV 5µs/DIV FRONT PAGE APPLICATION 12VIN TO 5VOUT AT 10mA VSYNC = 0V 8614 F03 Figure 3. Burst Mode Operation For some applications it is desirable for the LT8614 to operate in pulse-skipping mode, offering two major differences from Burst Mode operation. First is the clock stays awake at all times and all switching cycles are aligned to the clock. In this mode much of the internal circuitry is awake at all times, increasing quiescent current to several hundred µA. Second is that full switching frequency is reached at lower output load than in Burst Mode operation (see Figure 2b). To enable pulse-skipping mode, the SYNC pin is tied high either to a logic output or to the INTVCC pin. When a clock is applied to the SYNC pin the LT8614 will also operate in pulse-skipping mode. FB Resistor Network The output voltage is programmed with a resistor divider between the output and the FB pin. Choose the resistor values according to: ⎛ V ⎞ R1= R2 ⎜ OUT – 1⎟ ⎝ 0.970V ⎠ (1) Reference designators refer to the Block Diagram. 1% resistors are recommended to maintain output voltage accuracy. 14 ⎛ V ⎞ ⎛ V ⎞ ⎛ 1⎞ IQ = 1.7µA + ⎜ OUT ⎟ ⎜ OUT ⎟ ⎜ ⎟ ⎝ R1+R2 ⎠ ⎝ VIN ⎠ ⎝ n ⎠ (2) where 1.7µA is the quiescent current of the LT8614 and the second term is the current in the feedback divider reflected to the input of the buck operating at its light load efficiency n. For a 3.3V application with R1 = 1M and R2 = 412k, the feedback divider draws 2.3µA. With VIN = 12V and n = 80%, this adds 0.8µA to the 1.7µA quiescent current resulting in 2.5µA no-load current from the 12V supply. Note that this equation implies that the no-load current is a function of VIN; this is plotted in the Typical Performance Characteristics section. When using large FB resistors, a 4.7pF to 22pF phase-lead capacitor should be connected from VOUT to FB. Setting the Switching Frequency The LT8614 uses a constant frequency PWM architecture that can be programmed to switch from 200kHz to 3MHz by using a resistor tied from the RT pin to ground. A table showing the necessary RT value for a desired switching frequency is in Table 1. The RT resistor required for a desired switching frequency can be calculated using: RT = 46.5 – 5.2 fSW (3) where RT is in kΩ and fSW is the desired switching frequency in MHz. 8614fd For more information www.linear.com/LT8614 LT8614 Applications Information For transient operation, VIN may go as high as the absolute maximum rating of 42V regardless of the RT value, however the LT8614 will reduce switching frequency as necessary to maintain control of inductor current to assure safe operation. Table 1. SW Frequency vs RT Value fSW (MHz) RT (kΩ) 0.2 232 0.3 150 0.4 110 0.5 88.7 0.6 71.5 0.7 60.4 0.8 52.3 1.0 41.2 1.2 33.2 1.4 28.0 1.6 23.7 1.8 20.5 2.0 18.2 2.2 15.8 3.0 10.7 The LT8614 is capable of a maximum duty cycle of greater than 99%, and the VIN-to-VOUT dropout is limited by the RDS(ON) of the top switch. In this mode the LT8614 skips switch cycles, resulting in a lower switching frequency than programmed by RT. For applications that cannot allow deviation from the programmed switching frequency at low VIN/VOUT ratios use the following formula to set switching frequency: VIN(MIN) = Operating Frequency Selection and Trade-Offs Selection of the operating frequency is a trade-off between efficiency, component size, and input voltage range. The advantage of high frequency operation is that smaller inductor and capacitor values may be used. The disadvantages are lower efficiency and a smaller input voltage range. The highest switching frequency (fSW(MAX)) for a given application can be calculated as follows: fSW(MAX) = ( VOUT + VSW(BOT) tON(MIN) VIN – VSW(TOP) + VSW(BOT) ) (4) where VIN is the typical input voltage, VOUT is the output voltage, VSW(TOP) and VSW(BOT) are the internal switch drops (~0.45V, ~0.15V, respectively at maximum load) and tON(MIN) is the minimum top switch on-time (see the Electrical Characteristics). This equation shows that a slower switching frequency is necessary to accommodate a high VIN/VOUT ratio. VOUT + VSW(BOT) 1– fSW • tOFF(MIN) – VSW(BOT) + VSW(TOP) (5) where VIN(MIN) is the minimum input voltage without skipped cycles, VOUT is the output voltage, VSW(TOP) and VSW(BOT) are the internal switch drops (~0.45V, ~0.15V, respectively at maximum load), fSW is the switching frequency (set by RT), and tOFF(MIN) is the minimum switch off-time. Note that higher switching frequency will increase the minimum input voltage below which cycles will be dropped to achieve higher duty cycle. Inductor Selection and Maximum Output Current The LT8614 is designed to minimize solution size by allowing the inductor to be chosen based on the output load requirements of the application. During overload or short-circuit conditions the LT8614 safely tolerates operation with a saturated inductor through the use of a high speed peak-current mode architecture. A good first choice for the inductor value is: L= VOUT + VSW(BOT) fSW (6) where fSW is the switching frequency in MHz, VOUT is the output voltage, VSW(BOT) is the bottom switch drop (~0.15V) and L is the inductor value in μH. 8614fd For more information www.linear.com/LT8614 15 LT8614 Applications Information To avoid overheating and poor efficiency, an inductor must be chosen with an RMS current rating that is greater than the maximum expected output load of the application. In addition, the saturation current (typically labeled ISAT) rating of the inductor must be higher than the load current plus 1/2 of in inductor ripple current: 1 IL(PEAK) =ILOAD(MAX) + ΔIL 2 (7) where ∆IL is the inductor ripple current as calculated in Equation 9 and ILOAD(MAX) is the maximum output load for a given application. As a quick example, an application requiring 1A output should use an inductor with an RMS rating of greater than 1A and an ISAT of greater than 1.3A. During long duration overload or short-circuit conditions, the inductor RMS rating requirement is greater to avoid overheating of the inductor. To keep the efficiency high, the series resistance (DCR) should be less than 0.04Ω, and the core material should be intended for high frequency applications. The LT8614 limits the peak switch current in order to protect the switches and the system from overload faults. The top switch current limit (ILIM) is at least 8.5A at low duty cycles and decreases linearly to 7.2A at DC = 0.8. The inductor value must then be sufficient to supply the desired maximum output current (IOUT(MAX)), which is a function of the switch current limit (ILIM) and the ripple current. IOUT(MAX) =ILIM – ΔIL 2 (8) The peak-to-peak ripple current in the inductor can be calculated as follows: ΔIL = ⎞ VOUT ⎛ V • ⎜ 1– OUT ⎟ L • fSW ⎝ VIN(MAX) ⎠ (9) where fSW is the switching frequency of the LT8614, and L is the value of the inductor. Therefore, the maximum output current that the LT8614 will deliver depends on the switch current limit, the inductor value, and the input and output voltages. The inductor value may have to be increased if the inductor ripple current does not allow 16 sufficient maximum output current (IOUT(MAX)) given the switching frequency, and maximum input voltage used in the desired application. In order to achieve higher light load efficiency, more energy must be delivered to the output during the single small pulses in Burst Mode operation such that the LT8614 can stay in sleep mode longer between each pulse. This can be achieved by using a larger value inductor (i.e., 4.7µH), and should be considered independent of switching frequency when choosing an inductor. For example, while a lower inductor value would typically be used for a high switching frequency application, if high light load efficiency is desired, a higher inductor value should be chosen. See curve in Typical Performance Characteristics. The optimum inductor for a given application may differ from the one indicated by this design guide. A larger value inductor provides a higher maximum load current and reduces the output voltage ripple. For applications requiring smaller load currents, the value of the inductor may be lower and the LT8614 may operate with higher ripple current. This allows use of a physically smaller inductor, or one with a lower DCR resulting in higher efficiency. Be aware that low inductance may result in discontinuous mode operation, which further reduces maximum load current. For more information about maximum output current and discontinuous operation, see Linear Technology’s Application Note 44. Finally, for duty cycles greater than 50% (VOUT/VIN > 0.5), a minimum inductance is required to avoid sub-harmonic oscillation. See Application Note 19. Input Capacitors The VIN of the LT8614 should be bypassed with at least three ceramic capacitors for best performance. Two small ceramic capacitors of 1µF should be placed close to the part; one at the VIN1/GND1 pins and a second at VIN2/ GND2 pins. These capacitors should be 0402 or 0603 in size. For automotive applications requiring 2 series input capacitors, two small 0402 or 0603 may be placed at each side of the LT8614 near the VIN1/GND1 and VIN2/ GND2 pins. 8614fd For more information www.linear.com/LT8614 LT8614 Applications Information A third, larger ceramic capacitor of 2.2µF or larger should be placed close to VIN1 or VIN2. See layout section for more detail. X7R or X5R capacitors are recommended for best performance across temperature and input voltage variations. Note that larger input capacitance is required when a lower switching frequency is used. If the input power source has high impedance, or there is significant inductance due to long wires or cables, additional bulk capacitance may be necessary. This can be provided with a low performance electrolytic capacitor. A ceramic input capacitor combined with trace or cable inductance forms a high quality (under damped) tank circuit. If the LT8614 circuit is plugged into a live supply, the input voltage can ring to twice its nominal value, possibly exceeding the LT8614’s voltage rating. This situation is easily avoided (see Application Note 88). Output Capacitor and Output Ripple The output capacitor has two essential functions. Along with the inductor, it filters the square wave generated by the LT8614 to produce the DC output. In this role it determines the output ripple, thus low impedance at the switching frequency is important. The second function is to store energy in order to satisfy transient loads and stabilize the LT8614’s control loop. Ceramic capacitors have very low equivalent series resistance (ESR) and provide the best ripple performance. For good starting values, see the Typical Applications section. Use X5R or X7R types. This choice will provide low output ripple and good transient response. Transient performance can be improved with a higher value output capacitor and the addition of a feedforward capacitor placed between VOUT and FB. Increasing the output capacitance will also decrease the output voltage ripple. A lower value of output capacitor can be used to save space and cost but transient performance will suffer and may cause loop instability. See the Typical Applications in this data sheet for suggested capacitor values. When choosing a capacitor, special attention should be given to the data sheet to calculate the effective capacitance under the relevant operating conditions of voltage bias and temperature. A physically larger capacitor or one with a higher voltage rating may be required. Ceramic Capacitors Ceramic capacitors are small, robust and have very low ESR. However, ceramic capacitors can cause problems when used with the LT8614 due to their piezoelectric nature. When in Burst Mode operation, the LT8614’s switching frequency depends on the load current, and at very light loads the LT8614 can excite the ceramic capacitor at audio frequencies, generating audible noise. Since the LT8614 operates at a lower current limit during Burst Mode operation, the noise is typically very quiet to a casual ear. If this is unacceptable, use a high performance tantalum or electrolytic capacitor at the output. Low noise ceramic capacitors are also available. A final precaution regarding ceramic capacitors concerns the maximum input voltage rating of the LT8614. As previously mentioned, a ceramic input capacitor combined with trace or cable inductance forms a high quality (underdamped) tank circuit. If the LT8614 circuit is plugged into a live supply, the input voltage can ring to twice its nominal value, possibly exceeding the LT8614’s rating. This situation is easily avoided (see Application Note 88). Enable Pin The LT8614 is in shutdown when the EN pin is low and active when the pin is high. The rising threshold of the EN comparator is 1.0V, with 40mV of hysteresis. The EN pin can be tied to VIN if the shutdown feature is not used, or tied to a logic level if shutdown control is required. Adding a resistor divider from VIN to EN programs the LT8614 to regulate the output only when VIN is above a desired voltage (see the Block Diagram). Typically, this threshold, VIN(EN), is used in situations where the input supply is current limited, or has a relatively high source resistance. A switching regulator draws constant power from the source, so source current increases as source voltage drops. This looks like a negative resistance load to the source and can cause the source to current limit or latch low under low source voltage conditions. The VIN(EN) threshold prevents the regulator from operating at source 8614fd For more information www.linear.com/LT8614 17 LT8614 Applications Information voltages where the problems might occur. This threshold can be adjusted by setting the values R3 and R4 such that they satisfy the following equation: ⎛ R3 ⎞ VIN(EN) = ⎜ +1⎟ •1.0V ⎝ R4 ⎠ (10) where the LT8614 will remain off until VIN is above VIN(EN). Due to the comparator’s hysteresis, switching will not stop until the input falls slightly below VIN(EN). When operating in Burst Mode operation for light load currents, the current through the VIN(EN) resistor network can easily be greater than the supply current consumed by the LT8614. Therefore, the VIN(EN) resistors should be large to minimize their effect on efficiency at low loads. INTVCC Regulator An internal low dropout (LDO) regulator produces the 3.4V supply from VIN that powers the drivers and the internal bias circuitry. The INTVCC can supply enough current for the LT8614’s circuitry and must be bypassed to ground with a minimum of 1μF ceramic capacitor. Good bypassing is necessary to supply the high transient currents required by the power MOSFET gate drivers. To improve efficiency the internal LDO can also draw current from the BIAS pin when the BIAS pin is at 3.1V or higher. Typically the BIAS pin can be tied to the output of the LT8614, or can be tied to an external supply of 3.3V or above. If BIAS is connected to a supply other than VOUT, be sure to bypass with a local ceramic capacitor. If the BIAS pin is below 3.0V, the internal LDO will consume current from VIN. Applications with high input voltage and high switching frequency where the internal LDO pulls current from VIN will increase die temperature because of the higher power dissipation across the LDO. Do not connect an external load to the INTVCC pin. Output Voltage Tracking and Soft-Start The LT8614 allows the user to program its output voltage ramp rate by means of the TR/SS pin. An internal 2.2μA pulls up the TR/SS pin to INTVCC. Putting an external capacitor on TR/SS enables soft starting the output to prevent current surge on the input supply. During the softstart ramp the output voltage will proportionally track the 18 TR/SS pin voltage. For output tracking applications, TR/ SS can be externally driven by another voltage source. From 0V to 0.97V, the TR/SS voltage will override the internal 0.97V reference input to the error amplifier, thus regulating the FB pin voltage to that of TR/SS pin. When TR/SS is above 0.97V, tracking is disabled and the feedback voltage will regulate to the internal reference voltage. The TR/SS pin may be left floating if the function is not needed. An active pull-down circuit is connected to the TR/SS pin which will discharge the external soft-start capacitor in the case of fault conditions and restart the ramp when the faults are cleared. Fault conditions that clear the soft-start capacitor are the EN/UV pin transitioning low, VIN voltage falling too low, or thermal shutdown. Output Power Good When the LT8614’s output voltage is within the ±9% window of the regulation point, which is a VFB voltage in the range of 0.883V to 1.057V (typical), the output voltage is considered good and the open-drain PG pin goes high impedance and is typically pulled high with an external resistor. Otherwise, the internal pull-down device will pull the PG pin low. To prevent glitching both the upper and lower thresholds include 1.2% of hysteresis. The PG pin is also actively pulled low during several fault conditions: EN/UV pin is below 1V, INTVCC has fallen too low, VIN is too low, or thermal shutdown. Synchronization To select low ripple Burst Mode operation, tie the SYNC pin below 0.4V (this can be ground or a logic low output). To synchronize the LT8614 oscillator to an external frequency connect a square wave (with 20% to 80% duty cycle) to the SYNC pin. The square wave amplitude should have valleys that are below 0.4V and peaks above 2.4V (up to 6V). The LT8614 will not enter Burst Mode operation at low output loads while synchronized to an external clock, but instead will pulse skip to maintain regulation. The LT8614 may be synchronized over a 200kHz to 3MHz range. The RT resistor should be chosen to set the LT8614 switching frequency equal to or below the lowest synchronization 8614fd For more information www.linear.com/LT8614 LT8614 Applications Information input. For example, if the synchronization signal will be 500kHz and higher, the RT should be selected for 500kHz. The slope compensation is set by the RT value, while the minimum slope compensation required to avoid subharmonic oscillations is established by the inductor size, input voltage, and output voltage. Since the synchronization frequency will not change the slopes of the inductor current waveform, if the inductor is large enough to avoid subharmonic oscillations at the frequency set by RT, then the slope compensation will be sufficient for all synchronization frequencies. For some applications it is desirable for the LT8614 to operate in pulse-skipping mode, offering two major differences from Burst Mode operation. First is the clock stays awake at all times and all switching cycles are aligned to the clock. Second is that full switching frequency is reached at lower output load than in Burst Mode operation. These two differences come at the expense of increased quiescent current. To enable pulse-skipping mode, the SYNC pin is tied high either to a logic output or to the INTVCC pin. There is another situation to consider in systems where the output will be held high when the input to the LT8614 is absent. This may occur in battery charging applications or in battery-backup systems where a battery or some other supply is diode ORed with the LT8614’s output. If the VIN pin is allowed to float and the EN pin is held high (either by a logic signal or because it is tied to VIN), then the LT8614’s internal circuitry will pull its quiescent current through its SW pin. This is acceptable if the system can tolerate several μA in this state. If the EN pin is grounded the SW pin current will drop to near 1µA. However, if the VIN pin is grounded while the output is held high, regardless of EN, parasitic body diodes inside the LT8614 can pull current from the output through the SW pin and the VIN pin. Figure 4 shows a connection of the VIN and EN/UV pins that will allow the LT8614 to run only when the input voltage is present and that protects against a shorted or reversed input. D1 VIN VIN LT8614 EN/UV GND The LT8614 does not operate in forced continuous mode regardless of SYNC signal. Never leave the SYNC pin floating. 8614 F04 Figure 4. Reverse VIN Protection Shorted and Reversed Input Protection The LT8614 will tolerate a shorted output. Several features are used for protection during output short-circuit and brownout conditions. The first is the switching frequency will be folded back while the output is lower than the set point to maintain inductor current control. Second, the bottom switch current is monitored such that if inductor current is beyond safe levels switching of the top switch will be delayed until such time as the inductor current falls to safe levels. Frequency foldback behavior depends on the state of the SYNC pin: If the SYNC pin is low the switching frequency will slow while the output voltage is lower than the programmed level. If the SYNC pin is connected to a clock source or tied high, the LT8614 will stay at the programmed frequency without foldback and only slow switching if the inductor current exceeds safe levels. High Temperature Considerations For higher ambient temperatures, care should be taken in the layout of the PCB to ensure good heat sinking of the LT8614. The ground pins on the bottom of the package should be soldered to a ground plane. This ground should be tied to large copper layers below with thermal vias; these layers will spread heat dissipated by the LT8614. Placing additional vias can reduce thermal resistance further. The maximum load current should be derated as the ambient temperature approaches the maximum junction rating. Power dissipation within the LT8614 can be estimated by calculating the total power loss from an efficiency measurement and subtracting the inductor loss. The die temperature is calculated by multiplying the LT8614 power dissipation by the thermal resistance from junction to ambient. The LT8614 will stop switching and indicate a fault condition if safe junction temperature is exceeded. 8614fd For more information www.linear.com/LT8614 19 LT8614 Typical Applications 5V 4A Step-Down Converter VIN 5.8V TO 42V 4.7µF EN/UV VIN1 1µF 0603 GND1 GND2 LT8614 PG 10nF VIN2 BST SYNC/MODE TR/SS 0.1µF 4.7µH VOUT 5V 4A SW BIAS 1µF 4.7pF 1M FB INTVCC 41.2k 1µF 0603 RT 243k GND fSW = 1MHz L: IHLP2525CZ-01 47µF 1210 X5R/X7R 8614 TA08 3.3V, 4A Step-Down Converter VIN 4.1V TO 42V 4.7µF EN/UV VIN1 1µF 0603 GND1 GND2 LT8614 PG 10nF VIN2 SYNC/MODE TR/SS BST 0.1µF 4.7µH VOUT 3.3V 4A SW BIAS 1µF 41.2k 1µF 0603 4.7pF 1M FB INTVCC RT 412k GND fSW = 1MHz L: IHLP2525CZ-01 47µF 1210 X5R/X7R 8614 TA05 Ultralow EMI 5V, 4A Step-Down Converter VIN 5.8V TO 42V FB1 BEAD L2 0.22µH 4.7µF 1206 10µF 1210 4.7µF 1206 1µF 0603 VIN1 EN/UV GND1 PG 10nF VIN2 GND2 LT8614 SYNC/MODE TR/SS BST 4.7pF INTVCC RT VOUT 5V 4A 1M FB GND fSW = 2MHz FB1 BEAD: MPZ2012S300A L: IHLP2525CZ-01 L2: IHLP1616AB-01 20 0.1µF 2.2µH SW BIAS 1µF 18.2k 1µF 0603 243k 47µF 1210 X5R/X7R 8614 TA02 8614fd For more information www.linear.com/LT8614 LT8614 Typical Applications 2MHz 5V, 4A Step-Down Converter VIN 5.8V TO 42V 4.7µF EN/UV VIN1 1µF 0603 GND1 GND2 LT8614 PG 10nF VIN2 SYNC/MODE TR/SS BST 0.1µF 2.2µH VOUT 5V 4A SW BIAS 1µF 4.7pF 1M FB INTVCC 18.2k 1µF 0603 RT 243k GND fSW = 2MHz L: IHLP2525CZ-01 47µF 1210 X5R/X7R 8614 TA03 2MHz 3.3V, 4A Step-Down Converter VIN 4.1V TO 42V 4.7µF VIN1 1µF 0603 EN/UV GND1 PG 10nF VIN2 GND2 LT8614 SYNC/MODE TR/SS BST 0.1µF 1.5µH 4.7pF INTVCC RT VOUT 3.3V 4A SW BIAS 1µF 18.2k 1µF 0603 1M FB GND fSW = 2MHz L: IHLP2020CZ-01 412k 47µF 1210 X5R/X7R 8614 TA06 8614fd For more information www.linear.com/LT8614 21 LT8614 Package Description Please refer to http://www.linear.com/product/LT8614#packaging for the most recent package drawings. UDC Package Variation: UDC20(18) Package 20(18)-LeadUDC Plastic QFN (3mm × 4mm) Variation: (Reference LTC DWGUDC20(18) # 05-08-1956 Rev C) 20(18)-Lead Plastic QFN (3mm × 4mm) (Reference LTC DWG 05-08-1956 Exposed Pad#Variation AARev C) Exposed Pad Variation AA 0.055 BSC 0.70 ±0.05 3.50 ±0.05 2.10 ±0.05 1.50 REF 0.770 BSC 0.220 ±0.05 0.356 ±0.05 0.400 ±0.05 PACKAGE OUTLINE 0.25 ±0.05 0.50 BSC 2.50 REF 3.10 ±0.05 4.50 ±0.05 RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS APPLY SOLDER MASK TO AREAS THAT ARE NOT SOLDERED 3.00 ±0.10 0.75 ±0.05 1.50 REF PIN 1 ID 0.12 × 45° 0.40 ±0.10 1 PIN 1 TOP MARK (NOTE 5) 4.00 ±0.10 0.220 ±0.05 2.127 ±0.10 2 2.50 REF 0.770 BSC 0.356 ±0.05 0.400 ±0.05 (UDC20(18)) QFN 1116 REV C 0.200 REF 0.00 – 0.05 R = 0.110 TYP 0.25 ±0.05 0.50 BSC BOTTOM VIEW—EXPOSED PAD NOTE: 1. DRAWING IS NOT A JEDEC PACKAGE OUTLINE 2. DRAWING NOT TO SCALE 3. ALL DIMENSIONS ARE IN MILLIMETERS 4. DIMENSIONS OF EXPOSED PAD ON BOTTOM OF PACKAGE DO NOT INCLUDE MOLD FLASH. MOLD FLASH, IF PRESENT, SHALL NOT EXCEED 0.15mm ON ANY SIDE 5. SHADED AREA IS ONLY A REFERENCE FOR PIN 1 LOCATION ON THE TOP AND BOTTOM OF PACKAGE 22 8614fd For more information www.linear.com/LT8614 LT8614 Revision History REV DATE DESCRIPTION A 03/14 Clarified package description. PAGE NUMBER 2 Clarified Applications Information. 13 Clarified applications components. 20, 21, 24 Clarified rev of package drawing. 22 Clarified Related Parts list. 24 B 05/15 Added H- Grade version 2, 3 C 08/16 Clarified Order Information 2 Clarified Minimum On-Time Condition 3 Clarified RT Programming Switching Frequency Graph 7 Clarified Bias Pin Description Clarified Components on Typical Applications D 06/17 9 20, 21 Clarified the Features section. 1 Added MP Grade. 2 Added MP Grade (Notes 2 and 3). 3 Clarified the Operating Frequency Selection and Trade-Offs section. 15 Clarified components in the Typical Applications section. 24 8614fd 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. For more information www.linear.com/LT8614 23 LT8614 Typical Applications 12V, 4A Step-Down Converter VIN 12.8V TO 42V 4.7µF VIN1 1µF 0603 EN/UV GND1 PG 10nF VIN2 1µF 0603 GND2 LT8614 BST SYNC/MODE TR/SS 0.1µF 4.7µH 1µF 4.7pF 1M FB INTVCC 41.2k VOUT 12V 4A SW BIAS RT 88.7k GND fSW = 1MHz L: IHLP2525CZ-01 47µF 1210 X5R/X7R 8614 TA04 2MHz 1.8V, 4A Step-Down Converter VIN 3.4V TO 22V (42V TRANSIENT) 4.7µF VIN1 1µF 0603 EN/UV GND1 PG 10nF VIN2 LT8614 SYNC/MODE TR/SS BST SW BIAS 1µF 18.2k 1µF 0603 GND2 FB INTVCC RT 0.1µF 1µH EXTERNAL SOURCE >3.1V OR GND VOUT 1.8V 4A 10pF 866k 1M GND fSW = 2MHz L: IHLP2020CZ-01 100µF 1210 X5R/X7R 8614 TA07 Related Parts PART NUMBER DESCRIPTION COMMENTS LT8610 42V, 2.5A, 96% Efficiency, 2.2MHz Synchronous Micropower Step-Down DC/DC Converter with IQ = 2.5µA VIN: 3.4V to 42V, VOUT(MIN) = 0.97V, IQ = 2.5µA, ISD < 1µA, MSOP-16E Package LT8610A/LT8610AB 42V, 3.5A, 96% Efficiency, 2.2MHz Synchronous Micropower Step-Down DC/DC Converter with IQ = 2.5µA VIN: 3.4V to 42V, VOUT(MIN) = 0.97V, IQ = 2.5µA, ISD < 1µA, MSOP-16E Package LT8611 42V, 2.5A, 96% Efficiency, 2.2MHz Synchronous Micropower Step-Down DC/DC Converter with IQ = 2.5µA and Input/Output Current Limit/Monitor VIN: 3.4V to 42V, VOUT(MIN) = 0.97V, IQ = 2.5µA, ISD < 1µA, 3mm × 5mm QFN-24 Package LT8612 42V, 6A, 96% Efficiency, 2.2MHz Synchronous Micropower Step-Down DC/DC Converter with IQ = 3µA VIN: 3.4V to 42V, VOUT(MIN) = 0.97V, IQ = 2.5µA, ISD < 1µA, 3mm × 6mm QFN Package LT3971 38V, 1.2A, 2.2MHz High Efficiency Micropower Step-Down DC/DC Converter with IQ = 2.8µA VIN: 4.2V to 38V, VOUT(MIN) = 1.21V, IQ = 2.8µA, ISD < 1µA, 3mm × 3mm DFN-10 and MSOP-10E Packages LT3991 55V, 1.2A, 2.2MHz High Efficiency Micropower Step-Down DC/DC Converter with IQ = 2.8µA VIN: 4.2V to 55V, VOUT(MIN) = 1.21V, IQ = 2.8µA, ISD < 1µA, 3mm × 3mm DFN-10 and MSOP-10E Packages LT3970 40V, 350mA, 2.2MHz High Efficiency Micropower Step-Down DC/DC Converter with IQ = 2.5µA VIN: 4.2V to 40V, VOUT(MIN) = 1.21V, IQ = 2.5µA, ISD < 1µA, 3mm × 2mm DFN-10 and MSOP-10 Packages LT3990 62V, 350mA, 2.2MHz High Efficiency MicroPower Step-Down DC/DC Converter with IQ = 2.5µA VIN: 4.2V to 62V, VOUT(MIN) = 1.21V, IQ = 2.5µA, ISD < 1µA, 3mm × 3mm DFN-10 and MSOP-6E Packages 24 8614fd LT 0617 REV D • PRINTED IN USA For more information www.linear.com/LT8614 www.linear.com/LT8614  LINEAR TECHNOLOGY CORPORATION 2013