LT8608/LT8608B 42V, 1.5A Synchronous Step-Down Regulator with 2.5µA Quiescent Current FEATURES DESCRIPTION Wide Input Voltage Range: 3.0V 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 High Efficiency 2MHz Synchronous Operation: nn >92% Efficiency at 0.5A, 5V OUT from 12VIN nn 1.5A Continuous Output Current nn Fast Minimum Switch-On Time: 35ns nn Adjustable and Synchronizable: 200kHz to 2.2MHz nn Spread Spectrum Frequency Modulation for Low EMI nn Allows Use of Small Inductors nn Low Dropout nn Peak Current Mode Operation nn Accurate 1V Enable Pin Threshold nn Internal Compensation nn Output Soft-Start and Tracking nn Small 10-Lead MSOP Package or 8-Lead 2mm × 2mm DFN Package The LT®8608 is a compact, high efficiency, high speed synchronous monolithic step-down switching regulator that consumes only 1.7µA of quiescent current. The LT8608 can deliver 1.5A of continuous current. Low ripple Burst Mode operation enables high efficiency down to very low output currents while keeping the output ripple below 10mV. Internal compensation with peak current mode topology allows the use of small inductors and results in fast transient response and good loop stability. The EN/ UV pin has an accurate 1V threshold and can be used to program VIN undervoltage lockout or to shut down the LT8608. The PG flag signals when VOUT is within ±8.5% of the programmed output voltage as well as fault conditions. nn The MSOP package includes a SYNC pin to synchronize to an external clock, or to select Burst Mode operation or pulse-skipping with or without spread-spectrum; the TR/SS pin programs soft-start or tracking. The DFN package omits these pins and can be purchased in pulseskipping or Burst Mode operation varieties. APPLICATIONS General Purpose Step Down nn Low EMI Step Down nn PACKAGE SYNC FUNCTIONALITY LT8608MSE MSE Programmable LT8608DFN DFN Burst Mode Operation LT8608BDFN DFN Pulse-Skipping Mode All registered trademarks and trademarks are the property of their respective owners. TYPICAL APPLICATION 12VIN to 5VOUT Efficiency IN 100 5V, 2MHz Step-Down OUT 95 4.7µF ON OFF VIN EN/UV SYNC BST 0.1µF 2.2µH SW 10pF LT8608 INTVCC TR/SS RT 1µF 18.2k GND PG FB VOUT 5V 1.5A 1M EFFICIENCY (%) 90 VIN 5.5V TO 42V 85 80 75 70 65 60 22µF 187k 8608 TA01a 55 50 fSW = 2MHz 0 0.25 0.50 0.75 1.00 IOUT (A) 1.25 1.50 8608 TA01b Rev C Document Feedback For more information www.analog.com 1 LT8608/LT8608B ABSOLUTE MAXIMUM RATINGS (Note 1) VIN, EN/UV, PG...........................................................42V FB, TR/SS ...................................................................4V SYNC Voltage ..............................................................6V Operating Junction Temperature Range (Note 2) LT8608E ............................................ –40°C to 125°C LT8608I.............................................. –40°C to 125°C LT8608H............................................. –40°C to 150°C Storage Temperature Range................... –65°C to 150°C PIN CONFIGURATION TOP VIEW TOP VIEW BST SW INTVCC RT SYNC 1 2 3 4 5 10 9 8 7 6 11 GND EN/UV VIN PG TR/SS FB SW 2 9 GND INTVCC 3 7 VIN 6 PG 5 FB RT 4 MSE PACKAGE 10-LEAD PLASTIC MSOP θJA = 40°C/W, θJC = 10°C/W EXPOSED PAD (PIN 11) IS GND, MUST BE SOLDERED TO PCB ORDER INFORMATION 8 EN/UV BST 1 DC PACKAGE 8-LEAD (2mm × 2mm) PLASTIC DFN http://www.linear.com/product/LT8608#orderinfo LEAD FREE FINISH TAPE AND REEL PART MARKING* PACKAGE DESCRIPTION TEMPERATURE RANGE LT8608EMSE#PBF LT8608EMSE#TRPBF LTGVZ 10-Lead Plastic MSOP –40°C to 125°C LT8608IMSE#PBF LT8608IMSE#TRPBF LTGVZ 10-Lead Plastic MSOP –40°C to 125°C LT8608HMSE#PBF LT8608HMSE#TRPBF LTGVZ 10-Lead Plastic MSOP –40°C to 150°C LT8608EDC#PBF LT8608EDC#TRPBF LGXB 8-Lead (2mm × 2mm) Plastic DFN –40°C to 125°C LT8608IDC#PBF LT8608IDC#TRPBF LGXB 8-Lead (2mm × 2mm) Plastic DFN –40°C to 125°C LT8608HDC#PBF LT8608HDC#TRPBF LGXB 8-Lead (2mm × 2mm) Plastic DFN –40°C to 150°C LT8608BEDC#PBF LT8608BEDC#TRPBF LGXC 8-Lead (2mm × 2mm) Plastic DFN –40°C to 125°C LT8608BIDC#PBF LT8608BIDC#TRPBF LGXC 8-Lead (2mm × 2mm) Plastic DFN –40°C to 125°C LT8608BHDC#PBF LT8608BHDC#TRPBF LGXC 8-Lead (2mm × 2mm) Plastic DFN –40°C to 150°C Consult ADI 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 TYP MAX 2.5 3.0 3.2 V l VIN Quiescent Current VIN Current in Regulation 2 UNITS VEN/UV = 0V, VSYNC = 0V VEN/UV = 2V, Not Switching, VSYNC = 0V or LT8608 DFN, VIN ≤ 36V l 1 1.7 4 12 µA µA VIN = 6V, VOUT = 2.7V, Output Load = 100µA VIN = 6V, VOUT = 2.7V, Output Load = 1mA l l 56 500 90 700 µA µA Rev C For more information www.analog.com LT8608/LT8608B 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 TYP MAX UNITS Feedback Reference Voltage MSOP Package VIN = 6V, ILOAD = 100mA VIN = 6V, ILOAD = 100mA l 0.774 0.762 0.778 0.778 0.782 0.798 V V DFN Package VIN = 6V, ILOAD = 100mA VIN = 6V, ILOAD = 100mA l 0.773 0.753 0.778 0.778 0.783 0.803 V V Feedback Voltage Line Regulation VIN = 4.0V to 40V l ±0.02 ±0.06 %/V Feedback Pin Input Current VFB = 1V l Minimum On-Time ILOAD = 1A, SYNC = 0V or LT8608 DFN ILOAD = 1A, SYNC = 1.9V or LT8608B DFN l l Minimum Off Time Oscillator Frequency Top Power NMOS On-Resistance ±20 nA 35 35 65 60 ns ns 93 130 ns MSOP Package RT = 221k, ILOAD = 500mA RT = 60.4k, ILOAD = 500mA RT = 18.2k, ILOAD = 200mA l l l 155 640 1.90 200 700 2.00 245 760 2.10 kHz kHz MHz DFN Package RT = 221k, ILOAD = 500mA RT = 60.4k, ILOAD = 500mA RT = 18.2k, ILOAD = 200mA l l l 130 610 1.85 200 700 2.00 270 790 2.15 kHz kHz MHz ILOAD = 0.5A 350 Top Power NMOS Current Limit l 2.1 Bottom Power NMOS On-Resistance 2.9 mΩ 3.9 230 SW Leakage Current VIN = 36V l EN/UV Pin Threshold EN/UV Rising l 15 0.99 EN/UV Pin Hysteresis 1.05 A mΩ µA 1.11 50 V mV EN/UV Pin Current VEN/UV = 2V l ±20 nA PG Upper Threshold Offset from VFB VFB Rising l 5.0 8.5 13.0 % PG Lower Threshold Offset from VFB VFB Falling l 5.0 8.5 13.0 % PG Leakage VPG = 42V l ±200 nA PG Pull-Down Resistance VPG = 0.1V 1200 Ω Sync Low Input Voltage MSOP Only l Sync High Input Voltage INTVCC = 3.5V, MSOP Only l TR/SS Source Current MSOP Only l TR/SS Pull-Down Resistance Fault Condition, TR/SS = 0.1V, MSOP Only Spread Spectrum Modulation Frequency VSYNC = 3.3V, MSOP Only PG Hysteresis 0.5 550 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. Absolute Maximum Ratings are those values beyond which the life of a device may be impaired. Note 2: The LT8608E 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 LT8608I is guaranteed over the full –40°C to 125°C operating junction l 0.4 0.9 2.7 1 0.5 % V 3.2 V 2 3 µA 300 900 Ω 3 6 kHz temperature range. The LT8608H is guaranteed over the full –40°C to 150°C operating junction temperature range. High junction temperatures degrade operating lifetimes. Operating lifetime is derated at junction temperatures greater than 125°C. Note 3: 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. Rev C For more information www.analog.com 3 LT8608/LT8608B TYPICAL PERFORMANCE CHARACTERISTICS Efficiency (5V Output, Burst Mode Operation) Mode Operation) Efficiency (5V Output, Burst Mode 100 100 VIN = 12V 95 95 80 75 70 65 50 0.00 0.25 0.50 0.75 1.00 IOUT (A) 1.25 60 50 40 30 0 0.001 0.01 1.50 85 0.1 8608 G01 1 10 IOUT (mA) 100 1k 75 70 65 60 50 0.00 10k 50 40 30 0 0.001 0.01 SYNC = 0V OR LT8608 DFN L = 2.2µH fSW = 2MHz 0.1 1 10 IOUT (mA) 100 1k 8608 G03 0.3 779 778 777 0.2 0.1 0.0 –0.1 –0.2 –0.3 776 775 –50 10k –10 8608 G04 30 70 110 TEMPERATURE (°C) 150 –0.5 4.00 3.3 SYNC = 0V OR LT8608 DFN 3.1 INPUT CURRENT (µA) IIN (µA) 3.00 2.75 2.50 2.00 18 26 INPUT VOLTAGE (V) 34 42 2.7 2.5 2.3 2.1 1.9 1.7 2.25 –0.15 SYNC = 0V OR LT8608 DFN 2.9 3.25 –0.10 1.50 No-Load Supply Current 3.50 0.10 –0.05 0.50 0.75 1 1.25 OUTPUT CURRENT (A) vs Temperature (Not Switching) 3.75 0.15 0.00 0.25 8608G06 No-Load Supply Current (3.3V (3.3V Output) Output) 0.05 0 8608 G05 Line Regulation 10 1.50 –0.4 0.20 2 1.25 Load Regulation CHANGE IN VOUT (%) FB REGULATION VOLTAGE (mV) 60 10 0.75 1.00 IOUT (A) 0.4 VIN = 24V 20 0.50 0.5 VIN = 12V 80 70 0.25 8608 G02 780 90 SYNC = 0V OR LT8608 DFN L = 2.2µH fSW = 2MHz 55 FB Voltage 100 VIN = 24V 80 SYNC = 0V OR LT8608 DFN L = 2.2µH fSW = 2MHz 10 Efficiency (3.3V Output, 2MHz, Burst Operation) Mode Mode Operation) –0.20 VIN = 24V 70 20 SYNC = 0V OR LT8608 DFN L = 2.2µH fSW = 2MHz 55 VIN = 12V 90 EFFICIENCY (%) VIN = 24V EFFICIENCY (%) EFFICIENCY (%) 85 EFFICIENCY (%) VIN = 12V 80 60 CHANGE IN VOUT (%) 100 90 90 1.5 2 10 18 26 INPUT VOLTAGE (V) 34 42 8608 G08 8608 G07 4 Efficiency (3.3V Output, 2MHz, Burst Operation) Mode Mode Operation) Operation) Mode Operation) 1.3 –50 –10 30 70 110 TEMPERATURE (°C) 150 8608 G09 Rev C For more information www.analog.com LT8608/LT8608B TYPICAL PERFORMANCE CHARACTERISTICS Top FET Current Limit vs TopDuty Fet Cycle Current Limit vs Duty Cycle Top FET Current Limit 3.1 DUTY CYCLE = 0 SWITCH DROP (mV) 2.75 2.50 2.8 2.7 2.25 2.6 0 20 40 60 DUTY CYCLE (%) 80 –10 30 70 110 TEMPERATURE (°C) 700 39 400 300 200 110 105 37 36 35 34 33 32 100 30 –50 –30 –10 10 30 50 70 90 110 130 150 TEMPERATURE (°C) 2 80 –50 –30 –10 10 30 50 70 90 110 130 150 TEMPERATURE (°C) 8608 G15 Switching Frequency vs Temperature Switching Frequency Vs Temperature SWITHCING FREQUENCY (kHz) 500 375 250 125 2500 2020 2250 2015 2000 2010 2005 2000 1995 1990 1985 1980 VOUT = 3.3V 2 8608 G16 Burst Burst Frequency Frequency vs vs Load Load Current Current 2025 SWITHCING FREQUENCY (kHz) L = XFL4020–222MEC 625 DROPOUT VOLTAGE (mV) 90 8608 G14 Dropout Voltage vs Load Current 0.25 0.50 0.75 1 1.25 1.50 1.75 LOAD CURRENT (A) 95 85 8608 G13 0 100 31 BOT SW 0.25 0.50 0.75 1 1.25 1.50 1.75 SWITCH CURRENT (A) Minimum Off-Time vs Temperature Minimum Off-Time Vs Temperature MINIMUM OFF-TIME (ns) 500 0 300 8608 G12 IOUT = 1A 38 600 MINIMUM ON-TIME (ns) SWITCH DROP (mV) 40 750 350 Minimum On-Time vs Temperature Minimum On-Time Vs Temperature 800 TOP SW 400 8608 G11 Switch Drop vs Switch Current 0 450 TOP SW BOT SW 200 –50 –30 –10 10 30 50 70 90 110 130 150 TEMPERATURE (°C) 150 8608 G10 0 SWITCH CURRENT = 1A 250 2.5 –50 100 Switch Drop Drop vs vs Temperature Temperature Switch 500 2.9 2.00 550 3.0 3.00 ISW (A) TOP FET CURRENT LIMIT (A) 3.25 Top FET Current Limit Vs Temperature vs Temperature 1975 –50 30 70 110 TEMPERATURE (°C) 1500 1250 1000 750 SYNC = 0V OR LT8608 DFN L = 2.2µH VIN = 12V VOUT = 3.3V 500 250 RT = 18.2k –10 1750 150 8608 G17 0 0 100 200 300 400 LOAD CURRENT (mA) 500 8608 G18 Rev C For more information www.analog.com 5 LT8608/LT8608B TYPICAL PERFORMANCE CHARACTERISTICS Minimum Load to Full Frequency (SYNC Float to 1.9V) (MSOP Package) L = 2.2µH VOUT = 5V RT = 18.2k FREQUENCY (kHz) LOAD CURRENT (mA) 100 75 50 25 2500 1.0 2250 0.9 2000 0.8 1750 0.7 FB VOLTAGE (V) 125 Frequency Foldback 1500 1250 1000 0 0 5 10 15 20 25 30 INPUT VOLTAGE (V) 35 0.6 0.5 0.4 750 0.3 500 0.2 0.1 SYNC = 0V OR LT8608 DFN RT = 18.2k 250 0 0 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 FB VOLTAGE (V) 40 8608 G19 Steady State Case Temperature Rise vs Load Current (5VOUT) (5V out) IN 3.25 50 VIN = 6V VIN = 12V VIN= 36V 45 2.4 40 3.00 CASE TEMP RISE (°C) 2.3 2.2 VIN UVLO (V) SOFT-START CURRENT (µA) 0.1 0.2 0.4 0.5 0.6 0.7 0.8 1.0 1.1 1.2 SS VOLTAGE (V) 8608 G21 VIN UVLO UVLO V 2.5 2.1 2.0 1.9 2.75 2.50 1.8 1.7 35 30 25 20 15 10 2.25 L = 2.2µH fSW = 2MHz 5 1.6 1.5 –50 –30 –10 10 30 50 70 90 110 130 150 TEMPERATURE (°C) Start-Up Start–UpDropout Droupout VIN = 12V VIN = 36V 40 INPUT VOLTAGE (V) 25 20 15 10 L = 2.2µH fSW = 2MHz 5 0 0.00 0.25 0.50 0.75 1.00 IOUT (A) 1.25 1.50 8608 G25 1.25 1.50 8608 G24 Start-Up Start-UpDropout Droupout 6 6 6 6 5 5 5 5 RLOAD = 50Ω 4 4 VIN VOUT 3 3 2 2 1 0 0 1 2 3 4 5 INPUT VOLTAGE (V) 6 7 7 RLOAD = 5Ω 4 4 VIN VOUT 3 3 2 2 1 1 1 0 0 8608 G26 0 1 2 3 4 5 INPUT VOLTAGE (V) 6 7 OUTPUT VOLTAGE (V) 30 0.75 1.00 IOUT (A) 7 OUTPUT VOLTAGE (V) 35 0.50 7 7 INPUT VOLTAGE (V) 45 0.25 8608 G23 Steady State Case Temperature Rise vs Load (3.3VOUT) 50 0 0.00 2.00 –50 –30 –10 10 30 50 70 90 110 130 150 TEMPERATURE (°C) 8608 G22 CASE TEMP RISE (°C) 0 8608 G20 Soft-Start Current vs Temperature (MSOP Package) Soft Start Current Vs Temperature 6 Soft-Start Tracking Soft-Start Tracking (MSOP Package) 0 8608 027 Rev C For more information www.analog.com LT8608/LT8608B TYPICAL PERFORMANCE CHARACTERISTICS Switching Waveforms Switching Waveforms Switching Waveforms IL IL 1A/DIV IL 1A/DIV 200mA/DIV SW SW 5V/DIV 5V/DIV 200ns/DIV SW 2V/DIV 10μs/DIV 8608 G28 8608 G29 12VIN TO 5VOUT AT 3mA 12VIN TO 3.3 VOUT AT 1A 2MHz 200ns/DIV 8608 G30 36VIN TO 3.3VOUT AT 1A 2MHz Transient Response Transient Response ILOAD 500mA/DIV ILOAD 500mA/DIV VOUT 50mV/DIV VOUT 50mV/DIV 50µs/DIV 8608 G31 VIN = 12V 0.5A TO 1A COUT = 47μF fSW = 2MHz 50µs/DIV 8608 G32 VIN = 24V 0.5A TO 1A COUT = 47μF fSW = 2MHz Rev C For more information www.analog.com 7 LT8608/LT8608B PIN FUNCTIONS BST: 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. Do not place a resistor in series with this pin. SW: The SW pin is the output of the internal power switches. Connect this pin to the inductor and boost capacitor. This node should be kept small on the PCB for good performance. INTVCC: Internal 3.5V Regulator Bypass Pin. The internal power drivers and control circuits are powered from this voltage. INTVCC max output current is 20mA. Voltage on INTVCC will vary between 2.8V and 3.5V. Decouple this pin to power ground with at least a 1μF low ESR ceramic capacitor. Do not load the INTVCC pin with external circuitry. RT: A resistor is tied between RT and ground to set the switching frequency. When synchronizing, the RT resistor should be chosen to set the LT8608 switching frequency equal to or below the lowest synchronization input. SYNC (MSOP Only): 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. Leave floating for pulse-skipping mode with no spread spectrum modulation. Tie to INTVCC or tie to a voltage between 3.2V and 5.0V for pulse-skipping mode with spread spectrum modulation. When in pulseskipping mode, the IQ will increase to several mA. There is no SYNC pin in the LT8608 DFN package. The LT8608 DFN package internally ties SYNC to ground. The LT8608B DFN package internally floats SYNC. FB: The LT8608 regulates the FB pin to 0.778V. Connect the feedback resistor divider tap to this pin. 8 TR/SS (MSOP Only): Output Tracking and Soft-Start Pin. This pin allows user control of output voltage ramp rate during start-up. A TR/SS voltage below 0.778V forces the LT8608 to regulate the FB pin to equal the TR/SS pin voltage. When TR/SS is above 0.778V, the tracking function is disabled and the internal reference resumes control of the error amplifier. An internal 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 a 300Ω MOSFET during shutdown and fault conditions; use a series resistor if driving from a low impedance output. There is no TR/SS pin in the LT8608 DFN or LT8608B and the node is internally floated. PG: The PG pin is the open-drain output of an internal comparator. PG remains low until the FB pin is within ±8.5% of the final regulation voltage, and there are no fault conditions. PG is valid when VIN is above 3.2V, regardless of EN/UV pin state. VIN: The VIN pin supplies current to the LT8608 internal circuitry and to the internal topside power switch. This pin must be locally bypassed. Be sure to place the positive terminal of the input capacitor as close as possible to the VIN pins, and the negative capacitor terminal as close as possible to the GND pins. EN/UV: The LT8608 is shut down when this pin is low and active when this pin is high. The hysteretic threshold voltage is 1.05V going up and 1.00V 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 LT8608 will shut down. GND: Exposed Pad Pin. The exposed pad must be connected to the negative terminal of the input capacitor and soldered to the PCB in order to lower the thermal resistance. Rev C For more information www.analog.com LT8608/LT8608B BLOCK DIAGRAM VIN VIN CIN – + INTERNAL 0.778V REF R3 (OPT) EN/UV R4 (OPT) PG 1V + – SHDN ±8.5% SLOPE COMP ERROR AMP + + – RPG R2 CSS RT R1 FB TR/SS VC SHDN TSD INTVCC UVLO VIN UVLO 2µA INTVCC CVCC OSCILLATOR 200kHz TO 2.2MHz VOUT CFF 3.5V REG BST BURST DETECT SWITCH LOGIC AND ANTISHOOT THROUGH CBST M1 L SW VOUT COUT M2 GND SHDN TSD VIN UVLO RT SYNC 8608 BD Rev C For more information www.analog.com 9 LT8608/LT8608B OPERATION The LT8608 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 0.778V 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 excess current flowing through the bottom switch, the next clock cycle will be delayed until switch current returns to a safe level. If the EN/UV pin is low, the LT8608 is shut down and draws 1µA from the input. When the EN/UV pin is above 1.05V, the switching regulator becomes active. To optimize efficiency at light loads, the LT8608 enters Burst Mode operation during 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 from the input supply when regulating with no load. The SYNC pin is tied low to use Burst Mode operation and can be floated 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 mA. The SYNC pin may be tied high for spread spectrum modulation mode, and the LT8608 will operate similar to pulse-skipping mode but vary the clock frequency to reduce EMI. The LT8608 DFN has no SYNC pin and will always operate in Burst Mode operation. The LT8608B DFN has no SYNC pin and will always operate in pulse-skipping mode. Comparators monitoring the FB pin voltage will pull the PG pin low if the output voltage varies more than ±8.5% (typical) from the set point, or if a fault condition is present. The oscillator reduces the LT8608’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. When a clock is applied to the SYNC pin the frequency foldback is disabled. APPLICATIONS INFORMATION Achieving Ultralow Quiescent Current To enhance efficiency at light loads, the LT8608 enters into 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 LT8608 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 LT8608 consumes 1.7μA. 10 As the output load decreases, the frequency of single current pulses decreases (see Figure 1) and the percentage of time the LT8608 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. Rev C For more information www.analog.com LT8608/LT8608B APPLICATIONS INFORMATION Burst Frequency vs Load Current 2500 SWITHCING FREQUENCY (kHz) VOUT 20mV/DIV L = 2.2µH VIN = 12V VOUT = 3.3V SYNC = 0V 2250 2000 1750 SW 5V/DIV INDUCTOR CURRENT 1500 1250 500mA/DIV 1000 20μs/DIV 8608 F03 Figure 3. Burst Mode Operation 750 500 250 0 0 100 200 300 400 LOAD CURRENT (mA) 500 VOUT 20mV/DIV SW 5V/DIV 8608 F01 Figure 1. SW Burst Mode Frequency vs Load 500mA/DIV 125 L = 2.2µH VOUT = 5V RT = 18.2k LOAD CURRENT (mA) 100 50 25 0 5 10 500ns/DIV 8608 F04 Figure 4. Burst Mode Operation (Zoomed In) 75 0 INDUCTOR CURRENT 15 20 25 30 INPUT VOLTAGE (V) 35 40 8608 F02 Figure 2. Full Switching Frequency Minimum Load vs VIN in Pulse Skipping Mode (MSOP Only) While in Burst Mode operation the current limit of the top switch is approximately 550mA resulting in output voltage ripple shown in Figure 3 and Figure 4. 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 1. The output load at which the LT8608 reaches the programmed frequency varies based on input voltage, output voltage, and inductor choice. For some applications it is desirable for the LT8608 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 as shown in Figure 2. To enable pulse-skipping mode the SYNC pin is floated. To achieve spread spectrum modulation with pulse-skipping mode, the SYNC pin is tied high. While a clock is applied to the SYNC pin the LT8608 will also operate in pulse-skipping mode. The LT8608 DFN is programmed for Burst Mode operation and can not enter pulse-skipping mode. The LT8608B DFN is programmed for pulse-skipping and cannot enter Burst Mode operation. Rev C For more information www.analog.com 11 LT8608/LT8608B APPLICATIONS INFORMATION FB Resistor Network Operating Frequency Selection and Trade-Offs The output voltage is programmed with a resistor divider between the output and the FB pin. Choose the resistor values according to: 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. ⎛ V ⎞ R1= R2 ⎜ OUT – 1⎟ ⎝ 0.778V ⎠ 1% resistors are recommended to maintain output voltage accuracy. The total resistance of the FB resistor divider should be selected to be as large as possible when good low load efficiency is desired: The resistor divider generates a small load on the output, which should be minimized to optimize the quiescent current at low loads. When using large FB resistors, a 10pF phase lead capacitor should be connected from VOUT to FB. Setting the Switching Frequency The LT8608 uses a constant frequency PWM architecture that can be programmed to switch from 200kHz to 2.2MHz 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. When in spread spectrum modulation mode, the frequency is modulated upwards of the frequency set by RT. Table 1. SW Frequency vs RT Value 12 fSW (MHz) RT (kΩ) 0.2 221 0.300 143 0.400 110 0.500 86.6 0.600 71.5 0.700 60.4 0.800 52.3 0.900 46.4 1.000 40.2 1.200 33.2 1.400 27.4 1.600 23.7 1.800 20.5 2.000 18.2 2.200 16.2 The highest switching frequency (fSW(MAX)) for a given application can be calculated as follows: fSW (M AX ) = VOUT + VSW (BOT ) t ON(M IN ) ( VIN – VSW ( TOP ) + VSW (BOT ) ) where VIN is the typical input voltage, VOUT is the output voltage, VSW(TOP) and VSW(BOT) are the internal switch drops (~0.55V, ~0.35V, respectively at max load) and tON(MIN) is the minimum top switch on-time (see Electrical Characteristics). This equation shows that slower switching frequency is necessary to accommodate a high VIN/ VOUT ratio. For transient operation VIN may go as high as the Abs Max rating regardless of the RT value, however the LT8608 will reduce switching frequency as necessary to maintain control of inductor current to assure safe operation. The LT8608 is capable of maximum duty cycle approaching 100%, and the VIN to VOUT dropout is limited by the RDS(ON) of the top switch. In this mode the LT8608 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(M IN ) = VOUT + VSW (BOT ) 1– fSW • t OFF (M IN ) – VSW (BOT ) + VSW ( TOP ) 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 (~0.55V, ~0.35V, respectively at max 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. Rev C For more information www.analog.com LT8608/LT8608B APPLICATIONS INFORMATION Inductor Selection and Maximum Output Current The LT8608 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 LT8608 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 where fSW is the switching frequency in MHz, VOUT is the output voltage, VSW(BOT) is the bottom switch drop (~0.35V) and L is the inductor value in μH. 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(M AX ) + ∆ L 2 As a quick example, an application requiring 0.5A output should use an inductor with an RMS rating of greater than 0.5A and an ISAT of greater than 0.8A. 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 LT8608 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 2.1A at low duty cycles and decreases linearly to 1.55A at D = 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: ∆IL 2 ∆IL = ⎞ VOUT ⎛ V ⎜⎜ 1– OUT ⎟⎟ L • fSW ⎝ VIN(M AX ) ⎠ where fSW is the switching frequency of the LT8608, and L is the value of the inductor. Therefore, the maximum output current that the LT8608 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 sufficient maximum output current (IOUT(MAX)) given the switching frequency, and maximum input voltage used in the desired application. For more information about maximum output current and discontinuous operation, see Analog Device’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 Capacitor where ∆IL is the inductor ripple current as calculated several paragraphs below and ILOAD(MAX) is the maximum output load for a given application. IOUT (M AX ) = ILIM – The peak-to-peak ripple current in the inductor can be calculated as follows: Bypass the input of the LT8608 circuit with a ceramic capacitor of X7R or X5R type. Y5V types have poor performance over temperature and applied voltage, and should not be used. A 4.7μF to 10μF ceramic capacitor is adequate to bypass the LT8608 and will easily handle the ripple current. 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. 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 LT8608 and to force this very high frequency switching current into a tight local loop, minimizing EMI. A 4.7μF capacitor is capable of this task, but only if it is placed close to the LT8608 (see the PCB Layout section). A second precaution regarding the ceramic input capacitor concerns the maximum input voltage rating of the LT8608. Rev C For more information www.analog.com 13 LT8608/LT8608B APPLICATIONS INFORMATION A ceramic input capacitor combined with trace or cable inductance forms a high quality (under damped) tank circuit. If the LT8608 circuit is plugged into a live supply, the input voltage can ring to twice its nominal value, possibly exceeding the LT8608’s voltage rating. This situation is easily avoided (see Analog Devices Application Note 88). loads the LT8608 can excite the ceramic capacitor at audio frequencies, generating audible noise. Since the LT8608 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. Output Capacitor and Output Ripple A final precaution regarding ceramic capacitors concerns the maximum input voltage rating of the LT8608. As previously mentioned, a ceramic input capacitor combined with trace or cable inductance forms a high quality (under damped) tank circuit. If the LT8608 circuit is plugged into a live supply, the input voltage can ring to twice its nominal value, possibly exceeding the LT8608’s rating. This situation is easily avoided (see Analog Devices Application Note 88). The output capacitor has two essential functions. Along with the inductor, it filters the square wave generated by the LT8608 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 LT8608’s control loop. Ceramic capacitors have very low equivalent series resistance (ESR) and provide the best ripple performance. A good starting value is: C OUT = 100 VOUT • fSW where fSW is in MHz, and COUT is the recommended output capacitance in μF. 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 LT8608 due to their piezoelectric nature. When in Burst Mode operation, the LT8608’s switching frequency depends on the load current, and at very light 14 Enable Pin The LT8608 is in shutdown when the EN pin is low and active when the pin is high. The rising threshold of the EN comparator is 1.05V, with 50mV 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 LT8608 to regulate the output only when VIN is above a desired voltage (see 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 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⎟ • 1V ⎝ R4 ⎠ where the LT8608 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). Rev C For more information www.analog.com LT8608/LT8608B APPLICATIONS INFORMATION When 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 LT8608. Therefore, the VIN(EN) resistors should be large to minimize their effect on efficiency at low loads. is considered good and the open-drain PG pin goes high impedance and is typically pulled high with an external resistor. Otherwise, the internal drain pull-down device will pull the PG pin low. To prevent glitching both the upper and lower thresholds include 0.5% of hysteresis. INTVCC Regulator 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. An internal low dropout (LDO) regulator produces the 3.5V supply from VIN that powers the drivers and the internal bias circuitry. The INTVCC can supply enough current for the LT8608’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. Applications with high input voltage and high switching frequency 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 (MSOP Only) The LT8608 allows the user to program its output voltage ramp rate by means of the TR/SS pin. An internal 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 soft-start ramp the output voltage will proportionally track the TR/SS pin voltage. For output tracking applications, TR/SS can be externally driven by another voltage source. From 0V to 0.778V, the TR/SS voltage will override the internal 0.778V reference input to the error amplifier, thus regulating the FB pin voltage to that of TR/SS pin. When TR/SS is above 0.778V, tracking is disabled and the feedback voltage will regulate to the internal reference voltage. 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 softstart capacitor are the EN/UV pin transitioning low, VIN voltage falling too low, or thermal shutdown. The LT8608 and LT8608B DFN do not have TR/SS pin or functionality. Output Power Good When the LT8608’s output voltage is within the ±8.5% window of the regulation point, which is a VFB voltage in the range of 0.716V to 0.849V (typical), the output voltage Synchronization (MSOP Only) 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 LT8608 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 0.9V and peaks above 2.7V (up to 5V). The LT8608 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 LT8608 may be synchronized over a 200kHz to 2.2MHz range. The RT resistor should be chosen to set the LT8608 switching frequency equal to or below the lowest synchronization 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 LT8608 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 as shown in Figure 2 in an earlier section. These two differences come at the expense of increased quiescent current. To enable pulse-skipping mode the SYNC pin is floated. For more information www.analog.com Rev C 15 LT8608/LT8608B APPLICATIONS INFORMATION For some applications, reduced EMI operation may be desirable, which can be achieved through spread spectrum modulation. This mode operates similar to pulse skipping mode operation, with the key difference that the switching frequency is modulated up and down by a 3kHz triangle wave. The modulation has the frequency set by RT as the low frequency, and modulates up to approximately 20% higher than the frequency set by RT. To enable spread spectrum mode, tie SYNC to INTVCC or drive to a voltage between 3.2V and 5V. The LT8608 does not operate in forced continuous mode regardless of SYNC signal. The LT8608 DFN is programmed for Burst Mode operation and cannot enter pulse skipping mode. The LT8608B DFN is programmed for pulse-skipping and cannot enter Burst Mode operation. the LT8608’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 0.7µA. However, if the VIN pin is grounded while the output is held high, regardless of EN, parasitic body diodes inside the LT8608 can pull current from the output through the SW pin and the VIN pin. Figure 5 shows a connection of the VIN and EN/UV pins that will allow the LT8608 to run only when the input voltage is present and that protects against a shorted or reversed input. D1 VIN LT8608 EN/UV GND 8608 F05 Shorted and Reversed Input Protection The LT8608 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. This allows for tailoring the LT8608 to individual applications and limiting thermal dissipation during short circuit conditions. Frequency foldback behavior depends on the state of the SYNC pin: If the SYNC pin is low or high, or floated 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, the LT8608 will stay at the programmed frequency without foldback and only slow switching if the inductor current exceeds safe levels. There is another situation to consider in systems where the output will be held high when the input to the LT8608 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 LT8608’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 16 VIN Figure 5. Reverse VIN Protection PCB Layout For proper operation and minimum EMI, care must be taken during printed circuit board layout. Figure 7 shows the recommended component placement with trace, ground plane and via locations. Note that large, switched currents flow in the LT8608’s VIN pins, GND pins, and the input capacitor (C1). The loop formed by the input capacitor should be as small as possible by placing the capacitor adjacent to the VIN and GND pins. When using a physically large input capacitor the resulting loop may become too large in which case using a small case/value capacitor placed close to the VIN and GND pins plus a larger capacitor further away is preferred. These components, along with the inductor and output capacitor, 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 traces will shield them from the SW and BOOST nodes. The exposed pad on the bottom of the package must be soldered to ground so that the pad is connected to ground electrically and also Rev C For more information www.analog.com LT8608/LT8608B APPLICATIONS INFORMATION Figure 7 shows the basic guidelines for a layout example that can pass CISPR25 radiated emission test with class 5 limits. Thermal Considerations For higher ambient temperatures, care should be taken in the layout of the PCB to ensure good heat sinking of the LT8608. Figure 7 shows the recommended component placement with trace, ground plane, and via locations. The exposed pad on the bottom of the package must 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 LT8608. 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 LT8608 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 LT8608 power dissipation by the thermal resistance from junction to ambient. The LT8608 will stop switching and indicate a fault condition if safe junction temperature is exceeded. Temperature rise of the LT8608 is worst when operating at high load, high VIN, and high switching frequency. If the case temperature is too high for a given application, then either VIN, switching frequency or load current can be decreased to reduce the temperature to an acceptable level. Figure 6 shows how case temperature rise can be managed by reducing VIN. (5V out) 50 45 40 CASE TEMP RISE (°C) acts as a heat sink thermally. To keep thermal resistance low, extend the ground plane as much as possible, and add thermal vias under and near the LT8608 to additional ground planes within the circuit board and on the bottom side. VIN = 6V VIN = 12V VIN= 36V 35 30 25 20 15 10 L = 2.2µH fSW = 2MHz 5 0 0.00 0.25 0.50 0.75 1.00 IOUT (A) 1.25 1.50 8608 F06 Figure 6. Case Temperature Rise vs Load Current Rev C For more information www.analog.com 17 LT8608/LT8608B APPLICATIONS INFORMATION GROUND PLANE ON LAYER 2 COUT L CIN CBST 1 CVCC CIN(OPT) RT RPG R4 R3 R1 CFF GND VIA VIN VIA VOUT VIA EN/UV VIA R2 CSS OTHER SIGNAL VIA 8609S F07 Figure 7. PCB Layout (Not to Scale) 18 Rev C For more information www.analog.com LT8608/LT8608B TYPICAL APPLICATIONS 3.3V Step Down VIN 3.9V TO 42V C1 0.1µF VIN C2 4.7µF EN/UV BST SYNC SW L1 2.2µH R4 100k LT8608 INTVCC C3 1µF PG C5 10pF TR/SS C6 10nF R1 18.2k RT GND FB 8608 TA03 fSW = 2MHz R3 309k R2 1M L1 = XFL4020-222ME VOUT 3.3V 1.5A POWER GOOD C4 22µF X7R 1206 5V Step Down VIN 5.6V TO 42V C1 0.1µF VIN C2 4.7µF EN/UV BST SYNC SW L1 2.2µH R4 100k LT8608 INTVCC C3 1µF PG C5 10pF TR/SS C6 10nF R1 18.2k RT GND FB 8608 TA04 fSW = 2MHz R3 187k R2 1M L1 = XFL4020-222ME VOUT 5V 1.5A POWER GOOD C4 22µF X7R 1206 12V Step Down VIN 12.7V TO 42V C1 0.1µF VIN C2 4.7µF EN/UV BST SYNC SW L1 10µH R4 100k LT8608 INTVCC C3 1µF C5 10pF TR/SS C6 10nF R1 40.2k FSW = 1MHz PG RT GND FB 8608 TA05 R3 69.8k R2 1M L1 = XAL4040-103ME VOUT 12V 1.5A POWER GOOD C4 22µF X7R 1210 Rev C For more information www.analog.com 19 LT8608/LT8608B TYPICAL APPLICATIONS 1.8V 2MHz Step-Down Converter VIN 3.1V TO 20V (42V TRANSIENT) C2 4.7µF PSKIP R1 18.2k L1 2.2µH VOUT 1.8V 1.5A SW INTVCC C6 10nF M1 NFET BST EN/UV SYNC C3 1µF C1 0.1µF VIN LT8608 (MSOP) R4 100k PG C5 10pF TR/SS RT GND FB 8608 TA06 fSW = 2MHz R3 768k R2 1M L1 = XFL4020-222ME POWER GOOD C4 22µF X7R 1206 Ultralow EMI 5V 1.5A Step-Down Converter VIN 5.8V TO 42V L2 BEAD L3 4.7µH C8 4.7µF C7 4.7µF VIN C2 4.7µF BST EN/UV SYNC INTVCC SW LT8608 (MSOP) C1 0.1µF R4 100K PG C5 10pF TR/SS C3 1µF C6 10nF FB RT R1 60.4k GND fSW = 700kHz L1 4.7µH R3 187k R2 1M L1 = XFL4020-472ME 8608 TA07 VOUT 5V 1.5A POWER GOOD C4 22µF X7R 1206 C2, C4, C7, C8 X7R 1206 20 Rev C For more information www.analog.com LT8608/LT8608B PACKAGE DESCRIPTION Please refer to http://www.linear.com/product/LT8608#packaging for the most recent package drawings. MSE Package 10-Lead Plastic MSOP, Exposed Die Pad (Reference LTC DWG # 05-08-1664 Rev I) BOTTOM VIEW OF EXPOSED PAD OPTION 1.88 ±0.102 (.074 ±.004) 5.10 (.201) MIN 1 0.889 ±0.127 (.035 ±.005) 1.68 ±0.102 (.066 ±.004) 0.05 REF 10 0.305 ± 0.038 (.0120 ±.0015) TYP RECOMMENDED SOLDER PAD LAYOUT 3.00 ±0.102 (.118 ±.004) (NOTE 3) DETAIL “B” CORNER TAIL IS PART OF DETAIL “B” THE LEADFRAME FEATURE. FOR REFERENCE ONLY NO MEASUREMENT PURPOSE 10 9 8 7 6 DETAIL “A” 0° – 6° TYP 1 2 3 4 5 GAUGE PLANE 0.53 ±0.152 (.021 ±.006) DETAIL “A” 0.18 (.007) 0.497 ±0.076 (.0196 ±.003) REF 3.00 ±0.102 (.118 ±.004) (NOTE 4) 4.90 ±0.152 (.193 ±.006) 0.254 (.010) 0.29 REF 1.68 (.066) 3.20 – 3.45 (.126 – .136) 0.50 (.0197) BSC 1.88 (.074) SEATING PLANE 0.86 (.034) REF 1.10 (.043) MAX 0.17 – 0.27 (.007 – .011) TYP 0.50 (.0197) BSC NOTE: 1. DIMENSIONS IN MILLIMETER/(INCH) 2. DRAWING NOT TO SCALE 3. DIMENSION DOES NOT INCLUDE MOLD FLASH, PROTRUSIONS OR GATE BURRS. MOLD FLASH, PROTRUSIONS OR GATE BURRS SHALL NOT EXCEED 0.152mm (.006") PER SIDE 4. DIMENSION DOES NOT INCLUDE INTERLEAD FLASH OR PROTRUSIONS. INTERLEAD FLASH OR PROTRUSIONS SHALL NOT EXCEED 0.152mm (.006") PER SIDE 5. LEAD COPLANARITY (BOTTOM OF LEADS AFTER FORMING) SHALL BE 0.102mm (.004") MAX 6. EXPOSED PAD DIMENSION DOES INCLUDE MOLD FLASH. MOLD FLASH ON E-PAD SHALL NOT EXCEED 0.254mm (.010") PER SIDE. 0.1016 ±0.0508 (.004 ±.002) MSOP (MSE) 0213 REV I Rev C For more information www.analog.com 21 LT8608/LT8608B PACKAGE DESCRIPTION Please refer to http://www.linear.com/product/LT8608#packaging for the most recent package drawings. DC8 Package 8-Lead Plastic DFN (2mm × 2mm) (Reference LTC DWG # 05-08-1939 Rev Ø) Exposed Pad Variation AA 1.8 REF 0.90 REF 0.23 REF 0.85 ±0.05 2.60 ±0.05 PACKAGE OUTLINE 0.335 REF 0.25 ±0.05 0.45 BSC RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS APPLY SOLDER MASK TO AREAS THAT ARE NOT SOLDERED 2.00 ±0.05 (4 SIDES) PIN 1 BAR TOP MARK (SEE NOTE 6) 2.00 SQ ±0.05 1.8 REF 5 8 0.23 0.335 REF REF 0.55 ±0.05 PIN 1 NOTCH R = 0.15 (DC8MA) DFN 0113 REV Ø 4 0.200 REF 0.75 ±0.05 1 0.23 ±0.05 0.45 BSC BOTTOM VIEW—EXPOSED PAD 0.00 – 0.05 NOTE: 1. DRAWING IS NOT A JEDEC PACKAGE OUTLINE 2. DRAWING NOT TO SCALE 3. ALL DIMENSIONS ARE IN MILLIMETERS 4. DIMENSIONS OF EXPOSED PAD ON BOTTOM OF PACKAGE DO NOT INCLUDE MOLD FLASH. MOLD FLASH, IF PRESENT, SHALL NOT EXCEED 0.15mm ON ANY SIDE 5. EXPOSED PAD SHALL BE SOLDER PLATED 6. SHADED AREA IS ONLY A REFERENCE FOR PIN 1 LOCATION ON THE TOP AND BOTTOM OF PACKAGE 22 Rev C For more information www.analog.com LT8608/LT8608B REVISION HISTORY REV DATE DESCRIPTION A 12/16 Clarified minimum on time to 35ns Clarified condition on feedback voltage line regulation Clarified transient response graphs B 01/18 Added DFN Package Option Added H-Grade Temperature Options Clarified Graphs for MSOP Package Option Only Clarified Pin Functions for MSOP and DFN Package Options Clarified Operation and Applications Information Section for MSOP and DFN Package Options Added Figure 6 Clarified Typical Applications for MSOP Package Option C 05/18 Added B version Added table to clarify versions Modified text in Description to add DFN functionality Added B version to Order Information Clarified Minimum On-Time Conditions Clarified Efficiency graphs Clarified No-Load Supply Current graphs Clarified Burst Mode Operation vs Output Current graph Clarified Frequency Foldback graph Clarified Pin Functions on SYNC and TR/SS Clarified Operation third and fifth paragraph Clarified last paragraph to include DFN B version and Figure 1 and Figure 3 Clarified Applications Information to include DFN B version PAGE NUMBER 1 2 7 1-3 2 6 8 10-11, 15-17 18 20 All 1 1 2 3 4 4 5 6 8 10 11 16 Rev C Information furnished by Analog Devices is believed to be accurate and reliable. However, no responsibility is assumed by Analog Devices for its use, nor for any infringements of patents or other rights of third parties that may result from its use. Specifications subject to change without notice. No license For is granted implication or otherwise under any patent or patent rights of Analog Devices. moreby information www.analog.com 23 LT8608/LT8608B TYPICAL APPLICATION 3.3V and 1.8V with Ratio Tracking VIN 3.9V TO 42V C2 4.7µF C1 0.1µF VIN R1 18.2k R4 100k LT8608 INTVCC C6 10nF VOUT 3.3V, 1.5A SW SYNC C3 1µF L1 2.2µH BST EN/UV PG C5 10pF TR/SS RT GND FB R3 309k R2 1M POWER GOOD C4 47µF fSW = 2MHz C8 4.7µF R9 31.6k L2 2.2µH BST EN/UV SW SYNC INTVCC C12 1µF C7 0.1µF VIN R8 100k LT8608 PG C11 10pF TR/SS R10 10k R5 18.2k RT GND FB 8608 TA02 R7 768k R6 1M VOUT 1.8V 1.5A POWER GOOD C10 47µF fSW = 2MHz C2, C8 X7R 1206 C4, C10, X7R 1210 RELATED PARTS PART NUMBER DESCRIPTION COMMENTS LT8609/ LT8609A 42V, 2A/3A Peak, 93% Efficiency, 2.2MHz Synchronous MicroPower Step-Down DC/DC Converter with IQ = 2.5µA VIN = 3.2V to 42V, VOUT(MIN) = 0.8V, IQ = 2.5µA, ISD < 1µA, MSOP-10E Package LT8610A/ 8610AB 42V, 3.5A, 96% Efficiency, 2.2MHz Synchronous MicroPower Step-Down VIN = 3.4V to 42V, VOUT(MIN) = 0.97V, IQ = 2.5µA, ISD < 1µA, DC/DC Converter with IQ = 2.5µA MSOP-16E Package LT8610AC 42V, 3.5A, 96% Efficiency, 2.2MHz Synchronous MicroPower Step-Down VIN = 3V to 42V, VOUT(MIN) = 0.8V, IQ = 2.5µA, ISD < 1µA, DC/DC Converter with IQ = 2.5µA MSOP-16E Package LT8610 42V, 2.5A, 96% Efficiency, 2.2MHz Synchronous MicroPower Step-Down VIN = 3.4V to 42V, VOUT(MIN) = 0.97V, IQ = 2.5µA, ISD < 1µA, DC/DC Converter with IQ = 2.5µA MSOP-16E Package LT8611 42V, 2.5A, 96% Efficiency, 2.2MHz Synchronous MicroPower Step-Down VIN = 3.4V to 42V, VOUT(MIN) = 0.97V, IQ = 2.5µA, ISD < 1µA, DC/DC Converter with IQ = 2.5µA and Input/Output Current Limit/Monitor 3mm × 5mm QFN-24 Package LT8616 42V, Dual 2.5A + 1.5A, 95% Efficiency, 2.2MHz Synchronous MicroPower Step-Down DC/DC Converter with IQ = 5µA LT8620 65V, 2.5A, 96% Efficiency, 2.2MHz Synchronous MicroPower Step-Down VIN = 3.4V to 65V, VOUT(MIN) = 0.97V, IQ = 2.5µA, ISD < 1µA, DC/DC Converter with IQ = 2.5µA MSOP-16E, 3mm × 5mm QFN-24 Packages LT8614 42V, 4A, 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, 3mm × 4mm QFN-18 Package LT8612 42V, 6A, 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 = 3.0µA, ISD < 1µA, 3mm × 6mm QFN-28 Package LT8640 42V, 5A/7A Peak, 96% Efficiency, 3MHz Synchronous MicroPower Step- VIN = 3.4V to 42V, VOUT(MIN) = 0.97V, IQ = 2.5µA, ISD < 1µA, Down DC/DC Converter with IQ = 2.5µA 3mm × 4mm QFN-18 Package LT8602 42V, Quad Output (2.5A+1.5A+1.5A+1.5A) 95% Efficiency, 2.2MHz Synchronous MicroPower Step-Down DC/DC Converter with IQ = 25µA 24 VIN = 3.4V to 42V, VOUT(MIN) = 0.8V, IQ = 5µA, ISD < 1µA, TSSOP-28E, 3mm × 6mm QFN-28 Packages VIN = 3V to 42V, VOUT(MIN)= 0.8V, IQ = 25µA, ISD < 1µA, 6mm × 6mm QFN-40 Package Rev C D16877 0-5/18(C) For more information www.analog.com www.analog.com ANALOG DEVICES, INC. 2016-2018