LT8608

LT8608 (Rev. D)

Document overview

• Manufacturer or author: Analog Devices; Inc.
• PDF pages: 24

Technical content

Rev. DFor more information www.analog.com TYPICAL APPLICATION FEATURES DESCRIPTION 42V, 1.5A Synchronous Step-Down Regulator with 2.5µA Quiescent Current 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. 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 pulse- skipping or Burst Mode operation varieties. PACKAGE SYNC FUNCTIONALITY LT8608MSE MSE Programmable LT8608DFN DFN Burst Mode Operation LT8608BDFN DFN Pulse-Skipping Mode

APPLICATIONS

n Wide Input Voltage Range: 3.0V to 42V n Ultralow Quiescent Current Burst Mode® Operation n <2.5µA IQ Regulating 12VIN to 3.3VOUT n Output Ripple <10mVP-P n High Efficiency 2MHz Synchronous Operation n >92% Efficiency at 0.5A, 5VOUT from 12VIN n 1.5A Continuous Output Current n Fast Minimum Switch-On Time: 35ns n Adjustable and Synchronizable: 200kHz to 2.2MHz n Spread Spectrum Frequency Modulation for Low EMI n Allows Use of Small Inductors n Low Dropout n Peak Current Mode Operation n Accurate 1V Enable Pin Threshold n Internal Compensation n Output Soft-Start and T racking n Small 10-Lead MSOP Package or 8-Lead 2mm × 2mm DFN Package n General Purpose Step Down n Low EMI Step Down All registered trademarks and trademarks are the property of their respective owners. 5V, 2MHz Step-Down 12VIN to 5VOUT Efficiency f SW = 2MHz I OUT (A) 0.25 0.50 0.75 1.00 1.25 1.50 100 EFFICIENCY (%) IN OUT

8608 TA01b

EN/UVON OFF 0.1µF 22µF 10pF 4.7µF VIN 5.5V TO 42V 1µF VOUT 1.5A 187k

8608 TA01a

2.2µH SYNC INTVCC TR/SS RT L T8608 GND SW PG FB 18.2k Document Feedback

Rev. D For more information www.analog.com PIN CONFIGURATION ABSOLUTE MAXIMUM RATINGS (Note 1) BST SW INTVCC RT SYNC EN/UV VIN PG TR/SS FB TOP VIEW GND 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 TOP VIEW BST SW INTVCC RT EN/UV VIN PG FB DC PACKAGE 8-LEAD (2mm × 2mm) PLASTIC DFN GND 3 6 ORDER INFORMATION LEAD FREE FINISH TAPE AND REEL PART MARKING* PACKAGE DESCRIPTION TEMPERATURE RANGE LT8608EMSE#PBF LT8608EMSE#TRPBF L TGVZ 10-Lead Plastic MSOP –40°C to 125°C LT8608IMSE#PBF LT8608IMSE#TRPBF L TGVZ 10-Lead Plastic MSOP –40°C to 125°C LT8608HMSE#PBF LT8608HMSE#TRPBF L TGVZ 10-Lead Plastic MSOP –40°C to 150°C LT8608EDC#TRMPBF LT8608EDC#TRPBF LGXB 8-Lead (2mm × 2mm) Plastic DFN –40°C to 125°C LT8608IDC#TRMPBF LT8608IDC#TRPBF LGXB 8-Lead (2mm × 2mm) Plastic DFN –40°C to 125°C LT8608HDC#TRMPBF LT8608HDC#TRPBF LGXB 8-Lead (2mm × 2mm) Plastic DFN –40°C to 150°C LT8608BEDC#TRMPBF LT8608BEDC#TRPBF LGXC 8-Lead (2mm × 2mm) Plastic DFN –40°C to 125°C LT8608BIDC#TRMPBF LT8608BIDC#TRPBF LGXC 8-Lead (2mm × 2mm) Plastic DFN –40°C to 125°C LT8608BHDC#TRMPBF LT8608BHDC#TRPBF LGXC 8-Lead (2mm × 2mm) Plastic DFN –40°C to 150°C Contact the factory for parts specified with wider operating temperature ranges. *The temperature grade is identified by a label on the shipping container. Tape and reel specifications. 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 TYP MAX UNITS Minimum Input Voltage l 2.5 3.0 3.2 V VIN Quiescent Current VEN/UV = 0V, VSYNC = 0V VEN/UV = 2V, Not Switching, VSYNC = 0V or LT8608 DFN, VIN ≤ 36V l 1.7 µA µA VIN Current in Regulation VIN = 6V, VOUT = 2.7V, Output Load = 100µA VIN = 6V, VOUT = 2.7V, Output Load = 1mA l l 500 700 µA µA Operating Junction Temperature Range (Note 2)

Rev. DFor more information www.analog.com ELECTRICAL CHARACTERISTICS The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. 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 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. 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 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 ±20 nA Minimum On-Time ILOAD = 1A, SYNC = 0V or LT8608 DFN ILOAD = 1A, SYNC = 1.9V or LT8608B DFN l l ns ns Minimum Off Time 93 130 ns Oscillator Frequency 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 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 Top Power NMOS On-Resistance ILOAD = 0.5A 350 mΩ Top Power NMOS Current Limit l 2.1 2.9 3.9 A Bottom Power NMOS On-Resistance 230 mΩ SW Leakage Current VIN = 36V l 15 µA EN/UV Pin Threshold EN/UV Rising l 0.99 1.05 1.11 V EN/UV Pin Hysteresis 50 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 Hysteresis 0.5 % PG Leakage VPG = 42V l ±200 nA PG Pull-Down Resistance VPG = 0.1V 550 1200 Ω Sync Low Input Voltage MSOP Only l 0.4 0.9 V Sync High Input Voltage INTVCC = 3.5V, MSOP Only l 2.7 3.2 V TR/SS Source Current MSOP Only l 1 2 3 µA TR/SS Pull-Down Resistance Fault Condition, TR/SS = 0.1V, MSOP Only 300 900 Ω Spread Spectrum Modulation Frequency VSYNC = 3.3V, MSOP Only l 0.5 3 6 kHz

Rev. D For more information www.analog.com TYPICAL PERFORMANCE CHARACTERISTICS No-Load Supply Current vs Temperature Efficiency (5V Output, Burst Mode Operation) Efficiency (3.3V Output, 2MHz, Burst Mode Operation) Efficiency (3.3V Output, 2MHz, Burst Mode Operation) FB Voltage Load Regulation Line Regulation No-Load Supply Current (3.3V Output) Efficiency (5V Output, Burst Mode Operation) I OUT (A) 0.00 0.25 0.50 0.75 1.00 1.25 1.50 100 EFFICIENCY (%) Mode Operation)

8608 G01

L = 2.2µH f SW = 2MHz V IN = 12V V IN = 24V SYNC = 0V OR L T8608 DFN f SW = 2MHz V IN = 12V V IN = 24V L = 2.2µH I OUT (A) 0.00 0.25 0.50 0.75 1.00 1.25 1.50 100 EFFICIENCY (%) Mode Operation)

8608 G03 SYNC = 0V OR L T8608 DFN

f SW = 2MHz V IN = 12V V IN = 24V L = 2.2µH I OUT (mA) 0.001 0.01 0.1 100 10k 100 EFFICIENCY (%) Mode Operation)

8608 G04

SYNC = 0V OR L T8608 DFN TEMPERATURE (°C) –50 –10 110 150 775 776 777 778 779 780 FB REGULATION VOL TAGE (mV) FB Voltage

8608 G05

OUTPUT CURRENT (A) 0.25 0.50 0.75 1.25 1.50 –0.5 –0.4 –0.3 –0.2 –0.1 0.0 0.1 0.2 0.3 0.4 0.5 CHANGE IN V OUT (%) Load Regulation 8608G06 INPUT VOL TAGE (V) –0.20 –0.15 –0.10 –0.05 0.00 0.05 0.10 0.15 0.20 CHANGE IN V OUT (%)

8608 G07

INPUT VOL TAGE (V) 2.00 2.25 2.50 2.75 3.00 3.25 3.50 3.75 4.00 I IN (µA) (3.3V Output)

8608 G08

SYNC = 0V OR L T8608 DFN TEMPERATURE (°C) –50 –10 110 150 1.3 1.5 1.7 1.9 2.1 2.3 2.5 2.7 2.9 3.1 3.3 INPUT CURRENT (µA) (Not Switching)

8608 G09 SYNC = 0V OR L T8608 DFN

V IN = 12V V IN = 24V I OUT (mA) 0.001 0.01 0.1 100 10k 100 EFFICIENCY (%) Mode Operation)

8608 G02

f SW = 2MHz L = 2.2µH SYNC = 0V OR L T8608 DFN

Rev. DFor more information www.analog.com TYPICAL PERFORMANCE CHARACTERISTICS Top FET Current Limit vs Duty Cycle Top FET Current Limit vs Temperature Switch Drop vs Temperature Switch Drop vs Switch Current Minimum On-Time vs Temperature Minimum Off-Time vs Temperature Dropout Voltage vs Load Current SWITCH CURRENT = 1A TOP SW BOT SW TEMPERATURE (°C) –50 –30 –10 110 130 150 200 250 300 350 400 450 500 550 SWITCH DROP (mV) Switch Drop vs Temperature

8608 G12

SWITCH CURRENT (A) 0.25 0.50 0.75 1.25 1.50 1.75 100 200 300 400 500 600 700 800 SWITCH DROP (mV) Switch Drop vs Switch Current

8608 G13

I OUT = 1A TEMPERATURE (°C) –50 –30 –10 110 130 150 MINIMUM ON-TIME (ns) Minimum On-Time Vs Temperature

8608 G14

TEMPERATURE (°C) –50 –30 –10 110 130 150 100 105 110 MINIMUM OFF-TIME (ns) Minimum Off-Time Vs Temperature

8608 G15

L = XFL4020–222MEC LOAD CURRENT (A) 0.25 0.50 0.75 1.25 1.50 1.75 125 250 375 500 625 750 DROPOUT VOL TAGE (mV) Dropout Voltage vs Load Current

8608 G16

VOUT = 3.3V Burst Frequency vs Load Current Switching Frequency vs Temperature R T = 18.2k TEMPERATURE (°C) –50 –10 110 150 1975 1980 1985 1990 1995 2000 2005 2010 2015 2020 2025 SWITHCING FREQUENCY (kHz) Switching Frequency Vs Temperature

8608 G17

LOAD CURRENT (mA) 100 200 300 400 500 250 500 750 1000 1250 1500 1750 2000 2250 2500 SWITHCING FREQUENCY (kHz) Burst Frequency vs Load Current

8608 G18

V IN = 12V L = 2.2µH V OUT = 3.3V SYNC = 0V OR L T8608 DFN DUTY CYCLE (%) 100 2.00 2.25 2.50 2.75 3.00 3.25 TOP FET CURRENT LIMIT (A) Top Fet Current Limit vs Duty Cycle

8608 G10

DUTY CYCLE = 0 TEMPERATURE (°C) –50 –10 110 150 2.5 2.6 2.7 2.8 2.9 3.0 3.1 I SW (A) Top FET Current Limit Vs Temperature

8608 G11

Rev. D For more information www.analog.com TYPICAL PERFORMANCE CHARACTERISTICS Minimum Load to Full Frequency (SYNC Float to 1.9V) (MSOP Package) Frequency Foldback Soft-Start T racking (MSOP Package) Steady State Case Temperature Rise vs Load Current (5VOUT)Soft-Start Current vs Temperature (MSOP Package) Steady State Case Temperature Rise vs Load (3.3VOUT) VIN UVLO Start-Up Dropout Start-Up Dropout INPUT VOL TAGE (V) 100 125 LOAD CURRENT (mA)

8608 G19

L = 2.2µH V OUT = 5V R T = 18.2k FB VOL TAGE (V) 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 RT = 18.2k 250 500 750 1000 1250 1500 1750 2000 2250 2500 FREQUENCY (kHz)

8608 G20

SYNC = 0V OR L T8608 DFN SS VOL TAGE (V) 0.1 0.2 0.4 0.5 0.6 0.7 0.8 1.0 1.1 1.2 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 FB VOL TAGE (V) Soft-Start T racking

8608 G21

TEMPERATURE (°C) –50 –30 –10 110 130 150 1.5 1.6 1.7 1.8 1.9 2.0 2.1 2.2 2.3 2.4 2.5 SOFT-START CURRENT (µA) Soft Start Current Vs Temperature

8608 G22

TEMPERATURE (°C) –50 –30 –10 110 130 150 2.00 2.25 2.50 2.75 3.00 3.25 V IN UVLO (V) V IN UVLO

8608 G23

V IN = 6V V IN = 12V V IN = 36V I OUT (A) 0.00 0.25 0.50 0.75 1.00 1.25 1.50 CASE TEMP RISE (°C) (5V out)

8608 G24

f SW = 2MHz L = 2.2µH f SW = 2MHz L = 2.2µH V IN = 12V V IN = 36V I OUT (A) 0.00 0.25 0.50 0.75 1.00 1.25 1.50 CASE TEMP RISE (°C)

8608 G25

R LOAD = 50Ω INPUT VOL TAGE (V) INPUT VOL TAGE (V) OUTPUT VOL TAGE (V) Start–Up Droupout

8608 G26

V IN V OUT V IN V OUT R LOAD = 5Ω INPUT VOL TAGE (V) INPUT VOL TAGE (V) OUTPUT VOL TAGE (V) Start-Up Droupout 8608 027

Rev. DFor more information www.analog.com TYPICAL PERFORMANCE CHARACTERISTICS Switching Waveforms Switching Waveforms T ransient Response T ransient Response Switching Waveforms 50µs/DIV ILOAD 500mA/DIV VOUT 50mV/DIV

8608 G31

VIN = 12V 0.5A TO 1A COUT = 47/uni03BCF fSW = 2MHz 50µs/DIV ILOAD 500mA/DIV VOUT 50mV/DIV

8608 G32

VIN = 24V 0.5A TO 1A COUT = 47/uni03BCF fSW = 2MHz IL 1A/DIV SW 5V/DIV

8608 G28

12VIN TO 3.3 VOUT AT 1A 2MHz IL 200mA/DIV SW 5V/DIV

8608 G29

36VIN TO 3.3VOUT AT 1A 2MHz IL 1A/DIV SW 2V/DIV

8608 G30

Rev. D For more information www.analog.com 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 pulse- skipping 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. TR/SS (MSOP Only): Output T racking 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 volt- age. 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 volt- age 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 con- nected to the negative terminal of the input capacitor and soldered to the PCB in order to lower the thermal resistance.

Rev. DFor more information www.analog.com BLOCK DIAGRAM ++– SLOPE COMP INTERNAL 0.778V REF OSCILLATOR 200kHz TO 2.2MHz BURST DETECT 3.5V REG CBST COUT VOUT 8608 BD SW L BST SWITCH LOGIC AND ANTI- SHOOT THROUGH ERROR AMP SHDN ±8.5% VC SHDN TSD INTVCC UVLO VIN UVLO SHDN TSD VIN UVLO EN/UV (OPT) (OPT) 1V + – INTVCC GND PG FB RT CSS VOUT RPG CFF TR/SS 2µA RT SYNC VINVIN CIN CVCC

Rev. D For more information www.analog.com 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 volt- age 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 oper- ates 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 oper- ate 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 fold- back 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 capaci- tor followed by sleep periods where the output power is supplied by the output capacitor. While in sleep mode the LT8608 consumes 1.7μA. As the output load decreases, the frequency of single cur- rent 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 convert- ers. 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.

Figure 2. Full Switching Frequency Minimum Load vs VIN in Figure 1. SW Burst Mode Frequency vs Load

8608 F01

8608 F02

tion with pulse-skipping mode, the SYNC pin is tied high. for pulse-skipping and cannot enter Burst Mode operation. voltage, output voltage, and inductor choice. Figure 4. Burst Mode Operation (Zoomed In) Figure 3. Burst Mode Operation

8608 F03

8608 F04

the quiescent current at low loads. should be connected from VOUT to FB. Table 1. SW Frequency vs RT Value are lower efficiency and a smaller input voltage range. control of inductor current to assure safe operation. (set by RT), and tOFF(MIN) is the minimum switch off-time. to achieve higher duty cycle.

Rev. DFor more information www.analog.com APPLICATIONS INFORMATION Inductor Selection and Maximum Output Current The LT8608 is designed to minimize solution size by al- lowing 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) rat- ing of the inductor must be higher than the load current plus 1/2 of in inductor ripple current: IL(PEAK ) = ILOAD(MAX ) + 1 where ∆IL is the inductor ripple current as calculated several paragraphs below and ILOAD(MAX) is the maximum output load for a given application. 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: IOUT(MAX ) = ILIM – ∆IL The peak-to-peak ripple current in the inductor can be calculated as follows: ∆IL = VOUT L • fSW 1– VOUT VIN(MAX ) 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 Bypass the input of the LT8608 circuit with a ceramic capaci- tor 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 sup- ply 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. D For more information www.analog.com APPLICATIONS INFORMATION A ceramic input capacitor combined with trace or cable inductance forms a high quality (under damped) tank cir- cuit. 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). Output Capacitor and Output Ripple 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: COUT = 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. T ransient 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 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. A final precaution regarding ceramic capacitors concerns the maximum input voltage rating of the LT8608. As pre- viously 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 Ap- plication Note 88). 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: VIN(EN) = R3 + 1⎛ ⎠⎟ • 1V 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. DFor more information www.analog.com 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. INTVCC Regulator 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 T racking 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 capaci- tor 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 regulat- ing 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 soft- start 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 APPLICATIONS INFORMATION 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. 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 (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 val- leys 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 subhar- monic oscillations is established by the inductor size, input voltage, and output voltage. Since the synchroniza- tion 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 synchro- nization frequencies. For some applications it is desirable for the LT8608 to operate in pulse-skipping mode, offering two major differ- ences 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.

and cannot enter Burst Mode operation. during short circuit conditions. switching if the inductor current exceeds safe levels.

8608 F05

Figure 5. Reverse VIN Protection

a fault condition if safe junction temperature is exceeded. planes within the circuit board and on the bottom side. placement with trace, ground plane, and via locations. temperature approaches the maximum junction rating. Figure 6. Case Temperature Rise vs Load Current

8608 F06

Figure 7. PCB Layout (Not to Scale)

Rev. DFor more information www.analog.com TYPICAL APPLICATIONS 3.3V Step Down 5V Step Down 12V Step Down 0.1µF 4.7µF 1µF 22µF X7R 1206 10pF 2.2µH 18.2k 309k 100k 10nF VIN EN/UV SYNC L T8608 INTVCC TR/SS RT GND FB PG SW BST VIN 3.9V TO 42V POWER GOOD fSW = 2MHz L1 = XFL4020-222ME VOUT 3.3V 1.5A

8608 TA03

0.1µF 4.7µF 1µF 22µF X7R 1206 10pF 2.2µH 18.2k 187k 100k 10nF VIN EN/UV SYNC L T8608 INTVCC TR/SS RT GND FB PG SW BST VIN 5.6V TO 42V POWER GOOD fSW = 2MHz L1 = XFL4020-222ME VOUT 1.5A

8608 TA04

0.1µF 4.7µF 1µF 22µF X7R 1210 10pF 10µH 40.2k 69.8k 100k 10nF VIN EN/UV SYNC L T8608 INTVCC TR/SS RT GND FB PG SW BST VIN 12.7V TO 42V POWER GOOD FSW = 1MHz L1 = XAL4040-103ME VOUT 12V 1.5A

8608 TA05

Rev. D For more information www.analog.com TYPICAL APPLICATIONS 1.8V 2MHz Step-Down Converter Ultralow EMI 5V 1.5A Step-Down Converter 0.1µF 4.7µF 1µF 22µF X7R 1206 10pF 2.2µH 18.2k 768k 100k 10nF VIN EN/UV SYNC L T8608 (MSOP) INTVCC TR/SS RT GND FB PG SW BST VIN 3.1V TO 20V (42V TRANSIENT) POWER GOOD fSW = 2MHz L1 = XFL4020-222ME VOUT 1.8V 1.5A

8608 TA06

0.1µF 4.7µF 1µF 22µF X7R 1206 10pF 4.7µH 60.4k 187k 100K 10nF BEAD 4.7µH 4.7µF 4.7µF VIN EN/UV SYNC L T8608 (MSOP) INTVCC TR/SS RT GND FB PG SW BST VIN 5.8V TO 42V POWER GOOD fSW = 700kHz L1 = XFL4020-472ME VOUT 1.5A

8608 TA07

C2, C4, C7, C8 X7R 1206

Rev. DFor more information www.analog.com PACKAGE DESCRIPTION MSOP (MSE) 0213 REV I 0.53 ±0.152 (.021 ±.006) SEATING PLANE 0.18 (.007) 1.10 (.043) MAX 0.17 –/uni00A00.27 (.007 – .011) TYP 0.86 (.034) REF 0.50 (.0197) BSC 1 2 3 4 5 4.90 ±0.152 (.193 ±.006) 0.497 ±0.076 (.0196 ±.003) REF8910 7 6 3.00 ±0.102 (.118 ±.004) (NOTE 3) 3.00 ±0.102 (.118 ±.004) (NOTE 4) 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. 0.254 (.010) 0° – 6° TYP DETAIL “A” DETAIL “A” GAUGE PLANE 5.10 (.201) MIN 3.20 – 3.45 (.126 – .136) 0.889 ±0.127 (.035 ±.005) RECOMMENDED SOLDER PAD LAYOUT 1.68 ±0.102 (.066 ±.004) 1.88 ±0.102 (.074 ±.004) 0.50 (.0197) BSC 0.305 ± 0.038 (.0120 ±.0015) TYP BOTTOM VIEW OF EXPOSED PAD OPTION 1.68 (.066) 1.88 (.074) 0.1016 ±0.0508 (.004 ±.002) DETAIL “B” DETAIL “B” CORNER TAIL IS PART OF THE LEADFRAME FEATURE. FOR REFERENCE ONL Y NO MEASUREMENT PURPOSE

0.05 REF

0.29 REF 10-Lead Plastic MSOP, Exposed Die Pad (Reference LTC DWG # 05-08-1664 Rev I)

Rev. D For more information www.analog.com PACKAGE DESCRIPTION 2.00 ±0.05 (4 SIDES) 2.00 SQ ±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 0.55 ±0.05 BOTTOM VIEW—EXPOSED PAD 0.23 REF0.335 REF

0.335 REF

0.75 ±0.05 PIN 1 BAR TOP MARK (SEE NOTE 6)

0.200 REF

0.00 – 0.05 (DC8MA) DFN 0113 REV Ø 0.23 ±0.05

0.45 BSC

0.25 ±0.05 RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS APPLY SOLDER MASK TO AREAS THAT ARE NOT SOLDERED 0.90 REF 0.23 REF 0.85 ±0.05

1.8 REF

2.60 ±0.05 PACKAGE OUTLINE R = 0.15 8-Lead Plastic DFN (2mm × 2mm) (Reference LTC DWG # 05-08-1939 Rev Ø) Exposed Pad Variation AA

Rev. DFor more information www.analog.com 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 is granted by implication or otherwise under any patent or patent rights of Analog Devices.

REVISION HISTORY

REV DATE DESCRIPTION PAGE NUMBER 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 1-3 10-11, 15-17 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 All D 03/19 Corrected TRM Part # in Order Information 2

Rev. D For more information www.analog.com D16877 0-3/19(D) www.analog.com  ANALOG DEVICES, INC. 2016-2019 RELATED PARTS TYPICAL APPLICATION 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, LT8610A/ 8610AB 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, LT8610AC 42V, 3.5A, 96% Efficiency, 2.2MHz Synchronous MicroPower Step-Down DC/DC Converter with IQ = 2.5µA VIN = 3V to 42V, VOUT(MIN) = 0.8V, IQ = 2.5µA, ISD < 1µA, 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, 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 LT8616 42V, Dual 2.5A + 1.5A, 95% Efficiency, 2.2MHz Synchronous MicroPower Step-Down DC/DC Converter with IQ = 5µA VIN = 3.4V to 42V, VOUT(MIN) = 0.8V, IQ = 5µA, ISD < 1µA, TSSOP-28E, 3mm × 6mm QFN-28 Packages LT8620 65V, 2.5A, 96% Efficiency, 2.2MHz Synchronous MicroPower Step-Down DC/DC Converter with IQ = 2.5µA VIN = 3.4V to 65V, VOUT(MIN) = 0.97V, IQ = 2.5µA, ISD < 1µA, 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- 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 Synchronous MicroPower Step-Down DC/DC Converter with IQ = 25µA VIN = 3V to 42V, VOUT(MIN)= 0.8V, IQ = 25µA, ISD < 1µA, 6mm × 6mm QFN-40 Package 3.3V and 1.8V with Ratio T racking 0.1µF 4.7µF 31.6k R10 10k C10 47µF C11 10pF 2.2µH 18.2k 768k 100k C12 1µF VIN EN/UV SYNC L T8608 INTVCC TR/SS RT GND FB PG SW BST POWER GOOD fSW = 2MHz VOUT 1.8V 1.5A

8608 TA02

C2, C8 X7R 1206 C4, C10, X7R 1210 0.1µF 4.7µF 1µF 47µF 10pF 2.2µH 18.2k 309k 100k 10nF VIN EN/UV L T8608 INTVCC TR/SS RT GND FB PG SW BST VIN 3.9V TO 42V POWER GOOD fSW = 2MHz VOUT 3.3V , 1.5A SYNC