LT8610AC/LT8610AC-1 42V, 3.5A Synchronous Step-Down Regulator with 2.5A Quiescent Current Features n n n n n n n n n n Description LT8610 Feature Set, Plus: 3V Minimum Input Voltage 800mV Feedback Voltage 3.5A Maximum Output Current Fast 30ns Minimum Switch-On Time Improved Burst Mode Efficiency Improved EMI Wide Input Voltage Range: 3V to 42V Ultralow Quiescent Current Burst Mode(R) Operation: 2.5A IQ Regulating 12VIN to 3.3VOUT High Efficiency Synchronous Operation: 95% Efficiency at 1A, 5VOUT from 12VIN 93% Efficiency at 1A, 3.3VOUT from 12VIN Low Dropout Under All Conditions: 200mV at 1A Safely Tolerates Inductor Saturation in Overload Adjustable and Synchronizable Frequency: LT8610AC: 200kHz to 2.2MHz LT8610AC-1: 1.5MHz to 2.2MHz Accurate 1.015V Enable Pin Threshold Output Soft-Start and Tracking Small Thermally Enhanced 16-Lead MSOP Package Applications n n Automotive and Industrial Supplies GSM Power Supplies The LT(R)8610AC/LT8610AC-1 is a compact, high efficiency, high speed synchronous monolithic step-down switching regulator that consumes only 2.5A of quiescent current. Compared to the LT8610AB, the LT8610AC/LT8610AC-1 has a lower feedback voltage of 800mV and a lower minimum VIN of 3V. The LT8610AC/LT8610AC-1 has the same additional features of the LT8610AB as compared to the LT8610: higher maximum output current of 3.5A, faster minimum on time of 30ns, and higher light load efficiency. The LT8610AC/LT8610AC-1 has a SYNC pin for synchronization to an external clock. A capacitor on the TR/ SS pin programs the output voltage ramp rate during startup. The PG flag signals when VOUT is within 8% of the programmed output voltage as well as fault conditions. The LT8610AC-1 has internal compensation optimized for high switching frequencies resulting in improved transient response compared to the LT8610AC. The LT8610AC-1 also has a feature which will soft-start the part when exiting dropout or brownout conditions reducing output voltage overshoot. FEEDBACK VOLTAGE OUTPUT MIN ON MIN VIN CURRENT TIME IMPROVED EMI LT8610AC 0.80V 3.0V 3.5A 30ns Yes LT8610AB* 0.97V 3.4V 3.5A 30ns Yes LT8610A* 0.97V 3.4V 3.5A 30ns Yes LT8610* 0.97V 3.4V 2.5A 50ns - *See LT8610A/LT8610AB or LT8610 data sheet. L, LT, LTC, LTM, Burst Mode, Linear Technology and the Linear logo are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners. Typical Application Efficiency at 5VOUT 5V 3.5A Step-Down Converter 4.7F 10nF 1F VIN BST EN/UV LT8610AC PG SW SYNC BIAS TR/SS FB 90 0.1F 4.7H VOUT 5V 47F 3.5A x2 1M 80 EFFICIENCY (%) VIN 5.5V TO 42V 100 60.4k fSW = 700kHz 60 50 10pF INTVCC RT 70 VIN = 12V VIN = 24V VIN = 36V 40 GND 191k 30 0.1 1 10 100 LOAD CURRENT (mA) 8610ac1 TA01a 1000 8610ac1 TA01b 8610acfa For more information www.linear.com/LT8610AC 1 LT8610AC/LT8610AC-1 Absolute Maximum Ratings (Note 1) Pin Configuration VIN, EN/UV, PG...........................................................42V BIAS...........................................................................30V BST Pin Above SW Pin................................................4V FB, TR/SS, RT, INTVCC ................................................4V SYNC Voltage ..............................................................6V Operating Junction Temperature Range (Note 2) LT8610ACE/LT8610ACE-1....................... -40 to 125C LT8610ACI/LT8610ACI-1......................... -40 to 125C LT8610ACH/LT8610ACH-1...................... -40 to 150C Storage Temperature Range.......................-65 to 150C TOP VIEW SYNC TR/SS RT EN/UV VIN VIN NC GND 1 2 3 4 5 6 7 8 16 15 14 13 12 11 10 9 17 GND FB PG BIAS INTVCC BST SW SW SW MSE PACKAGE 16-LEAD PLASTIC MSOP JA = 40C/W, JC(PAD) = 10C/W EXPOSED PAD (PIN 17) IS GND, MUST BE SOLDERED TO PCB Order Information LEAD FREE FINISH TAPE AND REEL PART MARKING* PACKAGE DESCRIPTION TEMPERATURE RANGE LT8610ACEMSE#PBF LT8610ACEMSE#TRPBF 8610AC 16-Lead Plastic MSOP -40C to 125C LT8610ACIMSE#PBF LT8610ACIMSE#TRPBF 8610AC 16-Lead Plastic MSOP -40C to 125C LT8610ACHMSE#PBF LT8610ACHMSE#TRPBF 8610AC 16-Lead Plastic MSOP -40C to 150C LT8610ACEMSE-1#PBF LT8610ACEMSE-1#TRPBF 610AC1 16-Lead Plastic MSOP -40C to 125C LT8610ACIMSE-1#PBF LT8610ACIMSE-1#TRPBF 610AC1 16-Lead Plastic MSOP -40C to 125C LT8610ACHMSE-1#PBF LT8610ACHMSE-1#TRPBF 610AC1 16-Lead Plastic MSOP -40C to 150C Consult LTC Marketing for parts specified with wider operating temperature ranges. *The temperature grade is identified by a label on the shipping container.Consult LTC Marketing for information on non-standard lead based finish parts. For more information on lead free part marking, go to: http://www.linear.com/leadfree/ For more information on tape and reel specifications, go to: http://www.linear.com/tapeandreel/ Electrical Characteristics The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25C. PARAMETER CONDITIONS Minimum Input Voltage VIN Quiescent Current TYP MAX l MIN 2.6 3.0 V l 1.0 1.0 3 8 A A l 1.7 1.7 4 10 A A 0.28 0.5 mA 30 225 60 500 A A 0.800 0.800 0.806 0.812 V V 0.004 0.02 %/V 20 nA VEN/UV = 0V VEN/UV = 2V, Not Switching, VSYNC = 0V VEN/UV = 2V, Not Switching, VSYNC = 2V VIN Current in Regulation VOUT = 0.8V, VIN = 6V, Output Load = 100A VOUT = 0.8V, VIN = 6V, Output Load = 1mA l l Feedback Reference Voltage VIN = 6V, ILOAD = 0.5A VIN = 6V, ILOAD = 0.5A l Feedback Voltage Line Regulation VIN = 4.0V to 42V, ILOAD = 1A l Feedback Pin Input Current VFB = 1V 0.794 0.788 -20 UNITS 8610acfa 2 For more information www.linear.com/LT8610AC LT8610AC/LT8610AC-1 Electrical Characteristics The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25C. PARAMETER CONDITIONS MIN TYP MAX UNITS INTVCC Voltage ILOAD = 0mA, VBIAS = 0V ILOAD = 0mA, VBIAS = 3.3V 3.23 3.25 3.4 3.29 3.57 3.35 V V 2.37 2.47 2.57 V INTVCC Undervoltage Lockout BIAS Pin Current Consumption VBIAS = 3.3V, ILOAD = 1A, 2MHz Minimum On-Time ILOAD = 1A, SYNC = 0V ILOAD = 1A, SYNC = 3.3V 9.2 l l 15 15 Minimum Off-Time Oscillator Frequency RT = 221k, ILOAD = 1A (LT8610AC Only) RT = 60.4k, ILOAD = 1A (LT8610AC Only) RT = 18.2k, ILOAD = 1A (LT8610AC/LT8610AC-1) Top Power NMOS On-Resistance ISW = 1A 45 45 ns ns 95 125 ns 210 700 2.00 240 735 2.15 kHz kHz MHz l l l 180 665 1.85 l 5 6.7 4.3 120 Top Power NMOS Current Limit Bottom Power NMOS On-Resistance VINTVCC = 3.4V, ISW = 1A Bottom Power NMOS Current Limit VINTVCC = 3.4V 3.4 SW Leakage Current VIN = 42V, VSW = 0V, 42V -1.5 EN/UV Pin Threshold EN/UV Rising m 8 65 l 0.955 EN/UV Pin Hysteresis 1.015 VEN/UV = 2V PG Upper Threshold Offset from VFB PG Lower Threshold Offset from VFB -20 A m 5.4 A 1.5 A 1.075 50 EN/UV Pin Current V mV 20 nA VFB Falling l 5 8.0 11 % VFB Rising l -11 -8.0 -5 % 40 nA 680 2000 1.1 2.0 1.4 2.4 V V 2.0 3.2 A PG Hysteresis 0.4 PG Leakage VPG = 3.3V PG Pull-Down Resistance VPG = 0.1V SYNC Threshold SYNC Falling SYNC Rising -40 l 0.8 1.6 TR/SS Source Current TR/SS Pull-Down Resistance mA 30 30 l Fault Condition, TR/SS = 0.1V Note 1: Stresses beyond those listed under Absolute Maximum Ratings may cause permanent damage to the device. Exposure to any Absolute Maximum Rating condition for extended periods may affect device reliability and lifetime. Note 2: The LT8610ACE/LT8610ACE-1 is guaranteed to meet performance specifications from 0C to 125C junction temperature. Specifications over the -40C to 125C operating junction temperature range are assured by design, characterization, and correlation with statistical process controls. The LT8610ACI/LT8610ACI-1 is guaranteed over the full -40C to 125C 1.0 230 % operating junction temperature range. The LT8610ACH/LT8610ACH-1 is guaranteed over the full -40C to 150C operating junction temperature range. High junction temperatures degrade operating lifetimes. Operating lifetime is derated at junction temperatures greater than 125C. Note 3: This IC includes overtemperature protection that is intended to protect the device during overload conditions. Junction temperature will exceed 150C when overtemperature protection is active. Continuous operation above the specified maximum operating junction temperature will reduce lifetime. 8610acfa For more information www.linear.com/LT8610AC 3 LT8610AC/LT8610AC-1 Typical Performance Characteristics Efficiency at 5VOUT 100 95 75 fSW = 700kHz (LT8610AC) L = IHLP-2525EZ-01, 4.7H 70 65 EFFICIENCY (%) EFFICIENCY (%) 80 90 70 fSW = 700kHz (LT8610AC) L = IHLP-2525EZ-01, 4.7H 60 50 40 60 VIN = 12V VIN = 24V VIN = 36V 55 0 0.5 1.5 2 2.5 1 LOAD CURRENT (A) 3 20 0.01 0.1 1 10 100 LOAD CURRENT (mA) 90 fSW = 700kHz (LT8610AC) L = IHLP-2525EZ-01, 4.7H 50 40 1 10 100 LOAD CURRENT (mA) 80 75 EN Pin Thresholds 0.96 0.94 95 65 35 TEMPERATURE (C) 5 0.800 0.796 0.792 125 155 8610ac1 G07 95 65 35 TEMPERATURE (C) -25 5 155 Line Regulation 0.15 VOUT = 3.3V VIN = 12V VOUT = 3.3V ILOAD = 0.5A 0.12 0.09 0.10 0.05 0 -0.05 -0.10 0.06 0.03 0 -0.03 -0.06 -0.15 -0.09 -0.20 -0.12 -0.25 125 8610ac1 G06 CHANGE IN VOUT (%) EN FALLING -25 0.804 0.784 -55 0.15 CHANGE IN VOUT (%) EN THRESHOLD (V) 1.02 0.92 -55 0.808 Load Regulation 0.20 3.5 0.788 VIN = 12V VIN = 24V (LT8610AC) 0.25 3 0.812 8610ac1 G05 1.04 0.98 1.5 2 2.5 1 LOAD CURRENT (A) 8610ac1 G03 60 0.25 0.50 0.75 1.00 1.25 1.50 1.75 2.00 2.25 SWITCHING FREQUENCY (MHz) 1000 EN RISING 0.5 0 Reference Voltage 85 65 8610ac1 G04 1.00 VIN = 12V VIN = 24V VIN = 36V 0.816 70 VIN = 12V VIN = 24V VIN = 36V 0.1 65 50 1000 FB PIN VOLTAGE (V) 80 EFFICIENCY (%) EFFICIENCY (%) VOUT = 3.3V 95 L = IHLP-2525EZ-01, 4.7H 70 fSW = 700kHz (LT8610AC) L = IHLP-2525EZ-01, 4.7H 70 55 100 90 20 0.01 75 Efficiency vs Frequency at 1A Efficiency at 3.3VOUT 30 80 8610ac1 G02 8610ac1 G01 60 85 60 VIN = 12V VIN = 24V VIN = 36V 30 3.5 Efficiency at 3.3VOUT 95 80 85 100 100 90 90 50 Efficiency at 5VOUT EFFICIENCY (%) 100 TA = 25C, unless otherwise noted. 0 0.5 1.5 2 2.5 1 LOAD CURRENT (A) 3 3.5 8610ac1 G08 -0.15 0 5 10 15 20 25 30 35 INPUT VOLTAGE (V) 40 45 8610ac1 G09 8610acfa 4 For more information www.linear.com/LT8610AC LT8610AC/LT8610AC-1 Typical Performance Characteristics 25 VOUT = 3.3V IN REGULATION 4.5 4.0 3.0 2.5 2.0 1.5 7.5 7.0 CURRENT LIMIT (A) 3.5 8.0 VOUT = 3.3V VIN = 12V IN REGULATION 20 INPUT CURRENT (A) INPUT CURRENT (A) Top FET Current Limit vs Duty Cycle No Load Supply Current No Load Supply Current 5.0 TA = 25C, unless otherwise noted. 15 10 1.0 3.0 3.5 0 5 10 15 20 25 30 35 INPUT VOLTAGE (V) 40 45 -25 65 5 95 35 TEMPERATURE (C) 6.75 5.25 5.75 5.50 5.25 4.50 4.25 4.00 3.75 5.00 3.50 3.25 5 35 65 TEMPERATURE (C) 95 125 -55 -25 95 5 35 65 TEMPERATURE (C) 125 41 TOP SW BOT SW 150 100 50 120 39 37 35 33 31 VOUT = 3.3V VOUT = 0.8V 27 0.5 1 1.5 2 SWITCH CURRENT (A) 2.5 3 8610ac1 G16 BOT SW -25 65 5 95 35 TEMPERATURE (C) 25 -55 -25 125 155 Minimum Off-Time 125 29 0 100 8610ac1 G15 MINIMUM OFF-TIME (ns) 350 MINIMUM ON-TIME (ns) ILOAD = 1.5A 43 VSYNC = 5V 200 TOP SW Minimum On-Time 400 250 150 0 -55 155 45 300 SWITCH CURRENT = 1A 8610ac1 G14 8610ac1 G13 Switch Drop 1.0 50 3.00 450 0.8 200 4.75 4.75 -25 0.4 0.6 DUTY CYCLE Switch Drop 250 SWITCH DROP (mV) CURRENT LIMIT (A) 6.00 0.2 0 8610ac1 G12 5.00 30% DC 4.50 -55 155 Bottom FET Current Limit 5.50 6.25 125 8610ac1 G11 7.00 6.50 CURRENT LIMIT (A) 4.5 0 -55 Top FET Current Limit SWITCH DROP (mV) 5.0 4.0 8610ac1 G10 0 6.0 5.5 5 0.5 0 6.5 5 65 95 35 TEMPERATURE (C) 125 155 8610ac1 G17 VOUT = 3.3V ILOAD = 0.5A 115 110 105 100 95 90 85 80 75 -50 -25 95 65 35 TEMPERATURE (C) 5 125 155 8610ac1 G18 8610acfa For more information www.linear.com/LT8610AC 5 LT8610AC/LT8610AC-1 Typical Performance Characteristics 700 730 600 500 400 300 200 800 RT = 60.4k (LT8610AC) SWITCHING FREQUENCY (kHz) 740 720 710 700 690 680 670 100 0 0.5 1.5 2 2.5 1 LOAD CURRENT (A) 3 660 -55 -25 3.5 5 65 95 35 TEMPERATURE (C) Minimum Load to Full Frequency 40 30 20 10 600 200 0 100 200 300 400 500 600 700 800 LOAD CURRENT (mA) 8610ac1 G21 Soft-Start Tracking 0.8 500 400 300 200 0.6 0.4 0.2 100 5 10 20 15 25 30 INPUT VOLTAGE (V) 35 0 40 0 0.1 0.2 0.3 0.4 0.5 0.6 FB VOLTAGE (V) 0 0.8 PG High Thresholds PG THRESHOLD OFFSET FROM VREF (%) VSS = 0.5V 2.2 2.1 2.0 1.9 1.8 1.7 95 65 35 TEMPERATURE (C) 5 125 155 8610ac1 G25 0 0.2 0.8 0.6 0.4 TR/SS VOLTAGE (V) PG Low Thresholds -6.0 9.5 9.0 8.5 8.0 FB RISING 7.5 FB FALLING 7.0 6.5 6.0 -55 1.2 1.0 8610ac1 G24 10.0 2.3 1.6 -50 -25 0.7 8610ac1 G23 Soft-Start Current SS PIN CURRENT (A) 300 1.0 8610ac1 G22 2.4 400 0 155 FB VOLTAGE (V) 50 0 125 VOUT = 3.3V VIN = 12V VSYNC = 0V RT = 60.4k (LT8610AC) 700 SWITCHING FREQUENCY (kHz) LOAD CURRENT (mA) 60 500 Frequency Foldback 800 VOUT = 3.3V fSW = 700kHz PULSE-SKIPPING MODE (LT8610AC) 70 600 8610ac1 G20 8610ac1 G19 80 VIN = 12V VOUT = 3.3V L = 4.7H (LT8610AC) 700 100 PG THRESHOLD OFFSET FROM VREF (%) 0 Burst Frequency Switching Frequency 800 SWITCHING FREQUENCY (kHz) DROPOUT VOLTAGE (mV) Dropout Voltage TA = 25C, unless otherwise noted. -25 95 65 35 TEMPERATURE (C) 5 125 155 8610ac1 G26 -6.5 -7.0 -7.5 FB RISING -8.0 FB FALLING -8.5 -9.0 -9.5 -10.0 -55 -25 5 65 95 35 TEMPERATURE (C) 125 155 8610ac1 G27 8610acfa 6 For more information www.linear.com/LT8610AC LT8610AC/LT8610AC-1 Typical Performance Characteristics TA = 25C, unless otherwise noted. RT Programmed Switching Frequency 250 Minimum Input Voltage 3.0 (LT8610AC) 225 2.9 MINIMUM INPUT VOLTAGE (V) RT PIN RESISTOR (k) 200 175 150 125 100 75 50 25 2.8 2.7 2.6 2.5 2.4 2.3 2.2 2.1 0 0.2 0.6 1.4 1.8 1 SWITCHING FREQUENCY (MHz) 2.0 -55 -25 2.2 8610ac1 G28 Bias Pin Current 155 Bias Pin Current 12 VBIAS = 5V 6.0 VOUT = 5V ILOAD = 1A 5.5 fSW = 700kHz (LT8610AC) VBIAS = 5V VOUT = 5V VIN = 12V ILOAD = 1A (LT8610AC) 10 BIAS PIN CURRENT (mA) BIAS PIN CURRENT (mA) 125 8610ac1 G29 6.5 5.0 4.5 4.0 3.5 8 6 4 2 3.0 2.5 95 65 35 TEMPERATURE (C) 5 5 10 15 20 25 30 35 INPUT VOLTAGE (V) 40 0 45 0 0.5 1 1.5 2 SWITCHING FREQUENCY (MHz) 8610ac1 G30 8610ac1 G31 Burst Mode Operation Switching Waveforms IL 1A/DIV Switching Waveforms IL 1A/DIV VSW 5V/DIV IL 500mA/DIV VSW 5V/DIV VSW 10V/DIV VOUT 20mV/DIV 500ns/DIV 12VIN TO 5VOUT AT 1A 2.5 20s/DIV 8610ac1 G32 8610ac1 G33 VSYNC = 0V COUT = 47F L = 4.7H 500ns/DIV 36VIN TO 5VOUT AT 1A 8610ac1 G34 8610acfa For more information www.linear.com/LT8610AC 7 LT8610AC/LT8610AC-1 Typical Performance Characteristics LT8610AC Transient Response VOUT 200mV/DIV LT8610AC Transient Response VOUT 200mV/DIV IL 2A/DIV IL 2A/DIV 50s/DIV 1.5A TO 3.5A TRANSIENT 12VIN, 3.3VOUT COUT = 47F x 2 8610ac1 G35 50s/DIV 200mA TO 2A TRANSIENT 12VIN, 3.3VOUT COUT = 47F x 2 Start-Up Dropout Performance 8610ac1 G36 Start-Up Dropout Performance VIN 1V/DIV VIN 1V/DIV VOUT 1V/DIV TA = 25C, unless otherwise noted. VIN VOUT 1V/DIV VOUT 100ms/DIV 2.5 LOAD (2A IN REGULATION) 8610ac1 G37 8610ac1 G38 LT8610AC-1 Transient Response VOUT 200mV/DIV VOUT 200mV/DIV IOUT 2A/DIV IOUT 2A/DIV 8610ac1 G39 VOUT 100ms/DIV 20 LOAD (250mA IN REGULATION) LT8610AC-1 Transient Response 50s/DIV 1.5A TO 3.5A TRANSIENT 12VIN, 3.3VOUT COUT = 47F x 2 VIN 50s/DIV 100mA TO 2A TRANSIENT 12VIN, 3.3VOUT COUT = 47F x 2 8610ac1 G40 8610acfa 8 For more information www.linear.com/LT8610AC LT8610AC/LT8610AC-1 Pin Functions SYNC (Pin 1): External Clock Synchronization Input. Ground this pin for low ripple Burst Mode operation at low output loads. Tie to a clock source for synchronization to an external frequency. Apply a DC voltage of 3V or higher or tie to INTVCC for pulse-skipping mode. When in pulseskipping mode, the IQ will increase to several hundred A. Do not float this pin. TR/SS (Pin 2): Output Tracking and Soft-Start Pin. This pin allows user control of output voltage ramp rate during start-up. A TR/SS voltage below 0.8V forces the LT8610AC/ LT8610AC-1 to regulate the FB pin to equal the TR/SS pin voltage. When TR/SS is above 0.8V, the tracking function is disabled and the internal reference resumes control of the error amplifier. An internal 2.2A pull-up current from INTVCC on this pin allows a capacitor to program output voltage slew rate. This pin is pulled to ground with an internal 230 MOSFET during shutdown and fault conditions; use a series resistor if driving from a low impedance output. This pin may be left floating in the LT8610AC if the tracking function is not needed. A minimum of 100pF of external capacitance must be used on the LT8610AC-1. RT (Pin 3): A resistor is tied between RT and ground to set the switching frequency. EN/UV (Pin 4): The LT8610AC/LT8610AC-1 is shut down when this pin is low and active when this pin is high. The hysteretic threshold voltage is 1.015V going up and 0.97V 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 LT8610AC/ LT8610AC-1 will shut down. VIN (Pins 5, 6): The VIN pins supply current to the LT8610AC/LT8610AC-1 internal circuitry and to the internal topside power switch. These pins must be tied together and 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. NC (Pin 7): No Connect. This pin is not connected to internal circuitry. SW (Pins 9, 10, 11): The SW pins are the outputs of the internal power switches. Tie these pins together and connect them to the inductor and boost capacitor. This node should be kept small on the PCB for good performance. BST (Pin 12): This pin is used to provide a drive voltage, higher than the input voltage, to the topside power switch. Place a 0.1F boost capacitor as close as possible to the IC. INTVCC (Pin 13): Internal 3.4V Regulator Bypass Pin. The internal power drivers and control circuits are powered from this voltage. INTVCC maximum output current is 20mA. Do not load the INTVCC pin with external circuitry. INTVCC current will be supplied from BIAS if VBIAS > 3.1V, otherwise current will be drawn from VIN. Voltage on INTVCC will vary between 2.8V and 3.4V when VBIAS is between 3.0V and 3.6V. Decouple this pin to power ground with at least a 1F low ESR ceramic capacitor placed close to the IC. BIAS (Pin 14): The internal regulator will draw current from BIAS instead of VIN when BIAS is tied to a voltage higher than 3.1V. For output voltages of 3.3V and above this pin should be tied to VOUT. If this pin is tied to a supply other than VOUT use a 1F local bypass capacitor on this pin. PG (Pin 15): The PG pin is the open-drain output of an internal comparator. PG remains low until the FB pin is within 8% of the final regulation voltage, and there are no fault conditions. PG is valid when VIN is above 3.0V, regardless of EN/UV pin state. FB (Pin 16): The LT8610AC/LT8610AC-1 regulates the FB pin to 0.800V. Connect the feedback resistor divider tap to this pin. Also, connect a phase lead capacitor between FB and VOUT. Typically, this capacitor is 4.7pF to 10pF. GND (Pin 8, Exposed Pad Pin 17): Ground. These pins are the return path of the internal bottom-side switch and must be tied together. Place the negative terminal of the input capacitor as close to the GND pin and exposed pad as possible. The exposed pad must be soldered to the PCB in order to lower the thermal resistance. 8610acfa For more information www.linear.com/LT8610AC 9 LT8610AC/LT8610AC-1 Block Diagram VIN VIN 5, 6 CIN R3 OPT 4 R4 OPT 15 EN/UV PG 1V + - SHDN 8% R2 R1 16 CSS OPT 2 RT ERROR AMP + + - VOUT C1 - + INTERNAL 0.80V REF 3 1 FB TR/SS SLOPE COMP INTVCC OSCILLATOR BST VC BURST DETECT SHDN TSD INTVCC UVLO VIN UVLO 2.2A BIAS 3.4V REG SWITCH LOGIC AND ANTISHOOT THROUGH 14 13 CVCC 12 CBST M1 L SW 9-11 VOUT COUT M2 GND SHDN TSD VIN UVLO 8 RT SYNC GND 17 8610ac1 BD 8610acfa 10 For more information www.linear.com/LT8610AC LT8610AC/LT8610AC-1 Operation The LT8610AC/LT8610AC-1 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 0.8V reference. When the load current increases it causes a reduction in the feedback voltage relative to the reference leading the error amplifier to raise the VC voltage until the average inductor current matches the new load current. When the top power switch turns off, the synchronous power switch turns on until the next clock cycle begins or inductor current falls to zero. If overload conditions result in more than 4.3A 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 LT8610AC/LT8610AC-1 is shut down and draws 1A from the input. When the EN/UV pin is above 1.015V, the switching regulator will become active. To optimize efficiency at light loads, the LT8610AC/ LT8610AC-1 operates in Burst Mode operation in light load situations. Between bursts, all circuitry associated with controlling the output switch is shut down, reducing the input supply current to 1.7A. In a typical application, 2.5A 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 tied to a logic high to use pulse-skipping mode. If a clock is applied to the SYNC pin the part will synchronize to an external clock frequency and operate in pulse-skipping mode. While in pulse-skipping mode the oscillator operates continuously and positive SW transitions are aligned to the clock. During light loads, switch pulses are skipped to regulate the output and the quiescent current will be several hundred A. To improve efficiency across all loads, supply current to internal circuitry can be sourced from the BIAS pin when biased at 3.3V or above. Else, the internal circuitry will draw current from VIN. The BIAS pin should be connected to VOUT if the LT8610AC/LT8610AC-1 output is programmed at 3.3V or above. Comparators monitoring the FB pin voltage will pull the PG pin low if the output voltage varies more than 8% (typical) from the set point, or if a fault condition is present. The oscillator reduces the LT8610AC/LT8610AC-1's operating frequency when the voltage at the FB pin is low. This frequency foldback helps to control the inductor current when the output voltage is lower than the programmed value which occurs during start-up or overcurrent conditions. When a clock is applied to the SYNC pin or the SYNC pin is held DC high, the frequency foldback is disabled and the switching frequency will slow down only during overcurrent conditions. The LT8610AC-1 differs from the LT8610AC in that the internal compensation is optimized for higher switching frequency operation and a TR/SS pin discharge circuit is added to reduce output voltage overshoot when exiting dropout or brownout conditions. The internal compensation change increases the bandwidth of the control loop, which improves transient response. As a consequence the minimum programmable switching frequency of the LT8610AC-1 is increased to 1.5MHz. The SS discharge circuit regulates the TR/SS pin to the same voltage as the FB pin during dropout or brownout. This feature provides some soft-starting when the part exits dropout or brownout which reduces output voltage overshoot. Both of these changes are intended to reduce output voltage overshoot in 2MHz switching frequency applications. 8610acfa For more information www.linear.com/LT8610AC 11 LT8610AC/LT8610AC-1 Applications Information Achieving Ultralow Quiescent Current To enhance efficiency at light loads, the LT8610AC/ LT8610AC-1 operates in low ripple Burst Mode operation, which keeps the output capacitor charged to the desired output voltage while minimizing the input quiescent current and minimizing output voltage ripple. In Burst Mode operation the LT8610AC/LT8610AC-1 delivers single 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 LT8610AC/LT8610AC-1 consumes 1.7A. As the output load decreases, the frequency of single current pulses decreases (see Figure 1a) and the percentage of time the LT8610AC/LT8610AC-1 is in sleep mode increases, resulting in much higher light load efficiency than for typi- cal converters. By maximizing the time between pulses, the converter quiescent current approaches 2.5A 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. While in Burst Mode operation the current limit of the top switch is approximately 1.3A for the LT8610AC/ LT8610AC-1 resulting in output voltage ripple shown in Figure 2. An increase in output capacitance proportionally decreases the output voltage ripple (Table 1). 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 1a. The output load at which the LT8610AC/LT8610AC-1 reaches the Burst Frequency VIN = 12V VOUT = 3.3V L = 4.7H (LT8610AC) 700 600 500 400 300 200 60 50 40 30 20 10 100 0 VOUT = 3.3V fSW = 700kHz PULSE-SKIPPING MODE (LT8610AC) 70 LOAD CURRENT (mA) SWITCHING FREQUENCY (kHz) 800 Minimum Load to Full Frequency 80 0 0 100 200 300 400 500 600 700 800 LOAD CURRENT (mA) (1a) 5 8610ac1 F01a 10 20 15 25 30 INPUT VOLTAGE (V) (1b) 35 40 8610ac1 F01b Figure 1. SW Frequency vs Load Information in Burst Mode Operation (1a) and Pulse-Skipping Mode (1b) VSW 5V/DIV IL 500mA/DIV VOUT 20mV/DIV VSYNC = 0V COUT = 47F L = 4.7H 20s/DIV 8610ac1 F02 Figure 2. Burst Mode Operation 8610acfa 12 For more information www.linear.com/LT8610AC LT8610AC/LT8610AC-1 Applications Information programmed frequency varies based on input voltage, output voltage, and inductor choice. Inductor value has a very strong effect on Burst Mode efficiency. Larger value inductors allow more charge to be transferred to the output per pulse, which increases both efficiency and output voltage ripple. If higher efficiency is needed in a Burst Mode application, increasing inductor value can be a quick solution. Table 1. Output Voltage Ripple vs Output Capacitance for LT8610AC when VIN = 12V, VOUT = 3.3V, and L = 4.7H OUTPUT CAPACITANCE OUTPUT RIPPLE 47F 40mV 47F x2 20mV 47F x4 10mV For some applications it is desirable for the LT8610AC/ LT8610AC-1 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 A. Second is that full switching frequency is reached at lower output load than in Burst Mode operation (see Figure 1b). To enable pulse-skipping mode, the SYNC pin is tied high either to a logic output or to the INTVCC pin. When a clock is applied to the SYNC pin the LT8610AC/LT8610AC-1 will also operate in pulseskipping mode. FB Resistor Network The output voltage is programmed with a resistor divider between the output and the FB pin. Choose the resistor values according to: V R1= R2 OUT - 1 0.80V (1) Reference designators refer to the Block Diagram. 1% resistors are recommended to maintain output voltage accuracy. If low input quiescent current and good light-load efficiency are desired, use large resistor values for the FB resistor divider. The current flowing in the divider acts as a load current, and will increase the no-load input current to the converter, which is approximately: V V 1 IQ = 1.7A + OUT OUT R1+R2 VIN n (2) where 1.7A is the quiescent current of the LT8610AC/ LT8610AC-1 and the second term is the current in the feedback divider reflected to the input of the buck operating at its light load efficiency n. For a 3.3V application with R1 = 1M and R2 = 316k, the feedback divider draws 2.5A. With VIN = 12V and n = 80%, this adds 0.8A to the 1.7A quiescent current resulting in 2.5A no-load current from the 12V supply. Note that this equation implies that the no-load current is a function of VIN; this is plotted in the Typical Performance Characteristics section. When using large FB resistors, a 4.7pF to 10pF phase-lead capacitor should be connected from VOUT to FB. Setting the Switching Frequency The LT8610AC 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 2A. The LT8610AC-1 can be programmed to switch from 1.5MHz to 2.2MHz. The minimum allowed programmable switching frequency is higher for the LT8610AC-1 compared to the LT8610AC because the LT8610AC-1 has internal compensation which is optimized for higher switching frequencies to improve transient response by increasing control loop bandwidth. A table showing the necessary RT value for a desired switching frequency is shown in Table 2B. The RT resistor required for a desired switching frequency can be calculated using: RT = 46.5 - 5.2 fSW (3) where RT is in k and fSW is the desired switching frequency in MHz. 8610acfa For more information www.linear.com/LT8610AC 13 LT8610AC/LT8610AC-1 Applications Information slower switching frequency is necessary to accommodate a high VIN/VOUT ratio. Table 2A. LT8610AC SW Frequency vs RT Value fSW (MHz) RT (k) 0.2 232 0.3 150 0.4 110 0.5 88.7 0.6 71.5 0.7 60.4 0.8 52.3 1.0 41.2 1.2 33.2 1.4 28.0 1.6 23.7 1.8 20.5 2.0 18.2 2.2 15.8 For transient operation, VIN may go as high as the absolute maximum rating of 42V regardless of the RT value, however the LT8610AC/LT8610AC-1 will reduce switching frequency as necessary to maintain control of inductor current to assure safe operation. The LT8610AC/LT8610AC-1 is capable of a maximum duty cycle of greater than 99%, and the VIN-to-VOUT dropout is limited by the RDS(ON) of the top switch. In this mode the LT8610AC/LT8610AC-1 skips switch cycles, resulting in a lower switching frequency than programmed by RT. For applications that cannot allow deviation from the programmed switching frequency at low VIN/VOUT ratios use the following formula to set switching frequency: VIN(MIN) = Table 2B. LT8610AC-1 SW Frequency vs RT Value fSW (MHz) RT (k) 1.4 28.0 1.6 23.7 1.8 20.5 2.0 18.2 2.2 15.8 Operating Frequency Selection and Trade-Offs Selection of the operating frequency is a trade-off between efficiency, component size, and input voltage range. The advantage of high frequency operation is that smaller inductor and capacitor values may be used. The disadvantages are lower efficiency and a smaller input voltage range. The highest switching frequency (fSW(MAX)) for a given application can be calculated as follows: fSW(MAX) = ( VOUT + VSW(BOT) tON(MIN) VIN - VSW(TOP) + VSW(BOT) ) (4) where VIN is the typical input voltage, VOUT is the output voltage, VSW(TOP) and VSW(BOT) are the internal switch drops (~0.42V, ~0.21V, respectively at maximum load) and tON(MIN) is the minimum top switch on-time (see the Electrical Characteristics). This equation shows that a VOUT + VSW(BOT) 1- fSW * tOFF(MIN) - VSW(BOT) + VSW(TOP) (5) where VIN(MIN) is the minimum input voltage without skipped cycles, VOUT is the output voltage, VSW(TOP) and VSW(BOT) are the internal switch drops (~0.42V, ~0.21V, respectively at maximum load), fSW is the switching frequency (set by RT), and tOFF(MIN) is the minimum switch off-time. Note that higher switching frequency will increase the minimum input voltage below which cycles will be dropped to achieve higher duty cycle. Inductor Selection and Maximum Output Current The LT8610AC/LT8610AC-1 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 LT8610AC/ LT8610AC-1 safely tolerates operation with a saturated inductor through the use of a high speed peak-current mode architecture. A good first choice for the inductor value is: L= VOUT + VSW(BOT) fSW (6) where fSW is the switching frequency in MHz, VOUT is the output voltage, VSW(BOT) is the bottom switch drop (~0.21V) and L is the inductor value in H. 8610acfa 14 For more information www.linear.com/LT8610AC LT8610AC/LT8610AC-1 Applications Information To avoid overheating and poor efficiency, an inductor must be chosen with an RMS current rating that is greater than the maximum expected output load of the application. In addition, the saturation current (typically labeled ISAT) rating of the inductor must be higher than the load current plus 1/2 of in inductor ripple current: 1 IL(PEAK) =ILOAD(MAX) + IL 2 (7) where IL is the inductor ripple current as calculated in Equation 9 and ILOAD(MAX) is the maximum output load for a given application. As a quick example, an application requiring 1A output should use an inductor with an RMS rating of greater than 1A and an ISAT of greater than 1.3A. During long duration overload or short-circuit conditions, the inductor RMS rating requirement is greater to avoid overheating of the inductor. To keep the efficiency high, the series resistance (DCR) should be less than 0.04, and the core material should be intended for high frequency applications. The LT8610AC/LT8610AC-1 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 6A at low duty cycles and decreases linearly to 5A at DC = 0.8. The inductor value must then be sufficient to supply the desired maximum output current (IOUT(MAX)), which is a function of the switch current limit (ILIM) and the ripple current. IOUT(MAX) =ILIM - IL 2 (8) The peak-to-peak ripple current in the inductor can be calculated as follows: IL = VOUT L * fSW V * 1- OUT VIN(MAX) (9) where fSW is the switching frequency of the LT8610AC/ LT8610AC-1, and L is the value of the inductor. Therefore, the maximum output current that the LT8610AC/ LT8610AC-1 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. The optimum inductor for a given application may differ from the one indicated by this design guide. A larger value inductor provides a higher maximum load current and reduces the output voltage ripple. For applications requiring smaller load currents, the value of the inductor may be lower and the LT8610AC/LT8610AC-1 may operate with higher ripple current. This allows use of a physically smaller inductor, or one with a lower DCR resulting in higher efficiency. Be aware that low inductance may result in discontinuous mode operation, which further reduces maximum load current. Inductor value has a very strong effect on Burst Mode efficiency. Larger value inductors allow more charge to be transferred to the output per pulse, which increases both efficiency and output voltage ripple. If higher efficiency is needed in a Burst Mode application, increasing inductor value can be a quick solution. For more information about maximum output current and discontinuous operation, see Linear Technology's Application Note 44. Finally, for duty cycles greater than 50% (VOUT/VIN > 0.5), a minimum inductance is required to avoid sub-harmonic oscillation. See Application Note 19. Input Capacitor Bypass the input of the LT8610AC/LT8610AC-1 circuit with a ceramic capacitor of X7R or X5R type placed as close as possible to the VIN and PGND pins. Y5V types have poor performance over temperature and applied voltage, and should not be used. A 4.7F to 10F ceramic capacitor is adequate to bypass the LT8610AC/LT8610AC-1 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. 8610acfa For more information www.linear.com/LT8610AC 15 LT8610AC/LT8610AC-1 Applications Information 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 LT8610AC/LT8610AC-1 and to force this very high frequency switching current into a tight local loop, minimizing EMI. A 4.7F capacitor is capable of this task, but only if it is placed close to the LT8610AC/LT8610AC-1 (see the PCB Layout section). A second precaution regarding the ceramic input capacitor concerns the maximum input voltage rating of the LT8610AC/LT8610AC-1. A ceramic input capacitor combined with trace or cable inductance forms a high quality (under damped) tank circuit. If the LT8610AC/LT8610AC-1 circuit is plugged into a live supply, the input voltage can ring to twice its nominal value, possibly exceeding the LT8610AC/LT8610AC-1's voltage rating. This situation is easily avoided (see Linear Technology 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 LT8610AC/LT8610AC-1 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 LT8610AC/LT8610AC-1's control loop. Ceramic capacitors have very low equivalent series resistance (ESR) and provide the best ripple performance. For good starting values, see the Typical Applications section. Use X5R or X7R types. This choice will provide low output ripple and good transient response. Transient performance can be improved with a higher value output capacitor and the addition of a feedforward capacitor placed between VOUT and FB. Increasing the output capacitance will also decrease the output voltage ripple. A lower value of output capacitor can be used to save space and cost but transient performance will suffer and may cause loop instability. See the Typical Applications in this data sheet for suggested capacitor values. When choosing a capacitor, special attention should be given to the data sheet to calculate the effective capacitance under the relevant operating conditions of voltage bias and temperature. A physically larger capacitor or one with a higher voltage rating may be required. Ceramic Capacitors Ceramic capacitors are small, robust and have very low ESR. However, ceramic capacitors can cause problems when used with the LT8610AC/LT8610AC-1 due to their piezoelectric nature. When in Burst Mode operation, the LT8610AC/LT8610AC-1's switching frequency depends on the load current, and at very light loads the LT8610AC/ LT8610AC-1 can excite the ceramic capacitor at audio frequencies, generating audible noise. Since the LT8610AC/ LT8610AC-1 operates at a lower current limit during Burst Mode operation, the noise is typically very quiet to a casual ear. If this is unacceptable, use a high performance tantalum or electrolytic capacitor at the output. Low noise ceramic capacitors are also available. A final precaution regarding ceramic capacitors concerns the maximum input voltage rating of the LT8610AC/ LT8610AC-1. As previously mentioned, a ceramic input capacitor combined with trace or cable inductance forms a high quality (underdamped) tank circuit. If the LT8610AC/ LT8610AC-1 circuit is plugged into a live supply, the input voltage can ring to twice its nominal value, possibly exceeding the LT8610AC/LT8610AC-1's rating. This situation is easily avoided (see Linear Technology Application Note 88). Enable Pin The LT8610AC/LT8610AC-1 is in shutdown when the EN pin is low and active when the pin is high. The rising threshold of the EN comparator is 1.015V, with 45mV 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 LT8610AC/LT8610AC-1 to regulate the output only when VIN is above a desired voltage (see the Block Diagram). Typically, this threshold, VIN(EN), is used in situations where the input supply is current limited, or has a relatively high source resistance. A switching regulator draws constant power from the source, so source current increases as source voltage drops. This looks like a negative resistance load to the source and can cause the source to current limit or latch low under low source voltage conditions. The VIN(EN) threshold prevents the regulator from operating at source voltages where the problems might occur. This 8610acfa 16 For more information www.linear.com/LT8610AC LT8610AC/LT8610AC-1 Applications Information threshold can be adjusted by setting the values R3 and R4 such that they satisfy the following equation: R3 VIN(EN) = +1 *1.015V R4 (10) where the LT8610AC/LT8610AC-1 will remain off until VIN is above VIN(EN). Due to the comparator's hysteresis, switching will not stop until the input falls slightly below VIN(EN). When operating in Burst Mode operation for light load currents, the current through the VIN(EN) resistor network can easily be greater than the supply current consumed by the LT8610AC/LT8610AC-1. Therefore, the VIN(EN) resistors should be large to minimize their effect on efficiency at low loads. INTVCC Regulator An internal low dropout (LDO) regulator produces the 3.4V supply from VIN that powers the drivers and the internal bias circuitry. The INTVCC can supply enough current for the LT8610AC/LT8610AC-1's circuitry and must be bypassed to ground with a minimum of 1F ceramic capacitor. Good bypassing is necessary to supply the high transient currents required by the power MOSFET gate drivers. To improve efficiency the internal LDO can also draw current from the BIAS pin when the BIAS pin is at 3.1V or higher. Typically the BIAS pin can be tied to the output of the LT8610AC/LT8610AC-1, or can be tied to an external supply of 3.3V or above. If BIAS is connected to a supply other than VOUT, be sure to bypass with a local ceramic capacitor. If the BIAS pin is below 3.0V, the internal LDO will consume current from VIN. Applications with high input voltage and high switching frequency where the internal LDO pulls current from VIN will increase die temperature because of the higher power dissipation across the LDO. Do not connect an external load to the INTVCC pin. Output Voltage Tracking and Soft-Start The LT8610AC/LT8610AC-1 allows the user to program its output voltage ramp rate by means of the TR/SS pin. An internal 2.2A 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 0.8V, the TR/SS voltage will override the internal 0.8V reference input to the error amplifier, thus regulating the FB pin voltage to that of TR/SS pin. When TR/SS is above 0.8V, tracking is disabled and the feedback voltage will regulate to the internal reference voltage. The LT8610AC-1 has an additional TR/SS pin feature not included in the LT8610AC. During dropout or brownout, when the FB pin is below the regulated value, the LT8610AC-1 regulates the TR/SS pin to the same voltage as the FB pin. When the dropout or brownout condition goes away, the LT8610AC-1 output voltage will slowly ramp-up with the soft-start voltage preventing output voltage overshoot. Scope shots of the LT8610AC-1 recovering from dropout and short-circuit are show in Fig. 3. In the LT8610AC, the TR/SS pin may be left floating if the function is not needed. In the LT8610AC-1, the TR/SS pin must have at least 100pF of external capacitance. VIN 5V/DIV VOUT 1V/DIV VSS 1V/DIV 20ms/DIV 12VIN 3.3VIN 12VIN WITH 3.3VOUT AT 1A 8610ac1 F03a (3a) VSS 1V/DIV VOUT 1V/DIV IOUT 2A/DIV 10ms/DIV 5.5A BROWNOUT WITH 12VIN, 3.3VOUT AT 1A LOAD 8610ac1 F03b (3b) Figure 3. LT8610AC-1 Soft Starting to Eliminate Output Voltage Overshoot When Exiting Dropout (3a) and Brownout (3b) 8610acfa For more information www.linear.com/LT8610AC 17 LT8610AC/LT8610AC-1 Applications Information An active pull-down circuit is connected to the TR/SS pin which will discharge the external soft-start capacitor in the case of fault conditions and restart the ramp when the faults are cleared. Fault conditions that clear the soft-start capacitor are the EN/UV pin transitioning low, VIN voltage falling too low, or thermal shutdown. Output Power Good When the LT8610AC/LT8610AC-1's output voltage is within the 8% window of the regulation point, which is a VFB voltage in the range of 0.736V to 0.864V (typical), the output voltage is considered good and the open-drain PG pin goes high impedance and is typically pulled high with an external resistor. Otherwise, the internal pull-down device will pull the PG pin low. To prevent glitching both the upper and lower thresholds include 0.4% of hysteresis. The PG pin is also actively pulled low during several fault conditions: EN/UV pin is below 1.015V, INTVCC has fallen too low, VIN is too low, or thermal shutdown. Synchronization To select low ripple Burst Mode operation, tie the SYNC pin below 0.4V (this can be ground or a logic low output). To synchronize the LT8610AC/LT8610AC-1 oscillator to an external frequency connect a square wave (with 20% to 80% duty cycle) to the SYNC pin. The square wave amplitude should have valleys that are below 0.4V and peaks above 2.4V (up to 6V). The LT8610AC/LT8610AC-1 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 LT8610AC may be synchronized over a 200kHz to 2.2MHz range, while the LT8610AC-1 can only be synchronized over a 1.5MHz to 2.2MHz range. The RT resistor should be chosen to set the LT8610AC/ LT8610AC-1 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 LT8610AC/ LT8610AC-1 to operate in pulse-skipping mode, offering two major differences from Burst Mode operation. First is the clock stays awake at all times and all switching cycles are aligned to the clock. Second is that full switching frequency is reached at lower output load than in Burst Mode operation. These two differences come at the expense of increased quiescent current. To enable pulse-skipping mode, the SYNC pin is tied high either to a logic output or to the INTVCC pin. The LT8610AC/LT8610AC-1 does not operate in forced continuous mode regardless of SYNC signal. Never leave the SYNC pin floating. Shorted and Reversed Input Protection The LT8610AC/LT8610AC-1 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. The LT8610AC-1 has a feature where the TR/SS pin voltage will be regulated to the same voltage as the FB pin. This means the part will soft-start out of an output short-circuit or brownout condition preventing output voltage overshoot. Frequency foldback behavior depends on the state of the SYNC pin: If the SYNC pin is low the switching frequency will slow while the output voltage is lower than the programmed level. If the SYNC pin is connected to a clock source or tied high, the LT8610AC/LT8610AC-1 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 LT8610AC/ LT8610AC-1 is absent. This may occur in battery charg8610acfa 18 For more information www.linear.com/LT8610AC LT8610AC/LT8610AC-1 Applications Information ing applications or in battery-backup systems where a battery or some other supply is diode ORed with the LT8610AC/LT8610AC-1's output. If the VIN pin is allowed to float and the EN pin is held high (either by a logic signal or because it is tied to VIN), then the LT8610AC/ LT8610AC-1's internal circuitry will pull its quiescent current through its SW pin. This is acceptable if the system can tolerate several A in this state. If the EN pin is grounded the SW pin current will drop to near 1A. However, if the VIN pin is grounded while the output is held high, regardless of EN, parasitic body diodes inside the LT8610AC/LT8610AC-1 can pull current from the output through the SW pin and the VIN pin. Figure 4 shows a connection of the VIN and EN/UV pins that will allow the LT8610AC/LT8610AC-1 to run only when the input voltage is present and that protects against a shorted or reversed input. GND 1 16 TR/SS 2 15 RT 3 14 BIAS 4 13 INTVCC 5 12 6 11 7 10 8 9 SYNC EN/UV VIN VOUT FB PG BST SW GND D1 VIN VIN LT8610AC EN/UV GND 8610ac1 F04 VOUT Figure 4. Reverse VIN Protection PCB Layout VOUT LINE TO BIAS For proper operation and minimum EMI, care must be taken during printed circuit board layout. Figure 5 shows the recommended component placement with trace, ground plane and via locations. Note that large, switched currents flow in the LT8610AC/LT8610AC-1'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 VIAS TO GROUND PLANE 8610ac1 F05 OUTLINE OF LOCAL GROUND PLANE Figure 5. Recommended PCB Layout for the LT8610AC 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 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 LT8610AC/LT8610AC-1 to additional ground planes within the circuit board and on the bottom side. Unlike the LT8610, the LT8610AC/LT8610AC-1 has pin 7 as an NC (no connect) pin. This pin can be soldered to GND to have an LT8610 compatible PCB layout. Alternatively, pin 7 can be left unconnected to help meet PCB clearance and creepage requirements between the VIN and GND traces. 8610acfa For more information www.linear.com/LT8610AC 19 LT8610AC/LT8610AC-1 Applications Information For higher ambient temperatures, care should be taken in the layout of the PCB to ensure good heat sinking of the LT8610AC/LT8610AC-1. 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 LT8610AC/LT8610AC-1. 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 LT8610AC/LT8610AC-1 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 LT8610AC/ LT8610AC-1 power dissipation by the thermal resistance from junction to ambient. The LT8610AC/LT8610AC-1 will stop switching and indicate a fault condition if safe junction temperature is exceeded. 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 an example of how case temperature can be managed by reducing VIN, switching frequency, or load. 140 CASE TEMPERATURE RISE (C) High Temperature Considerations TA = 25C 120 fSW = 2MHz ILOAD = 3.5A 100 80 60 40 20 0 fSW = 1MHz ILOAD = 3.5A (LT8610AC) fSW = 2MHz ILOAD = 2.5A 8 20 16 24 28 INPUT VOLTAGE (V) 12 32 36 8610ac1 F06 Figure 6. LT8610AC Case Temperature Rise Temperature rise of the LT8610AC/LT8610AC-1 is worst when operating at high load, high VIN, and high switching Typical Applications 5V 2MHz Step-Down Converter VIN 5.5V TO 42V 4.7F VIN BST EN/UV LT8610AC-1 SW SYNC 12V Step-Down Converter VIN 12.5V TO 42V 0.1F 2.2H 47F* 1210 X5R BIAS 10nF 100k 1F TR/SS INTVCC RT 18.2k fSW = 2MHz L: COILCRAFT XAL 5030 PG FB VOUT 5V 3.5A 4.7F BST EN/UV LT8610AC SYNC TR/SS INTVCC RT 10pF 41.2k 191k 8610ac1 TA02 fSW = 1MHz PG FB 1M VOUT 12V 3.5A 47F* 1210 X5R BIAS 100k 1F GND 0.1F 10H SW 10nF POWER GOOD 1M VIN POWER GOOD 10pF GND 71.5k 8610ac1 TA09 L: VISHAY IHLP-2525CZ-01 *Consider doubling output capacitance if application requires low output voltage ripple in Burst Mode operation. 8610acfa 20 For more information www.linear.com/LT8610AC LT8610AC/LT8610AC-1 Typical Applications 5V Step-Down Converter VIN 5.5V TO 42V VIN 4.7F BST EN/UV LT8610AC 0.1F 10H SW SYNC BIAS TR/SS PG 10nF 100k 1F INTVCC RT FB 1M VOUT 5V 3.5A 100F* 1210 X5R POWER GOOD 10pF GND 110k 191k fSW = 400kHz 8610ac1 TA03 L: VISHAY IHLP-2525CZ-01 1.8V 2MHz Step-Down Converter VIN 3.0V TO 15V (42V TRANSIENT) VIN 4.7F BST EN/UV PG LT8610AC SYNC 0.1F 1H SW 100F* 1210 X5R BIAS 10nF TR/SS 1F INTVCC RT 18.2k fSW = 2MHz FB VOUT 1.8V 3.5A 1M 4.7pF GND 806k 8610ac1 TA06 L: VISHAY IHLP-2020BZ-01 *Consider doubling output capacitance if application requires low output voltage ripple in Burst Mode operation. 8610acfa For more information www.linear.com/LT8610AC 21 LT8610AC/LT8610AC-1 Typical Applications 3.3V 2MHz Step-Down Converter VIN 3.8V TO 27V (42V TRANSIENT) VIN 4.7F BST EN/UV PG LT8610AC-1 SW 0.1F 2.2H 47F* 1210 X5R BIAS SYNC 10nF TR/SS 1F 1M FB INTVCC RT VOUT 3.3V 3.5A 4.7pF GND 18.2k 316k fSW = 2MHz 8610ac1 TA04 L: COILCRAFT XAL 5030 1.8V Step-Down Converter VIN 3.0V TO 42V 4.7F VIN BST EN/UV PG LT8610AC SYNC 0.1F 4.7H SW 47F* x3 1210 X5R BIAS 10nF 1F TR/SS INTVCC RT 110k fSW = 400kHz FB VOUT 1.8V 3.5A 1M 4.7pF GND 806k 8610ac1 TA07 L: VISHAY IHLP-2020BZ-01 *Consider doubling output capacitance if application requires low output voltage ripple in Burst Mode operation. 8610acfa 22 For more information www.linear.com/LT8610AC LT8610AC/LT8610AC-1 Typical Applications 3.3V Step-Down Converter VIN 3.8V TO 42V 4.7F VIN BST 0.1F 8.2H EN/UV LT8610AC PG SW 100F* 1210 X5R BIAS SYNC 10nF 1F TR/SS INTVCC RT VOUT 3.3V 3.5A 1M FB 4.7pF GND 110k 316k fSW = 400kHz 8610ac1 TA05 L: VISHAY IHLP-2525BD-01 Ultralow EMI 5V 2.5A Step-Down Converter VIN 5.5V TO 42V FB1 BEAD 4.7F 4.7H 4.7F 4.7F VIN EN/UV PG 10nF 1F BST LT8610AC 0.1F 4.7H SW SYNC BIAS TR/SS FB 1M 47F* 1210 X5R VOUT 5V 3.5A 10pF INTVCC RT GND 52.3k fSW = 800kHz 191k 8610ac1 TA11 FB1: TDK MPZ2012S101A L: VISHAY IHLP-2020BZ-01 *Consider doubling output capacitance if application requires low output voltage ripple in Burst Mode operation. 8610acfa For more information www.linear.com/LT8610AC 23 LT8610AC/LT8610AC-1 Package Description Please refer to http://www.linear.com/designtools/packaging/ for the most recent package drawings. MSE Package 16-Lead Plastic MSOP, Exposed Die Pad (Reference LTC DWG # 05-08-1667 Rev F) BOTTOM VIEW OF EXPOSED PAD OPTION 2.845 0.102 (.112 .004) 5.10 (.201) MIN 2.845 0.102 (.112 .004) 0.889 0.127 (.035 .005) 8 1 1.651 0.102 (.065 .004) 1.651 0.102 3.20 - 3.45 (.065 .004) (.126 - .136) 0.305 0.038 (.0120 .0015) TYP 16 0.50 (.0197) BSC 4.039 0.102 (.159 .004) (NOTE 3) RECOMMENDED SOLDER PAD LAYOUT 0.254 (.010) 0.35 REF 0.12 REF DETAIL "B" CORNER TAIL IS PART OF DETAIL "B" THE LEADFRAME FEATURE. FOR REFERENCE ONLY 9 NO MEASUREMENT PURPOSE 0.280 0.076 (.011 .003) REF 16151413121110 9 DETAIL "A" 0 - 6 TYP 3.00 0.102 (.118 .004) (NOTE 4) 4.90 0.152 (.193 .006) GAUGE PLANE 0.53 0.152 (.021 .006) DETAIL "A" 1.10 (.043) MAX 0.18 (.007) SEATING PLANE 0.17 - 0.27 (.007 - .011) TYP 1234567 8 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. 0.86 (.034) REF 0.1016 0.0508 (.004 .002) MSOP (MSE16) 0213 REV F 8610acfa 24 For more information www.linear.com/LT8610AC LT8610AC/LT8610AC-1 Revision History REV DATE DESCRIPTION A 05/15 Added the LT8610AC-1 version PAGE NUMBER all Added the LT8610AC-1 version and Description 1 Added the LT8610AC-1 version to Order Information 2 Added the LT8610AC-1 version to Electrical Characteristics and Note 2 3 Added the LT8610AC-1 version to Pin Functions 7 Added the LT8610AC-1 version to Operation section 11 Added the LT8610AC-1 version to Applications section Added the LT8610AC-1 Typical Application 12 to 22 21, 22, 26 8610acfa Information furnished by Linear Technology Corporation is believed to be accurate and reliable. However, no responsibility is assumed for its use. Linear Technology Corporation makes no representaFor more www.linear.com/LT8610AC tion that the interconnection of its information circuits as described herein will not infringe on existing patent rights. 25 LT8610AC/LT8610AC-1 Typical Application 3.3V and 1.8V with Ratio Tracking VIN 3.8V TO 42V 4.7F VIN BST EN/UV PG LT8610AC VIN 3.8V TO 27V 0.1F 5.6H SW 47F* 1210 X5R SYNC 10nF 1F BIAS TR/SS INTVCC RT FB Ultralow IQ 2.5V, 3.3V Step-Down with LDO VIN 4.7F VOUT1 3.3V 3.5A PG LT8610AC-1 SW 255k TR/SS 1F INTVCC RT GND FB GND fSW = 2MHz L: VISHAY IHLP-2020BZ-01 VIN BST EN/UV PG LT8610AC 0.1F 3.3H BIAS TR/SS 10k 1F INTVCC RT FB IN 316k OUT LT3008-2.5 VOUT2 2.5V 2.2F 20mA SHDN SENSE 8610ac1 TA10 VOUT2 1.8V 100F* 3.5A 1210 X5R SW SYNC 30.1k 1M 4.7pF 18.2k 80.6k VOUT1 3.3V 3.5A 47F* 1210 X5R BIAS SYNC fSW = 500kHz 4.7F 0.1F 2.2H 10nF 4.7pF 88.7k BST EN/UV 100k 4.7pF GND 88.7k fSW = 500kHz 80.6k 8610ac1 TA08 L: VISHAY IHLP-2020CZ-01, 5.6H L: VISHAY IHLP-2020CZ-01, 3.3H *Consider doubling output capacitance if application requires low output voltage ripple in Burst Mode operation. Related Parts PART NUMBER DESCRIPTION COMMENTS LT8610A/ LT8610AB 42V, 3.5A, 96% Efficiency, 2.2MHz Synchronous Micropower Step-Down DC/DC Converter with IQ = 2.5A VIN: 3.4V to 42V, VOUT(MIN) = 0.97V, IQ = 2.5A, ISD < 1A, MSOP-16E Package LT8610 42V, 2.5A, 96% Efficiency, 2.2MHz Synchronous Micropower Step-Down DC/DC Converter with IQ = 2.5A VIN: 3.4V to 42V, VOUT(MIN) = 0.97V, IQ = 2.5A, ISD < 1A, MSOP-16E Package LT8611 42V, 2.5A, 96% Efficiency, 2.2MHz Synchronous Micropower Step-Down DC/DC Converter with IQ = 2.5A and Input/Output Current Limit/Monitor VIN: 3.4V to 42V, VOUT(MIN) = 0.97V, IQ = 2.5A, ISD < 1A, 3mm x 5mm QFN-24 Package LT8620 65V, 2.5A, 96% Efficiency, 2.2MHz Synchronous Micropower Step-Down DC/DC Converter with IQ = 2.5A VIN: 3.4V to 65V, VOUT(MIN) = 0.97V, IQ = 2.5A, ISD < 1A, MSOP-16E and 3mm x 5mm QFN-24 Packages LT8614 42V, 4A, 96% Efficiency, 2.2MHz Synchronous Micropower Step-Down DC/DC Converter with IQ = 2.5A VIN: 3.4V to 42V, VOUT(MIN) = 0.97V, IQ = 2.5A, ISD < 1A, 3mm x 4mm QFN Package LT8612 42V, 6A, 96% Efficiency, 2.2MHz Synchronous Micropower Step-Down DC/DC Converter with IQ = 3A VIN: 3.4V to 42V, VOUT(MIN) = 0.97V, IQ = 3A, ISD < 1A, 3mm x 6mm QFN Package LT3975 4.3V, 2.5A, 2.2MHz Micropower Step-Down DC/DC Converter with IQ = 2.7A VIN: 4.3V to 42V, VOUT(MIN) = 1.2V, IQ = 2.7A, ISD < 1A, MSOP-16E Package LT3976 40V, 5A, 2.2MHz Micropower Step-Down DC/DC Converter with IQ = 3.3A VIN: 4.3V to 40V, VOUT(MIN) = 1.2V, IQ = 3.3A, ISD < 1A, MSOP-16E and 3mm x 5mm QFN-24 Packages 8610acfa 26 Linear Technology Corporation 1630 McCarthy Blvd., Milpitas, CA 95035-7417 For more information www.linear.com/LT8610AC (408) 432-1900 FAX: (408) 434-0507 www.linear.com/LT8610AC LT 0515 REV A * PRINTED IN USA LINEAR TECHNOLOGY CORPORATION 2014