LTC3610 24V, 12A Monolithic Synchronous Step-Down DC/DC Converter Description Features ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ 12A Output Current Wide VIN Range = 4V to 24V Internal N-Channel MOSFETs True Current Mode Control Optimized for High Step-Down Ratios tON(MIN) ≤100ns Extremely Fast Transient Response Stable with Ceramic COUT ±1% 0.6V Voltage Reference Power Good Output Voltage Monitor Adjustable On-Time/Switching Frequency Adjustable Current Limit Programmable Soft-Start Output Overvoltage Protection Optional Short-Circuit Shutdown Timer Low Shutdown IQ: 15µA Available in a 9mm × 9mm 64-Pin QFN Package Applications ■ ■ Point of Load Regulation Distributed Power Systems L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners. Protected by U.S. Patents including 5481178, 6100678, 6580258, 5847554, 6304066. The LTC®3610 is a high efficiency, monolithic synchronous step-down DC/DC converter that can deliver up to 12A output current from a 4V to 24V (28V maximum) input supply. It uses a valley current control architecture to deliver very low duty cycle operation at high frequency with excellent transient response. The operating frequency is selected by an external resistor and is compensated for variations in VIN and VOUT. The LTC3610 can be configured for discontinuous or forced continuous operation at light load. Forced continuous operation reduces noise and RF interference while discontinuous mode provides high efficiency by reducing switching losses at light loads. Fault protection is provided by internal foldback current limiting, an output overvoltage comparator and an optional short-circuit shutdown timer. Soft-start capability for supply sequencing is accomplished using an external timing capacitor. The regulator current limit is user programmable. A power good output voltage monitor indicates when the output is in regulation. The LTC3610 is available in a compact 9mm × 9mm QFN package. Typical Application High Efficiency Step-Down Converter VON ION RUN/SS VIN 100pF Efficiency vs Load Current 604k 10µF ×3 LTC3610 SW 470pF 31.83k 0.22µF ITH SGND INTVCC 30.1k FCB VRNG PGOOD EXTVCC 100µF ×2 BOOST 95 4.7µF 10000 VIN = 5V 90 VOUT 2.5V 12A 85 1000 VIN = 12V 80 75 100 70 POWER LOSS 12V 65 60 POWER LOSS 5V 55 PGND 50 0.01 VFB 3610 TA01a VIN 4V TO 24V 9.5k VOUT = 2.5V 0.1 1 LOAD CURRENT (A) 10 POWER LOSS (mW) 0.47µH 100 EFFICIENCY (%) 0.1µF 10 1 3610 TA01b 3610ff LTC3610 Absolute Maximum Ratings Pin Configuration (Note 1) PGND 1 49 SGND 50 SGND 51 SVIN 52 SVIN 53 INTVCC 54 INTVCC 55 SW 56 PGND 57 PGND 58 PGND 59 PGND 60 PGND 61 PGND 62 PGND 63 PGND TOP VIEW 64 PGND Input Supply Voltage (VIN, ION)................... 28V to –0.3V Boosted Topside Driver Supply Voltage (BOOST)................................................. 34V to –0.3V SW Voltage............................................. 28V to –0.3V INTVCC, EXTVCC, (BOOST – SW), RUN/SS, PGOOD Voltages....................................... 7V to –0.3V FCB, VON, VRNG Voltages............. INTVCC + 0.3V to –0.3V ITH, VFB Voltages........................................ 2.7V to –0.3V Operating Temperature Range (Note 4).............................................. –40°C to 125°C Junction Temperature (Note 2).............................. 125°C Storage Temperature Range.................... –55°C to 125°C 48 SGND 65 PGND PGND 2 47 SGND PGND 3 46 SGND SW 4 45 SGND SW 5 44 EXTVCC SW 6 43 VFB SW 7 42 SGND 66 SW SW 8 41 ION SW 9 40 SGND SW 10 39 FCB 68 SGND SW 11 PVIN 12 PVIN 13 38 ITH 37 VRNG 36 PGOOD 67 PVIN PVIN 14 PVIN 15 35 VON 34 SGND 33 SGND SGND 32 SGND 31 RUN/SS 30 BOOST 29 SGND 28 NC 27 SW 26 PVIN 25 PVIN 24 PVIN 23 PVIN 22 PVIN 21 PVIN 20 PVIN 19 PVIN 17 PVIN 18 PVIN 16 WP PACKAGE 64-LEAD (9mm × 9mm) QFN MULTIPAD TJMAX = 125°C, θJA = 28°C/W Order Information LEAD FREE FINISH LTC3610EWP#PBF TAPE AND REEL LTC3610EWP#TRPBF PART MARKING* LTC3610WP PACKAGE DESCRIPTION LTC3610IWP#PBF LTC3610IWP#TRPBF LTC3610WP LEAD BASED FINISH LTC3610EWP TAPE AND REEL LTC3610EWP#TR PART MARKING* LTC3610WP 64-Lead (9mm × 9mm) Plastic QFN PACKAGE DESCRIPTION LTC3610IWP LTC3610IWP#TR LTC3610WP 64-Lead (9mm × 9mm) Plastic QFN 64-Lead (9mm × 9mm) Plastic QFN TEMPERATURE RANGE –40°C to 125°C –40°C to 125°C TEMPERATURE RANGE –40°C to 125°C –40°C to 125°C 64-Lead (9mm × 9mm) Plastic QFN Consult LTC Marketing for parts specified with wider operating temperature ranges.*The temperature grade is identified by a label on the shipping container. For more information on lead free part marking, go to: http://www.linear.com/leadfree/ For more information on tape and reel specifications, go to: http://www.linear.com/tapeandreel/ Electrical Characteristics The ● denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C (Note 4). VIN = 15V unless otherwise noted. SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS Main Control Loop VIN Operating Input Voltage Range 4 IQ Input DC Supply Current Normal Shutdown Supply Current VFB Feedback Reference Voltage ITH = 1.2V (Note 3) ΔVFB(LINEREG) Feedback Voltage Line Regulation VIN = 4V to 28V, ITH = 1.2V (Note 3) ● 0.594 24 V 900 15 2000 30 µA µA 0.600 0.606 V 0.002 %/V 3610ff LTC3610 Electrical Characteristics The ● denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C (Note 4). VIN = 15V unless otherwise noted. SYMBOL PARAMETER CONDITIONS ΔVFB(LOADREG) Feedback Voltage Load Regulation ITH = 0.5V to 1.9V (Note 3) MIN IFB Feedback Input Current VFB = 0.6V ITH = 1.2V (Note 3) gm(EA) Error Amplifier Transconductance VFCB Forced Continuous Threshold IFCB Forced Continuous Pin Current VFCB = 0.6V tON On-Time ION = 60µA, VON = 1.5V ION = 60µA, VON = 0V tON(MIN) Minimum On-Time ION = 180µA, VON = 0V tOFF(MIN) Minimum Off-Time ION = 30µA, VON = 1.5V IVALLEY(MAX) Maximum Valley Current VRNG = 0.5V, VFB = 0.56V, FCB = 0V VRNG = 0V, VFB = 0.56V, FCB = 0V IVALLEY(MIN) Maximum Reverse Valley Current VRNG = 0.5V, VFB = 0.64V, FCB = 0V VRNG = 0V, VFB = 0.64V, FCB = 0V ΔVFB(OV) Output Overvoltage Fault Threshold TYP MAX UNITS –0.05 –0.3 % –5 ±50 nA mS ● 1.4 1.7 2 ● 0.54 0.6 0.66 V –1 –2 µA 250 120 310 ns ns 60 100 ns 290 500 ns 170 ● ● 7 10 16 19 A A –6 –9 A A 7 10 0.8 13 % VRUN/SS(ON) RUN Pin Start Threshold 1.5 2 V VRUN/SS(LE) RUN Pin Latchoff Enable Threshold RUN/SS Pin Rising 4 4.5 V VRUN/SS(LT) RUN Pin Latchoff Threshold RUN/SS Pin Falling 3.5 4.2 V ● IRUN/SS(C) Soft-Start Charge Current VRUN/SS = 0V –0.5 –1.2 –3 µA IRUN/SS(D) Soft-Start Discharge Current VRUN/SS = 4.5V, VFB = 0V 0.8 1.8 3 µA VIN(UVLO) Undervoltage Lockout VIN Falling ● 3.4 3.9 V VIN(UVLOR) Undervoltage Lockout Release VIN Rising ● 3.5 4 V RDS(ON) Top Switch On-Resistance Bottom Switch On-Resistance 12 6.5 16 10 mΩ mΩ 5 5.5 V –0.1 ±2 % Internal VCC Regulator VINTVCC Internal VCC Voltage 6V < VIN < 28V, VEXTVCC = 4V ΔVLDO(LOADREG) Internal VCC Load Regulation ICC = 0mA to 20mA, VEXTVCC = 4V VEXTVCC EXTVCC Switchover Voltage ICC = 20mA, VEXTVCC Rising ΔVEXTVCC EXTVCC Switch Drop Voltage ICC = 20mA, VEXTVCC = 5V ΔVEXTVCC(HYS) EXTVCC Switchover Hysteresis ● 4.7 ● 4.5 4.7 150 V 300 500 mV mV PGOOD Output ΔVFBH PGOOD Upper Threshold VFB Rising 7 10 13 % ΔVFBL PGOOD Lower Threshold VFB Falling –7 –10 –13 % ΔVFB(HYS) PGOOD Hysteresis VFB Returning 1 2.5 % VPGL PGOOD Low Voltage IPGOOD = 5mA 0.15 0.4 V 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: TJ is calculated from the ambient temperature TA and power dissipation PD as follows: TJ = TA + (PD • 28°C/W) (θJA is simulated per JESD51-7 high effective thermal conductivity test board) θJC = 0.24°C/W (θJC is simulated when heat sink is applied at the bottom of the package). Note 3: The LTC3610 is tested in a feedback loop that adjusts VFB to achieve a specified error amplifier output voltage (ITH). Note 4: The LTC3610 is tested under pulsed load conditions such that TJ ≈ TA. The LTC3610E is guaranteed to meet performance specifications from 0°C to 125°C. Specifications over the –40°C to 125°C operating temperature range are assured by design, characterization and correlation with statistical process controls. The LTC3610I is guaranteed over the full –40°C to 125°C operating junction temperature range. Note that the maximum ambient temperature is determined by specific operating conditions in conjunction with board layout, the rated package thermal resistance and other environmental factors. 3610ff LTC3610 Typical Performance Characteristics Transient Response Transient Response (Discontinuous Mode) VOUT 100mV/DIV VOUT 100mV/DIV IL 5A/DIV IL 5A/DIV ILOAD 5A/DIV ILOAD 5A/DIV IL 5A/DIV 3610 G02 Efficiency vs Input Voltage 100 640 ILOAD = 10A 600 CONTINUOUS MODE 70 50 0.001 0.01 0.1 1 LOAD CURRENT (A) ILOAD = 10A 90 ILOAD = 1A 85 VIN = 12V VOUT = 2.5V EXTVCC = 5V FIGURE 6 CIRCUIT FREQUENCY (kHz) EFFICIENCY (%) 95 60 560 520 ILOAD = 0A 480 440 80 10 5 25 10 15 20 INPUT VOLTAGE (V) 400 Load Regulation Frequency vs Load Current 0.80 FCB = 0V FIGURE 6 CIRCUIT 5 10 3610 G05 3610 G04 15 20 INPUT VOLTAGE (V) 25 3610 G06 ITH Voltage vs Load Current 2.5 FIGURE 6 CIRCUIT FIGURE 6 CIRCUIT 0.60 CONTINUOUS MODE 2.0 DISCONTINUOUS MODE ITH VOLTAGE (V) 0.40 ΔVOUT (%) FREQUENCY (kHz) 3610 G03 Frequency vs Input Voltage FCB = 5V FIGURE 6 CIRCUIT DISCONTINUOUS MODE 80 650 600 550 500 450 400 350 300 250 200 150 100 50 0 40ms/DIV VIN = 12V VOUT = 2.5V RLOAD = 0.5Ω FIGURE 6 CIRCUIT ILOAD = 1A TO 7A VIN = 12V VOUT = 2.5V FCB = INTVCC FIGURE 6 CIRCUIT 100 EFFICIENCY (%) VOUT 1V/DIV 40µs/DIV Efficiency vs Load Current 90 RUN/SS 2V/DIV 3610 G01 40µs/DIV LOAD STEP 0A TO 8A VIN = 12V VOUT = 2.5V FCB = 0V FIGURE 6 CIRCUIT Start-Up 0.20 0 –0.20 –0.40 1.5 CONTINUOUS MODE 1.0 DISCONTINUOUS MODE 0.5 –0.60 0 2 4 6 8 LOAD CURRENT (A) 10 12 3610 G07 –0.80 0 2 4 6 8 LOAD CURRENT (A) 10 12 3610 G08 0 0 3 6 9 LOAD CURRENT (A) 12 3610 G09 3610ff LTC3610 Typical Performance Characteristics VRNG = 20 10000 1000 VVON = 0V 1V ON-TIME (ns) 0.5V 10 5 1000 0 0.5 1.0 1.5 2.0 ITH VOLTAGE (V) 2.5 10 3.0 400 100 150 100 50 100 20 18 16 14 12 0.6 0.7 0.8 VRNG VOLTAGE (V) 0.9 3 3610 G12 18 22 0.5 1 2 VON VOLTAGE (V) Maximum Valley Current Limit vs RUN/SS Voltage 24 10 125 0 3610 G11 Maximum Valley Current Limit vs VRNG Voltage 200 25 75 0 50 TEMPERATURE (°C) 10 ION CURRENT (µA) 3610 G10 IION = 30µA VVON = 0V 0 –50 –25 0 1 MAXIMUM VALLEY CURRENT LIMIT (A) 0 MAXIMUM VALLEY CURRENT LIMIT (A) ON-TIME (ns) 250 600 200 On-Time vs Temperature 300 ION = 30µA 100 –5 –10 On-Time vs VON Voltage 800 0.7V 15 LOAD CURRENT (A) On-Time vs ION Current ON-TIME (ns) 25 Load Current vs ITH Voltage and VRNG 15 12 9 6 3 0 1.65 1.90 2.15 2.40 2.65 2.90 3.15 3.40 RUN/SS VOLTAGE (V) 1.0 3610 G14 3610 G15 3610 G13 Maximum Valley Current Limit vs Temperature 15 15 10 5 –25 0 50 75 25 TEMPERATURE (°C) 100 125 3610 G16 MAXIMUM VALLEY CURRENT LIMIT (A) 18 MAXIMUM VALLEY CURRENT (A) MAXIMUM VALLEY CURRENT LIMIT (A) 20 0 –50 Maximum Valley Current Limit in Foldback Input Voltage vs Maximum Valley Current 16 14 12 10 8 6 4 4 7 10 13 16 19 22 INPUT VOLTAGE (V) 25 28 3610 G17 10 5 0 0 0.1 0.2 0.3 VFB (V) 0.4 0.5 0.6 3610 G18 3610ff LTC3610 Typical Performance Characteristics Error Amplifier gm vs Temperature Feedback Reference Voltage vs Temperature 2.0 0.62 0.59 INPUT CURRENT (µA) gm (mS) 0.60 1.6 1.4 1.2 0.58 –50 –25 75 0 25 50 TEMPERATURE (°C) 100 125 1.0 –50 0 50 75 25 TEMPERATURE (°C) –25 3610 G19 100 25 15 400 5 0 15 10 –0.20 5 –0.30 40 10 20 30 INTVCC LOAD CURRENT (mA) 0 400 50 500 3610 G22 700 600 800 FREQUENCY (kHz) 900 4 2 0 –50 1000 –25 0 50 75 25 TEMPERATURE (°C) 1 0 –1 –2 –50 PULL-UP CURRENT –25 0 50 75 25 TEMPERATURE (°C) 100 125 3610 G25 4.5 LATCHOFF ENABLE 4.0 LATCHOFF THRESHOLD 3.5 3.0 –50 –25 75 0 25 50 TEMPERATURE (°C) 125 3610 G24 UNDERVOLTAGE LOCKOUT THRESHOLD (V) RUN/SS PIN CURRENT (µA) RUN/SS PIN CURRENT (µA) PULL-DOWN CURRENT 100 Undervoltage Lockout Threshold vs Temperature 5.0 2 3610 G21 6 RUN/SS Pin Current vs Temperature 3 0 8 3610 G23 RUN/SS Pin Current vs Temperature 30 25 10 EXTVCC SWITCH RESISTANCE (Ω) IEXTVCC (mA) –0.10 10 20 15 INPUT VOLTAGE (V) EXTVCC Switch Resistance vs Temperature 20 0 5 3610 G20 0.10 0 10 EXTVCC = 5V 25 0.20 20 SHUTDOWN 600 IEXTVCC vs Frequency 0.30 ΔINTVCC (%) 30 800 0 125 35 1000 200 INTVCC Load Regulation –0.40 EXTVCC OPEN 1200 1.8 0.61 40 1400 SHUTDOWN CURRENT (µA) FEEDBACK REFERENCE VOLTAGE (V) Input and Shutdown Currents vs Input Voltage 100 125 3610 G26 4.0 3.5 3.0 2.5 2.0 –50 –25 75 0 25 50 TEMPERATURE (°C) 100 125 3610 G27 3610ff LTC3610 Pin Functions PGND (Pins 1, 2, 3, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65): Power Ground. Connect this pin closely to the (–) terminal of CVCC and the (–) terminal of CIN. SW (Pins 4, 5, 6, 7, 8, 9, 10, 11, 26, 55, 66): Switch Node Connection to the Inductor. The (–) terminal of the bootstrap capacitor CB also connects here. This pin swings from a diode voltage drop below ground up to VIN. PVIN (Pins 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 67): Main Input Supply. Decouple this pin to power PGND with the input capacitance CIN. NC (Pin 27): No Connection. SGND (Pins 28, 31, 32, 33, 34, 40, 42, 45, 46, 47, 48, 49, 50, 68): Signal Ground. All small-signal components and compensation components should connect to this ground, which in turn connects to PGND at one point. BOOST (Pin 29): Boosted Floating Driver Supply. The (+) terminal of the bootstrap capacitor CB connects here. This pin swings from a diode voltage drop below INTVCC up to VIN + INTVCC. RUN/SS (Pin 30): Run Control and Soft-Start Input. A capacitor to ground at this pin sets the ramp time to full output current (approximately 3s/µF) and the time delay for overcurrent latchoff (see Applications Information). Forcing this pin below 0.8V shuts down the device. VON (Pin 35): On-Time Voltage Input. Voltage trip point for the on-time comparator. Tying this pin to the output voltage or an external resistive divider from the output makes the on-time proportional to VOUT. The comparator input defaults to 0.7V when the pin is grounded and defaults to 2.4V when the pin is tied to INTVCC. Tie this pin to INTVCC in high VOUT applications to use a lower RON value. PGOOD (Pin 36): Power Good Output. Open-drain logic output that is pulled to ground when the output voltage is not within ± 10% of the regulation point. VRNG (Pin 37): Current Limit Range Input. The voltage at this pin adjusts maximum valley current and can be set from 0.5V to 0.7V by a resistive divider from INTVCC. It defaults to 0.7V if the VRNG pin is tied to ground which results in a typical 19A current limit. ITH (Pin 38): Current Control Threshold and Error Amplifier Compensation Point. The current comparator threshold increases with this control voltage. The voltage ranges from 0V to 2.4V with 0.8V corresponding to zero sense voltage (zero current). FCB (Pin 39): Forced Continuous Input. Tie this pin to ground to force continuous synchronous operation at low load, to INTVCC to enable discontinuous mode operation at low load or to a resistive divider from a secondary output when using a secondary winding. ION (Pin 41): On-Time Current Input. Tie a resistor from VIN to this pin to set the one-shot timer current and thereby set the switching frequency. VFB (Pin 43): Error Amplifier Feedback Input. This pin connects the error amplifier input to an external resistive divider from VOUT. EXTVCC (Pin 44): External VCC Input. When EXTVCC exceeds 4.7V, an internal switch connects this pin to INTVCC and shuts down the internal regulator so that controller and gate drive power is drawn from EXTVCC. Do not exceed 7V at this pin and ensure that EXTVCC < VIN. SVIN (Pins 51, 52): Supply Pin for Internal PWM Controller. INTVCC (Pins 53, 54): Internal 5V Regulator Output. The driver and control circuits are powered from this voltage. Decouple this pin to power ground with a minimum of 4.7µF low ESR tantalum or ceramic capacitor. 3610ff LTC3610 Functional Diagram RON VON ION 35 41 FCB EXTVCC 39 44 SVIN 51, 52 4.7V 0.7V 2.4V + 1µA PVIN – 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 67 0.6V REF 0.6V 5V REG INTVCC + – 53, 54 F 29 VVON tON = (10pF) IION S Q FCNT CB M1 ON SW + ICMP – L1 DB VOUT 4, 5, 6, 7, 8, 9, 10, 11, 26, 55, 66 SWITCH LOGIC IREV – SHDN 1.4V VRNG BOOST R 20k + CIN COUT OV M2 + CVCC 37 PGND × (0.5 TO 2) 1, 2, 3, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65 0.7V 36 PGOOD 1 240k + 1V Q2 Q4 UV – Q6 ITHB R2 0.54V 43 Q3 Q1 VFB + R1 OV + – – 0.8V – SS + RUN SHDN 1.2µA EA 27 + – – ×3.3 SGND 28, 31, 32, 33, 34, 40, 42, 45, 46, 47, 48, 49, 50, 68 0.66V NC 6V + 0.6V 38 ITH 0.4V 30 3610 FD RUN/SS CSS 3610ff LTC3610 Operation Main Control Loop The LTC3610 is a high efficiency monolithic synchronous, step-down DC/DC converter utilizing a constant on-time, current mode architecture. It operates from an input voltage range of 4V to 24V and provides a regulated output voltage at up to 12A of output current. The internal synchronous power switch increases efficiency and eliminates the need for an external Schottky diode. In normal operation, the top MOSFET is turned on for a fixed interval determined by a one-shot timer OST. When the top MOSFET is turned off, the bottom MOSFET is turned on until the current comparator ICMP trips, restarting the one-shot timer and initiating the next cycle. Inductor current is determined by sensing the voltage between the PGND and SW pins using the bottom MOSFET on-resistance. The voltage on the ITH pin sets the comparator threshold corresponding to inductor valley current. The error amplifier, EA, adjusts this voltage by comparing the feedback signal VFB from the output voltage with an internal 0.6V reference. If the load current increases, it causes a drop in the feedback voltage relative to the reference. The ITH voltage then rises until the average inductor current again matches the load current. At light load, the inductor current can drop to zero and become negative. This is detected by current reversal comparator IREV which then shuts off M2 (see Functional Diagram), resulting in discontinuous operation. Both switches will remain off with the output capacitor supplying the load current until the ITH voltage rises above the zero current level (0.8V) to initiate another cycle. Discontinuous mode operation is disabled by comparator F when the FCB pin is brought below 0.6V, forcing continuous synchronous operation. The operating frequency is determined implicitly by the top MOSFET on-time and the duty cycle required to maintain regulation. The one-shot timer generates an on-time that is proportional to the ideal duty cycle, thus holding frequency approximately constant with changes in VIN. The nominal frequency can be adjusted with an external resistor, RON. Overvoltage and undervoltage comparators OV and UV pull the PGOOD output low if the output feedback voltage exits a ±10% window around the regulation point. Furthermore, in an overvoltage condition, M1 is turned off and M2 is turned on and held on until the overvoltage condition clears. Foldback current limiting is provided if the output is shorted to ground. As VFB drops, the buffered current threshold voltage ITHB is pulled down by clamp Q3 to a 1V level set by Q4 and Q6. This reduces the inductor valley current level to one sixth of its maximum value as VFB approaches 0V. Pulling the RUN/SS pin low forces the controller into its shutdown state, turning off both M1 and M2. Releasing the pin allows an internal 1.2µA current source to charge up an external soft-start capacitor, CSS. When this voltage reaches 1.5V, the controller turns on and begins switching, but with the ITH voltage clamped at approximately 0.6V below the RUN/SS voltage. As CSS continues to charge, the soft-start current limit is removed. INTVCC/EXTVCC Power Power for the top and bottom MOSFET drivers and most of the internal controller circuitry is derived from the INTVCC pin. The top MOSFET driver is powered from a floating bootstrap capacitor CB. This capacitor is recharged from INTVCC through an external Schottky diode, DB, when the top MOSFET is turned off. When the EXTVCC pin is grounded, an internal 5V low dropout regulator supplies the INTVCC power from VIN. If EXTVCC rises above 4.7V, the internal regulator is turned off, and an internal switch connects EXTVCC to INTVCC. This allows a high efficiency source connected to EXTVCC, such as an external 5V supply or a secondary output from the converter, to provide the INTVCC power. Voltages up to 7V can be applied to EXTVCC for additional gate drive. If the input voltage is low and INTVCC drops below 3.5V, undervoltage lockout circuitry prevents the power switches from turning on. 3610ff LTC3610 Applications Information The basic LTC3610 application circuit is shown on the front page of this data sheet. External component selection is primarily determined by the maximum load current. The LTC3610 uses the on-resistance of the synchronous power MOSFET for determining the inductor current. The desired amount of ripple current and operating frequency also determines the inductor value. Finally, CIN is selected for its ability to handle the large RMS current into the converter and COUT is chosen with low enough ESR to meet the output voltage ripple and transient specification. VON and PGOOD Operating Frequency The choice of operating frequency is a trade-off between efficiency and component size. Low frequency operation improves efficiency by reducing MOSFET switching losses but requires larger inductance and/or capacitance in order to maintain low output ripple voltage. The operating frequency of LTC3610 applications is determined implicitly by the one-shot timer that controls the on-time tON of the top MOSFET switch. The on-time is set by the current into the ION pin and the voltage at the VON pin according to: VVON (10pF ) IION The LTC3610 has an open-drain PGOOD output that indicates when the output voltage is within ±10% of the regulation point. The LTC3610 also has a VON pin that allows the on-time to be adjusted. Tying the VON pin high results in lower values for RON which is useful in high VOUT applications. The VON pin also provides a means to adjust the on-time to maintain constant frequency operation in applications where VOUT changes and to correct minor frequency shifts with changes in load current. VRNG Pin and ILIMIT Adjust For a step-down converter, this results in approximately constant frequency operation as the input supply varies: The VRNG pin is used to adjust the maximum inductor valley current, which in turn determines the maximum average output current that the LTC3610 can deliver. The maximum output current is given by: IOUT(MAX) = IVALLEY(MAX) + 1/2 ΔIL The IVALLEY(MAX) is shown in the figure “Maximum Valley Current Limit vs VRNG Voltage” in the Typical Performance Characteristics. An external resistor divider from INTVCC can be used to set the voltage on the VRNG pin from 0.5V to 1V, or it can be simply tied to ground force a default value equivalent to 0.7V. When setting current limit, ensure that the junction temperature does not exceed the maximum rating of 125°C. Do not float the VRNG pin. tON = Tying a resistor, RON, from VIN to the ION pin yields an on-time inversely proportional to VIN. The current out of the ION pin is: IION = f= VIN RON VOUT [ HZ ] VVON RON(10pF ) To hold frequency constant during output voltage changes, tie the VON pin to VOUT or to a resistive divider from VOUT when VOUT > 2.4V. The VON pin has internal clamps that limit its input to the one-shot timer. If the pin is tied below 0.7V, the input to the one-shot is clamped at 0.7V. Similarly, if the pin is tied above 2.4V, the input is clamped at 2.4V. In high VOUT applications, tying VON to INTVCC so that the comparator input is 2.4V results in a lower value for RON. Figures 1a and 1b show how RON relates to switching frequency for several common output voltages. 3610ff 10 LTC3610 Applications Information SWITCHING FREQUENCY (kHz) 1000 VOUT = 3.3V VOUT = 1.5V 100 100 VOUT = 2.5V 1000 RON (kΩ) 10000 3610 F01a Figure 1a. Switching Frequency vs RON (VON = 0V) SWITCHING FREQUENCY (kHz) 1000 VOUT = 12V VOUT = 5V VOUT = 3.3V 100 100 1000 RON (kΩ) as current increases, constant frequency operation can be maintained. This is accomplished with a resistive divider from the ITH pin to the VON pin and VOUT. The values required will depend on the parasitic resistances in the specific application. A good starting point is to feed about 25% of the voltage change at the ITH pin to the VON pin as shown in Figure 2a. Place capacitance on the VON pin to filter out the ITH variations at the switching frequency. The resistor load on ITH reduces the DC gain of the error amp and degrades load regulation, which can be avoided by using the PNP emitter follower of Figure 2b. 10000 3610 F01b Figure 1b. Switching Frequency vs RON (VON = INTVCC) Because the voltage at the ION pin is about 0.7V, the current into this pin is not exactly inversely proportional to VIN, especially in applications with lower input voltages. To correct for this error, an additional resistor RON2 connected from the ION pin to the 5V INTVCC supply will further stabilize the frequency. 5V RON2 = RON 0.7 V Changes in the load current magnitude will also cause frequency shift. Parasitic resistance in the MOSFET switches and inductor reduce the effective voltage across the inductance, resulting in increased duty cycle as the load current increases. By lengthening the on-time slightly Minimum Off-Time and Dropout Operation The minimum off-time, tOFF(MIN), is the smallest amount of time that the LTC3610 is capable of turning on the bottom MOSFET, tripping the current comparator and turning the MOSFET back off. This time is generally about 250ns. The minimum off-time limit imposes a maximum duty cycle of tON/(tON + tOFF(MIN)). If the maximum duty cycle is reached, due to a dropping input voltage for example, then the output will drop out of regulation. The minimum input voltage to avoid dropout is: VIN(MIN) = VOUT tON + tOFF(MIN) tON A plot of maximum duty cycle vs frequency is shown in Figure 3. Setting the Output Voltage The LTC3611 develops a 0.6V reference voltage between the feedback pin, VFB, and the signal ground as shown in Figure 6. The output voltage is set by a resistive divider according to the following formula: ⎛ R2 ⎞ VOUT = 0.6V ⎜ 1+ ⎟ ⎝ R1⎠ To improve the frequency response, a feedforward capacitor C1 may also be used. Great care should be taken to route the VFB line away from noise sources, such as the inductor or the SW trace. 3610ff 11 LTC3610 Applications Information RVON1 30k VOUT RVON2 100k CVON 0.01µF RC VON LTC3610 A reasonable starting point is to choose a ripple current that is about 40% of IOUT(MAX). The largest ripple current occurs at the highest VIN. To guarantee that ripple current does not exceed a specified maximum, the inductance should be chosen according to: ITH CC (2a) RVON1 3k VOUT 10k INTVCC CVON 0.01µF RVON2 10k RC Q1 2N5087 VON LTC3610 ITH CC 3610 F02 (2b) Figure 2. Correcting Frequency Shift with Load Current Changes 2.0 SWITCHING FREQUENCY (MHz) frequency with small ripple current. However, achieving this requires a large inductor. There is a trade-off between component size, efficiency and operating frequency. ⎛ V ⎞⎛ VOUT ⎞ OUT 1− L=⎜ ⎟⎜ ⎟ ⎝ f ΔIL(MAX) ⎠ ⎝ VIN(MAX) ⎠ Once the value for L is known, the type of inductor must be selected. High efficiency converters generally cannot afford the core loss found in low cost powdered iron cores. A variety of inductors designed for high current, low voltage applications are available from manufacturers such as Sumida, Panasonic, Coiltronics, Coilcraft and Toko. CIN and COUT Selection 1.5 The input capacitance CIN is required to filter the square wave current at the drain of the top MOSFET. Use a low ESR capacitor sized to handle the maximum RMS current. DROPOUT REGION 1.0 0.5 0 0 0.25 0.50 0.75 DUTY CYCLE (VOUT/VIN) 1.0 3610 F03 Figure 3. Maximum Switching Frequency vs Duty Cycle Inductor Selection Given the desired input and output voltages, the inductor value and operating frequency determine the ripple current: ⎛V ⎞⎛ V ⎞ ΔIL = ⎜ OUT ⎟ ⎜ 1− OUT ⎟ VIN ⎠ ⎝ f L ⎠⎝ Lower ripple current reduces core losses in the inductor, ESR losses in the output capacitors and output voltage ripple. Highest efficiency operation is obtained at low IRMS ≅IOUT(MAX) VOUT VIN VIN –1 VOUT This formula has a maximum at VIN = 2VOUT, where IRMS = IOUT(MAX)/2. This simple worst-case condition is commonly used for design because even significant deviations do not offer much relief. Note that ripple current ratings from capacitor manufacturers are often based on only 2000 hours of life which makes it advisable to derate the capacitor. The selection of COUT is primarily determined by the ESR required to minimize voltage ripple and load step transients. The output ripple ΔVOUT is approximately bounded by: ⎛ 1 ⎞ ΔVOUT ≤ ΔIL ⎜ ESR + 8fCOUT ⎟⎠ ⎝ 3610ff 12 LTC3610 Applications Information Since ΔIL increases with input voltage, the output ripple is highest at maximum input voltage. Typically, once the ESR requirement is satisfied, the capacitance is adequate for filtering and has the necessary RMS current rating. Multiple capacitors placed in parallel may be needed to meet the ESR and RMS current handling requirements. Dry tantalum, special polymer, aluminum electrolytic and ceramic capacitors are all available in surface mount packages. Special polymer capacitors offer very low ESR but have lower capacitance density than other types. Tantalum capacitors have the highest capacitance density but it is important to only use types that have been surge tested for use in switching power supplies. Aluminum electrolytic capacitors have significantly higher ESR, but can be used in cost-sensitive applications providing that consideration is given to ripple current ratings and long-term reliability. Ceramic capacitors have excellent low ESR characteristics but can have a high voltage coefficient and audible piezoelectric effects. The high Q of ceramic capacitors with trace inductance can also lead to significant ringing. When used as input capacitors, care must be taken to ensure that ringing from inrush currents and switching does not pose an overvoltage hazard to the power switches and controller. To dampen input voltage transients, add a small 5µF to 50µF aluminum electrolytic capacitor with an ESR in the range of 0.5Ω to 2Ω. High performance through-hole capacitors may also be used, but an additional ceramic capacitor in parallel is recommended to reduce the effect of their lead inductance. Top MOSFET Driver Supply (CB, DB) An external bootstrap capacitor, CB, connected to the BOOST pin supplies the gate drive voltage for the topside MOSFET. This capacitor is charged through diode DB from INTVCC when the switch node is low. When the top MOSFET turns on, the switch node rises to VIN and the BOOST pin rises to approximately VIN + INTVCC. The boost capacitor needs to store about 100 times the gate charge required by the top MOSFET. In most applications an 0.1µF to 0.47µF, X5R or X7R dielectric capacitor is adequate. Discontinuous Mode Operation and FCB Pin The FCB pin determines whether the bottom MOSFET remains on when current reverses in the inductor. Tying this pin above its 0.6V threshold enables discontinuous operation where the bottom MOSFET turns off when inductor current reverses. The load current at which current reverses and discontinuous operation begins depends on the amplitude of the inductor ripple current and will vary with changes in VIN. Tying the FCB pin below the 0.6V threshold forces continuous synchronous operation, allowing current to reverse at light loads and maintaining high frequency operation. In addition to providing a logic input to force continuous operation, the FCB pin provides a means to maintain a flyback winding output when the primary is operating in discontinuous mode. The secondary output VOUT2 is normally set as shown in Figure 4 by the turns ratio N of the transformer. However, if the controller goes into discontinuous mode and halts switching due to a light primary load current, then VOUT2 will droop. An external resistor divider from VOUT2 to the FCB pin sets a minimum voltage VOUT2(MIN) below which continuous operation is forced until VOUT2 has risen above its minimum: ⎛ R4 ⎞ VOUT2(MIN) = 0.6V ⎜ 1+ ⎟ ⎝ R3 ⎠ Fault Conditions: Current Limit and Foldback The LTC3610 has a current mode controller which inherently limits the cycle-by-cycle inductor current not only in steady state operation but also in transient. To further limit current in the event of a short circuit to ground, the LTC3610 includes foldback current limiting. If the output falls by more than 25%, then the maximum sense voltage is progressively lowered to about one sixth of its full value. 3610ff 13 LTC3610 Applications Information SW GND 14 15 SGND SVIN SGND SVIN INTVCC SW INTVCC PGND PGND PGND PGND PGND PGND PGND PGND SW SGND SW FCB SW ITH PVIN VRNG PVIN PGOOD PVIN VON PVIN SGND PVIN SGND PVIN 16 LTC3610 48 47 46 45 44 43 42 R4 41 40 OPTIONAL EXTVCC CONNECTION 5V < VOUT2 < 7V 39 38 37 R3 36 35 34 33 SGND 13 ION SGND 12 SGND SW RUN/SS CIN + VIN SW BOOST 11 VFB SGND 10 EXTVCC SW NC 9 SW SW 8 SGND PVIN 7 SW PVIN 6 SGND PVIN + 5 SGND PGND PVIN COUT T1 1:N SGND PGND PVIN 4 PGND PVIN 3 • VOUT1 + • CSEC 1µF 2 PVIN 1 PVIN IN4148 VOUT2 PGND 64 63 62 61 60 59 58 57 56 55 54 53 52 51 50 49 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 3610 F04 SGND SW Figure 4. Secondary Output Loop and EXTVCC Connection INTVCC Regulator and EXTVCC Connection An internal P-channel low dropout regulator produces the 5V supply that powers the drivers and internal circuitry within the LTC3610. The INTVCC pin can supply up to 50mA RMS and must be bypassed to ground with a minimum of 4.7µF tantalum or ceramic capacitor. Good bypassing is necessary to supply the high transient currents required by the MOSFET gate drivers. The EXTVCC pin can be used to provide MOSFET gate drive and control power from the output or another external source during normal operation. Whenever the EXTVCC pin is above 4.7V the internal 5V regulator is shut off and an internal 50mA P-channel switch connects the EXTVCC pin to INTVCC. INTVCC power is supplied from EXTVCC until this pin drops below 4.5V. Do not apply more than 7V to the EXTVCC pin and ensure that EXTVCC ≤ VIN. The following list summarizes the possible connections for EXTVCC: 1. EXTVCC grounded. INTVCC is always powered from the internal 5V regulator. 2. EXTVCC connected to an external supply. A high efficiency supply compatible with the MOSFET gate drive requirements (typically 5V) can improve overall efficiency. 3. EXTVCC connected to an output derived boost network. The low voltage output can be boosted using a charge pump or flyback winding to greater than 4.7V. The system will start-up using the internal linear regulator until the boosted output supply is available. Soft-Start and Latchoff with the RUN/SS Pin The RUN/SS pin provides a means to shut down the LTC3610 as well as a timer for soft-start and overcurrent latchoff. Pulling the RUN/SS pin below 0.8V puts the LTC3610 into a low quiescent current shutdown (IQ < 30µA). Releasing the pin allows an internal 1.2µA current source to charge up the external timing capacitor CSS. If RUN/SS has been pulled all the way to ground, there is a delay before starting of about: tDELAY = 1.5V C = (1.3s/µF ) CSS 1.2µA SS 3610ff 14 LTC3610 Applications Information When the voltage on RUN/SS reaches 1.5V, the LTC3610 begins operating with a clamp on ITH of approximately 0.9V. As the RUN/SS voltage rises to 3V, the clamp on ITH is raised until its full 2.4V range is available. This takes an additional 1.3s/µF, during which the load current is folded back until the output reaches 75% of its final value. After the controller has been started and given adequate time to charge up the output capacitor, CSS is used as a short-circuit timer. After the RUN/SS pin charges above 4V, if the output voltage falls below 75% of its regulated value, then a short-circuit fault is assumed. A 1.8µA current then begins discharging CSS. If the fault condition persists until the RUN/SS pin drops to 3.5V, then the controller turns off both power MOSFETs, shutting down the converter permanently. The RUN/SS pin must be actively pulled down to ground in order to restart operation. The overcurrent protection timer requires that the soft‑start timing capacitor, CSS, be made large enough to guarantee that the output is in regulation by the time CSS has reached the 4V threshold. In general, this will depend upon the size of the output capacitance, output voltage and load current characteristic. A minimum soft-start capacitor can be estimated from: CSS > COUT VOUT RSENSE (10 –4 [F/V s]) Generally 0.1µF is more than sufficient. Overcurrent latchoff operation is not always needed or desired. Load current is already limited during a short-circuit by the current foldback circuitry and latchoff operation can prove annoying during troubleshooting. The feature can be overridden by adding a pull-up current greater than 5µA to the RUN/SS pin. The additional current prevents the discharge of CSS during a fault and also shortens the soft-start period. Using a resistor to VIN as shown in Figure 5a is simple, but slightly increases shutdown current. Connecting a resistor to INTVCC as shown in Figure 5b eliminates the additional shutdown current, but requires a diode to isolate CSS. Any pull-up network must be able to pull RUN/SS above the 4.2V maximum threshold of the latchoff circuit and overcome the 4µA maximum discharge current. INTVCC RSS* VIN 3.3V OR 5V D1 RUN/SS RSS* CSS D2* RUN/SS 2N7002 CSS 3610 F05 *OPTIONAL TO OVERRIDE OVERCURRENT LATCHOFF (5a) (5b) Figure 5. RUN/SS Pin Interfacing with Latchoff Defeated Efficiency Considerations The percent efficiency of a switching regulator is equal to the output power divided by the input power times 100%. It is often useful to analyze individual losses to determine what is limiting the efficiency and which change would produce the most improvement. Although all dissipative elements in the circuit produce losses, four main sources account for most of the losses in LTC3610 circuits: 1. DC I2R losses. These arise from the resistance of the internal resistance of the MOSFETs, inductor and PC board traces and cause the efficiency to drop at high output currents. In continuous mode the average output current flows through L, but is chopped between the top and bottom MOSFETs. The DC I2R loss for one MOSFET can simply be determined by [RDS(ON) + RL] • IO. 2. Transition loss. This loss arises from the brief amount of time the top MOSFET spends in the saturated region during switch node transitions. It depends upon the input voltage, load current, driver strength and MOSFET capacitance, among other factors. The loss is significant at input voltages above 20V and can be estimated from: Transition Loss ≅ (1.7A–1) VIN2 IOUT CRSS f 3. INTVCC current. This is the sum of the MOSFET driver and control currents. This loss can be reduced by supplying INTVCC current through the EXTVCC pin from a high efficiency source, such as an output derived boost network or alternate supply if available. 3610ff 15 LTC3610 Applications Information 4. CIN loss. The input capacitor has the difficult job of filtering the large RMS input current to the regulator. It must have a very low ESR to minimize the AC I2R loss and sufficient capacitance to prevent the RMS current from causing additional upstream losses in fuses or batteries. Other losses, including COUT ESR loss, Schottky diode D1 conduction loss during dead time and inductor core loss generally account for less than 2% additional loss. When making adjustments to improve efficiency, the input current is the best indicator of changes in efficiency. If you make a change and the input current decreases, then the efficiency has increased. If there is no change in input current, then there is no change in efficiency. Checking Transient Response The regulator loop response can be checked by looking at the load transient response. Switching regulators take several cycles to respond to a step in load current. When a load step occurs, VOUT immediately shifts by an amount equal to ΔILOAD (ESR), where ESR is the effective series resistance of COUT. ΔILOAD also begins to charge or discharge COUT generating a feedback error signal used by the regulator to return VOUT to its steady-state value. During this recovery time, VOUT can be monitored for overshoot or ringing that would indicate a stability problem. The ITH pin external components shown in Figure 6 will provide adequate compensation for most applications. For a detailed explanation of switching control loop theory see Application Note 76. Design Example As a design example, take a supply with the following specifications: VIN = 5V to 24V (12V nominal), VOUT = 2.5V ± 5%, IOUT(MAX) = 12A, f = 550kHz. First, calculate the timing resistor with VON = VOUT: 1 RON = = 182k 550kHz )(10pF ) ( and choose the inductor for about 40% ripple current at the maximum VIN: 2.5V ⎛ 2.5V ⎞ L= 1− = 0.86µH (550kHz )(0.4)(12A ) ⎜⎝ 28V ⎟⎠ Selecting a standard value of 0.82µH results in a maximum ripple current of: 2.5V ⎛ 2.5V ⎞ ΔIL = 1– = 4.4A 550kHz ) ( 0.82µH) ⎜⎝ 12V ⎟⎠ ( Next, set up VRNG voltage and check the ILIMIT. Tying VRNG to 0.5V will set the typical current limit to 16A, and tying VRNG to GND will result in a typical current around 19A. CIN is chosen for an RMS current rating of about 5A at 85°C. The output capacitors are chosen for a low ESR of 0.013Ω to minimize output voltage changes due to inductor ripple current and load steps. The ripple voltage will be only: ΔVOUT(RIPPLE)= ΔIL(MAX) (ESR) = (4.4A) (0.013Ω) = 57mV However, a 0A to 10A load step will cause an output change of up to: ΔVOUT(STEP) = ΔILOAD (ESR) = (10A) (0.013Ω) = 130mV An optional 22µF ceramic output capacitor is included to minimize the effect of ESL in the output ripple. The complete circuit is shown in Figure 6. How to Reduce SW Ringing As with any switching regulator, there will be voltage ringing on the SW node, especially for high input voltages. The ringing amplitude and duration is dependent on the switching speed (gate drive), layout (parasitic inductance) and MOSFET output capacitance. This ringing contributes to the overall EMI, noise and high frequency ripple. One way to reduce ringing is to optimize layout. A good layout minimizes parasitic inductance. Adding RC snubbers from SW to GND is also an effective way to reduce ringing. Finally, adding a resistor in series with the BOOST pin will slow down the MOSFET turn-on slew rate to dampen ringing, but at the cost of reduced efficiency. Note that since the IC is buffered from the high frequency transients by PCB and bondwire inductances, the ringing by itself is normally not a concern for controller reliability. 3610ff 16 LTC3610 Applications Information INTVCC CVCC 4.7µF 6.3V GND CF 0.1µF 25V SW RF1 1Ω VIN 10 11 VIN VIN 5V TO 24V C6 10µF 35V + (OPTIONAL) 12 13 14 15 16 SGND SVIN SGND SVIN INTVCC SW INTVCC PGND PGND PGND PGND PGND PGND PGND PGND SGND SW ION LTC3610 SW SGND SW FCB SW ITH PVIN VRNG PVIN PGOOD PVIN VON PVIN SGND PVIN PVIN CIN 10µF 35V 3× GND SW 48 47 46 EXTVCC C4 0.01µF R1 9.5k 1% 45 44 43 C1 (OPTIONAL) R2 30.1k 1% (OPTIONAL) C2 VOUT RON 182k 1% 42 41 40 CON 0.01µF 39 VIN (OPTIONAL) R5 31.84k 38 CC1 470pF 37 36 R3 0Ω 35 RPG1 100k 34 33 SGND 9 VFB SGND 8 SW RUN/SS (OPTIONAL) GND EXTVCC BOOST 7 SGND SW SGND 6 SW NC 5 SW L1 0.8µH PVIN + PVIN COUT1 220µF 2× SGND PVIN C5 22µF 6.3V PGND PVIN 4 SGND PVIN VOUT 2.5V AT 12A SGND PGND PVIN 3 PGND PVIN 2 PVIN 1 PGND 64 63 62 61 60 59 58 57 56 55 54 53 52 51 50 49 INTVCC C3 CC2 100pF (OPTIONAL) RVON 0Ω VOUT 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 SGND SW CIN: TAIYO YUDEN GMK325BJ106MM-B COUT: SANYO 10TPE220ML L1: CDEP85NP-R80MC-50 C5: MURATA GRM31CR60J226KE19 INTVCC DB CMDSH-3 RSS1 510k CB1 0.22µF CSS 0.1µF 3610 F06 VIN (OPTIONAL) SW Figure 6. Design Example: 5V to 24V Input to 2.5V/12A at 550kHz PC Board Layout Checklist When laying out a PC board follow one of the two suggested approaches. The simple PC board layout requires a dedicated ground plane layer. Also, for higher currents, a multilayer board is recommended to help with heat sinking of power components. • The ground plane layer should not have any traces and it should be as close as possible to the layer with the LTC3610. • Place CIN and COUT all in one compact area, close to the LTC3610. It may help to have some components on the bottom side of the board. • Keep small-signal components close to the LTC3610. • Ground connections (including LTC3610 SGND and PGND) should be made through immediate vias to the ground plane. Use several larger vias for power components. • Use a compact plane for the switch node (SW) to improve cooling of the MOSFETs and to keep EMI down. • Use planes for VIN and VOUT to maintain good voltage filtering and to keep power losses low. • Flood all unused areas on all layers with copper. Flooding with copper reduces the temperature rise of power components. Connect these copper areas to any DC net (VIN, VOUT, GND or to any other DC rail in your system). 3610ff 17 LTC3610 Applications Information CVCC SW SGND SVIN SGND SVIN INTVCC SW PGND PGND PGND PGND PGND PGND PGND PGND INTVCC SGND SW FCB SW ITH PVIN VRNG PVIN PGOOD PVIN VON PVIN SGND PVIN PVIN 16 LTC3610 SW 48 47 46 45 R1 44 R2 43 42 41 RON 40 39 RC 38 CC1 37 36 35 34 CC2 33 SGND 15 ION SGND CIN 14 SW RUN/SS 13 SGND BOOST 12 SW SGND 11 VFB NC 9 10 EXTVCC SW SW VOUT SW PVIN 8 SGND PVIN 7 SW PVIN 6 SGND PVIN COUT 5 SGND PGND PVIN 4 SGND PGND PVIN 3 PGND PVIN 2 PVIN 1 PGND 64 63 62 61 60 59 58 57 56 55 54 53 52 51 50 49 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 DB CB CSS RF 3610 F07 Figure 7. LTC3610 Layout Diagram When laying out a printed circuit board without a ground plane, use the following checklist to ensure proper operation of the controller. These items are also illustrated in Figure 7. • Keep the high dV/dT SW, BOOST and TG nodes away from sensitive small-signal nodes. • Segregate the signal and power grounds. All smallsignal components should return to the SGND pin at one point, which is then tied to the PGND pin. • Connect the top driver boost capacitor, CB, closely to the BOOST and SW pins. • Connect the input capacitor(s), CIN, close to the IC. This capacitor carries the MOSFET AC current. • Connect the INTVCC decoupling capacitor, CVCC, closely to the INTVCC and PGND pins. • Connect the VIN pin decoupling capacitor, CF, closely to the VIN and PGND pins. 3610ff 18 LTC3610 Typical Applications 3.3V Input to 1.5V/12A at 750kHz INTVCC CVCC 4.7µF 6.3V GND VIN2 = 5V CF 0.1µF 25V SW VIN VIN 3.3V C6 10µF 35V + (OPTIONAL) 12 13 14 15 16 SGND SVIN SGND SVIN INTVCC SW INTVCC PGND PGND PGND PGND PGND PGND PGND PGND LTC3610 SW SGND SW FCB SW ITH PVIN VRNG PVIN PGOOD PVIN VON PVIN SGND PVIN PVIN CIN 10µF 3× GND ION 48 47 46 EXTVCC C4 0.01µF R1 20.43k 1% 45 44 43 C1 (OPTIONAL) R2 30.1k 1% (OPTIONAL) C2 VOUT RON 113k 1% 42 41 40 CON 0.01µF (OPTIONAL) 39 38 VIN R5 11.15k CC1 470pF 37 36 35 RPG1 100k 34 33 CC2 100pF INTVCC (OPTIONAL) SGND 11 SGND SW SGND 10 SW RUN/SS 9 VFB BOOST 8 SW SGND 7 (OPTIONAL) GND EXTVCC NC 6 SW SW 5 PVIN L1 0.36µH SGND PVIN + SGND SW PVIN COUT1 220µF 2× PGND PVIN C5 22µF 6.3V SGND PVIN 4 SGND PGND PVIN 3 VOUT 1.5V AT 12A PGND PVIN 2 PVIN 1 PGND 64 63 62 61 60 59 58 57 56 55 54 53 52 51 50 49 RVON VOUT 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 SGND CB1 0.22µF CIN: TAIYO YUDEN TMK432BJ106MM COUT1: SANYO 10TPE220ML L1: TOKO FDH1040-36M C5: TAIYO YUDEN JMK316BJ226ML-T INTVCC RSS1 510k CSS 0.1µF (OPTIONAL) CVON 3610 TA02 VIN (OPTIONAL) 3610ff 19 LTC3610 Typical Applications 5V to 24V Input to 1.2V/12A at 550kHz INTVCC CVCC 4.7µF 6.3V GND CF 0.1µF 25V SW RF1 1Ω VIN VIN VIN 5V TO 24V C6 10µF 35V + (OPTIONAL) 12 13 14 15 16 SGND SVIN SGND SVIN INTVCC SW INTVCC PGND PGND PGND PGND PGND PGND PGND PGND LTC3610 SW SGND SW FCB SW ITH PVIN VRNG PVIN PGOOD PVIN VON PVIN SGND PVIN PVIN CIN 10µF 25V 3× GND ION 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 CB1 0.22µF C5: TAIYO YUDEN JMK316BJ226ML-T CIN: TAIYO YUDEN TMK432BJ106MM COUT1: SANYO 10TPE220ML L1: WURTH 744310055 INTVCC DB CMDSH-3 48 47 46 EXTVCC C4 0.01µF R1 30k 1% 45 44 C2 VOUT (OPTIONAL) 43 RON 301k 1% 42 41 40 CON 0.01µF (OPTIONAL) 39 38 VIN R5 31.84k CC1 470pF 37 36 35 RPG1 100k 34 33 CC2 100pF INTVCC RVON VOUT SGND SGND RSS1 510k CSS 0.1µF (OPTIONAL) R2 30.1k 1% C1 SGND 11 SGND SW SGND 10 SW RUN/SS 9 VFB BOOST 8 SW SGND 7 (OPTIONAL) GND EXTVCC NC 6 SW SW 5 PVIN L1 0.52µH SGND PVIN + SGND SW PVIN COUT1 220µF 2× PGND PVIN C5 22µF 6.3V SGND PVIN 4 SGND PGND PVIN 3 VOUT 1.2V AT 12A PGND PVIN 2 PVIN 1 PGND 64 63 62 61 60 59 58 57 56 55 54 53 52 51 50 49 (OPTIONAL) CVON 3610 TA03 VIN (OPTIONAL) 3610ff 20 LTC3610 Typical Applications 5V to 24V Input to 1.8V/12A All Ceramic 1MHz INTVCC CVCC 4.7µF 6.3V GND CF 0.1µF 25V SW RF1 1Ω VIN CIN 10µF 25V 3× VIN 12 13 14 15 16 SGND SVIN SGND SVIN INTVCC SW PGND PGND PGND PGND PGND PGND PGND PGND INTVCC SGND SW FCB SW ITH PVIN VRNG PVIN PGOOD PVIN VON PVIN SGND PVIN PVIN VIN 5V TO 24V LTC3610 SW 48 47 46 EXTVCC C4 0.01µF R1 10k 1% 45 44 43 C1 (OPTIONAL) (OPTIONAL) R2 20k 1% C2 VOUT RON 102k 1% 42 41 40 CON 0.01µF (OPTIONAL) 39 38 VIN R5 18.7k CC1 560pF 37 36 35 RPG1 100k 34 33 CC2 100pF INTVCC (OPTIONAL) SGND 11 ION SGND 10 SGND SW RUN/SS 9 SW BOOST 8 VFB SGND (OPTIONAL) GND SW NC 7 EXTVCC SW 6 SW PVIN 5 SGND PVIN L1 0.47µH SGND SW PVIN COUT 100µF 2× PGND PVIN C5 22µF 6.3V SGND PVIN 4 SGND PGND PVIN 3 VOUT 1.8V AT 12A PGND PVIN 2 PVIN 1 PGND 64 63 62 61 60 59 58 57 56 55 54 53 52 51 50 49 RVON VOUT 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 SGND CB1 0.22µF COUT: TAIYO YUDEN JMK325BJ107MY L1: TOKIN MPLC0730 C5: TAIYO YUDEN JMK316BJ226ML-T INTVCC DB CMDSH-3 RSS1 510k CSS 0.1µF (OPTIONAL) CVON 3610 TA04 VIN (OPTIONAL) 3610ff 21 aaa C 2x 2.04 1.81 1.30 2.98 3.60 PIN 1 0.30 – 0.50 0.87 aaa C 2x PAD 1 CORNER 1.42 1.17 1.39 0.30 (2x) 0.53 (2x) 2.30 0.20 – 0.30 0.95 RECOMMENDED SOLDER PAD LAYOUT TOP VIEW 3.85 3.99 4.53 3.50 3.30 0.50 TOP VIEW 9.00 BSC 3.30 4.10 1.92 2.01 B 9.00 BSC 1.19 A 3.06 // ccc C 0.90 ± 0.10 0.20 REF 0.00 – 0.05 5 SEATING PLANE 3.06 4.10 1.92 2.01 33 3.30 48 1.19 0.20 – 0.30 32 0.95 0.30 (2x) 0.53 (2x) 1.39 DIMENSION b APPLIES TO METALLIZED TERMINAL AND IS MEASURED BETWEEN 0.15mm AND 0.30mm FROM THE TERMINAL TIP. COPLANARITY APPLIES TO THE TERMINALS AND ALL OTHER SURFACE METALLIZATION 5 6 SYMBOL TOLERANCE aaa 0.15 bbb 0.10 ccc 0.10 3.85 3.99 4.53 3.50 3.30 0.50 BOTTOM VIEW (BOTTOM METALLIZATION DETAILS) 1.42 1.17 49 50 51 52 53 54 NOTE: 1. DIMENSIONING AND TOLERANCING CONFORM TO ASME Y14.5M-1994 2. ALL DIMENSIONS ARE IN MILLIMETERS, ANGLES ARE IN DEGREES (°) 3. N IS THE TOTAL NUMBER OF TERMINALS 4. THE LOCATION OF THE TERMINAL #1 IDENTIFIER AND TERMINAL NUMBERING CONVENTION CONFORMS TO JEDEC PUBLICATION 95 SPP-002 6 NX 0.08 C NX b bbb M C A B 22 (Reference LTC DWG # 05-08-1812 Rev A) 64-Lead QFN Multipad (9mm × 9mm) WP# Package (Reference LTC DWG 05-08-1812 Rev A) WP Package 64-Lead QFN Multipad (9mm × 9mm) 17 0.87 64 1.30 2.04 1.81 WP64 QFN REV A 0707 16 3.60 2.98 0.30 – 0.50 1 LTC3610 Package Description 3610ff LTC3610 Revision History (Revision history begins at Rev F) REV DATE DESCRIPTION F 07/10 I-grade part added. Reflected throughout the data sheet. PAGE NUMBER 1 to 24 3610ff Information furnished by Linear Technology Corporation is believed to be accurate and reliable. However, no responsibility is assumed for its use. Linear Technology Corporation makes no representation that the interconnection of its circuits as described herein will not infringe on existing patent rights. 23 LTC3610 Typical Application 14V to 24V Input to 12V/5A at 500kHz CVCC 4.7µF 6.3V INTVCC CF 0.1µF 25V SW GND RF1 1Ω VIN VIN VIN 14V TO 24V C6 10µF 35V + 12 13 14 15 16 (OPTIONAL) SGND SVIN SGND SVIN INTVCC SW PGND PGND PGND PGND PGND PGND PGND PGND INTVCC SGND SW FCB SW ITH PVIN VRNG PVIN PGOOD PVIN VON PVIN SGND PVIN PVIN CIN 10µF 25V 3× GND ION LTC3610 SW 48 47 46 EXTVCC C4 0.01µF R1 1.58k 1% 45 44 43 C1 (OPTIONAL) R2 30.1k 1% (OPTIONAL) C2 VOUT RON 1M 1% 42 41 40 CON 0.01µF (OPTIONAL) 39 38 VIN R5 20k CC1 560pF 37 36 35 RPG1 100k 34 33 CC2 100pF INTVCC (OPTIONAL) SGND 11 SGND SW SGND 10 SW RUN/SS 9 VFB BOOST 8 SW SGND (OPTIONAL) GND EXTVCC NC 7 SGND SW SW 6 SW PVIN 5 PVIN L1 5.7µH + SGND PVIN COUT 180µF 16V PGND PVIN C5 22µF 25V SGND PVIN 4 SGND PGND PVIN 3 VOUT 12V AT 5A PGND PVIN 2 PVIN 1 PGND 64 63 62 61 60 59 58 57 56 55 54 53 52 51 50 49 RVON INTVCC 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 SGND CIN: TAIYO YUDEN TMK432BJ106MM COUT: SANYO 16SVP180MX L1: SUMIDA CDEP1055R7 RSS1 510k CB1 0.22µF INTVCC DB CMDSH-3 CSS 0.1µF (OPTIONAL) CVON 3610 TA05 VIN (OPTIONAL) RUN/SS Related Parts PART NUMBER DESCRIPTION COMMENTS LTC1778 No RSENSE Current Mode Synchronous Step-Down Controller Up to 97% Efficiency, VIN: 4V to 36V, 0.8V ≤ VOUT ≤ (0.9)(VIN), IOUT Up to 20A LTC3411 1.25A (IOUT), 4MHz, Synchronous Step-Down DC/DC Converter 95% Efficiency, VIN: 2.5V to 5.5V, VOUT = 0.8V, IQ = 60µA, ISD <1µA, MS Package LTC3412 2.5A (IOUT) 4MHz Synchronous Step-Down DC/DC Converter 95% Efficiency, VIN: 2.5V to 5.5V, VOUT(MIN) = 0.8V, IQ = 60mA, ISD <1mA, TSSOP16E LTC3414 4A (IOUT), 4MHz, Synchronous Step-Down DC/DC Converter 95% Efficiency, VIN: 2.25V to 5.5V, VOUT(MIN) = 0.8V, IQ = 64µA, ISD <1µA, TSSOP20E Package LTC3418 8A (IOUT), 4MHz, Synchronous Step-Down DC/DC Converter 95% Efficiency, VIN: 2.25V to 5.5V, VOUT(MIN) = 0.8V, Thermally Enhanced 38-Lead QFN Package LTC3770 Fast, No RSENSE Step-Down Synchronous Controller with Margining, Tracking, PLL ±0.67% 0.6V Reference Voltage; Programmable Margining; True Current Mode; 4V ≤ VIN ≤ 32V LTC3778 Low VOUT, No RSENSE Synchronous Step-Down Controller 0.6V ≤ VOUT ≤ (0.9) VIN, 4V ≤ VIN ≤ 36V, IOUT Up to 20A LT3800 60V Synchronous Step-Down Controller Current Mode, Output Slew Rate Control LTM4600HV 10A Complete Switch Mode Power Supply 92% Efficiency, VIN: 4.5V to 28V, VOUT = 0.6V, True Current Mode Control, Ultrafast Transient Response LTM4601HV 12A Complete Switch Mode Power Supply 92% Efficiency, VIN: 4.5V to 28V, VOUT = 0.6V, True Current Mode Control, Ultrafast Transient Response LTM4602HV 6A Complete Switch Mode Power Supply 92% Efficiency, VIN: 4.5V to 28V, VOUT = 0.6V, True Current Mode Control, Ultrafast Transient Response LTM4603HV 6A Complete Switch Mode Power Supply 93% Efficiency, VIN: 4.5V to 28V, with PLL, Output Tracking and Margining 3610ff 24 Linear Technology Corporation 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com LT 0710 REV F • PRINTED IN USA LINEAR TECHNOLOGY CORPORATION 2007