LTC3608 18V, 8A Monolithic Synchronous Step-Down DC/DC Converter Description Features n n n n n n n n n n n n n n n n n 8A Output Current Wide VIN Range = 4V to 18V Internal N-Channel MOSFETs True Current Mode Control Optimized for High Step-Down Ratios tON(MIN) ≤ 100nsec 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 7mm × 8mm 52-Lead QFN Package Applications Point of Load Regulation Distributed Power Systems n n 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®3608 is a high efficiency, monolithic synchronous step-down DC/DC converter that can deliver up to 8A output current from a 4V to 18V (20V 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 LTC3608 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 LTC3608 is available in a compact 7mm × 8mm QFN package. Typical Application Efficiency and Power Loss vs Load Current High Efficiency Step-Down Converter VON ION RUN/SS VIN 100pF 187k 100 0.8µH SW 1500pF 11.3k 0.22µF ITH BOOST SGND INTVCC 30.1k FCB VRNG PGOOD EXTVCC 100µF ×2 10000 EFFICIENCY 90 1000 85 80 75 70 100 POWER LOSS 65 10 60 VIN = 12V VOUT = 2.5V EXTVCC = 5V 55 4.7µF PGND 50 0.01 VFB 3608 TA01a VOUT 2.5V 8A 95 9.53k POWER LOSS (mW) 10µF ×3 LTC3608 VIN 4V TO 18V EFFICIENCY (%) 0.1µF VOUT 0.1 1 LOAD CURRENT (A) 1 10 3608 TA01b 3608fc LTC3608 41 SW 42 SW 43 SW 44 SW 45 SW 46 SW 47 SW 48 PVIN 49 PVIN TOP VIEW 50 PVIN Input Supply Voltage (SVIN, PVIN, ION)........ 20V to –0.3V Boosted Topside Driver Supply Voltage (BOOST)................................................. 26V to –0.3V SW Voltage............................................. 20V 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 Junction Temperature Range (Notes 2, 4)......................................... –40°C to 125°C Storage Temperature Range.................... –55°C to 125°C pIN CONFIGURATION 51 PVIN (Note 1) 52 PVIN Absolute Maximum Ratings PVIN 1 40 PGND PVIN 2 39 PGND PVIN 3 38 PGND 53 PVIN PVIN 4 PVIN 5 37 PGND 55 SW 36 PGND PVIN 6 35 PGND PVIN 7 34 PGND SW 8 33 SW NC 9 32 INTVCC SGND 10 31 INTVCC BOOST 11 30 SVIN 54 SGND RUN/SS 12 29 EXTVCC VON 13 28 NC SGND 26 NC 25 NC 24 VFB 23 ION 22 NC 21 SGND 20 FCB 19 ITH 18 VRNG 17 SGND 15 27 SGND PGOOD 16 SGND 14 WKG PACKAGE 52-LEAD (7mm × 8mm) QFN MULTIPAD TJMAX = 125°C, θJA = 29°C/W Order Information LEAD FREE FINISH TAPE AND REEL PART MARKING* PACKAGE DESCRIPTION TEMPERATURE RANGE LTC3608EWKG#PBF LTC3608EWKG#TRPBF LTC3608WKG 52-Lead (7mm × 8mm) Plastic QFN –40°C to 125°C LTC3608IWKG#PBF LTC3608IWKG#TRPBF LTC3608WKG 52-Lead (7mm × 8mm) Plastic QFN –40°C to 125°C Consult LTC Marketing for parts specified with wider operating temperature ranges. *The temperature grade is identified by a label on the shipping container. For more information on lead free part marking, go to: http://www.linear.com/leadfree/ For more information on tape and reel specifications, go to: http://www.linear.com/tapeandreel/ 3608fc LTC3608 Electrical Characteristics The l denotes the specifications which apply over the full operating junction temperature range, otherwise specifications are at TA = 25°C. VIN = 15V unless otherwise noted. SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS Main Control Loop SVIN Operating Input Voltage Range IQ Input DC Supply Current Normal Shutdown Supply Current VFB Feedback Reference Voltage 4 ITH = 1.2V, –40°C to 85°C (Note 3) ITH = 1.2V, –40°C to 125°C (Note 3) l 0.594 0.590 18 V 900 15 2000 30 µA µA 0.600 0.600 0.606 0.610 V V ΔVFB(LINEREG) Feedback Voltage Line Regulation VIN = 4V to 18V, ITH = 1.2V (Note 3) 0.002 ΔVFB(LOADREG) Feedback Voltage Load Regulation ITH = 0.5V to 1.9V (Note 3) –0.05 –0.3 %/V % IFB Feedback Input Current VFB = 0.6V –5 ±50 nA gm(EA) Error Amplifier Transconductance ITH = 1.2V (Note 3) mS 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 tOFF(MIN) l 1.4 1.7 2 l 0.54 0.6 0.66 V –1 –2 µA 220 280 110 340 ns ns ION = 180µA, VON = 0V 60 100 ns Minimum Off-Time ION = 30µA, VON = 1.5V 320 500 ns 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 VRUN/SS(ON) RUN Pin Start Threshold 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 INTVCC Falling l 3.4 3.9 V VIN(UVLOR) Undervoltage Lockout Release INTVCC Rising l 3.5 4 V RDS(ON) Top Switch On-Resistance Bottom Switch On-Resistance 10 8 19 14 mΩ mΩ l l l 5 8 11 16 A A 5.5 7.5 A A 7 10 13 % 0.8 1.5 2 V 3608fc LTC3608 Electrical Characteristics The l denotes the specifications which apply over the full operating junction temperature range, otherwise specifications are at TA = 25°C. VIN = 15V unless otherwise noted. SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS 5 5.5 V –0.1 ±2 % Internal VCC Regulator VINTVCC Internal VCC Voltage 6V < VIN < 18V, 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 l l 4.7 4.5 4.7 150 V 300 mV 500 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 • 29°C/W)(θJA is simulated per JESD51-7 high effective thermal conductivity test board) θJC = 1°C/W (θJC is simulated when heat sink is applied at the bottom of the package.) Note 3: The LTC3608 is tested in a feedback loop that adjusts VFB to achieve a specified error amplifier output voltage (ITH). The specification at 85°C is not tested in production. This specification is assured by design, characterization, and correlation to testing at 125°C. Note 4: The LTC3608 is tested under pulsed load conditions such that TJ ≈ TA. The LTC3608E is guaranteed to meet specifications from 0°C to 125°C junction temperature. Specifications over the –40°C to 125°C operating junction temperature range are assured by design, characterization and correlation with statistical process controls. The LTC3608I is guaranteed over the full –40°C to 125°C operating junction temperature range. Note that the maximum ambient temperature consistent with these specifications is determined by specific operating conditions in conjunction with board layout, the rated package thermal impedance and other environmental factors. typical performance characteristics Transient Response Transient Response VOUT 200mV/DIV Start-Up VOUT 200mV/DIV RUN/SS 2V/DIV IL 5A/DIV IL 5A/DIV VOUT 1V/DIV ILOAD 5A/DIV ILOAD 5A/DIV 20µs/DIV LOAD STEP 0A TO 8A VIN = 12V VOUT = 2.5V FCB = 0V FIGURE 6 CIRCUIT IL 5A/DIV 3608 G01 20µs/DIV VIN = 12V VOUT = 2.5V FCB = INTVCC FIGURE 6 CIRCUIT 3610 G02 40ms/DIV VIN = 12V VOUT = 2.5V RLOAD = 0.5Ω FIGURE 6 CIRCUIT 3608 G03 3608fc LTC3608 Typical Performance Characteristics Efficiency vs Load Current Efficiency vs Input Voltage 100 100 40 30 20 10 VIN = 12V FREQ = 550kHz 0 0.01 0.1 1 LOAD CURRENT (A) VOUT = 5V VOUT = 3.3V VOUT = 2.5V VOUT = 2.5V VOUT = 1.8V VOUT = 1.2V VOUT = 1V ILOAD = 1A 85 80 10 550 500 ILOAD = 1A 5 400 20 10 15 INPUT VOLTAGE (V) 0.80 5 10 15 INPUT VOLTAGE (V) 3608 G05 Load Regulation 2.5 FIGURE 6 CIRCUIT 20 3608 G06 ITH Voltage vs Load Current 0.60 600 CONTINUOUS MODE DISCONTINUOUS MODE ITH VOLTAGE (V) 400 300 2.0 0.40 ΔVOUT (%) 500 0.20 0 –0.20 200 –0.40 100 1.5 CONTINUOUS MODE 1.0 DISCONTINUOUS MODE 0.5 –0.60 0 2 4 6 LOAD CURRENT (A) 8 10 –0.80 25 VRNG = 20 0 3608 G07 Load Current vs ITH Voltage and VRNG 2 4 6 LOAD CURRENT (A) 8 0 10 On-Time vs ION Current 10000 5 LOAD CURRENT (A) 10 3608 G09 On-Time vs VON Voltage 1000 VVON = 0V 1V 0 3608 G08 ION = 30µA 800 0.7V ON-TIME (ns) 0.5V 10 5 0 1000 ON-TIME (ns) 15 100 600 400 200 –5 –10 ILOAD = 10A 450 3608 G04 700 FREQUENCY (kHz) ILOAD = 10A 90 Frequency vs Load Current LOAD CURRENT (A) FREQUENCY (kHz) 50 EFFICIENCY (%) EFFICIENCY (%) 60 FCB = 0V FIGURE 6 CIRCUIT 600 95 70 0 650 FCB = 5V FIGURE 6 CIRCUIT 90 80 Frequency vs Input Voltage 0 0.5 1.0 1.5 2.0 ITH VOLTAGE (V) 2.5 3.0 3608 G10 10 1 10 ION CURRENT (µA) 100 3608 G11 0 0 1 2 VON VOLTAGE (V) 3 3608 G12 3608fc LTC3608 Typical Performance Characteristics 200 150 100 50 0 –50 –25 25 75 0 50 TEMPERATURE (°C) 100 15 10 0.7 0.8 VRNG VOLTAGE (V) 0.9 MAXIMUM VALLEY CURRENT (A) 15 10 5 0 50 75 25 TEMPERATURE (°C) 100 3608 G14 3 3608 G15 Maximum Valley Current Limit in Foldback 125 16 14 12 10 8 6 4 4 8 3608 G16 20 12 16 INPUT VOLTAGE (V) 2.0 0.62 10 5 0 0 0.1 0.2 3608 G17 0.3 VFB (V) 0.4 0.5 0.6 3608 G18 Error Amplifier gm vs Temperature 1.8 0.61 gm (mS) FEEDBACK REFERENCE VOLTAGE (V) 6 15 Feedback Reference Voltage vs Temperature 0.60 0.59 0.58 –50 9 0 1.65 1.90 2.15 2.40 2.65 2.90 3.15 3.40 RUN/SS VOLTAGE (V) 1.0 18 –25 12 Input Voltage vs Maximum Valley Current 20 MAXIMUM VALLEY CURRENT LIMIT (A) 0.6 0.5 3608 G13 Maximum Valley Current Limit vs Temperature 0 –50 15 20 5 125 Maximum Valley Current Limit vs RUN/SS Voltage MAXIMUM VALLEY CURRENT LIMIT (A) ON-TIME (ns) 250 25 IION = 30µA VVON = 0V MAXIMUM VALLEY CURRENT LIMIT (A) 300 Maximum Valley Current Limit vs VRNG Voltage MAXIMUM VALLEY CURRENT LIMIT (A) On-Time vs Temperature 1.6 1.4 1.2 –25 75 0 25 50 TEMPERATURE (°C) 100 125 3608 G19 1.0 –50 –25 50 0 75 25 TEMPERATURE (°C) 100 125 3608 G20 3608fc LTC3608 Typical Performance Characteristics Input and Shutdown Currents vs Input Voltage 0.20 25 800 20 SHUTDOWN 600 15 400 10 EXTVCC = 5V 0 5 10 15 INPUT VOLTAGE (V) 20 14 0 –0.30 0 –0.40 FCB PIN CURRENT (µA) 4 2 2 0 40 10 20 30 INTVCC LOAD CURRENT (mA) 100 125 –0.25 2 –0.50 –0.75 –1.00 3608 G24 25 75 0 50 TEMPERATURE (°C) 100 UNDERVOLTAGE LOCKOUT THRESHOLD (V) RUN/SS PIN CURRENT (µA) 4.5 LATCHOFF ENABLE 4.0 LATCHOFF THRESHOLD 75 0 25 50 TEMPERATURE (°C) 900 1000 3608 G23 125 PULL-DOWN CURRENT 1 0 –1 –2 –50 PULL-UP CURRENT –25 3608 G25 0 50 75 25 TEMPERATURE (°C) 100 125 3608 G26 Undervoltage Lockout Threshold vs Temperature 5.0 –25 600 700 800 FREQUENCY (kHz) RUN/SS Pin Current vs Temperature 3 –1.50 –50 –25 500 3608 G22 0 RUN/SS Pin Current vs Temperature 3.0 –50 0 400 50 –1.25 3.5 8 FCB Pin Current vs Temperature 6 0 50 75 25 TEMPERATURE (°C) 10 4 3608 G21 8 –25 12 6 –0.20 5 VIN = 20V 16 –0.10 10 EXTVCC SWITCH RESISTANCE (Ω) 18 0.10 EXTVCC Switch Resistance vs Temperature 0 –50 20 RUN/SS PIN CURRENT (µA) 200 SHUTDOWN CURRENT (µA) INPUT CURRENT (µA) 35 30 1000 0 0.30 IEXTVCC (mA) EXTVCC OPEN 1200 40 ΔINTVCC (%) 1400 IEXTVCC vs Frequency INTVCC Load Regulation 100 125 3608 G27 4.0 3.5 3.0 2.5 2.0 –50 –25 75 0 25 50 TEMPERATURE (°C) 100 125 3608 G28 3608fc LTC3608 Pin Functions PVIN (Pins 1, 2, 3, 4, 5, 6, 7, 48, 49, 50, 51, 52, 53): Main Input Supply. Decouple this pin to power PGND with the input capacitance, CIN SW (Pins 8, 33, 41, 42, 43, 44, 45, 46, 47, 55): 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. SGND (Pins 10, 14, 15, 20, 26, 27, 54): 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 11): 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 12): 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 13): 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 16): 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 17): 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 16A current limit. ITH (Pin 18): 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 19): 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. NC (Pins 9, 21, 24, 25, 28): No Connection. ION (Pin 22): 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 23): Error Amplifier Feedback Input. This pin connects the error amplifier input to an external resistive divider from VOUT. EXTVCC (Pin 29): 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 (Pin 30): Supply Pin for Internal PWM Controller. INTVCC (Pins 31, 32): 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. PGND (Pins 34, 35, 36, 37, 38, 39, 40): Power Ground. Connect this pin closely to the (–) terminal of CVCC and the (–) terminal of CIN. 3608fc LTC3608 Functional Diagram RON VON ION 13 22 FCB EXTVCC 19 29 SVIN 30 4.7V 0.7V 2.4V + 1µA PVIN – 1, 2, 3, 4, 5, 6, 7, 48, 49, 50, 51, 52, 53 0.6V REF 0.6V CIN 5V REG INTVCC + – 31, 32 F 11 VVON tON = (10pF) IION R S Q FCNT SW + ICMP – L1 DB VOUT 8, 33, 41, 42, 43, 44, 45, 46, 47, 55 SWITCH LOGIC IREV – SHDN 1.4V COUT OV M2 + CVCC 17 PGND × (0.5 TO 2) 0.7V 34, 35, 36, 37, 38, 39, 40 16 PGOOD 1 240k + 1V Q2 Q4 – Q6 ITHB R2 0.54V UV 23 Q3 Q1 R1 SGND OV + – VFB + – 0.8V – SS + 1.2µA + – 6V + 0.6V 18 ITH 10, 14, 15, 20, 26, 27, 54 0.66V RUN SHDN EA ×3.3 – VRNG CB M1 ON 20k + BOOST 0.4V 12 27 NC 9, 21, 24, 25, 28 3608 FD RUN/SS CSS 3608fc LTC3608 Operation Main Control Loop The LTC3608 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 18V (20V maximum) and provides a regulated output voltage at up to 8A 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. 3608fc 10 LTC3608 Applications Information The basic LTC3608 application circuit is shown on the front page of this data sheet. External component selection is primarily determined by the maximum load current. The LTC3608 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 tradeoff 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 LTC3608 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 LTC3608 has an open-drain PGOOD output that indicates when the output voltage is within ±10% of the regulation point. The LTC3608 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. 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 V ION = IN RON 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 LTC3608 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 = f= VVON VOUT [H ] RON (10pF) Z 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. 3608fc 11 LTC3608 Applications Information 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. RON2 = 5V R 0.7V ON 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 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. SWITCHING FREQUENCY (kHz) 1000 VOUT = 3.3V VOUT = 1.5V 100 100 VOUT = 2.5V 1000 RON (kΩ) 10000 3608 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) 10000 3608 F01b Figure 1b. Switching Frequency vs RON (VON = INTVCC) 3608fc 12 LTC3608 Applications Information 2.0 CVON 0.01µF RVON2 100k RC VON LTC3608 ITH CC (2a) VOUT INTVCC RVON1 3k 10k CVON 0.01µF RVON2 10k RC Q1 2N5087 CC VON LTC3608 ITH 1.0 0.5 0 0.25 0.50 0.75 DUTY CYCLE (VOUT/VIN) 1.0 3608 F03 Figure 3. Maximum Switching Frequency vs Duty Cycle Minimum Off-time and Dropout Operation The minimum off-time, tOFF(MIN), is the smallest amount of time that the LTC3608 is capable of turning on the bottom MOSFET, tripping the current comparator and turning the MOSFET back off. This time is generally about 320ns. 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: tON + tOFF(MIN) tON A plot of Maximum Duty Cycle vs Frequency is shown in Figure 3. Setting the Output Voltage The LTC3608 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⎠ DROPOUT REGION 3608 F02 Figure 2. Correcting Frequency Shift with Load Current Changes 1.5 0 (2b) VIN(MIN) = VOUT SWITCHING FREQUENCY (MHz) VOUT RVON1 30k To improve the frequency response, a feed forward 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 line. 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 frequency with small ripple current. However, achieving this requires a large inductor. There is a tradeoff between component size, efficiency and operating frequency. 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: ⎛ V ⎞⎛ VOUT ⎞ OUT 1− L=⎜ ⎟⎜ ⎟ ⎝ f ΔIL(MAX) ⎠ ⎝ VIN(MAX) ⎠ 3608fc 13 LTC3608 Applications Information 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 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. 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 ⎟⎠ ⎝ 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 3608fc 14 LTC3608 Applications Information Fault Conditions: Current Limit and Foldback threshold forces continuous synchronous operation, allowing current to reverse at light loads and maintaining high frequency operation. The LTC3608 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 LTC3608 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. 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: An internal P-channel low dropout regulator produces the 5V supply that powers the drivers and internal circuitry within the LTC3608. 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. ⎛ R4 ⎞ VOUT2(MIN) = 0.6V ⎜ 1+ ⎟ ⎝ R3 ⎠ = SGND 52 NC SGND SVIN EXTVCC INTVCC SW PGND PGND PGND PGND INTVCC SGND PVIN FCB PVIN ITH PVIN VRNG PVIN PGOOD PVIN SGND 1 2 3 4 5 6 7 8 9 10 11 12 13 14 SW 26 25 24 23 22 21 20 19 18 17 16 R4 OPTIONAL EXTVCC CONNECTION 5V < VOUT2 < 7V R3 15 SGND = PGND SW VON 51 NC LTC3608 RUN/SS 50 SW BOOST 49 ION SGND 48 VFB SW NC VIN SW SW CIN + NC PVIN 47 SW PVIN 46 NC PVIN 45 SW PVIN 44 SGND PVIN 43 PGND PGND • + SW PVIN COUT 41 42 T1 1:N PVIN VOUT1 + PGND 40 39 38 37 36 35 34 33 32 31 30 29 28 27 VOUT2 CSEC 1µF SW GND IN4148 • INTVCC Regulator and EXTVCC Connection 3608 F04 SGND Figure 4. Secondary Output Loop and EXTVCC Connection 3608fc 15 LTC3608 Applications Information 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. 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]) Soft-Start and Latchoff with the RUN/SS Pin Generally 0.1µF is more than sufficient. The RUN/SS pin provides a means to shut down the LTC3608 as well as a timer for soft-start and overcurrent latchoff. Pulling the RUN/SS pin below 0.8V puts the LTC3608 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: 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. tDELAY = 1.5V C = (1.3s/µF ) CSS 1.2µA SS When the voltage on RUN/SS reaches 1.5V, the LTC3608 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 3608fc 16 LTC3608 Applications Information INTVCC RSS* VIN 3.3V OR 5V D1 RUN/SS RSS* CSS D2* RUN/SS 2N7002 CSS 3608 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 LTC3608 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. 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. 3608fc 17 LTC3608 Applications Information Design Example Next, set up VRNG voltage and check the ILIMIT. Tying VRNG to 0.5V will set the typical current limit to 11A, and tying VRNG to GND will result in a typical current around 16A. CIN is chosen for an RMS current rating of about 5A at 85°C. The output capacitors are chosen for a low ESR of 0.002Ω to minimize output voltage changes due to inductor ripple current and load steps. The ripple voltage will be only: As a design example, take a supply with the following specifications: VIN = 5V to 20V (12V nominal), VOUT = 2.5V ± 5%, IOUT = 8A, f = 550kHz. First, calculate the timing resistor with VON = VOUT: 2.5V RON = ≈ 187k 550kHz ) (10pF )(2.4V) ( and choose the inductor for about 40% ripple current at the maximum VIN: 2.5V ⎛ 2.5V ⎞ L= 1− = 1.24µH 550kHz ) ( 0.4) ( 8A ) ⎜⎝ 20V ⎟⎠ ( Selecting a standard value of 1.2µH results in a maximum ripple current of: 2.5V ⎛ 2.5V ⎞ ΔIL = 1– = 3A 550kHz ) (1.2µH) ⎜⎝ 12V ⎟⎠ ( ΔVOUT(RIPPLE)= ΔIL(MAX) (ESR) = (3A) (0.002Ω) = 6mV However, a 0A to 8A load step will cause an output change of up to: ΔVOUT(STEP) = ΔILOAD (ESR) = (8A) (0.002Ω) = 16mV 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. INTVCC CVCC 4.7µF 6.3V PGND CF RF1 0.47µF 1Ω 25V SW EXTVCC C4 0.01µF VIN SGND 51 52 NC SGND SVIN EXTVCC INTVCC SW INTVCC PGND PGND PGND PGND PGND SW SGND PVIN FCB PVIN ITH PVIN VRNG PVIN PGOOD PVIN SGND 26 25 24 R1 9.5k 1% C1 1 2 3 4 5 6 7 8 9 10 11 12 13 14 RON 187k 1% (OPTIONAL) 23 22 21 CON 0.01µF 20 INTVCC KEEP POWER GROUND AND SIGNAL GROUND SEPARATE. CONNECT AT ONE POINT. DB CMDSH-3 C2 VOUT CC1 1500pF 17 16 R3 0Ω 15 RPG1 100k INTVCC C3 CC2 100pF (OPTIONAL) SGND RVON 0Ω RSS1 510k CB1 0.22µF SW (OPTIONAL) VIN R5 11.3k 18 SW CIN: TAIYO YUDEN GMK325BJ106MM-B COUT: TDKC2012X5ROJ226M L1: CDEP85NP-R80MC-50 C5: MURATA GRM31CR60J226KE19 R2 30.1k 1% (OPTIONAL) 19 SGND (OPTIONAL) 50 LTC3608 VON + NC RUN/SS C6 10µF 35V 49 ION SW BOOST CIN 10µF 35V 3× GND 48 SW SGND 47 VIN VIN 5V TO 18V VFB NC 46 SW SW 45 NC PVIN 44 (OPTIONAL) GND SW PVIN 43 NC PVIN + SGND SW PVIN COUT1 100µF ×2 SW PVIN C5 22µF 6.3V 42 PVIN 41 L1 0.8µH PVIN VOUT 2.5V AT 8A PGND PGND 40 39 38 37 36 35 34 33 32 31 30 29 28 27 CSS 0.1µF VIN (OPTIONAL) VOUT 0.1µF 3608 F06 = PGND = SGND Figure 6. Design Example: 5V to 18V Input to 2.5V/8A at 550kHz 18 3608fc LTC3608 Applications Information 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. 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 LTC3608. • Place CIN and COUT all in one compact area, close to the LTC3608. It may help to have some components on the bottom side of the board. • 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). 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. • 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 input capacitor(s), CIN, close to the IC. This capacitor carries the MOSFET AC current. • Keep the high dV/dT SW, BOOST and TG nodes away from sensitive small-signal nodes. • Connect the INTVCC decoupling capacitor, CVCC, closely to the INTVCC and PGND pins. • Connect the top driver boost capacitor, CB, closely to the BOOST and SW pins. • Connect the VIN pin decoupling capacitor, CF , closely to the VIN and PGND pins. • Keep small-signal components close to the LTC3608. • Ground connections (including LTC3608 SGND and PGND) should be made through immediate vias to the ground plane. Use several larger vias for power components. 3608fc 19 LTC3608 Applications Information CVCC SW VIN NC SGND SVIN EXTVCC INTVCC SW INTVCC PGND PGND PGND PGND PGND SW SGND PVIN FCB PVIN ITH PVIN VRNG PVIN PGOOD PVIN SGND SGND 52 NC LTC3608 1 2 3 4 5 6 7 8 9 10 11 12 13 14 VON 51 SW RUN/SS 50 CIN ION BOOST 49 SW SGND 48 VFB NC 47 SW SW 46 NC PVIN VOUT SW PVIN 45 NC PVIN 44 SW PVIN 43 SGND PVIN COUT SW PVIN 42 PVIN 41 PGND PGND 40 39 38 37 36 35 34 33 32 31 30 29 28 27 26 25 24 23 R1 R2 RON 22 21 20 19 18 RC CC1 17 16 15 CC2 CB DB CSS 3608 F07 Figure 7. LTC3608 Layout Diagram 3608fc 20 LTC3608 Typical Applications 3.6V Input to 1.5V/8A at 750kHz VIN2 = 5V INTVCC CVCC 4.7µF 6.3V PGND EXTVCC C4 0.01µF CF 0.47µF 25V SW SGND 50 51 52 (OPTIONAL) NC SGND SVIN EXTVCC INTVCC SW PGND PGND PGND PGND PGND PGND INTVCC NC LTC3608 SW SGND PVIN FCB PVIN ITH PVIN VRNG PVIN PGOOD PVIN SGND 1 2 3 4 5 6 7 8 9 10 11 12 13 14 CIN: TAIYO YUDEN TMK432BJ106MM COUT1: TDKC4532X5ROJ107M L1: CDEP85NP-R20MC-50 C5: TAIYO YUDEN JMK316BJ226ML-T INTVCC KEEP POWER GROUND AND SIGNAL GROUND SEPARATE. CONNECT AT ONE POINT. 26 25 24 R1 20.5k 1% C1 RON 113k 1% (OPTIONAL) 23 22 21 CON 0.01µF 20 17 39.2k 16 (OPTIONAL) C2 VOUT VIN R5 6.19k 18 15 R2 30.1k 1% (OPTIONAL) 19 CC1 3300pF INTVCC 11k RPG1 100k SGND 49 SW VON GND ION RUN/SS + SW BOOST C6 10µF 10V VFB SGND CIN 10µF 3× 48 SW NC 47 VIN VIN 3.6V NC SW 46 SW PVIN 45 NC PVIN 44 (OPTIONAL) GND SGND SW PVIN 43 SW PVIN COUT1 100µF ×2 42 PVIN C5 22µF 6.3V L1 0.2µH PVIN 41 + PVIN VOUT 1.5V AT 8A PGND 40 39 38 37 36 35 34 33 32 31 30 29 28 27 CC2 100pF INTVCC SGND VOUT CB1 0.22µF VOUT RSS1 510k CSS 0.1µF 0.1µF VIN (OPTIONAL) 3608 TA02 = PGND = SGND Transient Response Efficiency Curve 100 95 90 EFFICIENCY (%) IL 5A/DIV VOUT 200mV/DIV 20µs/DIV LOAD STEP 1A-8A VIN = 3.6V VOUT = 1.5V FCB = 0V 3608 TA02a 85 DCM CCM 80 75 70 65 60 55 50 100 VIN = 3.6V FREQ = 750kHz 1000 1000 LOAD CURRENT (A) 10000 3608 TA02b 3608fc 21 LTC3608 Typical Applications 5V to 18V Input to 1.2V/8A at 550kHz RF1 1Ω INTVCC CVCC 4.7µF 6.3V PGND VIN2 EXTVCC C4 0.01µF CF 0.47µF 25V SW SGND 50 51 52 (OPTIONAL) NC SGND SVIN EXTVCC INTVCC SW INTVCC PGND PGND PGND PGND PGND PGND NC LTC3608 SW SGND PVIN FCB PVIN ITH PVIN VRNG PVIN PGOOD PVIN SGND 1 2 3 4 5 6 7 8 9 10 11 12 13 14 C5: TAIYO YUDEN JMK316BJ226ML-T CIN: TAIYO YUDEN TMK432BJ106MM COUT1: TDKC4532X5R107M L1: CDEP85NP-R50MC-125 R1 30k 1% 25 24 23 DB CMDSH-3 C1 RON 187k 1% (OPTIONAL) 22 21 CON 0.01µF 20 R2 30.1k 1% (OPTIONAL) C2 VOUT VIN (OPTIONAL) 19 R5 7.68k 18 CC1 1500pF 17 16 15 RPG1 100k CC2 100pF INTVCC SGND VOUT CB1 0.22µF INTVCC KEEP POWER GROUND AND SIGNAL GROUND SEPARATE. CONNECT AT ONE POINT. 26 SGND + SW VON C6 10µF 35V 49 ION RUN/SS CIN 10µF 25V 3× GND SW BOOST VIN 5V TO 18V 48 VFB SGND 47 VIN SW NC 46 NC SW 45 NC SW PVIN 44 (OPTIONAL) GND SW PVIN 43 PVIN + SGND PVIN COUT1 100µF ×2 SW PVIN C5 22µF 6.3V 42 PVIN 41 L1 0.5µH PVIN VOUT 1.2V AT 8A PGND 40 39 38 37 36 35 34 33 32 31 30 29 28 27 RSS1 510k CSS 0.1µF RVON 0.1µF VOUT (OPTIONAL) CVON VIN (OPTIONAL) 3608 TA03 = PGND = SGND Transient Response Efficiency vs Load Current 90 VIN = 12V 85 FREQ = 550kHz 80 EFFICIENCY (%) IL 5A/DIV VOUT 200mV/DIV 75 70 65 DCM OCM 60 20µs/DIV LOAD STEP 1A-8A VIN = 12V VOUT = 1.2V FCB = 0V 3608 TA03a 55 50 100 1000 1000 LOAD CURRENT (A) 10000 3608 TA03b 3608fc 22 LTC3608 Typical Applications 5V to 18V Input to 1.8V/8A All Ceramic 1MHz RF1 1Ω INTVCC EXTVCC C4 0.01µF CF 0.1µF 25V CVCC 4.7µF 6.3V PGND VIN SW SGND 52 NC SGND SVIN EXTVCC INTVCC SW INTVCC PGND PGND PGND PGND PGND PGND SW SGND PVIN FCB PVIN ITH PVIN VRNG PVIN PGOOD PVIN SGND 1 2 3 4 5 6 7 8 9 10 11 12 13 14 COUT: TDKC3225XROJ107M L1: VISHAY IHLP2525-R47 C5: TAIYO YUDEN JMK316BJ226ML-T = PGND R1 15k 1% 25 24 C1 DB CMDSH-3 RON 102k 1% (OPTIONAL) 23 22 21 CON 0.01µF 20 R2 30.1k 1% (OPTIONAL) C2 VOUT VIN (OPTIONAL) 19 R5 5.76k 18 CC1 1500pF 17 16 15 RPG1 100k CC2 100pF INTVCC SGND VOUT CB1 0.22µF INTVCC KEEP POWER GROUND AND SIGNAL GROUND SEPARATE. CONNECT AT ONE POINT. 26 SGND 51 NC LTC3608 VON 50 ION SW RUN/SS 49 CIN 10µF 25V 3× SW BOOST 48 VFB SGND 47 VIN VIN 5V TO 18V SW NC 46 NC SW 45 SW PVIN 44 (OPTIONAL) GND NC PVIN 43 SW PVIN + SGND PVIN COUT1 100µF ×2 SW PVIN C5 22µF 6.3V 42 PVIN 41 L1 0.47µH PVIN VOUT 1.8V AT 8A PGND 40 39 38 37 36 35 34 33 32 31 30 29 28 27 VOUT RSS1 510k CSS 0.1µF 0.1µF VIN (OPTIONAL) 3608 TA04 = SGND Transient Response Efficiency vs Load Current 90 80 EFFICIENCY (%) IL 5A/DIV VOUT 200mV/DIV 20µs/DIV LOAD STEP 1A-5A VIN = 12V VOUT = 1.8V FCB = 0V 3608 TA04a DCM CCM 70 60 50 40 30 100 VIN = 12V 1000 LOAD CURRENT (mA) 10000 3608 TA04b 3608fc 23 LTC3608 Package Description WKG Package 52-Lead QFN Multipad (7mm × 8mm) (Reference LTC DWG # 05-08-1768 Rev Ø) SEATING PLANE A 7.00 BSC 0.00 – 0.05 2.625 REF 41 B PAD 1 CORNER 4 2.90 REF 0.50 BSC 40 bbb M C A B 7 8.00 BSC PIN 1 ID 52 1 3.40 REF 3.20 ± 0.10 2.025 ± 0.10 2.925 ± 0.10 3.90 ± 0.10 3.40 REF 33 8 32 9 1.00 REF 10 aaa C 2x NX b TOP VIEW 0.90 ± 0.10 NX 0.08 C // ccc C 8 7.50 ± 0.05 2.90 REF 0.50 BSC 1.35 ± 0.10 1.775 REF 15 0.25 ± 0.05 BOTTOM VIEW (BOTTOM METALLIZATION DETAILS) MLP52 QFN REV Ø 0807 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 2.025 ± 0.10 3.40 REF 2.925 ± 0.10 3.90 ± 0.10 8.50 ± 0.05 4.275 ± 0.10 PACKAGE OUTLINE 0.40 ± 0.10 1.775 REF 1.35 ± 0.10 THE LOCATION OF THE TERMINAL #1 IDENTIFIER AND TERMINAL NUMBERING CONVENTION CONFORMS TO JEDEC PUBLICATION 95 SPP-002 5. ND AND NE REFER TO THE NUMBER OF TERMINALS ON EACH D AND E SIDE RESPECTIVELY 6. NJR REFER TO NON JEDEC REGISTERED 7 DIMENSION b APPLIES TO METALLIZED TERMINAL AND IS MEASURED BETWEEN 0.20mm AND 0.30mm FROM THE TERMINAL TIP. IF THE TERMINAL HAS THE OPTIONAL RADIUS ON THE OTHER END OF THE TERMINAL, THE DIMENSION b SHOULD NOT BE MEASURED IN THAT RADIUS AREA. 8 COPLANARITY APPLIES TO THE TERMINALS AND ALL OTHER SURFACE METALLIZATION 9 DRAWING SHOWN ARE FOR ILLUSTRATION ONLY 1.00 REF 0.25 ± 0.05 0.40 ± 0.10 19 2.625 REF PIN 1 2.25 ± 0.10 14 26 9 3.40 REF 2.25 ± 0.10 27 0.580 ± 0.10 aaa C 2x 3.20 ± 0.10 4.275 ± 0.10 SYMBOL TOLERANCE aaa 0.15 bbb 0.10 ccc 0.10 RECOMMENDED SOLDER PAD LAYOUT TOP VIEW 3608fc 24 LTC3608 Revision History (Revision history begins at Rev C) REV DATE DESCRIPTION C 06/10 Updated SW voltage range in Absolute Maximum Ratings. PAGE NUMBER 2 Note 4 updated. 4 3608fc 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. 25 LTC3608 Typical Application 14V to 18V Input to 12V/5A at 500kHz CVCC 4.7µF, 6.3V PGND INTVCC SW SGND 42 43 44 45 46 47 NC SGND SVIN EXTVCC INTVCC SW PGND PGND PGND PGND PGND VFB SW ION SW NC LTC3608 SW 48 SGND PVIN CIN: TAIYO YUDEN TMK432BJ106MM COUT: SANYO 16SVP180MX L1: CDEP85NP-4R3MC-88 PVIN PGOOD PVIN SGND 2 3 4 5 6 7 8 9 10 11 12 13 14 25 24 DB CMDSH-3 R1 3.16k 1% C1 22 21 CON 0.01µF 20 R5 24.9k 18 17 16 15 90.9k 90 EFFICIENCY (%) 20µs/DIV LOAD STEP 1A-8A VIN = 18V VOUT = 12V FCB = 0V VOUT VIN CC1 3300pF INTVCC RPG1 100k INTVCC CC2 100pF SGND INTVCC (OPTIONAL) VIN CVON (OPTIONAL) RUN/SS 3608 TA05 100 95 3608 TA05a C2 Efficiency Curve Transient Response VOUT 200mV/DIV (OPTIONAL) 10k = SGND IL 5A/DIV R2 60.4k 1% (OPTIONAL) 19 RSS1 510k CSS 0.1µF RON 1M 1% (OPTIONAL) 23 CB1 0.22µF INTVCC = PGND VON 1 KEEP POWER GROUND AND SIGNAL GROUND SEPARATE. CONNECT AT ONE POINT. 26 SGND VRNG RUN/SS ITH PVIN BOOST 52 PVIN SGND 50 FCB NC 49 51 (OPTIONAL) SW SW + NC PVIN C6 10µF 35V SW PVIN CIN 10µF 25V 3× GND NC PVIN VIN VIN 14V TO 18V SGND SW PVIN (OPTIONAL) GND SW PVIN L1 4.3µH PVIN + PVIN COUT1 180µF 16V INTVCC 41 C5 22µF 25V PGND PGND 40 39 38 37 36 35 34 33 32 31 30 29 28 27 VOUT 12V AT 5A EXTVCC C4 0.01µF VIN2 RF1 CF 0.1µF 1Ω 25V 85 80 DCM CCM 75 70 65 60 55 50 100 VIN = 18V FREQ = 500kHz 1000 1000 LOAD CURRENT (A) 10000 3608 TA05b 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 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 LTC3610 12A Current Mode Monolithic Synchronous Step-Down Converter Up to 24V Input (28V Maximum). Current Mode Extremely Fast Transient Response 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 LTM4603HV 6A Complete Switch Mode Power Supply 93% Efficiency, VIN: 4.5V to 28V, with PLL, Output Tracking and Margining with Ultrafast Transient Response 3608fc 26 Linear Technology Corporation 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 l FAX: (408) 434-0507 l www.linear.com LT 0610 REV C • PRINTED IN USA LINEAR TECHNOLOGY CORPORATION 2008