LTC3614

FEATURES n n n n n n n n n n n n n n n LTC3614 4A, 4MHz Monolithic Synchronous Step-Down DC/DC Converter DESCRIPTION The LTC®3614 is a low quiescent current monolithic synchronous buck regulator using a current mode, constant frequency architecture. The no-load DC supply current in sleep mode is only 75μA while maintaining the output voltage (Burst Mode operation) at no load, dropping to zero current in shutdown. The 2.25V to 5.5V input supply voltage range makes the LTC3614 ideally suited for single Li-Ion as well as fixed low voltage input applications. 100% duty cycle capability provides low dropout operation, extending the operating time in battery-powered systems. The operating frequency is externally programmable up to 4MHz, allowing the use of small surface mount inductors. For switching-noise-sensitive applications, the LTC3614 can be synchronized to an external clock at up to 4MHz. Forced continuous mode operation in the LTC3614 reduces noise and RF interference. Adjustable compensation allows the transient response to be optimized over a wide range of loads and output capacitors. The internal synchronous switch increases efficiency and eliminates the need for an external catch diode, saving external components and board space. The LTC3614 is offered in a leadless 24-pin 3mm × 5mm thermally enhanced QFN package. L, LT, LTC, LTM, Linear Technology, the Linear logo and Burst Mode are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners. Protected by U.S. Patents, including 6580258, 5481178, 5994885, 6304066, 6498466, 6611131. 4A Output Current 2.25V to 5.5V Input Voltage Range Low Output Ripple Burst Mode® Operation: IQ = 75μA ±1% Output Voltage Accuracy Output Voltage Down to 0.6V High Efficiency: Up to 95% Low Dropout Operation: 100% Duty Cycle Programmable Slew Rate on SW Node Reduces Noise and EMI Adjustable Switching Frequency: Up to 4MHz Optional Active Voltage Positioning (AVP) with Internal Compensation Selectable Pulse-Skipping/Forced Continuous/Burst Mode Operation with Adjustable Burst Clamp Programmable Soft-Start Inputs for Start-Up Tracking or External Reference DDR Memory Mode, IOUT = ±3A Available in a 24-Pin 3mm × 5mm QFN Thermally Enhanced Package Point-of-Load Supplies Distributed Power Supplies Portable Computer Systems DDR Memory Termination Handheld Devices APPLICATIONS n n n n n TYPICAL APPLICATION VIN 2.7V TO 5.5V SVIN PVIN 100 10μF 4 EFFICIENCY (%) 90 80 70 60 50 40 30 20 10 0 1 SRLIM/DDR RUN TRACK/SS RT/SYNC LTC3614 SW PGOOD SGND ITH PGND MODE VFB Efficiency and Power Loss vs Load Current 1 POWER LOSS (W) 330nH VOUT 2.5V 4A 47μF 2 0.1 0.01 665k 3614 TA01a 210k VOUT = 2.5V VIN = 2.8V VIN = 3.3V VIN = 5V 0 10 100 1000 OUTPUT CURRENT (mA) 10000 3614 TA01b 3614f 1 LTC3614 ABSOLUTE MAXIMUM RATINGS (Note 1) PIN CONFIGURATION TOP VIEW TRACK/SS MODE 20 PGOOD 19 RUN 18 SVIN 25 PGND 17 PVIN 16 SW 15 SW 14 SW 13 SW 9 NC 10 11 12 PVIN PVIN NC PVIN, SVIN Voltages...................................... –0.3V to 6V SW Voltage ..................................–0.3V to (PVIN + 0.3V) ITH, RT/SYNC Voltages ............... –0.3V to (SVIN + 0.3V) SRLIM, TRACK/SS Voltages ....... –0.3V to (SVIN + 0.3V) MODE, RUN, VFB Voltages .......... –0.3V to (SVIN + 0.3V) PGOOD Voltage ............................................ –0.3V to 6V Operating Junction Temperature Range (Notes 2, 11) .......................................... –40°C to 125°C Storage Temperature.............................. –65°C to 150°C 24 23 22 21 SRLIM/DDR 1 RT/SYNC 2 SGND 3 PVIN 4 SW 5 SW 6 SW 7 SW 8 UDD PACKAGE 24-LEAD (3mm 5mm) PLASTIC QFN TJMAX = 125°C, θJA = 38°C/W EXPOSED PAD (PIN 25) IS PGND, MUST BE SOLDERED TO PCB ORDER INFORMATION LEAD FREE FINISH LTC3614EUDD#PBF LTC3614IUDD#PBF TAPE AND REEL LTC3614EUDD#TRPBF LTC3614IUDD#TRPBF PART MARKING* LFVM LFVM PACKAGE DESCRIPTION 24-Lead (3mm × 5mm) Plastic QFN 24-Lead (3mm × 5mm) Plastic QFN TEMPERATURE RANGE –40°C to 125°C –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. Consult LTC Marketing for information on non-standard lead based finish parts. For more information on lead free part marking, go to: http://www.linear.com/leadfree/ For more information on tape and reel specifications, go to: http://www.linear.com/tapeandreel/ VFB ITH 3614f 2 LTC3614 ELECTRICAL CHARACTERISTICS SYMBOL VIN VUVLO VFB PARAMETER Operating Voltage Range Undervoltage Lockout Threshold Feedback Voltage Internal Reference SVIN Ramping Down SVIN Ramping Up (Note 3) VTRACK = SVIN, VDDR = 0V 0°C < TJ < 85°C –40°C < TJ < 125°C (Note 3) VTRACK = 0.3V, VDDR = SVIN (Note 3) VTRACK = 0.5V, VDDR = SVIN VFB = 0.6V SVIN = PVIN = 2.25V to 5.5V (Notes 3, 4) TRACK/SS = SVIN ITH from 0.5V to 0.9V (Notes 3, 4) VITH = SVIN (Note 5) VFB = 0.5V, VMODE = SVIN (Note 6) VFB = 0.7V, VMODE = 0V, ITH = SVIN (Note 5) VFB = 0.7V, VMODE = 0V (Note 4) Shutdown Current RDS(ON) ILIM Top Switch On-Resistance Bottom Switch On-Resistance Top Switch Current Limit SVIN = PVIN = 5.5V, VRUN = 0V PVIN = 3.3V (Note 10) PVIN = 3.3V (Note 10) Sourcing (Note 8), VFB = 0.5V Duty Cycle 1ms RUN STATE REDUCED SWITCHING FREQUENCY DOWN TRACKING STATE UP TRACKING STATE RUN STATE 3614 F08 RUN PIN VOLTAGE SVIN PIN VOLTAGE Figure 8. DDR Pin Not Tied to SVIN 0.45V VFB PIN 0.3V VOLTAGE 0V 0.45V TRACK/SS 0.3V PIN VOLTAGE 0.2V 0V VIN RUN PIN VOLTAGE 0V VIN 0V EXTERNAL VOLTAGE REFERENCE 0.45V SVIN PIN VOLTAGE TIME SHUTDOWN SOFT-START STATE STATE tSS > 1ms RUN STATE REDUCED SWITCHING FREQUENCY DOWN TRACKING STATE UP TRACKING STATE RUN STATE 3614 F09 Figure 9. DDR Pin Tied to SVIN. Example DDR Application 3614f 22 LTC3614 APPLICATIONS INFORMATION Thermal Considerations In most applications, the LTC3614 does not dissipate much heat due to its high efficiency. However, in applications where the LTC3614 is running at high ambient temperature with low supply voltage and high duty cycles, such as in dropout, the heat dissipated may exceed the maximum junction temperature of the part. If the junction temperature reaches approximately 160°C, both power switches will be turned off and the SW node will become high impedance. To prevent the LTC3614 from exceeding the maximum junction temperature, some thermal analysis is required. The temperature rise is given by: TRISE = (PD)(θJA) where PD is the power dissipated by the regulator and θJA is the thermal resistance from the junction of the die to the ambient temperature. The junction temperature, TJ, is given by: TJ = TA + TRISE where TA is the ambient temperature. As an example, consider the case when the LTC3614 is in dropout at an input voltage of 3.3V with a load current of 4A at an ambient temperature of 85°C. From the Typical Performance Characteristics graph of Switch Resistance, the RDS(ON) resistance of the P-channel switch is 0.038Ω. Therefore, power dissipated by the part is: PD = (IOUT)2 • RDS(ON) = 0.61W For the QFN package, the θJA is 38°C/W. Therefore, the junction temperature of the regulator operating at 85°C ambient temperature is approximately: TJ = 0.61W • 38°C/W + 85°C = 108°C We can safely assume that the actual junction temperature will not exceed the absolute maximum junction temperature of 125°C. Note that for very low input voltage, the junction temperature will be higher due to increased switch resistance, RDS(ON). It is not recommended to use full load current with high ambient temperature and low input voltage. To maximize the thermal performance of the LTC3614 the exposed pad should be soldered to a ground plane. See the PCB Layout Board Checklist. 3614f 23 LTC3614 PACKAGE DESCRIPTION Design Example As a design example, consider using the LTC3614 in an application with the following specifications: VIN = 2.25V to 5.5V, VOUT = 1.8V, IOUT(MAX) = 4A, IOUT(MIN) = 200mA, f = 2.6MHz. Efficiency is important at both high and low load current, so Burst Mode operation will be utilized. First, calculate the timing resistor: 3.8211Hz RT = k – 16k = 130kΩ 2.6MHz Next, calculate the inductor value for about 33% ripple current at maximum VIN: 1.8V ⎛ ⎞ ⎛ 1.8V ⎞ L=⎜ • 1– = 0.35µH ⎝ 2.6MHz • 1.3A ⎟ ⎜ 5.5V ⎟ ⎠⎝ ⎠ Using a standard value of 0.33μH inductor results in a maximum ripple current of: 1.8V ⎛ ⎞ ⎛ 1.8V ⎞ ΔIL = ⎜ • ⎜ 1– ⎟ = 1.41A ⎝ 2.6MHz • 0.33µH⎟ ⎝ 5.5V ⎠ ⎠ COUT will be selected based on the ESR that is required to satisfy the output voltage ripple requirement and the bulk capacitance needed for loop stability. For this design, a 100μF ceramic capacitor is used with a X5R or X7R dielectric. Assuming worst-case conditions of VIN = 2VOUT, CIN should be selected for a maximum current rating of: IRMS = 4A • 1.8V ⎛ 3.6V ⎞ •⎜ – 1 = 2ARMS ⎝ 1.8V ⎟ ⎠ 3.6V The standard value of 22nF guarantees the minimum soft-start up time of 5ms. Figure 1 shows the schematic for this design example. PC Board Layout Checklist When laying out the printed circuit board, the following checklist should be used to ensure proper operation of the LTC3614: 1. A ground plane is recommended. If a ground plane layer is not used, the signal and power grounds should be segregated with all small-signal components returning to the SGND pin at one point which is then connected to the PGND pin close to the LTC3614. 2. Connect the (+) terminal of the input capacitor(s), CIN, as close as possible to the PVIN pin, and the (–) terminal as close as possible to the exposed pad, PGND. This capacitor provides the AC current into the internal power MOSFETs. 3. Keep the switching node, SW, away from all sensitive small-signal nodes. 4. Flood all unused areas on all layers with copper. Flooding with copper will reduce the temperature rise of power components. Connect the copper areas to PGND (exposed pad) for best performance. 5. Connect the VFB pin directly to the feedback resistors. The resistor divider must be connected between VOUT and SGND. Finally, define the soft start-up time choosing the proper value for the capacitor and the resistor connected to TRACK/SS. If we set minimum tSS = 5ms and a resistor of 2MΩ, the following equation can be solved with the maximum SVIN = 5.5V : CSS = 5ms = 21.6nF 5.5V ⎞ ⎛ 2MΩ •In ⎜ ⎝ 5.5V – 0.6V ⎟ ⎠ Decoupling PVIN with four 10μF to 22μF capacitors is adequate for most applications. If we set R2 = 196k, the value of R1 can now be determined by solving the following equation. ⎛ 1.8V ⎞ R1 = 196k • ⎜ −1 ⎠ ⎝ 0.6V ⎟ A value of 392k will be selected for R1. 3614f 24 LTC3614 TYPICAL APPLICATIONS General Purpose Buck Regulator with Fast Compensation and Improved Step Response, 2.25MHz VIN 2.25V TO 5.5V 10μF 4 RF 24Ω CF 1μF RSS 4.7M CSS 10nF RC 43k CC 220pF CC1 10pF PGOOD R5A 1M R5B 1M L1: VISHAY IHLP-2525CZ-01 330nH R4 100k SVIN PVIN RUN TRACK/SS SRLIM/DDR RT/SYNC LTC3614 SW PGOOD SGND ITH PGND MODE VFB R2 196k L1 0.33μH CO2 100μF R1 392k C3 22pF 3614 TA02a VOUT 1.8V 4A Efficiency vs Output Current 100 90 80 EFFICIENCY (%) 70 60 50 40 30 20 10 0 1 VIN = 2.5V VIN = 3.3V VIN = 4V VIN = 5.5V 10 100 1000 OUTPUT CURRENT (mA) 10000 3614 TA02b Load Step Response in Forced Continuous Mode VOUT = 1.8V VOUT 100mV/DIV IOUT 2A/DIV 50μs/DIV VIN = 3.3V VOUT = 1.8V IOUT = 100mA TO 4A VMODE = 1.5V 3614 TA02c 3614f 25 LTC3614 TYPICAL APPLICATIONS Master and Slave for Coincident Tracking Outputs Using a 1MHz External Clock VIN 2.25V TO 5.5V 22μF 4 RF1 24Ω CF1 1μF 4.7M 10nF 1MHz CLOCK RC1 15k CC1 470pF CC2 10pF 1M R5 100k PGOOD SVIN PVIN RUN TRACK/SS SRLIM/DDR RT/SYNC LTC3614 SW PGOOD SGND ITH PGND MODE VFB R2 357k L1 0.68μH CHANNEL 1 MASTER VOUT1 1.8V 4A CO12 100μF R1 715k C3 22pF R3 464k R4 464k 1M 22μF 4 RF2 24Ω CF2 1μF RC2 15k CC3 470pF CC4 10pF R7 100k PGOOD SVIN PVIN RUN TRACK/SS SRLIM/DDR RT/SYNC LTC3614 SW PGOOD SGND ITH PGND MODE VFB R6 301k CHANNEL 2 SLAVE L2 0.68μH VOUT2 1.2V CO22 4A 100μF R5 301k C7 22pF 3614 TA03a L1, L2: VISHAY IHLP-2525CZ-01 680nH Coincident Start-Up Coincident Tracking Up/Down VOUT1 VOUT2 500mV/DIV 500mV/DIV VOUT1 VOUT2 2ms/DIV 3614 TA03b 200ms/DIV 3614 TA03c 3614f 26 LTC3614 PACKAGE DESCRIPTION UDD Package 24-Lead Plastic QFN (3mm × 5mm) (Reference LTC DWG # 05-08-1833) 0.70 ±0.05 3.50 ± 0.05 2.10 ± 0.05 1.50 REF 3.65 ± 0.05 1.65 ± 0.05 PACKAGE OUTLINE 0.25 ±0.05 0.50 BSC 3.50 REF 4.10 ± 0.05 5.50 ± 0.05 RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS APPLY SOLDER MASK TO AREAS THAT ARE NOT SOLDERED 0.75 ± 0.05 3.00 ± 0.10 R = 0.05 TYP 1.50 REF 23 PIN 1 NOTCH R = 0.20 OR 0.25 45° CHAMFER 24 0.40 ± 0.10 PIN 1 TOP MARK (NOTE 6) 3.65 ± 0.10 5.00 ± 0.10 3.50 REF 1.65 ± 0.10 1 2 (UDD24) QFN 0808 REV Ø 0.200 REF 0.00 – 0.05 R = 0.115 TYP 0.25 ± 0.05 0.50 BSC BOTTOM VIEW—EXPOSED PAD NOTE: 1. DRAWING IS NOT A JEDEC PACKAGE OUTLINE 2. DRAWING NOT TO SCALE 3. ALL DIMENSIONS ARE IN MILLIMETERS 4. DIMENSIONS OF EXPOSED PAD ON BOTTOM OF PACKAGE DO NOT INCLUDE MOLD FLASH. MOLD FLASH, IF PRESENT, SHALL NOT EXCEED 0.15mm ON ANY SIDE 5. EXPOSED PAD SHALL BE SOLDER PLATED 6. SHADED AREA IS ONLY A REFERENCE FOR PIN 1 LOCATION ON THE TOP AND BOTTOM OF PACKAGE 3614f 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. 27 LTC3614 TYPICAL APPLICATION DDR Termination With Ratiometric Tracking of VDD, 1MHz VIN 3.3V Ratiometric Start-Up VDD C1 22μF 4 R6 562k R7 187k PGOOD RC 6k R4 1M R5 1M CC 2.2nF CC1 10pF R3 100k VDD 1.8V R8 365k SVIN RUN TRACK/SS RT/SYNC PVIN SRLIM/DDR L1 0.33μH SW SGND PGND VFB R2 200k R1 200k C3 22pF 3614 TA04a 500mV/DIV VTT LTC3614 PGOOD C4 100μF VTT 0.9V C5 ±3A 47μF 500μs/DIV 3614 TA04b ITH MODE L1: COILCRAFT DO3316T RELATED PARTS PART NUMBER LTC3616 LTC3612 LTC3418 LTC3415 LTC3416 LTC3413 LTC3412A DESCRIPTION 5.5V, 6A (IOUT) 4MHz Synchronous Step-Down DC/DC Converter 5.5V, 3A (IOUT), 4MHz, Synchronous Step-Down DC/DC Converter 5.5V, 8A (IOUT), 4MHz, Synchronous Step-Down DC/DC Converter 5.5V, 7A (IOUT), 1.5MHz, Synchronous Step-Down DC/DC Converter 5.5V, 4A (IOUT), 4MHz, Synchronous Step-Down DC/DC Converter 5.5V, 3A (IOUT Sink/Source), 2MHz, Monolithic Synchronous Regulator for DDR/QDR Memory Termination 5.5V, 2.5A (IOUT), 4MHz, Synchronous Step-Down DC/DC Converter COMMENTS 95% Efficiency, VIN(MIN) = 2.25V, VIN(MAX) = 5.5V, VOUT(MIN) = 0.6V, IQ = 70μA, ISD < 1μA, 3mm × 5mm QFN24 Package 95% Efficiency, VIN(MIN) = 2.25V, VIN(MAX) = 5.5V, VOUT(MIN) = 0.6V, IQ = 70μA, ISD