LTC3538EDCB

LTC3538 800mA Synchronous Buck-Boost DC/DC Converter FEATURES ■ ■ DESCRIPTION The LTC®3538 is a highly efficient, low noise, buck-boost DC/DC converter that operates from input voltages above, below, and equal to the output voltage. The topology incorporated in the IC provides a continuous transfer function through all operating modes, making the product ideal for single Lithium Ion or multicell Alkaline or NiMH applications where the output voltage is within the battery voltage range. The LTC3538 is suited for use in Micro Hard Disk Drive (μHDD) applications with its 800mA current capability. Burst Mode® operation provides high efficiency at light loads. The LTC3538 includes two 0.17Ω N-channel and two 0.2Ω P-channel MOSFET switches. Operating frequency is internally set to 1MHz to minimize solution footprint while maximizing efficiency. Other features include VIN VOUT In addition to affecting output current ripple, the size of the inductor can also affect the stability of the feedback loop. In boost mode, the converter transfer function has a right half plane zero at a frequency that is inversely proportional to the value of the inductor. As a result, a large inductor can move this zero to a frequency low enough to degrade the phase margin of the feedback loop. It is recommended that the inductor value be chosen less than 10μH. For high efficiency, choose a ferrite inductor with a high frequency core material to reduce core loses. The inductor should have low ESR (equivalent series resistance) to reduce the I2R losses, and must be able to handle the peak inductor current without saturating. Molded chokes or chip inductors usually do not have enough core to support the peak inductor currents in the 1A to 2A region. To minimize radiated noise, use a shielded inductor. See Table 1 for a suggested list of inductor suppliers. Output Capacitor Selection The bulk value of the output filter capacitor is selected to reduce the ripple due to charge into the capacitor each cycle. The steady state ripple due to charge is given by: ΔVP-P, BOOST = ILOAD • (VOUT – VIN)/(COUT • VOUT • f)V ΔVP-P,BUCK = (VIN – VOUT) • VOUT/(8 • L • VIN • COUT • f2)V where COUT = output filter capacitor, F ILOAD = Output load current, A A IOUT _ BURST(BUCK) = 0.27A; VOUT < VIN The maximum average Burst Mode output current that can be delivered in the four-switch buck-boost region is limited to the boost equation specified above. INDUCTOR SELECTION To achieve high efficiency, a low ESR inductor should be utilized for the converter. The inductor must have a saturation rating greater than the worst case average inductor current plus half the ripple current. The peak-to-peak current ripple will be larger in buck and boost mode than in the buck-boost region. The peak-to-peak inductor current ripple for each mode can be calculated from the following formulas, where f is the frequency (1MHz typical) and L is the inductance in μH. ΔIL,P-P,BUCK = VOUT • ( VIN – VOUT ) / VIN f •L VOUT • ( VOUT – VIN ) / VOUT f •L A ΔIL,P-P,BOOST = where f = frequency (1MHz typical), Hz L = inductor, H Table 1. Inductor Vendor Information SUPPLIER Coilcraft CoEv Magnetics Murata Sumida TDK TOKO PHONE (847) 639-6400 (800) 227-7040 (814) 237-1431 (800) 831-9172 USA: (847) 956-0666 Japan: 81 (3) 3607-5111 (847) 803-6100 (847) 297-0070 FAX (847) 639-1469 (650) 361-2508 (814) 238-0490 USA: (847) 956-0702 Japan: 81(3) 3607-5144 (847) 803-6296 (847) 699-7864 WEB SITE www.coilcraft.com www.tycoelectronics.com www.murata.com www.sumida.com www.component.tdk.com www.tokoam.com 3538fb 10 LTC3538 OPERATION Since the output current is discontinuous in boost mode, the ripple in this mode will generally be much larger than the magnitude of the ripple in buck mode. Minimizing solution size is usually a priority. Please be aware that ceramic capacitors can exhibit a significant reduction in effective capacitance when a bias is applied. The capacitors exhibiting the highest reduction are those packaged in the smallest case size. Input Capacitor Selection Since VIN is the supply voltage for the IC it is recommended to place at least a 4.7μF low ESR ceramic bypass capaci, tor close to VIN and GND. It is also important to minimize any stray resistance from the converter to the battery or other power source. Optional Schottky Diodes Schottky diodes across the synchronous switches B and D are not required, but do provide a lower drop during the break-before-make time (typically 15ns), thus improving efficiency. Use a surface mount Schottky diode such as an MBRM120T3 or equivalent. Do not use ordinary rectifier diodes since their slow recovery times will compromise efficiency. Table 2. Capacitor Vendor Information SUPPLIER PHONE AVX Sanyo Taiyo Yuden TDK FAX WEB SITE (803) 448-9411 (803) 448-1943 www.avxcorp.com (619) 661-6322 (619) 661-1055 www.sanyovideo.com (408) 573-4150 (408) 573-4159 www.t-yuden.com (847) 803-6100 (847) 803-6296 www.component.tdk.com importantly, leakage and parasitic capacitance need to be minimized. During start-up, 1.5μA is typically sourced from VC. The leakage of an external pull-down device and compensation components tied to VC, must therefore be minimized to ensure proper start-up. Capacitance from the pull-down device should also be minimized as it can affect converter stability. An N-channel MOSFET such as the FDV301N or similar is recommended if an external discrete N-channel MOSFET is needed. PCB Layout Considerations The LTC3538 switches large currents at high frequencies. Special care should be given to the PCB layout to ensure stable, noise-free operation. Figure 3 depicts the recommended PCB layout to be utilized for the LTC3538. A few key guidelines follow: 1. All circulating current paths should be kept as short as possible. This can be accomplished by keeping the routes to all components (except the FB divider network) in Figure 3 as short and as wide as possible. Capacitor ground connections should via down to the ground plane in the shortest route possible. The bypass capacitor on VIN should be placed as close to the IC as possible and should have the shortest possible paths to ground. 2. The small signal ground pad (GND) should have a single point connection to the power ground. A convenient way to achieve this is to short this pin directly to the Exposed Pad as shown in Figure 3. 3. The components in bold and their connections should all be placed over a complete ground plane. 4. To prevent large circulating currents from disrupting the output voltage sensing, the ground for the resistor divider should be returned directly to the small signal ground (GND) as shown. 5. Use of vias in the attach pad will enhance the thermal environment of the converter especially if the vias extend to a ground plane region on the exposed bottom surface of the PCB. Shutdown MOSFET Selection A discrete external N-channel MOSFET, open-drain pulldown device or other suitable means can be used to put the part in shutdown by pulling VC below 0.25V. Since the error amplifier sources 13μA typically when active and 1.5μA in shutdown, a relatively high resistance pulldown device can be used to pull VC below 0.25V. More 3538fb 11 LTC3538 OPERATION ƒ FILTER _ POLE = 1 FB 8 VIN VIN 2 • VOUT • π • L • COUT Hz (in boost mode) where L is in Henries and COUT is in Farads. The output filter zero is given by: 1 ƒ FILTER _ ZERO = Hz 2 • π • RESR • COUT VOUT 2 VC 7 SW1 3 GND 6 SW2 4 BURST 5 VOUT where RESR is the equivalent series resistance of the output capacitor. A troublesome feature in boost mode is the right-half plane zero (RHP), given by: VIN 2 ƒ RHPZ = Hz 2 • π • IOUT • L • VOUT The loop gain is typically rolled off before the RHP zero frequency. A simple Type I compensation network can be incorporated to stabilize the loop, but at a cost of reduced bandwidth and slower transient response. To ensure proper phase margin using Type I compensation, the loop must be crossed over a decade before the LC double pole. Referring to Figure 4, the unity-gain frequency of the error amplifier with the Type I compensation is given by: 1 ƒ UG = Hz 2 • π • R1• CP1 3538 F03 VIA TO GND PLANE Figure 3. LTC3538 Recommended PCB Layout Closing the Feedback Loop The LTC3538 incorporates voltage mode PWM control. The control to output gain varies with operation region (buck, boost, buck-boost), but is usually no greater than 15. The output filter exhibits a double pole response, as given by: ƒ FILTER _ POLE = (in buck mode) 1 Hz 2 • π • L • COUT VOUT + – 1V R1 FB 1 R2 VC 2 CP1 3538 F04 Figure 4. Error Amplifier with Type I Compensation 3538fb 12 LTC3538 OPERATION Most applications demand an improved transient response to allow a smaller output filter capacitor. To achieve a higher bandwidth, Type III compensation is required, providing two zeros to compensate for the double-pole response of the output filter. Referring to Figure 5, the location of the poles and zeros are given by: 1 ƒ POLE1 ≅ Hz 2 • π • 32e3 • R1• CP1 (which is extremly close to DC) 1 Hz 2 • π • R Z • CP1 1 ƒ ZERO2 = Hz 2 • π • R1• CZ1 1 ƒ POLE2 = Hz 2 • π • R Z • CP2 ƒ ZERO1 = where resistance is in Ohms and capacitance is in Farads. VOUT + – 1V FB 1 R1 CZ1 CP2 R2 VC 2 RZ CP1 3538 F05 Figure 5. Error Amplifier with Type III Compensation 3538fb 13 LTC3538 TYPICAL APPLICATION High Efficiency 5V/500mA from USB Input L1 3.3μH VOUT 5V, 500mA LTC3538 SW1 USB 4.35V TO 5.25V CIN 10μF PWM BURST BURST GND VIN SW2 VOUT FB VC 330pF M1 15k R2 200k R1 806k 10k 33pF COUT 22μF 1Ω ON OFF 3538 TA03 CIN: TAIYO YUDEN JMK212BJ106MG COUT: TAIYO YUDEN JMK325BJ226MM L1: SUMIDA CDRH2D18/HP-3R3NC M1: μP OPEN DRAIN I/O OR FAIRCHILD FDV301N 3538fb 14 LTC3538 PACKAGE DESCRIPTION DCB Package 8-Lead Plastic DFN (2mm × 3mm) (Reference LTC DWG # 05-08-1718 Rev A) 0.70 ±0.05 3.50 ±0.05 2.10 ±0.05 1.35 ±0.05 1.65 ± 0.05 PACKAGE OUTLINE 0.25 ± 0.05 0.45 BSC 1.35 REF RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS APPLY SOLDER MASK TO AREAS THAT ARE NOT SOLDERED R = 0.115 TYP R = 0.05 5 TYP 2.00 ±0.10 (2 SIDES) 0.40 ± 0.10 8 1.35 ±0.10 3.00 ±0.10 (2 SIDES) PIN 1 BAR TOP MARK (SEE NOTE 6) 4 0.200 REF 0.75 ±0.05 1.35 REF BOTTOM VIEW—EXPOSED PAD 0.00 – 0.05 NOTE: 1. DRAWING IS NOT A JEDEC PACKAGE OUTLINE 2. DRAWING NOT TO SCALE 3. ALL DIMENSIONS ARE IN MILLIMETERS 4. DIMENSIONS OF EXPOSED PAD ON BOTTOM OF PACKAGE DO NOT INCLUDE MOLD FLASH. MOLD FLASH, IF PRESENT, SHALL NOT EXCEED 0.15mm ON ANY SIDE 5. EXPOSED PAD SHALL BE SOLDER PLATED 6. SHADED AREA IS ONLY A REFERENCE FOR PIN 1 LOCATION ON THE TOP AND BOTTOM OF PACKAGE 1 0.23 ± 0.05 0.45 BSC 1.65 ± 0.10 PIN 1 NOTCH R = 0.20 OR 0.25 × 45° CHAMFER (DCB8) DFN 0106 REV A 3538fb 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. 15 LTC3538 RELATED PARTS PART NUMBER LTC3407 LTC3410 LTC3411 LTC3412 LTC3421 LTC3422 LTC3425 LTC3427 LTC3429 LTC3440 LTC3441/LTC3443 LTC3442 LTC3522 LTC3525 LTC3526/LTC3526B LTC3530 LTC3531 LTC3532 LTC3533 DESCRIPTION 600mA (IOUT), 1.5MHz Dual Synchronous Step-Up DC/DC Converter COMMENTS VIN: 2.5V to 5.5V, VOUT(MIN) = 0.8V IQ = 40μA, ISD ≤1μA, SC70 Package 300mA (ISW), 2.25MHz Synchronous Step-Down DC/DC Converter in SC70 VIN: 2.5V to 5.5V, VOUT(MIN) = 0.8V IQ = 26μA, ISD ≤1μA, MS Package 1.25A (IOUT), 4MHz Synchronous Step-Down DC/DC Converter 2.5A (IOUT), 4MHz Synchronous Step-Down DC/DC Converter 3A (ISW), 3MHz Synchronous Step-Up DC/DC Converter 1.5A (ISW), 3MHz Synchronous Step-Up DC/DC Converter with Output Disconnect 5A (ISW), 8MHz Multiphase Synchronous Step-Up DC/DC Converter 500mA (ISW), 1.25MHz Step-Up DC/DC Converter with Output Disconnect in 2mm × 2mm DFN 600mA (ISW), 500KHz Synchronous Step-Up DC/DC Converter 600mA (IOUT), 2MHz Synchronous Buck-Boost DC/DC Converter 1.2A (IOUT), Synchronous Buck-Boost DC/DC Converters, LTC3441(1MHz), LTC3443 (600kHz) 1.2A (IOUT), 2MHz Synchronous Buck-Boost DC/DC Converter 400mA, Synchronous Buck-Boost and 200mA Buck Converters 400mA (ISW), Synchronous Step-Up DC/DC Converter with Output Disconnect 500mA (ISW), 1MHz Synchronous Step-Up DC/DC Converter with Output Disconnect in 2mm × 2mm DFN 600mA (IOUT), 2MHz Synchronous Buck-Boost DC/DC Converter 200mA (IOUT) Synchronous Buck-Boost DC/DC Converter 500mA (IOUT), 2MHz Synchronous Buck-Boost DC/DC Converter 2A (IOUT), 2MHz Synchronous Buck-Boost DC/DC Converter VIN: 2.625V to 5.5V, VOUT(MIN) = 0.8V IQ = 62μA, ISD ≤1μA, MS Package VIN: 2.625V to 5.5V, VOUT(MIN) = 0.8V IQ = 62μA, ISD ≤1μA, TSSOP16E Package VIN: 0.5V to 4.5V, VOUT(MAX) = 5.25V IQ = 12μA, ISD