LTC3538EDCB#TRPBF

LTC3538 800mA Synchronous Buck-Boost DC/DC Converter FEATURES ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ DESCRIPTION Regulated Output with Input Voltages Above, Below, or Equal to the Output 800mA Continuous Output Current from a Single Lithium-Ion/Polymer Cell Single Inductor 1.8V to 5.25V VOUT Range 2.4V to 5.5V VIN Range 1MHz Fixed Frequency Operation Output Disconnect in Shutdown 35μA Quiesecent Current in Burst Mode Operation VIN VOUT 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 ΔIL,P-P,BOOST = f •L 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 A f •L VOUT • ( VOUT – 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. ILOAD = Output load current, A A where f = frequency (1MHz typical), Hz L = inductor, H Table 1. Inductor Vendor Information SUPPLIER PHONE FAX WEB SITE Coilcraft (847) 639-6400 (847) 639-1469 www.coilcraft.com CoEv Magnetics (800) 227-7040 (650) 361-2508 www.tycoelectronics.com Murata (814) 237-1431 (800) 831-9172 (814) 238-0490 www.murata.com Sumida USA: (847) 956-0666 Japan: 81 (3) 3607-5111 USA: (847) 956-0702 Japan: 81(3) 3607-5144 www.sumida.com TDK (847) 803-6100 (847) 803-6296 www.component.tdk.com TOKO (847) 297-0070 (847) 699-7864 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. 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. Input Capacitor Selection PCB Layout Considerations Since VIN is the supply voltage for the IC it is recommended to place at least a 4.7μF, low ESR ceramic bypass capacitor close to VIN and GND. It is also important to minimize any stray resistance from the converter to the battery or other power source. 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: Optional Schottky Diodes 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. 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 FAX WEB SITE (803) 448-9411 (803) 448-1943 www.avxcorp.com Sanyo (619) 661-6322 (619) 661-1055 www.sanyovideo.com Taiyo Yuden (408) 573-4150 (408) 573-4159 www.t-yuden.com TDK (847) 803-6100 (847) 803-6296 www.component.tdk.com 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 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. 3538fb 11 LTC3538 OPERATION ƒ FILTER _ POLE = VIN 2• VOUT • π • L • COUT Hz (in boost mode) 1 FB 8 VIN 2 VC 7 SW1 3 GND 6 SW2 4 BURST 5 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 where RESR is the equivalent series resistance of the output capacitor. VOUT VOUT 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 3538 F03 VIA TO GND PLANE Figure 3. LTC3538 Recommended PCB Layout The loop gain is typically rolled off before the RHP zero frequency. 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 = 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 1 Hz 2• π • L • COUT (in buck mode) 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 + – R1 1V CZ1 FB 1 R2 CP2 VC RZ CP1 2 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 USB 4.35V TO 5.25V SW1 SW2 VIN VOUT CIN 10μF PWM 10k 33pF COUT 22μF FB BURST BURST VC GND ON OFF 1Ω R1 806k 330pF M1 15k R2 200k 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 1.35 ±0.05 3.50 ±0.05 1.65 ± 0.05 2.10 ±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 1.65 ± 0.10 3.00 ±0.10 (2 SIDES) PIN 1 NOTCH R = 0.20 OR 0.25 × 45° CHAMFER PIN 1 BAR TOP MARK (SEE NOTE 6) (DCB8) DFN 0106 REV A 4 0.200 REF 1 0.23 ± 0.05 0.45 BSC 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 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 DESCRIPTION COMMENTS LTC3407 600mA (IOUT), 1.5MHz Dual Synchronous Step-Up DC/DC Converter VIN: 2.5V to 5.5V, VOUT(MIN) = 0.8V IQ = 40μA, ISD ≤1μA, SC70 Package LTC3410 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 LTC3411 1.25A (IOUT), 4MHz Synchronous Step-Down DC/DC Converter VIN: 2.625V to 5.5V, VOUT(MIN) = 0.8V IQ = 62μA, ISD ≤1μA, MS Package LTC3412 2.5A (IOUT), 4MHz Synchronous Step-Down DC/DC Converter VIN: 2.625V to 5.5V, VOUT(MIN) = 0.8V IQ = 62μA, ISD ≤1μA, TSSOP16E Package LTC3421 3A (ISW), 3MHz Synchronous Step-Up DC/DC Converter VIN: 0.5V to 4.5V, VOUT(MAX) = 5.25V IQ = 12μA, ISD