Datasheet LTC3568 (Analog Devices) - 9

LTC3568 applicaTions inForMaTion
Table 1. Representative Surface Mount Inductors
VIN for high frequency decoupling, when not using an all
MANU- MAX DC
ceramic capacitor solution.
FACTURER PART NUMBER VALUE CURRENT DCR HEIGHT
Toko A914BYW-2R2M (D52LC) 2.2µH 2.05A 49mΩ 2mm
Output Capacitor (COUT) Selection
Toko A915Y-2R0M (D53LC-A) 2µH 3.3A 22mΩ 3mm The selection of COUT is driven by the required ESR to Toko A918CY-2R0M (D62LCB) 2µH 2.33A 24mΩ 2mm minimize voltage ripple and load step transients. Typically, Coilcraft D01608C-222 2.2µH 2.3A 70mΩ 3mm once the ESR requirement is satisfied, the capacitance Sumida CDRH2D18/HP1R7 1.7µH 1.8A 35mΩ 2mm is adequate for filtering. The output ripple (ΔVOUT) is Sumida CDRH4D282R2 2.2µH 2.04A 23mΩ 3mm determined by: Sumida CDC5D232R2 2.2µH 2.16A 30mΩ 2.5mm  1  TDK VLCF4020T-1R8N1R9 1.8µH 1.97A 46mΩ 2mm ΔVOUT ≈ ΔIL ESR +   f 8 OC Taiyo Yuden N06DB2R2M 2.2µH 3.2A 29mΩ 3.2mm  OUT  Taiyo Yuden N05DB2R2M 2.2µH 2.9A 32mΩ 2.8mm where f = operating frequency, C Cooper SD14-2R0 2µH 2.37A 45mΩ 1.45mm OUT = output capacitance and ΔIL = ripple current in the inductor. The output ripple is highest at maximum input voltage since ΔIL increases
Catch Diode Selection
with input voltage. With ΔIL = 0.4 • IOUT the output ripple will be less than 100mV at maximum V A catch diode is not necessary. IN and fO = 1MHz with:
Input Capacitor (CIN) Selection
ESRCOUT < 130mΩ In continuous mode, the input current of the converter is a Once the ESR requirements for COUT have been met, the square wave with a duty cycle of approximately VOUT/VIN. RMS current rating generally far exceeds the IRIPPLE(P-P) To prevent large voltage transients, a low equivalent series requirement, except for an all ceramic solution. resistance (ESR) input capacitor sized for the maximum In surface mount applications, multiple capacitors may RMS current must be used. The maximum RMS capacitor have to be paralleled to meet the capacitance, ESR or RMS current is given by: current handling requirement of the application. Aluminum V electrolytic, special polymer, ceramic and dry tantulum OUT V ( IN − V ) I OUT RMS ≈ IMAX capacitors are all available in surface mount packages. VIN The OS-CON semiconductor dielectric capacitor avail- where the maximum average output current IMAX equals able from Sanyo has the lowest ESR(size) product of any the peak current minus half the peak-to-peak ripple cur- aluminum electrolytic at a somewhat higher price. Special rent, IMAX = ILIM – ΔIL/2. polymer capacitors, such as Sanyo POSCAP, offer very low ESR, but have a lower capacitance density than other This formula has a maximum at VIN = 2VOUT, where types. Tantalum capacitors have the highest capacitance IRMS = IOUT/2. This simple worst case is commonly used density, but it has a larger ESR and it is critical that the to design because even significant deviations do not offer capacitors are surge tested for use in switching power much relief. Note that capacitor manufacturer’s ripple cur- supplies. An excellent choice is the AVX TPS series of rent ratings are often based on only 2000 hours lifetime. surface mount tantalums, avalable in case heights ranging This makes it advisable to further derate the capacitor, from 2mm to 4mm. Aluminum electrolytic capacitors have or choose a capacitor rated at a higher temperature than a significantly larger ESR, and is often used in extremely required. Several capacitors may also be paral eled to meet cost-sensitive applications provided that consideration the size or height requirements of the design. An additional is given to ripple current ratings and long term reliability. 0.1µF to 1µF ceramic capacitor is also recommended on 3568fa Document Outline Features Applications Description Typical Application Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Operation Applications Information Typical Applications Package Description Revision History Typical Application Related Parts