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LTC3548AIDDPBF Datasheet(PDF) 10 Page - Linear Technology |
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LTC3548AIDDPBF Datasheet(HTML) 10 Page - Linear Technology |
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10 / 20 page  LTC3548A 10 3548af APPLICATIONS INFORMATION Table 1. Representative Surface Mount Inductors MANU- FACTURER PART NUMBER VALUE MAX DC CURRENT DCR HEIGHT Taiyo Yuden CB2016T2R2M CB2012T2R2M CB2016T3R3M 2.2μH 2.2μH 3.3μH 510mA 530mA 410mA 0.13Ω 0.33Ω 0.27Ω 1.6mm 1.25mm 1.6mm Panasonic ELT5KT4R7M 4.7μH 950mA 0.2Ω 1.2mm Sumida CDRH2D18/LD 4.7μH 630mA 0.086Ω 2mm Murata LQH32CN4R7M23 4.7μH 450mA 0.2Ω 2mm Taiyo Yuden NR30102R2M NR30104R7M 2.2μH 4.7μH 1100mA 750mA 0.1Ω 0.19Ω 1mm 1mm FDK FDKMIPF2520D FDKMIPF2520D FDKMIPF2520D 4.7μH 3.3μH 2.2μH 1100mA 1200mA 1300mA 0.11Ω 0.1Ω 0.08Ω 1mm 1mm 1mm TDK VLF3010AT4R7- MR70 VLF3010AT3R3- MR87 VLF3010AT2R2- M1R0 4.7μH 3.3μH 2.2μH 700mA 870mA 1000mA 0.28Ω 0.17Ω 0.12Ω 1mm 1mm 1mm Input Capacitor (CIN) Selection In continuous mode, the input current of the converter is a square wave with a duty cycle of approximately VOUT/VIN. To prevent large voltage transients, a low equivalent series resistance (ESR) input capacitor sized for the maximum RMS current must be used. The maximum RMS capacitor current is given by: ΔVOUT ≈ΔIL ESR+ 1 8fO COUT ⎛ ⎝⎜ ⎞ ⎠⎟ where the maximum average output current IMAX equals the peak current minus half the peak-to-peak ripple cur- rent, IMAX = ILIM – ΔIL/2. This formula has a maximum at VIN = 2VOUT, where IRMS = IOUT/2. This simple worst-case is commonly used to design because even significant deviations do not offer much relief. Note that capacitor manufacturer’s ripple cur- rent ratings are often based on only 2000 hours lifetime. This makes it advisable to further derate the capacitor, or choose a capacitor rated at a higher temperature than required. Several capacitors may also be paralleled to meet the size or height requirements of the design. An additional 0.1μF to 1μF ceramic capacitor is also recommended on VIN for high frequency decoupling, when not using an all ceramic capacitor solution. Output Capacitor (COUT) Selection The selection of COUT is driven by the required ESR to minimize voltage ripple and load step transients. Typically, once the ESR requirement is satisfied, the capacitance is adequate for filtering. The output ripple ( ΔVOUT) is determined by: ΔVOUT ≈ΔIL ESR+ 1 8fO COUT ⎛ ⎝⎜ ⎞ ⎠⎟ where fO = operating frequency, COUT = output capacitance and ΔIL = ripple current in the inductor. The output ripple is highest at maximum input voltage since ΔIL increases with input voltage. With ΔIL = 0.3 • ILIM the output ripple will be less than 100mV at maximum VIN and fO = 2.25MHz with: ESRCOUT < 150mΩ Once the ESR requirements for COUT have been met, the RMS current rating generally far exceeds the IRIPPLE(P-P) requirement, except for an all ceramic solution. In surface mount applications, multiple capacitors may have to be paralleled to meet the capacitance, ESR or RMS current handling requirement of the application. Aluminum electrolytic, special polymer, ceramic and dry tantulum capacitors are all available in surface mount packages. The OS-CON semiconductor dielectric capacitor available from Sanyo has the lowest ESR (size) product of any aluminum electrolytic at a somewhat higher price. Special polymer capacitors, such as Sanyo POSCAP, of- fer very low ESR, but have a lower capacitance density than other types. Tantalum capacitors have the highest capacitance density. However, they also have a larger ESR and it is critical that they are surge tested for use in switching power supplies. An excellent choice is the |
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