Datasheet LTC3548A (Analog Devices) - 10

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