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LTC3548 Datasheet(PDF) 8 Page - Linear Technology |
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LTC3548 Datasheet(HTML) 8 Page - Linear Technology |
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8 / 16 page  LTC3548 8 3548fc 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. Table 1. Representation Surface Mount Inductors PART NUMBER VALUE (μH) DCR (Ω MAX) MAX DC CURRENT (A) SIZE W × L × H (mm3) Sumida CDRH3D16 2.2 3.3 4.7 0.075 0.110 0.162 1.20 1.10 0.90 3.8 × 3.8 × 1.8 Sumida CDRH2D11 1.5 2.2 0.068 0.170 0.900 0.780 3.2 × 3.2 × 1.2 Sumida CMD4D11 2.2 3.3 0.116 0.174 0.950 0.770 4.4 × 5.8 × 1.2 Murata LQH32CN 1.0 2.2 0.060 0.097 1.00 0.079 2.5 × 3.2 × 2.0 Toko D312F 2.2 3.3 0.060 0.260 1.08 0.92 2.5 × 3.2 × 2.0 Panasonic ELT5KT 3.3 4.7 0.17 0.20 1.00 0.95 4.5 × 5.4 × 1.2 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: ΔV OUT ≈ ΔIL ESR + 1 8f O COUT ⎛ ⎝⎜ ⎞ ⎠⎟ where f = 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 • IOUT(MAX) 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 The inductor value will also have an effect on Burst Mode operation. The transition from low current operation begins when the peak inductor current falls below a level set by the burst clamp. Lower inductor values result in higher ripple current which causes this to occur at lower load currents. This causes a dip in efficiency in the upper range of low current operation. In Burst Mode operation, lower inductance values will cause the burst frequency to increase. Inductor Core Selection Different core materials and shapes will change the size/ current and price/current relationship of an inductor. Toroid or shielded pot cores in ferrite or permalloy materials are small and do not radiate much energy, but generally cost more than powdered iron core inductors with similar elec- trical characteristics. The choice of which style inductor to use often depends more on the price vs size requirements and any radiated field/EMI requirements than on what the LTC3548 requires to operate. Table 1 shows some typi- cal surface mount inductors that work well in LTC3548 applications. 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: I RMS ≈ IMAX V OUT (VIN –VOUT ) V IN 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 APPLICATIONS INFORMATION |
Similar Part No. - LTC3548_15 |
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Similar Description - LTC3548_15 |
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