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LTC3413EFE Datasheet(PDF) 8 Page - Linear Technology |
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LTC3413EFE Datasheet(HTML) 8 Page - Linear Technology |
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8 / 16 page  LTC3413 8 3413fc APPLICATIONS INFORMATION Inductor Selection For a given input and output voltage, the inductor value and operating frequency determine the ripple current. The ripple current ΔIL increases with higher VIN or VOUT and decreases with higher inductance. Δ= ⎛ ⎝⎜ ⎞ ⎠⎟ I fL V V V L OUT OUT IN 1 1 ()( ) – Having a lower ripple current reduces the core losses in the inductor, the ESR losses in the output capacitors and the output voltage ripple. Highest efficiency operation is achieved at low frequency with small ripple current. This, however, requires a large inductor. A reasonable starting point for selecting the ripple current is ΔIL = 0.4(IMAX). The largest ripple current occurs at the highest VIN. To guarantee that the ripple current stays below a specified maximum, the inductor value should be chosen according to the following equation: L V fI V V OUT LMAX OUT IN MAX = Δ ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ () () – 1 Inductor Core Selection Once the value for L is known, the type of inductor must be selected. Actual core loss is independent of core size for a fixed inductor value, but it is very dependent on the inductance selected. As the inductance increases, core losses decrease. Unfortunately, increased inductance requires more turns of wire and therefore copper losses will increase. Ferrite designs have very low core losses and are used often at high switching frequencies, so design goals can concentrate on copper loss and preventing saturation. Ferrite core material saturates “hard,” which means that inductance collapses abruptly when the peak design current is exceeded. This results in an abrupt increase in inductor ripple current and consequent output voltage ripple. Do not allow the core to saturate! 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 don’t radiate much energy, but generally cost more than powdered iron core inductors with similar characteristics. The choice of which style inductor to use mainly depends on the price versus size requirements and any radiated field/EMI requirements. Table 1 shows some recommended surface mount induc- tors for LTC3413 applications. Table 1. Recommended Surface Mount Inductors MANUFACTURER PART NUMBER VALUE (μH) DCR (mΩ) Murata LQH55DNR47M01 0.47 13.0 Vishay/Dale IHLP252CZPJR47M01 0.47 4.2 Pulse P1166.681T 0.44 6.0 Cooper SD20-R47 0.47 20.0 CIN and COUT Selection The input capacitance, CIN,isneededtofilterthetrapezoidal wave current at the source of the top MOSFET. To prevent large voltage transients from occurring, a low ESR input capacitor sized for the maximum RMS current should be used. The maximum RMS current is given by: II V V V V RMS OUT MAX OUT IN IN OUT = () –1 This formula has a maximum at VIN = 2VOUT, where IRMS = IOUT/2.Thissimpleworst-caseconditioniscommonlyused for design because even significant deviations do not offer much relief. Note that ripple current ratings from capacitor manufacturers are often based on only 2000 hours of life which 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 size or height requirements in the design. The selection of COUT is determined by the effective series resistance (ESR) that is required to minimize voltage ripple |
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