Datasheet LTC3407-3 (Analog Devices) - 8

LTC3407-3
APPLICATIONS INFORMATION
A general LTC3407-3 application circuit is shown in what the LTC3407-3 requires to operate. Table 1 shows Figure 2. External component selection is driven by the some typical surface mount inductors that work well in load requirement, and begins with the selection of the LTC3407-3 applications. inductor L. Once the inductor is chosen, CIN and COUT can be selected.
Table 1. Representative Surface Mount Inductors PART VALUE DCR MAX DC SIZE Inductor Selection NUMBER (μH) (Ω MAX) CURRENT (A) W × L × H (mm3)
Although the inductor does not infl uence the operat- Sumida 2.2 0.075 1.20 3.8 × 3.8 × 1.8 CDRH3D16 3.3 0.110 1.10 ing frequency, the inductor value has a direct effect on 4.7 0.162 0.90 ripple current. The inductor ripple current ΔIL decreases Sumida 1.5 0.068 0.900 3.2 × 3.2 × 1.2 with higher inductance and increases with higher VIN or CDRH2D11 2.2 0.170 0.780 VOUT: Sumida 2.2 0.116 0.950 4.4 × 5.8 × 1.2 CMD4D11 3.3 0.174 0.770 V V I OUT OUT Murata 1.0 0.060 1.00 2.5 × 3.2 × 2.0 L = • 1– f V LQH32CN 2.2 0.097 0.79 O • L IN Toko 2.2 0.060 1.08 2.5 × 3.2 × 2.0 Accepting larger values of ΔIL allows the use of low D312F 3.3 0.260 0.92 inductances, but results in higher output voltage ripple, Panasonic 3.3 0.17 1.00 4.5 × 5.4 × 1.2 greater core losses, and lower output current capability. A ELT5KT 4.7 0.20 0.95 reasonable starting point for setting ripple current is ΔIL = 0.3 • ILIM, where ILIM is the peak switch current limit. The
Input Capacitor (CIN) Selection
largest ripple current ΔIL occurs at the maximum input In continuous mode, the input current of the converter is a voltage. To guarantee that the ripple current stays below a square wave with a duty cycle of approximately V specifi ed maximum, the inductor value should be chosen OUT/VIN. To prevent large voltage transients, a low equivalent series according to the following equation: resistance (ESR) input capacitor sized for the maximum V V RMS current must be used. The maximum RMS capacitor L OUT • 1– OUT f V current is given by: O • IL IN(MAX) The inductor value will also have an effect on Burst Mode V (V – V ) I OUT IN OUT operation. The transition from low current operation RMS ≈ IMAX VIN begins when the peak inductor current falls below a level set by the burst clamp. Lower inductor values result in where the maximum average output current IMAX equals higher ripple current which causes this to occur at lower the peak current minus half the peak-to-peak ripple cur- load currents. This causes a dip in effi ciency in the upper rent, IMAX = ILIM – ΔIL/2. range of low current operation. In Burst Mode operation, lower inductance values will cause the burst frequency This formula has a maximum at VIN = 2VOUT, where IRMS to increase. = IOUT/2. This simple worst-case is commonly used to design because even signifi cant deviations do not offer
Inductor Core Selection
much relief. Note that capacitor manufacturer’s ripple cur- Different core materials and shapes will change the size/ rent ratings are often based on only 2000 hours lifetime. current and price/current relationship of an inductor. Toroid This makes it advisable to further derate the capacitor, or shielded pot cores in ferrite or permalloy materials are or choose a capacitor rated at a higher temperature than small and don’t radiate much energy, but generally cost required. Several capacitors may also be paralleled to meet more than powdered iron core inductors with similar elec- the size or height requirements of the design. An additional trical characteristics. The choice of which style inductor 0.1μF to 1μF ceramic capacitor is also recommended on to use often depends more on the price vs size require- VIN for high frequency decoupling, when not using an all ments and any radiated fi eld/EMI requirements than on ceramic capacitor solution. 34073fb 8