Datasheet LTC3407-4 (Analog Devices) - 8

LTC3407-4
APPLICATIONS INFORMATION
begins when the peak inductor current falls below a level
Table 1. Representative Surface Mount Inductors
set by the burst clamp. Lower inductor values result in
PART VALUE DCR MAX DC SIZE
higher ripple current which causes this to occur at lower
NUMBER (μH) (Ω MAX) CURRENT (A) W × L × H (mm3)
load currents. This causes a dip in effi ciency in the upper Sumida 2.2 0.075 1.20 3.8 × 3.8 × 1.8 range of low current operation. In Burst Mode operation, CDRH3D16 3.3 0.110 1.10 lower inductance values will cause the burst frequency 4.7 0.162 0.90 to increase. Sumida 1.5 0.068 0.900 3.2 × 3.2 × 1.2 CDRH2D11 2.2 0.170 0.780
Inductor Core Selection
Sumida 2.2 0.116 0.950 4.4 × 5.8 × 1.2 Different core materials and shapes will change the size/ CMD4D11 3.3 0.174 0.770 current and price/current relationship of an inductor. Toroid Murata 1.0 0.060 1.00 2.5 × 3.2 × 2.0 or shielded pot cores in ferrite or permalloy materials are LQH32CN 2.2 0.097 0.79 small and don’t radiate much energy, but generally cost Toko 2.2 0.060 1.08 2.5 × 3.2 × 2.0 D312F 3.3 0.260 0.92 more than powdered iron core inductors with similar elec- Panasonic 3.3 0.17 1.00 4.5 × 5.4 × 1.2 trical characteristics. The choice of which style inductor ELT5KT 4.7 0.20 0.95 to use often depends more on the price vs size require- ments and any radiated fi eld/EMI requirements than on
Output Capacitor (COUT) Selection
what the LTC3407-4 requires to operate. Table 1 shows The selection of COUT is driven by the required ESR to some typical surface mount inductors that work well in minimize voltage ripple and load step transients. Typically, LTC3407-4 applications. once the ESR requirement is satisfi ed, the capacitance
Input Capacitor (C
is adequate for fi ltering. The output ripple (ΔV
IN) Selection
OUT) is In continuous mode, the input current of the converter is a determined by: square wave with a duty cycle of approximately VOUT/VIN. 1 To prevent large voltage transients, a low equivalent series VOUT IL ES R + 8f resistance (ESR) input capacitor sized for the maximum O COUT RMS current must be used. The maximum RMS capacitor where f = operating frequency, COUT = output capacitance current is given by: and ΔIL = ripple current in the inductor. The output ripple V V – V is highest at maximum input voltage since ΔI OUT ( IN OUT ) L increases IRMS ≈IMAX with input voltage. With ΔI V L = 0.3 • ILIM the output ripple IN will be less than 100mV at maximum VIN and fO = 2.25MHz where the maximum average output current IMAX equals with: the peak current minus half the peak-to-peak ripple cur- ESRCOUT < 150mΩ rent, IMAX = ILIM – ΔIL/2. Once the ESR requirements for COUT have been met, the This formula has a maximum at V RMS current rating generally far exceeds the I IN = 2VOUT, where IRMS RIPPLE(P-P) = I requirement, except for an all ceramic solution. OUT/2. This simple worst-case is commonly used to design because even signifi cant deviations do not offer In surface mount applications, multiple capacitors may much relief. Note that capacitor manufacturer’s ripple cur- have to be paralleled to meet the capacitance, ESR or rent ratings are often based on only 2000 hours lifetime. RMS current handling requirement of the application. This makes it advisable to further derate the capacitor, Aluminum electrolytic, special polymer, ceramic and dry or choose a capacitor rated at a higher temperature than tantulum capacitors are all available in surface mount required. Several capacitors may also be paralleled to meet packages. The OS-CON semiconductor dielectric capacitor the size or height requirements of the design. An additional available from Sanyo has the lowest ESR(size) product 0.1μF to 1μF ceramic capacitor is also recommended on of any aluminum electrolytic at a somewhat higher price. VIN for high frequency decoupling, when not using an all Special polymer capacitors, such as Sanyo POSCAP, ceramic capacitor solution. 34074fa 8