Datasheet LTC3409 (Analog Devices) - 10

LTC3409
APPLICATIONS INFORMATION
The basic LTC3409 application circuit is shown on the fi rst
Table 1. Representative Surface Mount Inductors
page of this data sheet. External component selection is
PART VALUE DCR MAX DC SIZE
driven by the load requirement and begins with the selec-
NUMBER (μH) (Ω MAX) CURRENT (A) W
×
L
×
H (mm3)
tion of L followed by C Sumida 2.2 0.041 0.85 3.2 IN and COUT. × 3.2 × 2.0 CDRH2D18/LD 3.3 0.054 0.75
Inductor Selection
Sumida 1.5 0.068 0.90 3.2 × 3.2 × 1.2 CDRH2D11 2.2 0.170 0.78 For most applications, the value of the inductor will fall Sumida 2.2 0.116 0.950 4.4 × 5.8 × 1.2 in the range of 1μH to 10μH. Its value is chosen based CMD4D11 3.3 0.174 0.770 on the desired ripple current. Large value inductors Murata 1.0 0.060 1.00 2.5 × 3.2 × 2.0 LQH32CN 2.2 0.097 0.79 lower ripple current and small value inductors result in Toko 2.2 0.060 1.08 2.5 × 3.2 × 2.0 higher ripple currents. Higher VIN or VOUT also increases D312F 3.3 0.260 0.92 the ripple current as shown in Equation 1. A reasonable Panasonic 3.3 0.17 1.00 4.5 × 5.4 × 1.2 starting point for setting ripple current is ΔIL = 240mA ELT5KT 4.7 0.20 0.95 (40% of 600mA). V
C
I = 1 OUT
IN and COUT Selection
L V f • L OUT 1– V IN (1) In continuous mode, the source current of the top MOSFET is a square wave of duty cycle VOUT/VIN. To prevent large The DC current rating of the inductor should be at least voltage transients, a low ESR input capacitor sized for the equal to the maximum load current plus half the ripple maximum RMS current must be used. The maximum RMS current to prevent core saturation. Thus, a 720mA rated capacitor current is given by: inductor should be enough for most applications (600mA + 120mA). For better effi ciency, choose a low DC resistance ⎡ 1/2 V ( )⎤ OUT VIN – VOUT ⎣ ⎦ inductor. The inductor value also has an effect on Burst CIN Required IRMS IOUT(MAX) V Mode operation. The transition to low current operation be- IN gins when the inductor current peaks fall to approximately This formula has a maximum at V 200mA. Lower inductor values (higher ΔI IN = 2VOUT, where L) will cause this I to occur at lower load currents, which can cause a dip in RMS = IOUT/2. This simple worst-case condition is common- ly used for design because even signifi cant deviations do effi ciency in the upper range of low current operation. In not offer much relief. Note that the capacitor manufacturer’s Burst Mode operation, lower inductance values will cause ripple current ratings are often based on 2000 hours of the burst frequency to increase. life. This makes it advisable to further derate the capacitor,
Inductor Core Selection
or choose a capacitor rated at a higher temperature than required. Always consult the manufacturer if there is any Different core materials and shapes will change the question. The selection of COUT is driven by the required size/current and price/current relationship of an induc- effective series resistance (ESR). Typically, once the ESR tor. Toroid or shielded pot cores in ferrite or permalloy requirement for COUT has been met, the RMS current materials are small and don’t radiate much energy, but rating generally far exceeds the IRIPPLE(P-P) requirement. generally cost more than powdered iron core inductors The output ripple DVOUT is determined by: with similar electrical characteristics. The choice of which style inductor to use often depends more on the price vs V = I size requirements and any radiated fi eld/EMI requirements OUT L ESR + 1 8 • f • C OUT than on what the LTC3409 requires to operate. Table 1 shows some typical surface mount inductors that work well in LTC3409 applications. 3409fc 10