Datasheet LTC3406AB (Analog Devices) - 9

LTC3406AB
applicaTions inForMaTion
The basic LTC3406AB application circuit is shown on the
Table 1. Representative Surface Mount Inductors
front page. External component selection is driven by the
PART VALUE DCR MAX DC SIZE
load requirement and begins with the selection of L fol-
NUMBER (µH) (
W
MAX) CURRENT (A) W
×
L
×
H (mm3)
lowed by CIN and COUT. Sumida 1.5 0.043 1.55 3.8 × 3.8 × 1.8 CDRH3D16 2.2 0.075 1.20
Inductor Selection
3.3 0.110 1.10 4.7 0.162 0.90 For most applications, the value of the inductor will fall in Sumida 2.2 0.116 0.950 3.5 × 4.3 × 0.8 the range of 1µH to 4.7µH. Its value is chosen based on the CMD4D06 3.3 0.174 0.770 4.7 0.216 0.750 desired ripple current. Large value inductors lower ripple Panasonic 3.3 0.17 1.00 4.5 × 5.4 × 1.2 current and small value inductors result in higher ripple ELT5KT 4.7 0.20 0.95 currents. Higher VIN or VOUT also increases the ripple cur- Murata 1.0 0.060 1.00 2.5 × 3.2 × 2.0 rent as shown in equation 1. A reasonable starting point LQH32CN 2.2 0.097 0.79 for setting ripple current is DI 4.7 0.150 0.65 L = 240mA (40% of 600mA).  V 
C
DI OUT
IN and COUT Selection
L = 1 (f) L() VOUT 1−  VIN  (1) In continuous mode, the source current of the top MOSFET The DC current rating of the inductor should be at least is a square wave of duty cycle VOUT/VIN. To prevent large equal to the maximum load current plus half the ripple voltage transients, a low ESR input capacitor sized for the current to prevent core saturation. Thus, a 720mA maximum RMS current must be used. The maximum RMS rated inductor should be enough for most applications capacitor current is given by: (600mA + 120mA). For better efficiency, choose a low V 1/2  ( ) DC-resistance inductor. C OUT VIN − VOUT  IN required IRMS @ IOMAX V IN
Inductor Core Selection
This formula has a maximum at VIN = 2VOUT, where Different core materials and shapes will change the IRMS = IOUT/2. This simple worst-case condition is com- size/current and price/current relationship of an induc- monly used for design because even significant deviations tor. Toroid or shielded pot cores in ferrite or permalloy do not offer much relief. Note that the capacitor manu- materials are small and don’t radiate much energy, but facturer’s ripple current ratings are often based on 2000 generally cost more than powdered iron core inductors hours of life. This makes it advisable to further derate the with similar electrical characteristics. The choice of which capacitor, or choose a capacitor rated at a higher tem- style inductor to use often depends more on the price perature than required. Always consult the manufacturer vs size requirements and any radiated field/EMI require- if there is any question. ments than on what the LTC3406AB requires to operate. The selection of C Table 1 shows some typical surface mount inductors that OUT is driven by the required effective series resistance (ESR). work well in LTC3406AB applications. 3406abfb For more information www.linear.com/LTC3406AB 9