Datasheet LTC3406AB-2 (Analog Devices) - 9

LTC3406AB-2
applicaTions inForMaTion
The basic LTC3406AB-2 application circuit is shown on
Table 1. Representative Surface Mount Inductors
the front page. External component selection is driven by
PART VALUE DCR MAX DC SIZE
the load requirement and begins with the selection of L
NUMBER (µH) (
W
MAX) CURRENT (A) W
×
L
×
H (mm3)
followed 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 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 capacitor current is given by: rated inductor should be enough for most applications 1/2 (600mA + 120mA). For better efficiency, choose a low V  OUT (VIN − VOUT ) DC-resistance inductor. CIN required IRMS ≅ IOMAX VIN
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 vs perature than required. Always consult the manufacturer size requirements and any radiated field/EMI requirements if there is any question. than on what the LTC3406AB-2 requires to operate. Table The selection of COUT is driven by the required effective 1 shows some typical surface mount inductors that work series resistance (ESR). well in LTC3406AB-2 applications. 3406ab2fa For more information www.linear.com/LTC3406AB-2 9