Datasheet LTC3406B-1.2 (Analog Devices) - 7

LTC3406B-1.2
U U W U APPLICATIO S I FOR ATIO
The basic LTC3406B-1.2 application circuit is shown in
Table 1. Representative Surface Mount Inductors
Figure 1. External component selection is driven by the
PART VALUE DCR MAX DC SIZE
load requirement and begins with the selection of L fol-
NUMBER (
µ
H) (
Ω
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 desired ripple current. Large value inductors lower ripple 4.7 0.216 0.750 current and small value inductors result in higher ripple Panasonic 3.3 0.17 1.00 4.5 × 5.4 × 1.2 ELT5KT 4.7 0.20 0.95 currents. Higher VIN or VOUT also increases the ripple Murata 1.0 0.060 1.00 2.5 × 3.2 × 2.0 current as shown in equation 1. A reasonable starting point LQH3C 2.2 0.097 0.79 for setting ripple current is ∆IL = 240mA (40% of 600mA). 4.7 0.150 0.65 1 ⎛ VOUT ⎞ ∆IL = VOUT ⎜1−
C
(f)( ) ⎟ L ⎝ V (1)
IN and COUT Selection
IN ⎠ 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 rated maximum RMS current must be used. The maximum inductor should be enough for most applications (600mA RMS capacitor current is given by: + 120mA). For better efficiency, choose a low DC-resis- / tance inductor. V V [ ( − V OUT IN OUT )]1 2 C required I ≅ I IN RMS OMAX V
Inductor Core Selection
IN Different core materials and shapes will change the size/ This formula has a maximum at VIN = 2VOUT, where current and price/current relationship of an inductor. IRMS = IOUT/2. This simple worst-case condition is com- Toroid or shielded pot cores in ferrite or permalloy mate- monly used for design because even significant deviations rials are small and don’t radiate much energy, but gener- do not offer much relief. Note that the capacitor ally cost more than powdered iron core inductors with manufacturer’s ripple current ratings are often based on similar electrical characteristics. The choice of which style 2000 hours of life. This makes it advisable to further derate inductor to use often depends more on the price vs size the capacitor, or choose a capacitor rated at a higher requirements and any radiated field/EMI requirements temperature than required. Always consult the manufac- than on what the LTC3406B-1.2 requires to operate. Table turer if there is any question. 1 shows some typical surface mount inductors that work The selection of COUT is driven by the required effective well in LTC3406B-1.2 applications. series resistance (ESR). sn3406b12 3406b12fs 7