Datasheet LTC3406, LTC3406-1.5, LTC3406-1.8 (Analog Devices) - 8

LTC3406 LTC3406-1.5/LTC3406-1.8
U U W U APPLICATIO S I FOR ATIO
The basic LTC3406 application circuit is shown in Figure 1. inductor to use often depends more on the price vs size External component selection is driven by the load require- requirements and any radiated field/EMI requirements ment and begins with the selection of L followed by CIN and than on what the LTC3406 requires to operate. Table 1 COUT. shows some typical surface mount inductors that work well in LTC3406 applications.
Inductor Selection Table 1. Representative Surface Mount Inductors
For most applications, the value of the inductor will fall in the range of 1µH to 4.7µH. Its value is chosen based on the
PART VALUE DCR MAX DC SIZE NUMBER (
µ
H) (
Ω
MAX) CURRENT (A) W
×
L
×
H (mm3)
desired ripple current. Large value inductors lower ripple Sumida 1.5 0.043 1.55 3.8 × 3.8 × 1.8 current and small value inductors result in higher ripple CDRH3D16 2.2 0.075 1.20 currents. Higher VIN or VOUT also increases the ripple 3.3 0.110 1.10 current as shown in equation 1. A reasonable starting point 4.7 0.162 0.90 for setting ripple current is ∆I Sumida 2.2 0.116 0.950 3.5 L = 240mA (40% of 600mA). × 4.3 × 0.8 CMD4D06 3.3 0.174 0.770 1 ⎛ V 4.7 0.216 0.750 OUT ⎞ ∆IL = VOUT ⎜1− (f)( ) ⎟ L Panasonic 3.3 0.17 1.00 4.5 ⎝ V (1) × 5.4 × 1.2 IN ⎠ ELT5KT 4.7 0.20 0.95 Murata 1.0 0.060 1.00 2.5 × 3.2 × 2.0 The DC current rating of the inductor should be at least LQH32CN 2.2 0.097 0.79 equal to the maximum load current plus half the ripple 4.7 0.150 0.65 current to prevent core saturation. Thus, a 720mA rated inductor should be enough for most applications (600mA
CIN and COUT Selection
+ 120mA). For better efficiency, choose a low DC-resis- tance inductor. In continuous mode, the source current of the top MOSFET is a square wave of duty cycle VOUT/VIN. To prevent large The inductor value also has an effect on Burst Mode voltage transients, a low ESR input capacitor sized for the operation. The transition to low current operation begins maximum RMS current must be used. The maximum when the inductor current peaks fall to approximately RMS capacitor current is given by: 200mA. Lower inductor values (higher ∆IL) will cause this to occur at lower load currents, which can cause a dip in / V V [ ( − V OUT IN OUT )]1 2 efficiency in the upper range of low current operation. In C required I I IN RMS ≅ OMAX Burst Mode operation, lower inductance values will cause VIN the burst frequency to increase. This formula has a maximum at VIN = 2VOUT, where I
Inductor Core Selection
RMS = IOUT/2. This simple worst-case condition is com- monly used for design because even significant deviations Different core materials and shapes will change the size/ do not offer much relief. Note that the capacitor current and price/current relationship of an inductor. manufacturer’s ripple current ratings are often based on Toroid or shielded pot cores in ferrite or permalloy mate- 2000 hours of life. This makes it advisable to further derate rials are small and don’t radiate much energy, but gener- the capacitor, or choose a capacitor rated at a higher ally cost more than powdered iron core inductors with temperature than required. Always consult the manufac- similar electrical characteristics. The choice of which style turer if there is any question. 3406fa 8