Datasheet LTC3406A (Analog Devices) - 8

LTC3406A
APPLICATIONS INFORMATION
The basic LTC3406A 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) (
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 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 for LQH32CN 2.2 0.097 0.79 setting ripple current is ∆I 4.7 0.150 0.65 L = 240mA (40% of 600mA). style inductor to use often depends more on the price vs I = 1 L (f) L() VOUT 1 VOUT V size requirements and any radiated fi eld/EMI requirements IN (1) than on what the LTC3406A requires to operate. Table 1 The DC current rating of the inductor should be at least shows some typical surface mount inductors that work equal to the maximum load current plus half the ripple well in LTC3406A applications. current to prevent core saturation. Thus, a 720mA rated inductor should be enough for most applications
CIN and COUT Selection
(600mA + 120mA). For better effi ciency, choose a low In continuous mode, the source current of the top MOSFET DC-resistance inductor. is a square wave of duty cycle VOUT/VIN. To prevent large voltage transients, a low ESR input capacitor sized for the The inductor value also has an effect on Burst Mode opera- maximum RMS current must be used. The maximum RMS tion. The transition to low current operation begins when capacitor current is given by: the inductor current peaks fall to approximately 100mA. Lower inductor values (higher ∆I 1/2 L) will cause this to occur V ( ) OUT VIN VOUT at lower load currents, which can cause a dip in effi ciency C IN required IRMS IOMAX V in the upper range of low current operation. In Burst Mode IN operation, lower inductance values will cause the burst This formula has a maximum at VIN = 2VOUT, where frequency to increase. IRMS = IOUT/2. This simple worst-case condition is commonly used for design because even signifi cant de-
Inductor Core Selection
viations do not offer much relief. Note that the capacitor Different core materials and shapes will change the manufacturer’s ripple current ratings are often based on size/current and price/current relationship of an induc- 2000 hours of life. This makes it advisable to further derate tor. Toroid or shielded pot cores in ferrite or permalloy the capacitor, or choose a capacitor rated at a higher tem- materials are small and don’t radiate much energy, but perature than required. Always consult the manufacturer generally cost more than powdered iron core inductors if there is any question. with similar electrical characteristics. The choice of which 3406afa 8