Datasheet LTC3409A (Analog Devices) - 10

LTC3409A
APPLICATIONS INFORMATION
The basic LTC3409A application circuit is shown on the
Table 1. Representative Surface Mount Inductors
fi rst page of this data sheet. External component selec-
MAX DC
tion is driven by the load requirement and begins with the
PART NUMBER & VALUE MAX DCR CURRENT SIZE MANUFACTURER (μH) (mΩ) (A) W
×
L
×
H (mm3)
selection of L followed by CIN and COUT. Sumida 1.7 44 1.85 3.2 × 3.2 × 2.0 CDRH2D18/HP
Inductor Selection
Sumida 2.2 41 0.85 3.2 × 3.2 × 2.0 For most applications, the value of the inductor will fall CDRH2D18/LD 3.3 54 0.75 in the range of 1μH to 10μH. Its value is chosen based Sumida 1.5 68 0.90 3.2 × 3.2 × 1.2 CDRH2D11 2.2 98 0.78 on the desired ripple current. Large value inductors Murata 1.0 60 1.00 2.5 × 3.2 × 2.0 lower ripple current and small value inductors result in LQH32C_33 2.2 97 0.79 higher ripple currents. Higher VIN or VOUT also increases TDK 1.5 78 1.20 2.6 × 2.8 × 1.0 the ripple current as shown in Equation 1. A reasonable VLF3010AT 2.2 120 1.00 starting point for setting ripple current is ΔIL = 240mA TDK 1.5 68 1.20 2.6 × 2.8 × 1.2 (40% of 600mA). VLF3012AT 2.2 100 1.00 Wurth WE-TPC 1.0 85 1.50 2.8 × 2.8 × 1.1 ⎛ V ⎞ T/TH 74402800 2.2 155 1.00 ΔI OUT L = 1 V (1) f •L OUT 1– ⎝⎜ V ⎠⎟ Wurth 74402900 2.2 110 1.15 2.8 × 2.8 × 1.35 IN 3.3 135 0.95 The DC current rating of the inductor should be at least
CIN and COUT Selection
equal to the maximum load current plus half the ripple current to prevent core saturation. Thus, a 720mA rated In continuous mode, the source current of the top MOSFET inductor should be enough for most applications (600mA + is a square wave of duty cycle VOUT/VIN. To prevent large 120mA). For better effi ciency, choose a low DC resistance voltage transients, a low ESR input capacitor sized for the inductor. The inductor value also has an effect on Burst maximum RMS current must be used. The maximum RMS Mode operation. The transition to low current operation be- capacitor current is given by: gins when the inductor current peaks fall to approximately ⎡ 1/2 V ⎣ ( )⎤ 200mA. Lower inductor values (higher ΔI OUT VIN – VOUT ⎦ L) will cause this C ≅ IN Required IRMS IOUT(MAX) to occur at lower load currents, which can cause a dip in V IN effi ciency in the upper range of low current operation. In Burst Mode operation, lower inductance values will cause This formula has a maximum at VIN = 2VOUT, where the burst frequency to increase. IRMS = IOUT/2. This simple worst-case condition is common- ly used for design because even signifi cant deviations do
Inductor Core Selection
not offer much relief. Note that the capacitor manufacturer’s ripple current ratings are often based on 2000 hours of Different core materials and shapes will change the life. This makes it advisable to further derate the capacitor, size/current and price/current relationship of an induc- or choose a capacitor rated at a higher temperature than tor. Toroid or shielded pot cores in ferrite or permalloy required. Always consult the manufacturer if there is any materials are small and don’t radiate much energy, but question. The selection of C generally cost more than powdered iron core inductors OUT is driven by the required effective series resistance (ESR). Typically, once the ESR with similar electrical characteristics. The choice of which requirement for C style inductor to use often depends more on the price vs OUT has been met, the RMS current rating generally far exceeds the I size requirements and any radiated fi eld/EMI requirements RIPPLE(P-P) requirement. than on what the LTC3409A requires to operate. Table 1 shows some typical surface mount inductors that work well in LTC3409A applications. 3409af 10