Datasheet LTC3544B (Analog Devices) - 10

LTC3544B applicaTions inForMaTion The basic LTC3544B application circuit is shown on the
Table 1. Representative Surface Mount Inductors
first page of this data sheet. External component selec-
VALUE DCR MAX DC
tion is driven by the load requirement and begins with the
PART NUMBER (µH) (
Ω
MAX) CURRENT (A) W
×
L
×
H (mm3)
selection of L followed by CIN and COUT. Sumida 10 0.47 0.48 3.0 × 2.8 × 1.0 CDH2D09B 6.4 0.32 0.6
Inductor Selection
4.7 0.218 0.7 3.3 0.15 0.85 For most applications, the value of the inductor will fall in Wurth 10 0.50 0.50 2.8 × 2.8 × 1.35 the range of 1µH to 10µH. Its value is chosen based on the TPC744029 6.8 0.38 0.65 4.7 0.210 0.80 desired ripple current. Large inductor values lower ripple 3.3 0.155 0.95 current and small inductor values result in higher ripple TDK 10 0.67 0.49 2.8 × 2.6 × 1.0 currents. Higher VIN or VOUT also increases the ripple VLF3010AT 6.8 0.39 0.61 current as shown in Equation 1. A reasonable starting 4.7 0.28 0.70 3.3 0.17 0.87 point for setting ripple current for the 300mA regulator is ΔIL = 120mA (40% of 300mA).
CIN and COUT Selection
1  V  ΔI = V OUT In continuous mode, a worst-case estimate for the input L OUT ( – (1) ƒ)( ) 1 L  VIN  current ripple can be determined my assuming that the source current of the top MOSFET is a square wave of duty The DC current rating of the inductor should be at least equal cycle VOUT/VIN, and amplitude IOUT(MAX). To prevent large to the maximum load current plus half the ripple current voltage transients, a low ESR input capacitor sized for the to prevent core saturation. Thus, a 360mA rated inductor maximum RMS current must be used. The maximum RMS should be enough for most applications (300mA + 60mA). capacitor current is given by: For better efficiency, choose a low DCR inductor. V (V – V OUT IN OUT )
Inductor Core Selection
I ≅ I RMS OUT M ( AX) VIN Different core materials and shapes wil change the size/current and price/current relationship of an induc- This formula has a maximum at VIN = 2VOUT, where IRMS tor. Toroid or shielded pot cores in ferrite or permalloy = IOUT/2. This simple worst-case condition is commonly materials are small and don’t radiate much energy, but used for design. Note that the capacitor manufacturer’s generally cost more than powdered iron core inductors ripple current ratings are often based on 2000 hours of with similar electrical characteristics. The choice of which life (non-ceramic capacitors). This makes it advisable to style inductor to use often depends more on the price vs. further de-rate the capacitor, or choose a capacitor rated size requirements and any radiated field/EMI requirements at a higher temperature than required. Always consult the than on what the LTC3544B requires to operate. Table 1 manufacturer if there is any question. shows typical surface mount inductors that work well in The selection of COUT is driven by the required effective LTC3544B applications. series resistance (ESR). Typically, once the ESR require- ment for COUT has been met, the RMS current rating 3544bfb 0 Document Outline Features Applications Description Typical Application Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Typical Performance Characteristics Pin Functions Functional Diagrams Operation Applications Information Package Description Revision History Related Parts