Datasheet LTC3544 (Analog Devices) - 10

LTC3544
APPLICATIONS INFORMATION
The basic LTC3544 application circuit is shown on the fi rst
Table 1. Representative Surface Mount Inductors
page of this data sheet. External component selection is
Value DCR MAX DC
driven by the load requirement and begins with the selec-
Part Number (μH) (
Ω
MAX) CURRENT (A) W
×
L
×
H (mm3)
tion 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 4.7 0.218 0.7
Inductor Selection
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 V VLF3010AT 6.8 0.39 0.61 IN or VOUT also increases the ripple 4.7 0.28 0.70 current as shown in Equation 1. A reasonable starting 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 ⎞ In continuous mode, a worst-case estimate for the input ΔI = V OUT L 1– (1) ƒ L OUT ⎝⎜ V ( )( ) ⎠⎟ current ripple can be determined by assuming that the IN source current of the top MOSFET is a square wave of The DC current rating of the inductor should be at least equal duty cycle VOUT/VIN, and amplitude IOUT(MAX). To prevent to the maximum load current plus half the ripple current large voltage transients, a low ESR input capacitor sized for to prevent core saturation. Thus, a 360mA rated inductor the maximum RMS current must be used. The maximum should be enough for most applications (300mA + 60mA). RMS capacitor current is given by: For better effi ciency, choose a low DCR inductor. V V – V ( ) OUT IN OUT I ≅ I RMS OUT MA ( X)
Inductor Core Selection
VIN Different core materials and shapes will change the This formula has a maximum at VIN = 2VOUT, where IRMS size/current and price/current relationship of an induc- = IOUT/2. This simple worst-case condition is commonly tor. Toroid or shielded pot cores in ferrite or permalloy used for design. Note that the capacitor manufacturer’s materials are small and don’t radiate much energy, but ripple current ratings are often based on 2000 hours of generally cost more than powdered iron core inductors life (non-ceramic capacitors). This makes it advisable to with similar electrical characteristics. The choice of which further de-rate the capacitor, or choose a capacitor rated style inductor to use often depends more on the price vs. at a higher temperature than required. Always consult the size requirements and any radiated fi eld/EMI requirements manufacturer if there is any question. than on what the LTC3544 requires to operate. Table 1 shows typical surface mount inductors that work well in The selection of COUT is driven by the required effective LTC3544 applications. series resistance (ESR). Typically, once the ESR require- ment for COUT has been met, the RMS current rating generally far exceeds the IRIPPLE(P-P) requirement. The output ripple ΔVOUT is determined by: ⎛ 1 ⎞ ΔVOUT ≅ ΔI ESR L + ⎝⎜ 8 • ƒ • COUT ⎠⎟ where f = operating frequency, COUT = output capacitance and ΔIL = ripple current in the inductor. For a fi xed output 3544fa 10