Datasheet LTC3542 (Analog Devices) - 9

LTC3542
APPLICATIONS INFORMATION Table 1. Representative Surface Mount Inductors MAX DC PART NUMBER VALUE CURRENT DCR MANUFACTURER (μH) (A) (
Ω
) SIZE (mm3)
Sumida CDRH2D11-2RM 2.2 0.780 0.098 3.2 × 3.2 × 1.2 CDRH3D16 2.2 1.2 0.075 3.8 × 3.8 × 1.8 CMD4D11 2.2 0.95 0.116 4.4 × 5.8 × 1.2 CDH2D09B 3.3 0.85 0.15 2.8 × 3 × 1 CLS4D09 4.7 0.75 0.15 4.9 × 4.9 × 1 Murata LQH32CN 2.2 0.79 0.097 2.5 × 3.2 × 1.55 LQH43CN 4.7 0.75 0.15 4.5 × 3.2 × 2.6 TDK IVLC453232 2.2 0.85 0.18 4.8 × 3.4 × 3.4 VLF3010AT- 2.2 1.0 0.12 2.8 × 2.6 × 1 2R2M1R0 design. An additional 0.1μF to 1μF ceramic capacitor is T495 series, and Sprague 593D and 595D series. Consult also recommended on VIN for high frequency decoupling, the manufacturer for other specifi c recommendations. when not using an all ceramic capacitor solution.
Ceramic Input and Output Capacitors Output Capacitor (COUT) Selection
Higher value, lower cost ceramic capacitors are now The selection of COUT is driven by the required ESR to becoming available in smaller case sizes. Their high minimize voltage ripple and load step transients. Typically, ripple current rating, high voltage rating and low ESR once the ESR requirement is satisfi ed, the RMS current are tempting for switching regulator use. However, the rating generally far exceeds the IRIPPLE(P-P) requirement, ESR is so low that it can cause loop stability problems. except for an all ceramic solution. The output ripple (ΔVOUT) Since the LTC3542’s control loop does not depend on is determined by: the output capacitor’s ESR for stable operation, ceramic capacitors can be used to achieve very low output ripple ⎛ 1 ⎞ ΔV and small circuit size. X5R or X7R ceramic capacitors are OUT ≈ ΔI ESR L + ⎝⎜ 8 • ƒ •C O OUT ⎠ ⎟ recommended because these dielectrics have the best temperature and voltage characteristics of all the ceramics where fO is the switching frequency, COUT is the output for a given value and size. capacitance and ΔIL is the inductor ripple current. For a fi xed output voltage, the output ripple is highest at maximum Great care must be taken when using only ceramic input input voltage since ΔI and output capacitors. When a ceramic capacitor is used L increases with input voltage. at the input and the power is being supplied through long If tantalum capacitors are used, it is critical that the capaci- wires, such as from a wall adapter, a load step at the output tors are surge tested for use in switching power supplies. can induce ringing at the VIN pin. At best, this ringing can An excellent choice is the AVX TPS series of surface mount couple to the output and be mistaken as loop instability. tantalums, available in case heights ranging from 2mm to At worst, the ringing at the input can be large enough to 4mm. These are specially constructed and tested for low damage the part. For more information, see Application ESR so they give the lowest ESR for a given volume. Other Note 88. The recommended capacitance value to use is capacitor types include Sanyo POSCAP, Kemet T510 and 10μF for both input and output capacitors. 3542fa 9