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LTC3542EDC Datasheet(PDF) 9 Page - Linear Integrated Systems |
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LTC3542EDC Datasheet(HTML) 9 Page - Linear Integrated Systems |
9 / 16 page LTC3542 9 3542fa design. An additional 0.1μF to 1μF ceramic capacitor is also recommended on VIN for high frequency decoupling, when not using an all ceramic capacitor solution. Output Capacitor (COUT) Selection The selection of COUT is driven by the required ESR to minimize voltage ripple and load step transients. Typically, once the ESR requirement is satisfied, the RMS current rating generally far exceeds the IRIPPLE(P-P) requirement, except for an all ceramic solution. The output ripple ( ΔVOUT) is determined by: ΔΔ V I ESR C OUT L OUT ≈+ ⎛ ⎝⎜ ⎞ ⎠⎟ 1 8• • ƒO where fO is the switching frequency, COUT is the output capacitance and ΔIListheinductorripplecurrent.Forafixed output voltage, the output ripple is highest at maximum input voltage since ΔIL increases with input voltage. If tantalum capacitors are used, it is critical that the capaci- tors are surge tested for use in switching power supplies. An excellent choice is the AVX TPS series of surface mount tantalums, available in case heights ranging from 2mm to 4mm. These are specially constructed and tested for low ESR so they give the lowest ESR for a given volume. Other capacitor types include Sanyo POSCAP, Kemet T510 and T495 series, and Sprague 593D and 595D series. Consult the manufacturer for other specific recommendations. Ceramic Input and Output Capacitors Higher value, lower cost ceramic capacitors are now becoming available in smaller case sizes. Their high ripple current rating, high voltage rating and low ESR are tempting for switching regulator use. However, the ESR is so low that it can cause loop stability problems. Since the LTC3542’s control loop does not depend on the output capacitor’s ESR for stable operation, ceramic capacitors can be used to achieve very low output ripple and small circuit size. X5R or X7R ceramic capacitors are recommended because these dielectrics have the best temperature and voltage characteristics of all the ceramics for a given value and size. Great care must be taken when using only ceramic input and output capacitors. When a ceramic capacitor is used at the input and the power is being supplied through long wires, such as from a wall adapter, a load step at the output can induce ringing at the VIN pin. At best, this ringing can couple to the output and be mistaken as loop instability. At worst, the ringing at the input can be large enough to damage the part. For more information, see Application Note 88. The recommended capacitance value to use is 10μF for both input and output capacitors. Table 1. Representative Surface Mount Inductors MANUFACTURER PART NUMBER VALUE (μH) MAX DC CURRENT (A) DCR ( 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- 2R2M1R0 2.2 1.0 0.12 2.8 × 2.6 × 1 APPLICATIONS INFORMATION |
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