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LT3048 Datasheet(PDF) 11 Page - Linear Technology |
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LT3048 Datasheet(HTML) 11 Page - Linear Technology |
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11 / 16 page  LT3048 Series 11 3048fa For more information www.linear.com/LT3048 APPLICATIONS INFORMATION Figure 3. Recommended PCB Layout Reducing Output Noise With A Bypass Capacitor The LT3048 relies on the power supply rejection of the linear regulator to reduce switching regulator noise at LDOOUT. The linear regulator also contributes thermal noise to the output. The thermal noise can be reduced, and transient response improved, by adding a capacitor between LDOOUT and BYP. A typical value is 1nF. This capacitor increases start-up time of the regulator. Recommended PCB Layout Figure 3 shows the recommended layout for LT3048 cir- cuits. Most important is careful placement of the BSTOUT bypass capacitor C2. High frequency AC current flows in a loop formed by C2, internal power transistor Q1 and boost diode D2. Keep this loop small. Also be sure to place an unbroken ground plane below this loop, on the highest copper layer below the surface. This prevents the AC loop fromcouplingtoLDOOUTandothernearbycircuitry.Keep the SW node as small as possible. LDO Stability and Output Capacitance The LT3048 linear regulator requires an output capacitor for stability. It is designed to be stable with most low ESR capacitors (typically ceramic, tantalum or low ESR electrolytic). A minimum output capacitor of 1μF with an ESR of 1Ω or less is recommended to prevent oscilla- tions. Larger values of output capacitance decrease peak C2 C4 C3 3048 F03 C1 L1 Figure 2. LT3048 Typical and Guaranteed Load Current vs. Output Voltage LDOOUT VOLTAGE (V) 0 45 35 25 15 40 30 20 10 0 5 8 16 4 12 20 3048 F02 22 6 14 2 10 18 VIN = 2.7V VIN = 3.3V VIN = 3.6V GUARANTEED L = 5.6µH DCR = 120m deviations and provide improved transient response for larger load current changes. Bypass capacitors, used to decouple individual components powered by the LT3048, increase the effective output capacitor value. Give consideration to the use of ceramic capacitors as they are manufactured with a variety of dielectrics, each with different behavior across temperature and applied voltage. The most common dielectrics used are specified with EIA codes of Z5U, Y5V, X5R and X7R. Typical volt- age and temperature coefficients are shown in Figures 4 and 5. The X5R and X7R dielectrics have more stable characteristics and are most suitable for use as the output capacitor. The X7R type has better stability across tem- perature, while the X5R is less expensive and is available in higher values. Care still must be exercised when using X5R and X7R capacitors; the codes only specify operating temperature range and maximum capacitance change over temperature. Capacitance change due to DC bias with X5R and X7R capacitors can be significant enough to drop capacitor values below appropriate levels. Capaci- tor DC bias characteristics tend to improve as case size increases, but expected capacitance at operating voltage should be verified. |
Similar Part No. - LT3048_15 |
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Similar Description - LT3048_15 |
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