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LT3082EDDPBF Datasheet(PDF) 10 Page - Linear Technology |
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LT3082EDDPBF Datasheet(HTML) 10 Page - Linear Technology |
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10 / 20 page  LT3082 10 3082f APPLICATIONS INFORMATION electrics, each with different behavior across temperature and applied voltage. The most common dielectrics used are specified with EIA temperature characteristic codes of Z5U, Y5V, X5R and X7R. The Z5U and Y5V dielectrics are good for providing high capacitances in a small package, but they tend to have strong voltage and temperature coefficients, as shown in Figures 3 and 4. When used with a 5V regulator, a 16V 10μF Y5V capacitor can exhibit an effective value as low as 1μF to 2μF for the DC bias voltage applied and over the operating temperature range. The X5R and X7R dielectrics result in more stable characteristics and are more suitable for use as the output capacitor. The X7R type has better stability across temperature, while the X5R is less expensive and is available in higher values. Care still must be exercised when using X5R and X7R capacitors. The X5R and X7R codes only specify operating temperature range and maximum capacitance change over temperature. Capacitance change due to DC bias with X5R and X7R capacitors is better than with Y5V and Z5U capacitors, but can still be significant enough to drop capacitor values below appropriate levels. Capacitor DC bias characteristics tend to improve as component case size increases, but expected capacitance at operating voltage should be verified. Voltage and temperature coefficients are not the only sources of problems. Some ceramic capacitors have a piezoelectric response. A piezoelectric device generates voltage across its terminals due to mechanical stress. In a ceramic capacitor, the stress can be induced by vibrations in the system or thermal transients. Stability and Input Capacitance Low ESR, ceramic input bypass capacitors are acceptable for applications without long input leads. However, applica- tions connecting a power supply to an LT3082 circuit’s IN and GND pins with long input wires combined with a low ESR, ceramic input capacitors are prone to voltage spikes, reliability concerns and application-specific board oscil- lations. The input wire inductance found in many battery powered applications, combined with the low ESR ceramic input capacitor, forms a high-Q LC resonant tank circuit. In some instances this resonant frequency beats against the output current dependent LDO bandwidth and interferes with proper operation. Simple circuit modifications/solu- tions are then required. This behavior is not indicative of LT3082 instability, but is a common ceramic input bypass capacitor application issue. The self-inductance, or isolated inductance, of a wire is directly proportional to its length. Wire diameter is not a major factor on its self-inductance. For example, the self- inductance of a 2-AWG isolated wire (diameter = 0.26") is about half the self-inductance of a 30-AWG wire (diameter = 0.01"). One foot of 30-AWG wire has about 465nH of self-inductance. One of two ways reduces a wire’s self-inductance. One method divides the current flowing towards the LT3082 between two parallel conductors. In this case, the farther apart the wires are from each other, the more the self-in- ductance is reduced; up to a 50% reduction when placed a few inches apart. Splitting the wires basically connects DC BIAS VOLTAGE (V) 3082 F03 20 0 –20 –40 –60 –80 –100 0 4 8 10 26 12 14 X5R Y5V 16 BOTH CAPACITORS ARE 16V, 1210 CASE SIZE, 10μF Figure 3. Ceramic Capacitor DC Bias Characteristics TEMPERATURE (°C) –50 40 20 0 –20 –40 –60 –80 –100 25 75 3082 F04 –25 0 50 100 125 Y5V X5R BOTH CAPACITORS ARE 16V, 1210 CASE SIZE, 10μF Figure 4. Ceramic Capacitor Temperature Characteristics |
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