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LT3092ISTTRPBF Datasheet(PDF) 11 Page - Linear Technology |
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LT3092ISTTRPBF Datasheet(HTML) 11 Page - Linear Technology |
11 / 22 page LT3092 11 3092fb APPLICATIONS INFORMATION capacitor on the lower impedance output, and the same restrictions do not apply. Capacitors in the range of 0.1μF to 1μF usually provide sufficient bypassing on the input, and the value of input capacitance may be increased without limit. If an application uses GND referred capacitors on the input or output (particularly the input), pay attention to the length of the lines powering and returning ground from the circuit. In the case where long power supply and return lines are coupled with low ESR input capacitors, application-specific voltage spikes, oscillations and reliability concerns may be seen. This is not an issue with LT3092 stability, but rather the low ESR capacitor forming a high-Q resonant tank circuit with the inductance of the input wires. Adding series resistance with the input of the LT3092, or with the input capacitor, often solves this. Resistor values of 0.1Ω to 1Ω are often sufficient to dampen this resonance. Give extra consideration to the use of ceramic capacitors. Ceramic capacitors are manufactured with a variety of di- 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 5 and 6. 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 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. DC BIAS VOLTAGE (V) 3092 F05 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 5. Ceramic Capacitor DC Bias Characteristics TEMPERATURE (°C) –50 40 20 0 –20 –40 –60 –80 –100 25 75 3092 F06 –25 0 50 100 125 Y5V X5R BOTH CAPACITORS ARE 16V, 1210 CASE SIZE, 10μF Figure 6. Ceramic Capacitor Temperature Characteristics |
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