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LT3085EMS8E Datasheet(PDF) 10 Page - Linear Technology |
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LT3085EMS8E Datasheet(HTML) 10 Page - Linear Technology |
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10 / 28 page  LT3085 10 3085fb APPLICATIONS INFORMATION Input Capacitance and Stability The LT3085 is designed to be stable with a minimum capacitance of 1μF at each input pin. Ceramic capacitors with low ESR are available for use to bypass these pins, but in cases where long wires connect the LT3085 inputs to a power supply (and also from ground of the LT3085 circuitry back to power supply ground), this causes insta- bilities. This happens due to the wire inductance forming an LC tank circuit with the input capacitor and not as a result of instability on the LT3085. The self-inductance, or isolated inductance, of a wire is directly proportional to its length. The diameter does not have a major influence on its self-inductance. As an ex- ample, the self-inductance of a 2-AWG isolated wire with a diameter of 0.26in. is approximately half the self-inductance of a 30-AWG wire with a diameter of 0.01in. One foot of 30-AWG wire has 465nH of self-inductance. The overall self-inductance of a wire is reduced in one of two ways. One is to divide the current flowing towards the LT3085 between two parallel conductors. In this case, the farther apart the wires are from each other, the more the self-inductance is reduced, up to a 50% reduc- tion when placed a few inches apart. Splitting the wires basically connects two equal inductors in parallel, but placing them in close proximity gives the wires mutual inductance adding to the self-inductance. The second and most effective way to reduce overall inductance is to place both forward- and return-current conductors (the wire for the input and the wire for ground) in very close proximity. Two 30-AWG wires separated by only 0.02in. used as forward- and return-current conductors reduce the overall self-inductance to approximately one-fifth that of a single isolated wire. If the LT3085 is powered by a battery mounted in close proximity on the same circuit board, a 2.2μF input capaci- tor is sufficient for stability. When powering from distant supplies, use a larger input capacitor based on a guide- line of 1μF plus another 1μF per 8 inches of wire length. As power supply impedance does vary, the amount of capacitance needed to stabilize your application will also vary. Extra capacitance placed directly on the output of the power supply requires an order of magnitude more capacitance as opposed to placing extra capacitance close to the LT3085. Using series resistance between the power supply and the input of the LT3085 also stabilizes the application. As little as 0.1Ω to 0.5Ω, often less, is all that is needed to provide damping in the circuit. If the extra impedance between the power supply and the input is unacceptable, placing the resistors in series with the capacitors will pro- vide damping to prevent the LC resonance from causing full-blown oscillation. Stability and Output Capacitance The LT3085 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 2.2μF with an ESR of 0.5Ω or less is recommended to prevent oscillations. Larger values of output capacitance decrease peak deviations and provide improved transient response for larger load current changes. Bypass capacitors, used to decouple individual components powered by the LT3085, increase the effective output capacitor value. For improvement in transient performance, place a capaci- tor across the voltage setting resistor. Capacitors up to 1μF can be used. This bypass capacitor reduces system noise as well, but start-up time is proportional to the time constant of the voltage setting resistor (RSET in Figure 1) and SET pin bypass capacitor. Extra consideration must be given to the use of ceramic capacitors. Ceramic capacitors 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 temperature characteristic codes of Z5U, Y5V, X5R and X7R. The Z5U and Y5V dielectrics are good for providing high capacitances |
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