Datasheet LT3085 (Analog Devices) - 10

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