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LT3082 Datasheet(PDF) 11 Page - Linear Technology |
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LT3082 Datasheet(HTML) 11 Page - Linear Technology |
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11 / 20 page  LT3082 11 3082f APPLICATIONS INFORMATION 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 input and GND wires) in very close proximity. Two 30-AWG wires separated by only 0.02", used as forward- and return-current conduc- tors, reduce the overall self-inductance to approximately one-fifth that of a single isolated wire. If wiring modifications are not permissible for the applica- tions, including series resistance between the power supply and the input of the LT3082 also stabilizes the application. As little as 0.1Ω to 0.5Ω, often less, is effective in damping the LC resonance. If the added impedance between the power supply and the input is unacceptable, adding ESR to the input capacitor also provides the necessary damping of the LC resonance. However, the required ESR is generally higher than the series impedance required. Paralleling Devices Higher output current is obtained by paralleling multiple LT3082s together. Tie the individual SET pins together and tie the individual IN pins together. Connect the outputs in common using small pieces of PC trace as ballast resistors to promote equal current sharing. PC trace resistance in mΩ/inch is shown in Table 2. Ballasting requires only a tiny area on the PCB. Table 2. PC Board Trace Resistance WEIGHT (oz) 10mil WIDTH 20mil WIDTH 1 54.3 27.1 2 27.1 13.6 Trace resistance is measured in mΩ/in The worst-case room temperature offset, only ±2mV between the SET pin and the OUT pin, allows the use of very small ballast resistors. As shown in Figure 5, each LT3082 has a small 50mΩ ballast resistor, which at full output current gives better than 80% equalized sharing of the current. The external resistance of 50mΩ (25mΩ for the two devices in paral- lel) adds only about 10mV of output regulation drop at an output of 0.4A. Even with an output voltage as low as 1V, this adds only 1% to the regulation. Of course, paralleling more than two LT3082s yields even higher output current. Spreading the devices on the PC board also spreads the heat. Series input resistors can further spread the heat if the input-to-output difference is high. Figure 5. Parallel Devices SET + – LT3082 10μA 50mΩ 50mΩ IN VIN 4.8V TO 40V VOUT, 3.3V 0.4A OUT 10μF 1μF 165k 3082 F05 SET + – LT3082 10μA IN OUT Quieting the Noise The LT3082 offers numerous noise performance advan- tages. Every linear regulator has its sources of noise. In general, a linear regulator’s critical noise source is the reference. In addition, consider the error amplifier’s noise contribution along with the resistor divider’s noise gain. Many traditional low noise regulators bond out the voltage reference to an external pin (usually through a large value resistor) to allow for bypassing and noise reduction. The LT3082 does not use a traditional voltage reference like other linear regulators. Instead, it uses a 10μA reference current. The 10μA current source generates noise current levels of 2.7pA/√Hz (0.7nARMS over the 10Hz to 100kHz bandwidth). The equivalent voltage noise equals the RMS noise current multiplied by the resistor value. The SET pin resistor generates spot noise equal to √4kTR (k = Boltzmann’s constant, 1.38 • 10–23J/°K, and T is abso- lute temperature) which is RMS summed with the voltage noise If the application requires lower noise performance, bypass the voltage/current setting resistor with a capacitor to GND. Note that this noise-reduction capacitor increases start-up time as a factor of the RC time constant. |
Similar Part No. - LT3082_15 |
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Similar Description - LT3082_15 |
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