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LT1585CM-3.45 Datasheet(PDF) 10 Page - Linear Technology |
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LT1585CM-3.45 Datasheet(HTML) 10 Page - Linear Technology |
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10 / 16 page  10 LT1584/LT1585/LT1587 158457a APPLICATIONS INFORMATION The use of capacitors with low ESR, low ESL, and good high frequency characteristics is critical in meeting the output voltage tolerances of these high speed microprocessors. These requirements dictate a combination of high quality, surface mount tantalum capacitors and ceramic capaci- tors. The location of the decoupling network is critical to transient response performance. Place the decoupling network as close as possible to the processor pins because trace runs from the decoupling capacitors to the processor pins are inductive. The ideal location for the decoupling network is actually inside the microprocessor socket cav- ity. In addition, use large power and ground plane areas to minimize distribution drops. A possible stability problem that occurs in monolithic linear regulators is current limit oscillations. The LT1585/LT1587 essentially have a flat current limit over the range of input supply voltage. The lower current limit rating and 7V maximum supply voltage rating for these devices permit this characteristic. Current limit oscillations are typically nonexistent, unless the input and output decoupling ca- pacitors for the regulators are mounted several inches from the terminals. The LT1584 differs from the LT1585/ LT1587 and provides current limit foldback as input-to- output differential voltage increases. This safe-area char- acteristic exhibits a negative impedance because increas- ing voltage causes output current to decrease. Negative resistance during current limit is not unique to the LT1584 devices and is present on many power IC regulators. The value of the negative resistance is a function of how fast the current limit is folded back as input-to-output voltage increases. This negative resistance can react with capaci- tors and inductors on the input and output to cause oscillation during current limit. Depending on the values of series resistances, the overall system may end up unstable. However, the oscillation causes no problem and the IC remains protected. In general, if this problem occurs and is unacceptable, increasing the amount of output capacitance helps dampen the system. Protection Diodes In normal operation, the LT1584/LT1585/LT1587 family does not require any protection diodes. Older three-termi- nal regulators require protection diodes between the out- put pin and the input pin or between the adjust pin and the output pin to prevent die overstress. On the adjustable LT1584/LT1585/LT1587, internal resis- tors limit internal current paths on the adjust pin. There- fore, even with bypass capacitors on the adjust pin, no protection diode is needed to ensure device safety under short-circuit conditions. A protection diode between the input and output pins is usually not needed. An internal diode between the input and output pins on the LT1584/LT1585/LT1587 family can handle microsecond surge currents of 50A to 100A. Even with large value output capacitors it is difficult to obtain those values of surge currents in normal operation. Only with large values of output capacitance, such as 1000 µF to 5000 µF, and with the input pin instantaneously shorted to ground can damage occur. A crowbar circuit at the input of the LT1584/LT1585/LT1587 can generate those levels of current, and a diode from output to input is then recom- mended. This is shown in Figure 2. Usually, normal power supply cycling or system “hot plugging and unplugging” will not generate current large enough to do any damage. The adjust pin can be driven on a transient basis ±7V with respect to the output, without any device degradation. As with any IC regulator, exceeding the maximum input-to- output voltage differential causes the internal transistors to break down and none of the protection circuitry is then functional. Figure 2 LT1584-3.3 D1 1N4002 (OPTIONAL) GND VIN VOUT IN OUT C1 10 µF C2 22 µF LT1584 D1 1N4002 (OPTIONAL) ADJ VIN VOUT IN OUT LT1584 • F02 C1 10 µF C2 22 µF R1 R2 CADJ + + + + + |
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