Datasheet LT1584, LT1585, LT1587 (Analog Devices) - 10

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