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LT1585A Fiches technique(PDF) 5 Page - ON Semiconductor |
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LT1585A Fiches technique(HTML) 5 Page - ON Semiconductor |
5 / 9 page LT1585A 5 MOTOROLA ANALOG IC DEVICE DATA 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 capacitors. 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 cavity. 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 LT1585A essentially has a flat current limit over the range of input supply voltage. The lower current limit rating and 12V maximum supply voltage rating for these devices permit this characteristic. Current limit oscillations are typically nonexistent, unless the input and output decoupling capacitors for the regulators are mounted several inches from the terminals. Protection Diodes In normal operation, the LT1585A does not require any protection diodes. Older 3–terminal regulators require protection diodes between the output pin and the input pin or between the adjust pin and the output pin to prevent die overstress. Built–in internal resistors limit internal current paths on the adjust pin. Therefore, 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 LT1585A 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 500µF, and with the input pin instantaneously shorted to ground can damage occur. A crowbar circuit at the input of the LT1585A can generate those levels of current, and a diode from output to input is then recommended. This is shown in Figure 10. 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. Vout Vin LT1585A ADJ IN OUT D1 1N4002 (Optional) + + + R2 R1 C2 10 mF C1 10 mF C Figure 10. Ripple Rejection A bypass capacitor from the adjust pin to ground reduces the output ripple by the ratio of VOUT /1.25 V. The impedance of the adjust pin capacitor at the ripple frequency should be less than the value of R1 (typically in the rage of 100 Ω to 120 Ω) in the feedback divider network in Figure 10. Therefore, the value of the required adjust pin capacitor is a function of the input ripple frequency. For example, if R1 equals 100 Ω and the ripple frequency equals 120Hz, the adjust pin capacitor should be 22 µF. At 10kHz, only 0.22µF is needed. Output Voltage The LT1585A adjustable regulator develops a 1.25V reference voltage between the output pin and the adjust pin (see Figure 11). Placing a resistor R1 between these two terminals causes a constant current to flow through R1 and down through R2 to set the overall output voltage. Normally, this current is the specified minimum load current of 10mA. The current out of the adjust pin adds to the current from R1 and is typically 55 µA. Its output voltage contribution is small and only needs consideration when very precise output voltage setting is required. |
Numéro de pièce similaire - LT1585A |
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Description similaire - LT1585A |
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