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FAN1585AD Datasheet(PDF) 6 Page - Fairchild Semiconductor |
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FAN1585AD Datasheet(HTML) 6 Page - Fairchild Semiconductor |
6 / 13 page FAN1585A PRODUCT SPECIFICATION 6 REV. 1.1.6 7/8/05 Applications Information General The FAN1585A and FAN1585A-1.5 are three-terminal regu- lators optimized for GTL+ VTT termination and logic appli- cations. These devices are short-circuit protected and offer thermal shutdown to turn off the regulator when the junction temperature exceeds about 150°C. The FAN1585A series provides low dropout voltage and fast transient response. Frequency compensation uses capacitors with low ESR while still maintaining stability. This is critical in addressing the needs of low voltage high speed microprocessor buses like GTL+. Stability The FAN1585A series requires an output capacitor as a part of the frequency compensation. It is recommended to use a 22µF solid tantalum or a 100 µF aluminum electrolytic on the output to ensure stability. The frequency compensation of these devices optimizes the frequency response with low ESR capacitors. In general, it is suggested to use capacitors with an ESR of <300m Ω. It is also recommended to use bypass capacitors such as a 22µF tantalum or a 100µF alumi- num on the adjust pin of the FAN1585A for low ripple and fast transient response. When these bypassing capacitors are not used at the adjust pin, larger values of output capacitors provide equally good results. Refer to Typical Performance Characteristics for graph of stability of output capacitance ESR vs load current. Protection Diodes In normal operation, the FAN1585A series does not require any protection diodes. For the FAN1585A, 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 condi- tions. A protection diode between the input and output pins is usually not needed. An internal diode between the input and the output pins on the FAN1585A series can handle micro- second 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 1000µF to 5000µF, and with the input pin instantaneously shorted to ground can damage occur. A crowbar circuit at the input can generate those levels of cur- rent; a diode from output to input is then recommended, as 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 func- tional. Figure 10. Optional Protection Ripple Rejection In applications that require improved ripple rejection, a bypass capacitor from the adjust pin of the FAN1585A to ground reduces the output ripple by the ratio of VOUT/1.25V. The impedance of the adjust pin capacitor at the ripple frequency should be less than the value of R1 (typically in the range of 100 Ω to 120Ω) 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 22µF. At 10kHz, only 0.22µF is needed. Output Voltage The FAN1585A regulator develops a 1.25V reference volt- age 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 35µA. Its output voltage contribution is small and only needs consideration when a very precise out- put voltage setting is required. FAN1585A ADJ CADJ R2 R1 + C2 22 µF VOUT + C1 22 µF + IN OUT D1 1N4002 (OPTIONAL) VIN FAN1585A-1.5 GND C2 22 µF VOUT C1 22 µF + + IN OUT D1 1N4002 (OPTIONAL) VIN |
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