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FAN1084MC15 Scheda tecnica(PDF) 6 Page - Fairchild Semiconductor |
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FAN1084MC15 Scheda tecnica(HTML) 6 Page - Fairchild Semiconductor |
6 / 13 page FAN1084 PRODUCT SPECIFICATION 6 REV. 1.0.8 11/10/03 Applications Information General The FAN1084, FAN1084-1.5, and FAN1084-3.3 are three- terminal regulators optimized for GTL+ VTT termination and logic applications. These devices are short-circuit pro- tected, and offer thermal shutdown to turn off the regulator when the junction temperature exceeds about 150°C. The FAN1084 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 micro- processor buses like GTL+. Stability The FAN1084 series requires an output capacitor as a part of the frequency compensation. It is recommended to use a 22µF solid tantalum or a 100µF 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 <0.2 Ω. It is also recommended to use bypass capacitors such as a 22µF tantalum or a 100µF aluminum on the adjust pin of the FAN1084 for low ripple and fast transient response. When these bypassing capacitors are not used at the adjust pin, smaller 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 FAN1084 series does not require any protection diodes. For the FAN1084, 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 FAN1084 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 1000 µF 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 current; a diode from output to input is then recom- mended, as 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 ouput, 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. Optional Protection Ripple Rejection In applications that require improved ripple rejection, a bypass capacitor from the adjust pin of the FAN1084 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 2. 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 FAN1084 regulator develops a 1.25V reference voltage between the ouput pin and the adjust pin (see Figure 3). 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. FAN1084 ADJ C2 22 µF VOUT + C1 10 µF + + IN OUT D1 1N4002 (OPTIONAL) VIN FAN1084-1.5 GND C2 22 µF VOUT + C1 10 µF + IN OUT D1 1N4002 (OPTIONAL) VIN R1 R2 CADJ |
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