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FAN1582D25X Datasheet(PDF) 6 Page - Fairchild Semiconductor |
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FAN1582D25X Datasheet(HTML) 6 Page - Fairchild Semiconductor |
6 / 12 page FAN1582 PRODUCT SPECIFICATION 6 REV. 1.1.2 2/26/03 pin of the FAN1582 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. Protection Diodes In normal operation, the FAN1582 series does not require any protection diodes. For the FAN1582, 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 usu- ally not needed. An internal diode between the input and the output pins on the FAN1582 series can handle microsecond surge currents of 50A to 100A. Even with large value output capacitors it is difficult to obtain those values of surge cur- rents 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 current; a diode from output to input is then recommended, as shown in Figure 1. 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 1. Optional Protection Diode Ripple Rejection In applications that require improved ripple rejection, a bypass capacitor from the adjust pin of the FAN1582 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 1. 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 FAN1582 regulator develops a 1.25V reference voltage between the output pin and the adjust pin (see Figure 2). 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 50µA. Its output voltage contribution is small and only needs consideration when a very precise output voltage setting is required. Figure 2. Basic Regulator Circuit Load Regulation The FAN1582 family provides true remote sensing, eliminat- ing output voltage errors due to trace resistance. To utilize remote sensing, connect the VSENSE pin directly to the load, rather than at the VOUT pin. If the load is more than 1" away from the FAN1582, it may be necessary to increase the load capacitance to ensure stability. Thermal Considerations The FAN1582 series protect themselves under overload con- ditions with internal power and thermal limiting circuitry. However, for normal continuous load conditions, do not exceed maximum junction temperature ratings. It is important to consider all sources of thermal resistance from junction-to- ambient. These sources include the junction-to-case resistance, the case-to-heat sink interface resistance, and the heat sink resistance. Thermal resistance specifications have been developed to more accurately reflect device temperature and ensure safe operating temperatures. The electrical character- FAN1582 Adj C2 22 µF VOUT + C1 10 µF + CADJ + Vcntl Vsense Vin Vout R1 R2 D1 1N4002 (OPTIONAL) VIN VOUT FAN1582–1.5, 2.5 Gnd C2 22 µF VOUT + C1 10 µF + Vcntl Vsense Vin Vout D1 1N4002 (OPTIONAL) VIN VCNTL C2 22 µF VIN VOUT VREF C1 10 µF Vcntl + + Vcntl Vsense Vin Adj Vout FAN1582 R1 R2 IADJ 50 µA VOUT = VREF (1+R2/R1) + IADJ (R2) |
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