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AFL12009SW Datasheet(PDF) 8 Page - International Rectifier |
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AFL12009SW Datasheet(HTML) 8 Page - International Rectifier |
8 / 12 page 8 www.irf.com AFL28XXS Series General Application Information The AFL28XXS series of converters are capable of providing large transient currents to user loads on demand. Because the nominal input voltage range in this series is relatively low, the resulting input current demands will be correspondingly large. It is important therefore, that the line impedance be kept very low to prevent steady state and transient input currents from degrading the supply voltage between the voltage source and the converter input. In applications requiring high static currents and large transients, it is recommended that the input leads be made of adequate size to minimize resistive losses, and that a good quality capacitor of approximately 100 µfd be connected directly across the input terminals to assure an adequately low impedance at the input terminals. Table I relates nominal resistance values and selected wire sizes. Input Filter Undervoltage Lockout The AFL28XXS series converters incorporate a two stage LC input filter whose elements dominate the input load impedance characteristic during the turn-on. The input circuit is as shown in Figure IV. Figure IV. Input Filter Circuit A minimum voltage is required at the input of the converter to initiate operation. This voltage is set to 14V ± 0.5V. To preclude the possibility of noise or other variations at the input falsely initiating and halting converter operation, a hysteresis of approximately 1.0V is incorporated in this circuit. Thus if the input voltage droops to 13V ± 0.5V, the converter will shut down and remain inoperative until the input voltage returns to ≈14V. Output Voltage Adjust Pin 1 Pin 2 900nH 130nH 6 µfd 11.2 µfd In addition to permitting close voltage regulation of remotely located loads, it is possible to utilize the converter sense pins to incrementally increase the output voltage over a limited range. The adjustments made possible by this method are intended as a means to “trim” the output to a voltage setting for some particular application, but are not intended to create an adjustable output converter. These output voltage setting variations are obtained by connecting an appropriate resistor value between the +sense and -sense pins while connecting the -sense pin to the output return pin as shown in Figure V. below. The range of adjustment and corresponding range of resistance values can be determined by use of the following equation. R = 100 - -.025 adj NOM OUT NOM • ⎧ ⎨ ⎩ ⎫ ⎬ ⎭ V VV Where V NOM = device nominal output voltage, and V OUT = desired output voltage Figure V. Connection for V OUT Adjustment Finding a resistor value for a particular output voltage, is simply a matter of substituting the desired output voltage and the nominal device voltage into the equation and solving for the corresponding resistor value. Enable 2 Share + Sense - Sense Return + V out To Load R ADJ AFL28xxS Note: R adj must be set ≥ 500Ω Attempts to adjust the output voltage to a value greater than 120% of nominal should be avoided because of the potential of exceeding internal component stress ratings and subsequent operation to failure. Under no circumstance should the external setting resistor be made less than 500 Ω. By remaining within this specified range of values, completely safe operation fully within normal component derating limits is assured. Examination of the equation relating output voltage and resistor value reveals a special benefit of the circuit topology utilized for remote sensing of output voltage in the AFL28XXS series of converters. It is apparent that as the resistance increases, the output voltage approaches the nominal set value of the device. In fact the calculated limiting value of output voltage as the adjusting resistor becomes very large is ≈ 25mV above nominal device voltage. The consequence is that if the +sense connection is unintentionally broken, an AFL28XXS has a fail-safe output voltage of Vout + 25mV, where the 25mV is independent of the nominal output voltage. It can be further demonstrated that in the event of both the + and - sense connections being broken, the output will be limited to Vout + 440mV. This 440mV is also essentially constant independent of the nominal output voltage. |
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