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AFL5028DX-CH Datasheet(PDF) 8 Page - International Rectifier |
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AFL5028DX-CH Datasheet(HTML) 8 Page - International Rectifier |
8 / 11 page 8 www.irf.com AFL50XXS Series Input Filter Undervoltage Lockout The AFL50XXS series converters incorporate a LC input filter whose elements dominate the input load impedance characteristic at turn-on. The input circuit is as shown in Figure IV. Figure IV. Input Filter Circuit Output Voltage Adjust 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 deter- mined 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 solv- ing for the corresponding resistor value. 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 poten- tial of exceeding internal component stress ratings and subsequent operation to failure. Under no circumstance should the external setting resistor be made less than 500W. By remaining within this specified range of values, com- pletely safe operation fully within normal component derat- ing limits is assured. Enable 2 Share + Sense - Sense Return + V out To Load R ADJ AFL50xxS Examination of the equation relating output voltage and re- sistor value reveals a special benefit of the circuit topology utilized for remote sensing of output voltage in the AFL50XXS 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 unin- tentionally broken, an AFL50XXS has a fail-safe output volt- age 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 440 mV is also essentially constant independent of the nominal output voltage. 0.75µ H Pin 1 Pin 2 2.7µ fd A minimum voltage is required at the input of the converter to initiate operation. This voltage is set to 26.5 ± 1.5 volts. To preclude the possibility of noise or other variations at the input falsely initiating and halting converter operation, a hys- teresis of approximately 2 volts is incorporated in this cir- cuit. Thus if the input voltage droops to 24.5 ± 1.5 volts, the converter will shut down and remain inoperative until the input voltage returns to ≈ 25 volts. |
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