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IMP692ACPA Datasheet(PDF) 8 Page - A1 PROs co., Ltd. |
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IMP692ACPA Datasheet(HTML) 8 Page - A1 PROs co., Ltd. |
8 / 10 page 8 IMP690A IMP690A , 692A , 692A , 802L, 802M, 805L , 802L, 802M, 805L Application Information Power Fail Hysteresis A noise margin can be added to the simple monitoring circuit of Figure 7 by adding positive feedback from the PFO pin. The cir- cuit of Figure 8 adds this positive “latching” effect by means of an additional resistor R3 connected between PFO and PFI which helps in pulling PFI in the direction of PFO and eliminating an indecision at the trip point. Resistor R3 is normally about 10 times higher in resistance than R2 to keep the hysteresis band reason- able and should be larger than 10k Ω to avoid excessive loading on the PFO pin. The calculations for the correct values of resistors to set the hysteresis thresholds are given in Figure 8. A capacitor can be added to offer additional noise rejection by low-pass filtering. Figure 8. Hysteresis added to PFI pin 690A_10.eps VTRIP VL VH VIN 0V PFO +5V 0V VIN GND IMP690A IMP692A IMP802L IMP802M IMP805L R2 PFI VCC R1 PFO +5V R3 To µP *Optional C1* V R R R V R R R R R V R R R TRIP IH L = + = + − + − = 1 25 1 25 1 25 5 1 25 1 25 2 2 2 2 3 1 2 3 1 3 2 . . . . . Monitoring Capabilities of the Power-Fail Input Although designed for power supply failure monitoring, the PFI pin can be used for monitoring any voltage condition that can be scaled by means of a resistive divider. An example is the negative power supply monitor configured in Figure 9. In this case a good negative supply will hold the PFI pin below 1.25V and the PFO pin will be at a logic “0”. As the negative voltage declines, the voltage at the PFI pin will rise until it exceeds 1.25V and the PFO pin will go to a logic “1”. Figure 9. Using PFI to monitor negative supply voltage 5 1 25 1 25 1 2 − = − . . R V R TRIP 690A_11.eps +5V GND VTRIP 0V PFO +5V 0V V– IMP690A IMP692A IMP802L IMP802M IMP805L R2 V– PFI V– = VTRIP VCC PFO R1 |
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