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ADM707AR Datasheet(PDF) 5 Page - Analog Devices |
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ADM707AR Datasheet(HTML) 5 Page - Analog Devices |
5 / 8 page ADM705–ADM708 REV. B –5– Power-Fail Comparator The power-fail comparator is an independent comparator that may be used to monitor the input power supply. The comparator’s inverting input is internally connected to a 1.25 V reference voltage. The noninverting input is available at the PFI input. This input may be used to monitor the input power supply via a resistive divider network. When the voltage on the PFI input drops below 1.25 V, the comparator output ( PFO) goes low, indicating a power failure. For early warning of power failure, the comparator may be used to monitor the preregulator input simply by choosing an appropriate resistive divider network. The PFO output can be used to interrupt the processor so that a shutdown procedure is implemented before the power is lost. INPUT POWER R1 R2 POWER-FAIL INPUT 1.25V PFI PFO POWER-FAIL OUTPUT ADM70x Figure 5. Power-Fail Comparator Adding Hysteresis to the Power-Fail Comparator For increased noise immunity, hysteresis may be added to the power-fail comparator. Since the comparator circuit is non- inverting, hysteresis can be added simply by connecting a resistor between the PFO output and the PFI input as shown in Figure 6. When PFO is low, resistor R3 sinks current from the summing junction at the PFI pin. When PFO is high, resistor R3 sources current into the PFI summing junction. This results in differing trip levels for the comparator. Further noise immu- nity may be achieved by connecting a capacitor between PFI and GND. 7V TO 15V INPUT POWER TO P NMI R3 5V VCC R1 R2 1.25V PFI PFO ADM70x ADM663 5V PFO 0V 0V VL VIN VH Figure 6. Adding Hysteresis to the Power-Fail Comparator V RR RR R VR R V RE V RR R H L CC MID = [ + + × =+ = + 125 1 23 23 1 125 1 125 2 125 125 12 2 .] . . – – . . Valid RESET Below 1 V V CC The ADM70x family of products is guaranteed to provide a valid reset level with VCC as low as 1 V; please refer to the Typi- cal Performance Characteristics. As VCC drops below 1 V, the internal transistor will not have sufficient drive to hold it ON so the voltage on RESET will no longer be held at 0 V. A pull-down resistor as shown in Figure 7 may be connected externally to hold the line low if it is required. ADM70x GND RESET R1 Figure 7. RESET Valid Below 1 V |
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