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ADM708TAN Datasheet(PDF) 6 Page - Analog Devices |
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ADM708TAN Datasheet(HTML) 6 Page - Analog Devices |
6 / 8 page REV. A –6– ADM706P/R/S/T, ADM708R/S/T V MID 1.25 R1 + R2 R2 Valid RESET Below 1 V V CC The ADM70x family of products are 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 Power-Fail Comparator The power-fail comparator is an independent comparator which may be used to monitor the input power supply. The compara- tor’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 compara- tor 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 proce- dure is implemented before the power is lost. INPUT POWER R1 R2 POWER-FAIL INPUT 1.25V PFI PFO ADM70x POWER-FAIL OUTPUT 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 nonin- verting, 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 sum- ming 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 immunity may be achieved by connecting a capacitor between PFI and GND. INPUT POWER R1 R2 1.25V PFI PFO TO P NMI ADM70x R3 ADM663A +3.3V VCC 3.3V PFO 0V 0V VL VIN VH Figure 6. Adding Hysteresis to the Power-Fail Comparator V H = 1.25 1 + R2 + R3 R2 × R3 R1 V L = 1.25 + R1 1.25 R2 – VCC –1.25 R 3 VCC RESET 400ms/DIV Figure 8. ADM706/ADM708 RESET Output Voltage vs. Supply Voltage VCC RESET 400ms/DIV Figure 9. RESET Output Voltage vs. Supply Voltage Typical Performance Characteristics |
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