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ADM8692AN Fiches technique(PDF) 10 Page - Analog Devices |
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ADM8692AN Fiches technique(HTML) 10 Page - Analog Devices |
10 / 16 page ADM8690–ADM8695 REV. 0 –10– +APPLICATION INFORMATION Increasing the Drive Current If the continuous output current requirements at VOUT exceed 100 mA, or if a lower VCC–VOUT voltage differential is desired, an external PNP pass transistor may be connected in parallel with the internal transistor. The BATT ON output (ADM8691/ ADM8693/ADM8695) can directly drive the base of the exter- nal transistor. PNP TRANSISTOR 0.1µF 0.1µF BATTERY +5V INPUT POWER VCC BATT ON VOUT VBATT ADM8691 ADM8693 ADM8695 Figure 17. Increasing the Drive Current Using a Rechargeable Battery for Backup If a capacitor or a rechargeable battery is used for backup then the charging resistor should be connected to VOUT since this eliminates the discharge path that would exist during power- down if the resistor is connected to VCC. 0.1µF 0.1µF RECHARGEABLE BATTERY +5V INPUT POWER VCC VOUT VBATT ADM869x R I = VOUT – VBATT R Figure 18. Rechargeable Battery 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 be- tween the PFO output and the PFI input as shown in Figure 19. When PFO is low, resistor R3 sinks current from the summing junction at the PFI pin. When PFO is high, the series combina- tion of R3 and R4 source current into the PFI summing junc- tion. This results in differing trip levels for the comparator. ADM869x 1.3V PFI PFO R1 R2 +7V TO +15V INPUT POWER R4 R3 VCC TO µP NMI +5V 7805 5V 0V PFO 0V VL VH VIN VH = 1.3V (1+ + ) VL = 1.3V (1+ – ) ASSUMING R4 < < R3 THEN R1 R2 R1 R3 R1 R2 R1 (5V – 1.3V) 1.3V (R3 + R4) R1 R2 HYSTERESIS VH – VL = 5V ( ) Figure 19. Adding Hysteresis to the Power Fail Comparator Monitoring the Status of the Battery The power fail comparator can be used to monitor the status of the backup battery instead of the power supply if desired. This is shown in Figure 20. The PFI input samples the battery volt- age and generates an active low PFO signal when the battery voltage drops below a chosen threshold. It may be necessary to apply a test load in order to determine the loaded battery volt- age. This can be done under processor control using CEOUT. Since CEOUT is forced high during the battery backup mode, the test load will not be applied to the battery while it is in use, even if the microprocessor is not powered. ADM869x PFO +5V INPUT POWER CEIN CEOUT PFI VBATT VCC BATTERY 20k Ω OPTIONAL TEST LOAD 10M Ω 10M Ω LOW BATTERY SIGNAL TO µP I/O PIN FROM µP I/O PIN APPLIES TEST LOAD TO BATTERY Figure 20. Monitoring the Battery Status Alternate Watchdog Input Drive Circuits The watchdog feature can be enabled and disabled under pro- gram control by driving WDI with a three-state buffer (Figure 21a). When three-stated, the WDI input will float, thereby dis- abling the watchdog timer. WDI ADM869x WATCHDOG STROBE CONTROL INPUT Figure 21a. Programming the Watchdog Input This circuit is not entirely foolproof, and it is possible that a software fault could erroneously three-state the buffer. This would then prevent the ADM869x from detecting that the mi- croprocessor is no longer operating correctly. In most cases a better method is to extend the watchdog period rather than dis- abling the watchdog. This may be done under program control using the circuit shown in Figure 21b. When the control input is high, the OSC SEL pin is low and the watchdog timeout is set by the external capacitor. A 0.01 µF capacitor sets a watchdog timeout delay of 100 seconds. When the control input is low, the OSC SEL pin is driven high, selecting the internal oscillator. The 100 ms or the 1.6 s period is chosen, depending on which di- ode in Figure 21b is used. With D1 inserted, the internal timeout is set at 100 ms; with D2 inserted the timeout is set at 1.6 s. D2 D1 CONTROL INPUT* ADM869x OSC SEL OSC IN *LOW = INTERNAL TIMEOUT HIGH = EXTERNAL TIMEOUT Figure 21b. Programming the Watchdog Input |
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