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ADM694AN Datasheet(PDF) 10 Page - Analog Devices |
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ADM694AN Datasheet(HTML) 10 Page - Analog Devices |
10 / 16 page ADM690–ADM695 REV. A –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 (ADM691/ ADM693/ADM695) can directly drive the base of the external transistor. V OUT V CC BATTERY +5V INPUT POWER 0.1 µF 0.1 µF BATT ON V BATT PNP TRANSISTOR ADM691 ADM693 ADM695 Figure 17. Increasing the Drive Current Using a Rechargeable Battery for Back Up If a capacitor or a rechargeable battery is used for back up 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. V OUT V CC RECHARGEABLE BATTERY +5V INPUT POWER 0.1 µF 0.1 µF V BATT ADM69x 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. ADM69x R 2 1.3V PFO R 1 7805 R 4 R 3 +7V TO +15V INPUT POWER +5V PFI V CC TO µP NMI 5V 0V 0V V L V H V IN PFO V H = 1.3V (1+ ––– + ––– ) V L = 1.3V (1+ ––– – –––––––––––––) ASSUMING R 4 < < R 3 THEN HYSTERESIS V H – V L = 5V (––– ) R 1 R 2 R 1 R 3 R 1 R 2 R 1 R 2 R 1 (5V – 1.3V) 1.3V (R 3 + R 4 ) 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. |
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