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SP703CP Datasheet(PDF) 10 Page - Sipex Corporation |
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SP703CP Datasheet(HTML) 10 Page - Sipex Corporation |
10 / 16 page SP703/704DS/07 SP703/704 Low Power Microprocessor Supervisory © Copyright 2000 Sipex Corporation 10 Power-Fail Comparator The Power-Fail Comparator can be used as an under-voltage detector to signal the failing of a power supply (it is completely separate from the rest of the circuitry and does not need to be dedicated to this function). The PFI input is compared to an internal 1.25V reference. If PFI is less than 1.25V, PFO goes low. The external voltage divider drives PFI to sense the unregu- lated DC input to the +5V regulator. The volt- age-divider ratio can be chosen such that the voltage at PFI falls below 1.25V just before the +5V regulator drops out. PFO then triggers an interrupt which signals the µP to prepare for power-down. When V BATT connects to VOUT, the power-fail comparator is turned off and PFO is forced low to conserve backup-battery power. Backup-Battery Switchover In the event of a brownout or power failure, it may be necessary to preserve the contents of RAM. With a backup battery installed at V BATT, the RAM is assured to have power if V CC fails. As long as V CC exceeds the reset threshold, V OUT connects to VCC through a 0.6Ω PMOS power switch. Once V CC falls below the reset threshold, V CC or VBATT, whichever is higher, switches to V OUT. V BATT connects to VOUT through a 5 Ω switch only when V CC is below the reset threshold and V BATT is greater than VCC. When V CC exceeds the reset threshold, it is connected to V OUT, regardless of the voltage applied to V BATT Figure 9. During this time, the diode (D1) between V BATT and VOUT will conduct current from V BATT to VOUT if VBATT is more than .6V above V OUT. When V BATT connects to VOUT, backup mode is activated and the internal circuitry will be powered from the battery Figure 10. When V CC is just below V BATT, in the backup mode the current drawn from V BATT will be typically 30 µA. When V CC drops to more than 1V below V BATT, the internal switchover comparator shuts off and the supply current falls to less than 0.6 µA. Reset Threshold = 4.65V in SP703 Reset Threshold = 4.40V in SP704 Figure 9. BACKUP-BATTERY Switchover Block Diagram SW1 D1 D2 D3 SW2 VBATT VCC GND VOUT N O I T I D N O C1 W S2 W S V C C d l o h s e r h T t e s e R >n e p Od e s o l C V C C d n a d l o h s e r h T t e s e R < V C C V > T T A B n e p Od e s o l C V C C d n a d l o h s e r h T t e s e R < V C C V < T T A B d e s o l Cn e p O |
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