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SP805MEN Datasheet(PDF) 10 Page - Sipex Corporation |
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SP805MEN Datasheet(HTML) 10 Page - Sipex Corporation |
10 / 16 page Date: 11/29/04 SP690A/692A Low Power Microprocessor Supervisory with Battery Switch-Over © Copyright 2004 Sipex Corporation 10 The SP805L/M active-high RESET output is the inverse of the SP690A/SP692A/SP802 RE- SET output, and is valid with V CC down to 1V. Some µP's, such as Intel's 80C51, require an active-high reset pulse. Watchdog Input The watchdog circuit monitors the µP's activity. If the µP does not toggle the watchdog input (WDI) within 1.6sec, a reset pulse is triggered. The internal 1.6sec timer is cleared by either a reset pulse or by floating the WDI input. As long as RESET is asserted or the WDI input is floating, the timer remains cleared and does not count. As soon as RESET is released and WDI is driven high or low, the timer starts counting. It can detect pulses as short as 50ns. 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 unregulated DC input to the +5V regulator. The voltage-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 13. 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 pow- ered from the battery Figure 14. 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 SP690A/802L/805L Reset Threshold = 4.40V in SP692A/802M/805M Figure 13. 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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