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ASM805LCSA Datasheet(PDF) 4 Page - Alliance Semiconductor Corporation |
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ASM805LCSA Datasheet(HTML) 4 Page - Alliance Semiconductor Corporation |
4 / 13 page 4 of 13 Notice: The information in this document is subject to change without notice ASM690A / 692A ASM802L / 802M ASM805L µP Power Supply Supervisor With Battery Backup Switch rev 1.0 October 2003 Watchdog Input As discussed in the Reset section, the Watchdog input is used to monitor microprocessor activity. It can be used to insure that the microprocessor is in a continually responsive state by requiring that the WDI pin be toggled every second. If the WDI pin is not toggled within the 1.6 second window (minimum tWD + tRS), a reset pulse will be asserted to return the microprocessor to the initial start-up state. Pulses as short as 50ns can be applied to the WDI pin. If this feature is not used, the WDI pin should be open circuited or the logic placed into a high-impedance state to allow the pin to float. Backup-Battery Switchover A power loss can be made less severe if the system RAM contents are preserved. This is achieved in the ASM690/692/ 802/805 by switching from the failed VCC to an alternate power source connected at VBATT when VCC is less than the reset threshold voltage (VCC < VRT), and VCC is less than VBATT. The VOUT pin is normally connected to VCC through a 2 Ω PMOS switch but a brown-out or loss of VCC will cause a switchover to VBATT by means of a 20Ω PMOS switch. Although both conditions (VCC < VRT and VCC <VBATT) must occur for the switchover to VBATT to occur, VOUT will be switched back to VCC when VCC exceeds VRT irrespective of the voltage at VBATT. It should be noted that an internal device diode (D1 in Figure 3) will be forward biased if VBATT exceeds VCC by more than a diode drop when VCC is switched to VOUT. Because of this it is recommended that VBATT be no greater than VRT +0.6V. Table 1. Pin Connections in Battery Backup Mode During the backup power mode, the internal circuitry of the supervisory circuit draws power from the battery supply. While VCC is still alive, the comparator circuits remain alive and the current drawn by the device is typically 35µA. When VCC drops more than 1.1V below VBATT, the internal switchover comparator, the PFI comparator and WDI comparator will shut off, reducing the quiescent current drawn by the IC to less than 1µA. Condition SW1/SW2 SW3/SW4 VCC > Reset Threshold open closed VCC < Reset Threshold VCC > VBATT open closed VCC < Reset Threshold VCC < VBATT closed open ASM690A/802A/805L Reset Threshold = 4.65V ASM692A /ASM802M Reset Threshold = 4.4V Pin Connection VOUT Connected to VBATT through internal PMOS switch VBATT Connected to VOUT PFI Disabled PFO Logic-LOW RESET Logic-LOW (except on ASM805 where it is HIGH) WDI Watchdog timer disabled SW1 SW2 D1 D2 SW3 SW4 D3 ASM690A ASM692A ASM802L ASM802M ASM805L VOUT VCC VBATT Figure 3: Internal device configuration of battery switch-over function |
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