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IMP692AESA Datasheet(PDF) 6 Page - IMP, Inc |
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IMP692AESA Datasheet(HTML) 6 Page - IMP, Inc |
6 / 10 page 6 IMP690A IMP690A , 692A , 692A , 802L, 802M, 805L , 802L, 802M, 805L Application Information 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 IMP690/692/802/805 by switching from the failed VCC to an alternate power source con- nected 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 2Ω 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 4) 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. Figure 4. Internal device configuration of battery switch-over function VCC > Reset Threshold VCC < Reset Threshold and VCC > VBATT VCC < Reset Threshold and VCC < VBATT IMP690A / IMP802L / IMP805L Reset Threshold = 4.65V IMP692A / IMP802M Reset Threshold = 4.4V Closed Open Open Closed Closed Open CONDITION SW1/SW2 SW3/SW4 690A_06.eps SW4 SW3 D2 D1 D3 IMP690A IMP692A IMP802L IMP802M IMP805L SW1 SW2 VCC VOUT VBATT 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 cur- rent drawn by the device is typically 35 µA. 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 1 µA. Backup Power Sources - Batteries Battery voltage selection is important to insure that the battery does not discharge through the parasitic device diode D1 (see Figure 4) when VCC is less than VBATT and VCC > VRT. Table 2. Maximum Battery Voltages Although most batteries that meet the requirements of Table 2 are acceptable, lithium batteries are very effective backup source due to their high-energy density and very low self-discharge rates. Battery Replacement while Powered Batteries can be replaced even when the device is in a powered state as long as VCC remains above the reset threshold voltage VRT. In the IMP devices, a floating VBATT pin will not cause a power- supply switchover as can occur in some other supervisory circuits. If VBATT is not used, the pin should be grounded. Part No. MAXIMUM Battery Voltage IMP690A 4.80 IMP802L 4.80 IMP805L 4.80 IMP692A 4.55 IMP802M 4.55 Pin Connection VOUT Connected to VBATT through internal PMOS switch VBATT Connected to VOUT PFI Disabled PFO Logic-LOW RESET Logic LOW (except on IMP805 where it is HIGH) WDI Watchdog timer disabled |
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