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
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 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