IMP690A , 692A , 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 t
WD
+ t
RS
),
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.
Table 1. Pin Connections in Battery Backup Mode
Pin
V
OUT
V
BATT
PFI
PFO
RESET
WDI
Connection
Connected to V
BATT
through internal PMOS switch
Connected to V
OUT
Disabled
Logic-LOW
Logic LOW (except on IMP805 where it is HIGH)
Watchdog timer disabled
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 V
CC
to an alternate power source con-
nected at V
BATT
when V
CC
is less than the reset threshold voltage
(V
CC
< V
RT
), and V
CC
is less than V
BATT
. The V
OUT
pin is normally
connected to V
CC
through a 2Ω PMOS switch but a brown-out or
loss of V
CC
will cause a switchover to V
BATT
by means of a 20Ω
PMOS switch. Although both conditions (V
CC
< V
RT
and V
CC
<
V
BATT
) must occur for the switchover to V
BATT
to occur, V
OUT
will
be switched back to V
CC
when V
CC
exceeds V
RT
irrespective of the
voltage at V
BATT
. It should be noted that an internal device diode
(D1 in
Figure 4)
will be forward biased if V
BATT
exceeds V
CC
by
more than a diode drop when V
CC
is switched to V
OUT
. Because of
this it is recommended that V
BATT
be no greater than V
RT
+0.6V.
V
BATT
V
CC
During the backup power mode, the internal circuitry of the
supervisory circuit draws power from the battery supply. While
V
CC
is still alive, the comparator circuits remain alive and the cur-
rent drawn by the device is typically 35µA. When V
CC
drops more
than 1.1V below V
BATT
, 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.
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 V
CC
is less than V
BATT
and V
CC
> V
RT
.
Table 2. Maximum Battery Voltages
Part No.
IMP690A
IMP802L
IMP805L
IMP692A
IMP802M
MAXIMUM Battery Voltage
4.80
4.80
4.80
4.55
4.55
SW1
SW2
D1 D2
SW3
SW4
IMP690A IMP802L
IMP692A IMP802M
D3
IMP805L
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 V
CC
remains above the reset threshold voltage V
RT
.
In the IMP devices, a floating V
BATT
pin will not cause a power-
supply switchover as can occur in some other supervisory
circuits. If V
BATT
is not used, the pin should be grounded.
V
OUT
CONDITION
V
CC
> Reset Threshold
V
CC
< Reset Threshold and
V
CC
> V
BATT
V
CC
< Reset Threshold and
V
CC
< V
BATT
SW1/SW2 SW3/SW4
Open
Closed
Open
Closed
Closed
Open
IMP690A /IMP802L / IMP805L Reset Threshold = 4.65V
IMP692A / IMP802M Reset Threshold = 4.4V
690A_06.eps
Figure 4. Internal device configuration of battery
switch-over function
6
IMP [ IMP, INC ]
