IMP802 IMP Inc, IMP802 Datasheet - Page 6

• Manufacturer Part Number: IMP802
• Description: P POWER SUPPLY SUPERVISOR WITH BATTERY BACKUP SWITCH
• Manufacturer: IMP Inc

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
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 V
nected at V
(V
connected to V
loss of V
PMOS switch. Although both conditions (V
V
be switched back to V
voltage at V
(D1 in Figure 4) will be forward biased if V
more than a diode drop when V
this it is recommended that V
6
BATT
CC
Application Information
< V
) must occur for the switchover to V
IMP690A / IMP802L /IMP805L Reset Threshold = 4.65V
IMP692A /IMP802M Reset Threshold = 4.4V
V
Figure 4. Internal device configuration of battery
RT
CC
BATT
CONDITION
V
V
V
V
V
CC
CC
CC
CC
CC
), and V
SW1
BATT
IMP690A
IMP692A
will cause a switchover to V
BATT
> Reset Threshold
< Reset Threshold and
> V
< Reset Threshold and
< V
CC
BATT
BATT
when V
. It should be noted that an internal device diode
through a 2 PMOS switch but a brown-out or
CC
SW2
IMP802L
IMP802M
is less than V
CC
switch-over function
CC
when V
is less than the reset threshold voltage
CC
BATT
D1
to an alternate power source con-
V
CC
CC
OUT
D2
be no greater than V
D3
BATT
is switched to V
exceeds V
SW1/SW2 SW3/SW4
Closed
Open
Open
. The V
IMP805L
BATT
SW3
BATT
CC
RT
BATT
OUT
by means of a 20
to occur, V
irrespective of the
< V
Closed
Closed
Open
exceeds V
pin is normally
OUT
SW4
RT
. Because of
V
RT
and V
CC
690A_06.eps
WD
+0.6V.
OUT
IMP690A
IMP690A
+ t
CC
CC
will
RS
by
),
<
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
V
rent drawn by the device is typically 35 A. When V
than 1.1V below V
PFI comparator and WDI comparator will shut off, reducing the
quiescent current drawn by the IC to less than 1 A.
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
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 V
In the IMP devices, a floating V
supply switchover as can occur in some other supervisory
circuits. If V
RESET
CC
PFO
Pin
V
V
PFI
WDI
OUT
BATT
Part No.
IMP690A
IMP802L
IMP805L
IMP692A
IMP802M
is still alive, the comparator circuits remain alive and the cur-
, 692A
, 692A
Connection
Connected to V
Connected to V
Disabled
Logic-LOW
Logic LOW (except on IMP805 where it is HIGH)
Watchdog timer disabled
BATT
is not used, the pin should be grounded.
CC
CC
BATT
is less than V
remains above the reset threshold voltage V
, 802L, 802M, 805L
, 802L, 802M, 805L
, the internal switchover comparator, the
MAXIMUM Battery Voltage
BATT
OUT
through internal PMOS switch
BATT
BATT
and V
pin will not cause a power-
4.80
4.80
4.80
4.55
4.55
CC
> V
RT
.
CC
drops more
RT
.