IMP690A , 692A , 802L, 802M, 805L
Application Information
Backup Power Sources - SuperCap
TM
Capacitor storage, with very high values of capacitance, can be
used as a back-up power source instead of batteries. SuperCap
TM
are capacitors with capacities in the fractional farad range. A 0.1
farad SuperCap
TM
would provide a useful backup power source.
Like the battery supply, it is important that the capacitor voltage
remain below the maximum voltages shown in
Table 2.
Although
the circuit of
Figure 5
shows the most simple way to connect the
SuperCap
TM
, this circuit cannot insure that an overvoltage condi-
tion will not occur since the capacitor will ultimately charge up to
V
CC
. To insure that an overvoltage condition does not occur,
the circuit of
Figure 6
is preferred. In this circuit configuration, the
diode-resistor pair clamps the capacitor voltage at one diode-
drop below V
CC
. V
CC
itself should be regulated within
±5%
of
5V for the IMP692A/802M or within
±10%
of 5V for the
IMP690A/802L/805L to insure that the storage capacitor does not
achieve an overvoltage state.
Note: SuperCap
TM
is a trademark of Baknor Industries
+5V
Operation Without a Backup Power Source
When operating without a back-up power source, the V
BATT
pin
should be connected to GND and V
OUT
should be connected to
V
CC
, since power source switchover will not occur. Connecting
V
OUT
to V
CC
eliminates the voltage drop due to the ON-resistance
of the PMOS switch.
Power-Fail Comparator
The Power Fail feature is an independent voltage monitoring
function that can be used for any number of monitoring activities.
The PFI function can provide an early sensing of power supply
failure by sensing the voltage of the unregulated DC ahead of
the regulated supply sensing seen by the backup-battery
switchover circuitry.
The PFI pin is compared to a 1.25V internal reference. If the volt-
age at the PFI pin is less than this reference voltage, the
pin
PFO
goes low. By sensing the voltage of the raw DC power supply, the
microprocessor system can prepare for imminent power-loss,
especially if the battery backup supply is not enabled. The input
voltage at the PFI pin results from a simple resistor voltage
divider as shown in
Figure 7.
+5V
V
CC
V
OUT
To Static RAM
V
BATT
RESET
(RESET)
To
µP
R
1
V
CC
IMP690A
IMP692A
PFI IMP802L PFO
IMP802M
IMP805L
GND
+
0.1F
GND
( ) IMP805L
690A_07.eps
R
2
Figure 5. Capacitor as a backup power source
+5V
+5V
V
CC
V
OUT
To Static RAM
PFO
0V
A
B
V
BATT
RESET
To
µP
A
5R
2
<
1.25V
R
1
+
R
2
B
5R
2
>
1.25V
R
1
+
R
2
690A_09.eps
+
100k
0.1F
IMP692A
IMP802M
GND
Figure 7. Simple Voltage divider sets PFI trip point
690A_08.eps
( ) IMP805L
Figure 6. Capacitor as back-up Power Source - Voltage
clamped to 0.5V below V
CC
7
A1PROS [ A1 PROS CO., LTD. ]
