IMP690A, 692A, 802L, 802M, 805L
Application Information
TM
Backup Power Sources - SuperCap
Operation Without a Backup Power Source
Capacitor storage, with very high values of capacitance, can be When operating without a back-up power source, the VBATT pin
used as a back-up power source instead of batteries. SuperCapTM should be connected to GND and VOUT should be connected to
are capacitors with capacities in the fractional farad range. A 0.1
farad SuperCapTM would provide a useful backup power source.
V
CC, since power source switchover will not occur. Connecting
VOUT to VCC eliminates the voltage drop due to the ON-resistance
Like the battery supply, it is important that the capacitor voltage of the PMOS switch.
remain below the maximum voltages shown in Table 2. Although
Power-Fail Comparator
the circuit of Figure 5 shows the most simple way to connect the
SuperCapTM, this circuit cannot insure that an overvoltage condi-
tion will not occur since the capacitor will ultimately charge up to
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.
VCC. 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 VCC. VCC 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.
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 PFO pin
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.
Note: SuperCapTM is a trademark of Baknor Industries
+5V
VCC
VOUT
To Static RAM
+5V
VBATT RESET
(RESET)
To µP
VCC
+
IMP690A
IMP692A
IMP802L
IMP802M
IMP805L
R1
R2
0.1F
PFI
PFO
GND
690A_07.eps
( ) IMP805L
GND
Figure 5. Capacitor as a backup power source
+5V
+5V
PFO
0V
A
B
VCC
VOUT
To Static RAM
VBATT RESET
To µP
5R
5R
2
R +R
1 2
2
IMP692A
IMP802M
< 1.25V
> 1.25V
A
B
+
R +R
1
2
100k
0.1F
690A_09.eps
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 VCC
7
IMP [ IMP, INC ]
