7

IMP690A

, 802L, 802M, 805L

Application Information

Capacitor storage, with very high values of capacitance, can be

are capacitors with capacities in the fractional farad range. A 0.1

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

tion will not occur since the capacitor will ultimately charge up to

the circuit of Figure 6 is preferred. In this circuit configuration, the

diode-resistor pair clamps the capacitor voltage at one diode-

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.

Figure 5. Capacitor as a backup power source

Figure 6. Capacitor as back-up Power Source - Voltage

690A_08.eps

+5V

GND

IMP692A

IMP802M

( ) IMP805L

100k

+

0.1F

To

µP

To Static RAM

RESET

+

690A_07.eps

+5V

GND

( ) IMP805L

0.1F

To

µP

To Static RAM

RESET

(RESET)

Operation Without a Backup Power Source

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



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.

Figure 7. Simple Voltage divider sets PFI trip point

5

1 25

2

1

2

R

R

R

V

+

< .

5

1 25

2

1

2

R

R

R

V

+

> .

690A_09.eps

+5V

GND

0V

PFO

+5V

A

A

B

B

IMP690A

IMP692A

IMP802L

IMP802M

IMP805L

PFI

PFO