ASM690A/692A, ASM802L/802M, ASM805L

http://onsemi.com

7

that the capacitor voltage remain below the maximum

voltages shown in Table 6. Although the circuit of Figure 6

shows the most simple way to connect the SuperCap, this

circuit cannot insure that an over voltage condition will not

To insure that an over voltage condition does not occur, the

circuit of Figure 7 is preferred. In this circuit configuration,

the diode−resistor pair clamps the capacitor voltage at one

within

±5% of 5 V for the ASM692A/802M or within ±10%

of 5 V for the ASM690A/802L/805L to insure that the

storage capacitor does not achieve an over voltage state.

Figure 6. Capacitor as a Backup Power Source

Figure 7. Capacitor as a Backup Power Source

Operation without a Backup Power Source

When operating without a back−up power source, the

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 1.25 V internal reference. If the voltage 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 8.

Figure 8. Simple Voltage Divider Sets PFI Trip Point

Power Fail Hysteresis

A noise margin can be added to the simple monitoring

circuit of Figure 8 by adding positive feedback from the

PFO pin. The circuit of Figure 9 adds this positive “latching”

effect by means of an additional resistor R3 connected

between PFO and PFI which helps in pulling PFI in the

direction of PFO and eliminating an indecision at the trip

point. Resistor R3 is normally about 10 times higher in

resistance than R2 to keep the hysteresis band reasonable

and should be larger than 10 k

W to avoid excessive loading

on the PFO pin. The calculations for the correct values of

resistors to set the hysteresis thresholds are given in

Figure 9. A capacitor can be added to offer additional noise

rejection by low−pass filtering.