SP802LEP datasheet(12/16 Pages) EXAR

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Date: 11/29/04

SP690A/692A Low Power Microprocessor Supervisory with Battery Switch-Over

Allowable Backup Power-Source

Batteries

Lithium batteries work very well as backup

batteries due to very low self-discharge rate and

high energy density. Single lithium batteries

with open-circuit voltages of 3.0V to 3.6V are

ideal. Any battery with an open-circuit voltage

less than the minimum reset threshold plus 0.3V

can be connected directly to the V

BATT input of

this series with no additional circuitry; see

FIGURE 12. However, batteries with open-

circuit voltages that are greater than this value

cannot be used for backup, as current is sourced

into V

OUT through the diode (D1 in Figure 13)

when V

CC is close to the reset threshold.

Operation Without a Backup Power

Source

If a backup power source is not used, ground

BATT and connect VOUT to VCC. Since there is

no need to switch over to any backup power

source, V

OUT does not need to be switched. A

direct connection to V

CC eliminates any voltage

drops across the switch which may push V

OUT

below V

CC.

Replacing the Backup Battery

The backup battery can be removed while V

remains valid, without danger of triggering

RESET/RESET. As long as V

CC stays above the

reset threshold, battery-backup mode cannot be

entered.

Adding Hysteresis to the Power-Fail

Comparator

Hysteresis adds a noise margin to the power-fail

comparator and prevents repeated triggering of

PFO when V

IN is close to its trip point. Figure 18

shows how to add hysteresis to the power-fail

comparator. Select the ratio of R

1 and R2 such

that PFI sees 1.25V when V

IN falls to its trip

point (V

TRIP).

3 adds the hysteresis.

It will

typically be an order of magnitude greater (about

10 times) than R

1 or R2. The current through R1

and R

2 should be at least 1µA to ensure that the

25nA (max) PFI input current does not shift the

trip point. R

3 should be larger than 10KΩ so it

does not load down the PFO pin. Capacitor C1

adds additional noise rejection.

Monitoring a Negative Voltage

The power-fail comparator can be used to monitor

a negative supply rail using the circuit of Figure

19. When the negative rail is valid, PFO is low.

When the negative supply voltage drops, PFO

goes high.

This circuit's accuracy is

affected by the PFI threshold tolerance, the V

voltage, and the resistors, R1 and R2.

Figure 18. Adding Hysteresis to the POWER-FAIL

Comparator

PFI

PFO

*C1

VIN

connect to µP

VCC

+5V

GND

PFO

VIN

+5V

VTRIP

*optional

VTRIP =

R1 + R2

VH =

R2 || R3

R1 + R2 || R3

1.25

VL - 1.25

5.0 - 1.25

1.25

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