Battery-Backup Mode

The MAX791 requires two conditions to switch to bat-

the status of the inputs and outputs in battery-backup

mode.

Battery On Output

The Battery On (BATT ON) output indicates the status

sis effect), BATT ON typically sinks 3.2mA at 0.1V satu-

ration voltage. In battery-backup mode, this terminal

to indicate battery-switchover status or to supply base

drive to an external pass transistor for higher-current

applications (see

Typical Operating Circuit).

Input Supply Voltage

large PMOS switch. The switch carries the entire cur-

rent load for currents less than 250mA. The parallel

diode carries any current in excess of 250mA. Both the

switch and the diode have impedances less than 1

Ω

each (Figure 10). The maximum continuous current is

250mA, but power-on transients may reach a maximum

of 1A.

Backup-Battery Input

except the PMOS switch and parallel diode are much

smaller. Accordingly, the on-resistances of the diode

and the switch are each approximately 10

Ω.

Continuous current should be limited to 25mA and

peak currents (only during power-up) limited to 250mA.

The reverse leakage of this input is less than 1µA over

temperature and supply voltage.

Output Supply Voltage

ed to the substrate of the IC and supplies all the current

to the external system and internal circuitry. All open-

a 0.1µF capacitor.

Microprocessor Supervisory Circuit

______________________________________________________________________________________

13

200ms TYP

RESET

THRESHOLD

RESET

CE IN

CE OUT

SECOND CE PULSE ABSENT WHEN VBATT < 2V

Figure 11. Backup-Battery Monitor Timing Diagram

MAX791

VBATT

1

3

2

0.1

µF