MAX791C(1995) 查看數據表（PDF） - Maxim Integrated

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MAX791C
(Rev.:1995)

Microprocessor Supervisory Circuit

Maxim Integrated 

Microprocessor Supervisory Circuit

VBATT 1

VCC 3

2

VOUT

0.1µF

MAX791

Figure 10. VCC and VBATT-to-VOUT Switch

RESET

THRESHOLD

VCC

RESET

CE IN

200ms TYP

CE OUT

SECOND CE PULSE ABSENT WHEN VBATT < 2V

Figure 11. Backup-Battery Monitor Timing Diagram

Battery-Backup Mode

The MAX791 requires two conditions to switch to bat-

tery-backup mode: 1) VCC must be below the reset

threshold; 2) VCC must be below VBATT. Table 1 lists

the status of the inputs and outputs in battery-backup

mode.

Battery On Output

The Battery On (BATT ON) output indicates the status

of the internal VCC/battery-switchover comparator,

which controls the internal VCC and VBATT switches.

For VCC greater that VBATT (ignoring the small hystere-

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

ration voltage. In battery-backup mode, this terminal

sources approximately 10µA from VOUT. Use BATT ON

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

The Input Supply Voltage (VCC) should be a regulated

+5V. VCC connects to VOUT via a parallel diode and a

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 1Ω

each (Figure 10). The maximum continuous current is

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

of 1A.

Backup-Battery Input

The Backup-Battery Input (VBATT) is similar to VCC,

except the PMOS switch and parallel diode are much

smaller. Accordingly, the on-resistances of the diode

and the switch are each approximately 10Ω.

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

The Output Supply Voltage (VOUT) is internally connect-

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

to the external system and internal circuitry. All open-

circuit outputs will, for example, assume the VOUT volt-

age in their high states rather than the VCC voltage. At

the maximum source current of 250mA, VOUT will typi-

cally be 200mV below VCC. Decouple this terminal with

a 0.1µF capacitor.

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