MAX791CPE 查看數據表（PDF） - Maxim Integrated

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MAX791CPE

Microprocessor Supervisory Circuit

Maxim Integrated 

Microprocessor Supervisory Circuit

VBATT 1

VCC 3

2

VOUT

0.1µF

RESET

THRESHOLD

VCC

RESET

CE IN

200ms TYP

MAX791

CE OUT

SECOND CE PULSE ABSENT WHEN VBATT < 2V

Figure 10. VCC and VBATT-to-VOUT Switch

Figure 11. Backup-Battery Monitor Timing Diagram

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 than VBATT (ignoring the small hys-

teresis effect), BATT ON typically sinks 3.2mA at 0.1V

saturation voltage. In battery-backup mode, this termi-

nal 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-cur-

rent 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 tem-

perature 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.

Low-Battery Monitor

The MAX791 low-battery voltage function monitors

VBATT. Low-battery detection of 2.0V ±0.15V is moni-

tored only during the reset-timeout period (200ms) that

occurs either after a normal power-up sequence or

after the MR reset input has been returned to its high

state. If the battery voltage is below 2.0V, the second

CE pulse is inhibited after reset timeout. If the battery

voltage is above 2.0V, all CE pulses are allowed

through the CE gate after the reset timeout period. To

use this function, after the 200ms reset delay, write 00

(HEX) to a location using the first CE pulse, and write

FF (HEX) to the same location using the second CE

pulse following RESET going inactive on power-up. The

contents of the memory then indicates a good battery

(FF) or a low battery (00) (Figure 11).

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