MAX791C/D MAXIM [Maxim Integrated Products], MAX791C/D Datasheet - Page 13

• Manufacturer Part Number: MAX791C/D
• Description: Microprocessor Supervisory Circuit
• Manufacturer: MAXIM [Maxim Integrated Products]

The MAX791 requires two conditions to switch to bat-
tery-backup mode: 1) V
threshold; 2) V
the status of the inputs and outputs in battery-backup
mode.
The Battery On (BATT ON) output indicates the status
of the internal V
which controls the internal V
For V
sis effect), BATT ON typically sinks 3.2mA at 0.1V satu-
ration voltage. In battery-backup mode, this terminal
sources approximately 10µA from V
to indicate battery-switchover status or to supply base
drive to an external pass transistor for higher-current
applications (see Typical Operating Circuit ).
The Input Supply Voltage (V
+5V. V
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.
Figure 10. V
CC
CC
VBATT
greater that VBATT (ignoring the small hystere-
V
connects to V
CC
CC
and VBATT-to-V
1
3
CC
CC
______________________________________________________________________________________
must be below VBATT. Table 1 lists
/battery-switchover comparator,
MAX791
OUT
CC
Battery-Backup Mode
OUT
Input Supply Voltage
CC
via a parallel diode and a
CC
must be below the reset
Microprocessor Supervisory Circuit
) should be a regulated
Switch
Battery On Output
and VBATT switches.
OUT
2
. Use BATT ON
0.1 F
V
OUT
The Backup-Battery Input (VBATT) is similar to V
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.
The Output Supply Voltage (V
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 V
age in their high states rather than the V
the maximum source current of 250mA, V
cally be 200mV below V
a 0.1µF capacitor.
Figure 11. Backup-Battery Monitor Timing Diagram
THRESHOLD
CE OUT
RESET
CE IN
V
RESET
CC
200ms TYP
SECOND CE PULSE ABSENT WHEN VBATT < 2V
CC
Output Supply Voltage
. Decouple this terminal with
Backup-Battery Input
OUT
) is internally connect-
CC
OUT
voltage. At
OUT
will typi-
volt-
CC
13
,