Mic ro p ro c e s s o r S u p e rvis o ry Circ u it
MAX791
RESET
THRESHOLD
1
3
VBATT
200ms TYP
V
CC
2
V
OUT
RESET
CE IN
0.1µF
V
CC
CE OUT
MAX791
SECOND CE PULSE ABSENT WHEN VBATT < 2V
Figure 11. Backup-Battery Monitor Timing Diagram
Figure 10. V and VBATT-to-V
Switch
CC
OUT
Ba t t e ry-Ba c k u p Mo d e
The MAX791 requires two conditions to switch to bat-
Ba c k u p -Ba t t e ry In p u t
The Backup-Battery Input (VBATT) is similar to V
,
CC
tery-backup mode: 1) V
must be below the reset
except the PMOS switch and parallel diode are much
smaller. Accordingly, the on-resistances of the diode
a nd the s witc h a re e a c h a p p roxima te ly 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.
CC
threshold; 2) V
must be below VBATT. Table 1 lists
CC
the status of the inputs and outputs in battery-backup
mode.
Ba t t e ry On Ou t p u t
The Battery On (BATT ON) output indicates the status
of the inte rna l V /b a tte ry-s witc hove r c omp a ra tor,
CC
Ou t p u t S u p p ly Vo lt a g e
which controls the internal V
and VBATT switches.
CC
For V greater that VBATT (ignoring the small hystere-
The Output Supply Voltage (V ) is internally connect-
OUT
CC
sis effect), BATT ON typically sinks 3.2mA at 0.1V satu-
ration voltage. In battery-backup mode, this terminal
ed to the substrate of the IC and supplies all the current
to the external system and internal circuitry. All open-
sources approximately 10µA from V . Use BATT ON
OUT
circuit outputs will, for example, assume the V
volt-
OUT
to indicate battery-switchover status or to supply base
drive to an external pass transistor for higher-current
applications (see Typical Operating Circuit).
age in their high states rather than the V
the maximum source current of 250mA, V
voltage. At
CC
will typi-
OUT
cally be 200mV below V . Decouple this terminal with
CC
a 0.1µF capacitor.
In p u t S u p p ly Vo lt a g e
The Input Supply Voltage (V ) should be a regulated
CC
+5V. V
connects to V
via a parallel diode and a
CC
OUT
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.
______________________________________________________________________________________ 13
MAXIM [ MAXIM INTEGRATED PRODUCTS ]
