Microprocessor Supervisory Circuits
MAX691A/MAX693A/MAX800L/MAX800M
+5V
VBATT
2.8V
V
CC
VBATT
+5V
V
CC
MAX691A
MAX693A
MAX800L
MAX800M
CE IN
CE OUT
GND
C
LOAD
2.0V to 5.5V
MAX691A
MAX693A
MAX800L
PFO
PFI
MAX800M
GND
LOW BATT
50Ω
OUTPUT
IMPEDANCE
Figure 6. CE Propagation Delay Test Circuit
Figure 7. Low-Battery Indicator
Table 2. Input and Output Status in Battery-Backup
Mode
PIN
1
2
3
4
5
6
7
8
9
NAME
VBATT
V
OUT
V
CC
GND
BATT ON
LOWLINE
OSC IN
OSC SEL
PFI
STATUS
Supply current is 1µA max.
V
OUT
is connected to VBATT through an
internal PMOS switch.
Battery switchover comparator monitors
V
CC
for active switchover.
GND 0V, 0V reference for all signals.
Logic high. The open-circuit output is
equal to V
OUT
.
Logic low*
OSC IN is ignored.
OSC SEL is ignored.
The power-fail comparator remains
active in the battery-backup mode for
V
CC
≥
VBATT - 1.2V typ.
The power-fail comparator remains
active in the battery-backup mode for
V
CC
≥
VBATT - 1.2V typ. Below this volt-
age, PFO is forced low.
Watchdog is ignored.
Logic high. The open-circuit voltage is
equal to V
OUT
.
High impedance
Logic high. The open-circuit voltage is
equal to V
OUT
.
Logic low*
High impedance*
Power-Fail Output
The Power-Fail Output (PFO) goes low when PFI goes
below 1.25V. It typically sinks 3.2mA with a saturation
voltage of 0.1V. With PFI above 1.25V,
PFO
is actively
pulled to V
OUT
.
Battery-Backup Mode
Two conditions are required to switch to battery-back-
up mode: 1) V
CC
must be below the reset threshold,
and 2) V
CC
must be below VBATT. Table 2 lists the sta-
tus of the inputs and outputs in battery-backup mode.
Battery-On Output
The Battery-On (BATT ON) output indicates the status
of the internal V
CC
/battery-switchover comparator,
which controls the internal V
CC
and VBATT switches.
For V
CC
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 V
OUT
. 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).
10
PFO
Input Supply Voltage
The Input Supply Voltage (V
CC
) should be a regulated
5V. V
CC
connects to V
OUT
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. The maximum continuous current is 250mA, but
power-on transients may reach a maximum of 1A.
11
12
13
14
15
16
WDI
CE
OUT
CE
IN
WDO
RESET
RESET
* V
CC
must be below the reset threshold to enter battery-backup
mode.
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MAXIM [ MAXIM INTEGRATED PRODUCTS ]
