Datasheet

MAX691A/MAX693A/MAX800L/MAX800M
Microprocessor Supervisory Circuits
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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).
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.
MAX691A
MAX693A
MAX800L
MAX800M
CE IN
C
LOAD
CE OUT
GND
+5V
50Ω
OUTPUT
IMPEDANCE
V
CC
VBATT
2.8V
MAX691A
MAX693A
MAX800L
MAX800M
PFI
PFO
GND
+5V
V
CC
VBATT
2.0V to 5.5V
LOW BATT
Figure 6. CE Propagation Delay Test Circuit Figure 7. Low-Battery Indicator
PIN
NAME STATUS
1 VBATT Supply current is 1µA max.
2 V
OUT
V
OUT
is connected to VBATT through an
internal PMOS switch.
3 V
CC
Battery switchover comparator monitors
V
CC
for active switchover.
4 GND GND 0V, 0V reference for all signals.
5 BATT ON
Logic high. The open-circuit output is
equal to V
OUT
.
6
LOWLINE
Logic low*
7 OSC IN OSC IN is ignored.
8 OSC SEL OSC SEL is ignored.
9 PFI
The power-fail comparator remains
active in the battery-backup mode for
V
CC
≥ VBATT - 1.2V typ.
10
PFO
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.
11 WDI Watchdog is ignored.
12
CE OUT
Logic high. The open-circuit voltage is
equal to V
OUT
.
13
CE IN
High impedance
14
WDO
Logic high. The open-circuit voltage is
equal to V
OUT
.
15
RESET
Logic low*
16 RESET High impedance*
Table 2. Input and Output Status in Battery-Backup
Mode
* V
CC
must be below the reset threshold to enter battery-backup
mode.