Datasheet

Power-Fail Input
Power-Fail Input (PFI) is the input to the power-fail com-
parator. It has a guaranteed input leakage of ±25nA max
over temperature. The typical comparator delay is 25μs
from V
IL
to V
OL
(power failing), and 60μs from V
IH
to
V
OH
(power being restored). If PFI is not used, connect
it to ground.
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-backup
mode: 1) V
CC
must be below the reset threshold, and 2)
V
CC
must be below VBATT. Table 2 lists the status 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 hysteresis
effect), BATT ON typically sinks 3.2mA at 0.1V saturation
voltage. In battery-backup mode, this terminal sources
approximately 10μA from V
OUT
. Use BATT ON to indi-
cate battery-switchover status or to supply base drive to
an external pass transistor for higher-current 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
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
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*
*V
CC
must be below the reset threshold to enter battery-back-
up mode.
Table 2. Input and Output Status in
Battery-Backup Mode
Figure 6. CE Propagation Delay Test Circuit Figure 7. Low-Battery Indicator
MAX691A/MAX693A/
MAX800L/MAX800M
Microprocessor Supervisory Circuits
www.maximintegrated.com
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