Datasheet

calculate the amount of capacitance required to allow
the shutdown routine to complete before reset is
asserted:
C
HOLD
= (I
LOAD
x t
SHDN
) / (V
LR(min)
)
where t
SHDN
is the time required for the system to com-
plete the shutdown routine (including the V
CC
to low-
line propagation delay), I
LOAD
is the current being
drained from the capacitor, and V
LR
is the low-line to
reset threshold.
Output Supply Voltage
The output supply (OUT) transfers power from V
CC
or
BATT to the µP, RAM, and other external circuitry. At
the maximum source current of 250mA, V
OUT
will typi-
cally be 220mV below V
CC
. Decouple OUT with a 0.1µF
capacitor to ground.
Battery-Backup Mode
Battery-backup mode preserves the contents of RAM in
the event of a brownout or power failure. With a backup
battery installed at BATT, the MAX801/MAX808 automati-
cally switches RAM to backup power when V
CC
falls.
Two conditions are required for switchover to battery-
backup mode: 1) V
CC
must be below the reset threshold;
2) V
CC
must be below V
BATT
. Table 1 lists the status of
inputs and outputs during battery-backup mode.
BATT is designed to conduct up to 20mA to OUT dur-
ing battery backup. The PMOS switch on-resistance is
approximately 12Ω. Figure 4 shows the two series pass
elements (between the BATT input and OUT) that
facilitate UL recognition. V
BATT
can exceed V
CC
during
normal operation without causing a reset.
MAX801L/M/N, MAX808L/M/N
8-Pin µP Supervisory Circuits
with ±1.5% Reset Accuracy
_______________________________________________________________________________________ 9
GND
V
CC
TO μP NMI
C
HOLD
C
HOLD
> I
LOAD
x t
SHDN
V
LR
4.5V to 5.5V
LOWLINE
MAX801
MAX808
REGULATOR
Figure 3. Using LOWLINE to Provide a Power-Fail Warning to
the µP
MAX801
MAX808
P
PP
OUT
V
CC
BATT
0.1μF
CONTROL
CIRCUITRY
Figure 4. V
CC
and BATT to OUT Switch
RESET
Logic low33
RESET
Logic high; the open-circuit
voltage is equal to V
CC
.
—
V
CC
PIN
5
GND
LOWLINE
Ground—0V reference for
all signals
44
WDI
WDI is ignored and goes
high impedance.
—6
BATT
Supply current is 1µA max for
V
BATT
≤ 2.8V.
77
CE IN
CE OUT
High impedance (MAX808)6—
Logic high. The open-circuit
output voltage is equal to
V
BATT
(MAX808).
5—
OUT
OUT is connected to BATT
through two internal PMOS
switches in series.
NAME
88
Battery switchover
comparator monitors V
CC
for active switchover.
11
Logic low22
MAX801 MAX808
STATUS
Table 1. Input and Output Status in
Battery-Backup Mode