Datasheet

Battery-Backup Mode
Battery backup preserves the contents of RAM in the
event of a brownout or power failure. With a backup
battery installed at BATT, the MAX807 automatically
switches RAM to backup power when V
CC
falls. Two
conditions are required for switchover to battery-back-
up 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.
Backup-Battery Input
The BATT input is similar to V
CC
, except the PMOS
switch is much smaller. This input is designed to con-
duct up to 20mA to OUT during battery backup. The
on-resistance of the PMOS switch is approximately
13Ω. Figure 12 shows the two series pass elements
between the BATT input and OUT that facilitate UL
approval. V
BATT
can exceed V
CC
during normal opera-
tion without causing a reset.
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 260mV below V
CC
. Decouple this terminal with
a 0.1µF capacitor.
BATT ON Output
The battery on (BATT ON) output indicates the status of
the internal battery switchover comparator, which con-
trols the internal V
CC
and BATT switches. For V
CC
greater than V
BATT
(ignoring the small hysteresis
effect), BATT ON typically sinks 3.2mA at 0.4V. In bat-
tery-backup mode, this output sources approximately
5mA. Use BATT ON to indicate battery switchover sta-
tus, or to supply gate or base drive for an external pass
transistor for higher current applications (see the
Typical Operating Circuit).
MAX807L/M/N
Full-Featured µP Supervisory Circuit with
±1.5% Reset Accuracy
______________________________________________________________________________________ 13
Figure 12. V
CC
and BATT-to-OUT Switch
MAX807
P
PP
OUT
V
CC
BATT
0.1µF
CONTROL
CIRCUITRY
Table 1. Input and Output Status in Battery-Backup Mode
The power-fail comparator remains active in battery-backup mode for V
CC
≥ 4V. Below 4V, PFO is forced low.PFO2
WDI is ignored and goes high impedanceWDI4
Battery switchover comparator monitors V
CC
for active switchover.V
CC
3
MR is ignoredMR6
Logic high; the open-circuit output voltage is equal to V
CC
.RESET8
Logic lowLOW LINE7
Ground—0V reference for all signalsGND5
Logic high. The open-circuit output voltage is equal to V
CC
.WDO10
High impedanceCE IN12
Logic high. The open-circuit output voltage is equal to V
BATT
.CE OUT11
Supply current is 1µA maximum for V
BATT
≤ 2.8V.BATT14
PIN
OUT is connected to BATT through two internal PMOS switches in series.OUT16
The power-fail comparator remains active in battery-backup mode for V
CC
≥ 4V.PFI1
FUNCTIONNAME
Logic high when V
BATT
exceeds 2.285V. Valid for V
CC
≥ 4V. Below 4V, BATT OK is forced low.BATT OK15
Logic high. The open-circuit output voltage is equal to V
BATT
.BATT ON13
Logic lowRESET9