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MAX807NCWE Datasheet(PDF) 13 Page - Maxim Integrated Products

Part No. MAX807NCWE
Description  Full-Featured uP Supervisory Circuit with /-1.5% Reset Accuracy
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Maker  MAXIM [Maxim Integrated Products]
Homepage  http://www.maxim-ic.com
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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 VCC falls. Two
conditions are required for switchover to battery-back-
up mode: 1) VCC must be below the reset threshold; 2)
VCC must be below VBATT. Table 1 lists the status of
inputs and outputs during battery-backup mode.
Backup-Battery Input
The BATT input is similar to VCC, 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
13
Ω. Figure 12 shows the two series pass elements
between the BATT input and OUT that facilitate UL
approval. VBATT can exceed VCC during normal opera-
tion without causing a reset.
Output Supply Voltage
The output supply (OUT) transfers power from VCC or
BATT to the µP, RAM, and other external circuitry. At
the maximum source current of 250mA, VOUT will typi-
cally be 260mV below VCC. 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 VCC and BATT switches. For VCC
greater than VBATT (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).
Full-Featured µP Supervisory Circuit with
±1.5% Reset Accuracy
______________________________________________________________________________________
13
Figure 12. VCC and BATT-to-OUT Switch
MAX807
P
PP
OUT
VCC
BATT
0.1
µF
CONTROL
CIRCUITRY
Table 1. Input and Output Status in Battery-Backup Mode
The power-fail comparator remains active in battery-backup mode for VCC
≥ 4V. Below 4V, PFO is forced low.
PFO
2
WDI is ignored and goes high impedance
WDI
4
Battery switchover comparator monitors VCC for active switchover.
VCC
3
MR is ignored
MR
6
Logic high; the open-circuit output voltage is equal to VCC.
RESET
8
Logic low
LOW LINE
7
Ground—0V reference for all signals
GND
5
Logic high. The open-circuit output voltage is equal to VCC.
WDO
10
High impedance
CE IN
12
Logic high. The open-circuit output voltage is equal to VBATT.
CE OUT
11
Supply current is 1µA maximum for VBATT
≤ 2.8V.
BATT
14
PIN
OUT is connected to BATT through two internal PMOS switches in series.
OUT
16
The power-fail comparator remains active in battery-backup mode for VCC
≥ 4V.
PFI
1
FUNCTION
NAME
Logic high when VBATT exceeds 2.285V. Valid for VCC
≥ 4V. Below 4V, BATT OK is forced low.
BATT OK
15
Logic high. The open-circuit output voltage is equal to VBATT.
BATT ON
13
Logic low
RESET
9




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