MAX802 MAXIM [Maxim Integrated Products], MAX802 Datasheet - Page 8

• Manufacturer Part Number: MAX802
• Description: 3.0V/3.3V Microprocessor Supervisory Circuits
• Manufacturer: MAXIM [Maxim Integrated Products]

In the event of a brownout or power failure, it may be
necessary to preserve the contents of RAM. With a
backup battery installed at VBATT, the devices auto-
matically switch RAM to backup power when V
falls.
This family of µP supervisors (designed for 3.3V and 3V
systems) doesn’t always connect VBATT to V
VBATT is greater than V
(through a 140Ω switch) when V
VBATT is greater than V
1.75V (typ) regardless of the VBATT voltage. This is
done to allow the backup battery (e.g., a 3.6V lithium
cell) to have a higher voltage than V
Switchover at V
mode is entered before V
minimum required to reliably retain data in CMOS RAM.
Switchover at higher V
backup-battery life. When V
deferred until V
(V
V
A logic low on
while
high. This input has an internal 70µA pull-up current, so
it can be left open if it is not used.
TTL or CMOS logic levels, or with open-drain/collector
outputs. Connect a normally open momentary switch
from
external debounce circuitry is not required.
3.0V/3.3V Microprocessor Supervisory Circuits
Table 1. Input and Output Status in
Battery-Backup Mode
8
PIN NAME
CC
RST
–
R
—
RESET
VBATT
V
–
P
WDI
E
_______________________________________________________________________________________
V
—
–
M
—
PFI
OUT
through a 3Ω PMOS power switch.
F
—
CC
—
S
) to ensure a stable supply. V
—
–
M
–
M
R
O
–
—
E
—
–
—
R
R
T
–
–
–
to GND to create a manual-reset function;
is low, and for t
Connected to VBATT through an internal
140Ω switch
Disconnected from V
The power-fail comparator is disabled when
V
Logic low when V
The watchdog timer is disabled
Disabled
Low logic
High impedance
Connected to V
CC
–
M
—
SW
< V
R
–
CC
asserts reset. Reset remains asserted
(2.40V) ensures that battery-backup
SW
Backup-Battery Switchover
rises above the reset threshold
CC
CC
OUT
OUT
CC
WP
CC
. VBATT connects to V
, or when V
voltages would decrease
CC
STATUS
gets too close to the 2.0V
< V
(200ms) after
OUT
recovers, switchover is
–
M
SW
CC
—
R
OUT
CC
–
or PFI < V
is below V
can be driven with
Manual Reset
.
is connected to
CC
falls below
–
M
—
PFT
OUT
R
–
SW
returns
when
and
OUT
CC
These µP supervisory circuits are not short-circuit
protected. Shorting V
up transients such as charging a decoupling
capacitor—destroys the device. Decouple both V
and VBATT pins to ground by placing 0.1µF capacitors
as close to the device as possible.
SuperCaps™ are capacitors with extremely high
capacitance values (e.g., order of 0.47F) for their size.
Figure 3 shows two ways to use a SuperCap as a
backup power source. The SuperCap may be
connected through a diode to the 3V input (Figure 3a)
or, if a 5V supply is also available, the SuperCap may
be charged up to the 5V supply (Figure 3b) allowing a
longer backup period. Since VBATT can exceed V
while V
special precautions when using these µP supervisors
with a SuperCap.
These µP supervisors were designed for battery-
backed applications. If a backup battery is not used,
connect both VBATT and V
different µP supervisor such as the MAX706T/S/R or
MAX708T/S/R.
The backup power source can be removed while V
remains valid, if VBATT is decoupled with a 0.1µF
capacitor to ground, without danger of triggering
RESET/
battery-backup mode cannot be entered.
The power-fail comparator has a typical input
hysteresis of 10mV. This is sufficient for most applica-
tions where a power-supply line is being monitored
through an external voltage divider (see the Monitoring
an Additional Power Supply section).
If additional noise margin is desired, connect a resistor
between
the ratio of R1 and R2 such that PFI sees 1.237V (V
when V
hysteresis and will typically be more than 10 times the
value of R1 or R2. The hysteresis window extends both
above (V
SuperCap is a trademark of Baknor Industries.
__________Applications Information
CC
–
IN
R
—
H
–
P
—
E
) and below (V
—
falls to its trip point (V
F
is above the reset threshold, there are no
—
S
—
O
–
E
—
T
and PFI as shown in Figure 4a. Select
–
Replacing the Backup Battery
. As long as V
to the Power-Fail Comparator
Operation without a Backup
as a Backup Power Source
OUT
L
) the original trip point (V
to ground—excluding power-
OUT
Adding Hysteresis
Using a SuperCap
CC
TRIP
stays above V
to V
Power Source
). R3 adds the
CC
, or use a
TRIP
PFT
SW
CC
CC
CC
).
)
,