MAX690T/S/R, 704T/S/R, 802T/S/R, 804–806T/S/R
Backup-Battery Switchover
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
CC
falls.
This family of µP supervisors (designed for 3.3V and 3V
systems) doesn’t always connect VBATT to V
OUT
when
VBATT is greater than V
CC
. VBATT connects to V
OUT
(through a 140Ω switch) when V
CC
is below V
SW
and
VBATT is greater than V
CC
, or when V
CC
falls below
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
CC
.
Switchover at V
SW
(2.40V) ensures that battery-backup
mode is entered before V
OUT
gets too close to the 2.0V
minimum required to reliably retain data in CMOS RAM.
Switchover at higher V
CC
voltages would decrease
backup-battery life. When V
CC
recovers, switchover is
deferred until V
CC
rises above the reset threshold
(V
RST
) to ensure a stable supply. V
OUT
is connected to
V
CC
through a 3Ω PMOS power switch.
Manual Reset
A logic low on
–
M
—
R
–
asserts reset. Reset remains asserted
while
–
M
—
R
–
is low, and for t
WP
(200ms) after
–
M
—
R
–
returns
high. This input has an internal 70µA pull-up current, so
it can be left open if it is not used.
–
M
—
R
–
can be driven with
TTL or CMOS logic levels, or with open-drain/collector
outputs. Connect a normally open momentary switch
from
–
M
—
R
–
to GND to create a manual-reset function;
external debounce circuitry is not required.
__________Applications Information
These µP supervisory circuits are not short-circuit
protected. Shorting V
OUT
to ground—excluding power-
up transients such as charging a decoupling
capacitor—destroys the device. Decouple both V
CC
and VBATT pins to ground by placing 0.1µF capacitors
as close to the device as possible.
Using a SuperCap
as a Backup Power Source
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
CC
while V
CC
is above the reset threshold, there are no
special precautions when using these µP supervisors
with a SuperCap.
Operation without a Backup
Power Source
These µP supervisors were designed for battery-
backed applications. If a backup battery is not used,
connect both VBATT and V
OUT
to V
CC
, or use a
different µP supervisor such as the MAX706T/S/R or
MAX708T/S/R.
Replacing the Backup Battery
The backup power source can be removed while V
CC
remains valid, if VBATT is decoupled with a 0.1µF
capacitor to ground, without danger of triggering
RESET/
–
R
—
E
—
S
—
E
—
T
–
. As long as V
CC
stays above V
SW
,
battery-backup mode cannot be entered.
Adding Hysteresis
to the Power-Fail Comparator
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
–
P
—
F
—
O
–
and PFI as shown in Figure 4a. Select
the ratio of R1 and R2 such that PFI sees 1.237V (V
PFT
)
when V
IN
falls to its trip point (V
TRIP
). R3 adds the
hysteresis and will typically be more than 10 times the
value of R1 or R2. The hysteresis window extends both
above (V
H
) and below (V
L
) the original trip point (V
TRIP
).
3.0V/3.3V Microprocessor Supervisory Circuits
8 _______________________________________________________________________________________
SuperCap is a trademark of Baknor Industries.
PIN NAME STATUS
V
OUT
Connected to VBATT through an internal
140Ω switch
V
CC
Disconnected from V
OUT
PFI
The power-fail comparator is disabled when
V
CC
< V
SW
–
P
—
F
—
O
–
Logic low when V
CC
< V
SW
or PFI < V
PFT
WDI The watchdog timer is disabled
–
M
—
R
–
Disabled
–
R
—
E
—
S
—
E
—
T
–
Low logic
RESET High impedance
VBATT Connected to V
OUT
Table 1. Input and Output Status in
Battery-Backup Mode
