In the event of a brownout or power failure, it may be
necessary to preserve the contents of RAM. With a back-
up battery installed at VBATT, the devices automatically
switch RAM to backup power when VCC falls.
This family of µP supervisors (designed for 3.3V and 3V
systems) doesn’t always connect VBATT to VOUT when
VBATT is greater than VCC. VBATT connects to VOUT
(through a 140Ω switch) when VCC is below VSW and
VBATT is greater than VCC, or when VCC 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 VCC.
Switchover at VSW (2.40V) ensures that battery-backup
mode is entered before VOUT gets too close to the 2.0V
minimum required to reliably retain data in CMOS RAM.
Switchover at higher VCC voltages would decrease
backup-battery life. When VCC recovers, switchover is
deferred until VCC rises above the reset threshold (VRST)
to ensure a stable supply. VOUT is connected to VCC
through a 3Ω PMOS power switch.
A logic low on MR asserts reset. Reset remains asserted
while MR is low, and for tWP (200ms) after MR returns
high. This input has an internal 70µA pullup current, so it
can be left open if it is not used. MR can be driven with
TTL or CMOS logic levels, or with open-drain/collector
outputs. Connect a normally open momentary switch from
MR to GND to create a manual-reset function; external
debounce circuitry is not required.
Table 1. Input and Output Status in
Connected to VBATT through an internal
Disconnected from VOUT
The power-fail comparator is disabled when
VCC < VSW
Logic low when VCC < VSW or PFI < VPFT
The watchdog timer is disabled
RESET Low logic
RESET High impedance
VBATT Connected to VOUT
These µP supervisory circuits are not short-circuit
protected. Shorting VOUT to ground—excluding power-up
transients such as charging a decoupling capacitor—
destroys the device. Decouple both VCC and VBATT
pins to ground by placing 0.1µF capacitors as close as
possible to the device.
Using a SuperCap as a Backup Power Source
SuperCaps are capacitors with extremely high capaci-
tance 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 VCC while VCC is a bove 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 VOUT to VCC, 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 VCC
remains valid, if VBATT is decoupled with a 0.1µF
capacitor to ground, without danger of triggering RESET/
RESET. As long as VCC stays above VSW, battery-back-
up mode cannot be entered.
Adding Hysteresis to the Power-Fail
The power-fail comparator has a typical input hysteresis
of 10mV. This is sufficient for most applications where a
power-supply line is being monitored through an external
voltage divider (see the Monitoring an Additional Power
If additional noise margin is desired, connect a resistor
between PFO and PFI as shown in Figure 4a. Select the
ratio of R1 and R2 such that PFI sees 1.237V (VPFT)
when VIN falls to its trip point (VTRIP). R3 adds the
hysteresis and will typically be more than 10 times the
value of R1 or R2. The hysteresis window extends both
above (VH) and below (VL) the original trip point (VTRIP).
Connecting an ordinary signal diode in series with R3, as
shown in Figure 4b, causes the lower trip point (VL) to
Maxim Integrated │ 8