Datasheet

drops to more than 1V below V
BATT
, the internal switcho-
ver comparator shuts off and the supply current falls to
less than 1μA.
Manual Reset
The manual reset input (MR) allows RESET to be acti-
vated by a pushbutton switch. The switch is effectively
debounced by the 140ms minimum reset pulse width.
Because it is TTL/CMOS compatible, MR can be driven
by an external logic line.
Applications Information
Using a Supercap as a
Backup Power Source
Supercaps are capacitors with extremely high capaci-
tance values (on the order of 0.1 Farad). When using
supercaps, if V
CC
exceeds the MAX703/MAX704 reset
thresholds (4.65V and 4.40V, respectively), V
BATT
may
not exceed V
CC
by more than 0.6V. Thus, with a 5%
tolerance on V
CC
, V
BATT
should not exceed V
CC
(min)
+ 0.6V = 5.35V. Similarly, with a 10% tolerance on V
CC
,
V
BATT
should not exceed 5.1V.
Figure 4’s supercap circuit uses the MAX703 with a ±5%
tolerance voltage supply. In this circuit, the supercap
rapidly charges to within a diode drop of V
CC
. However,
the diode leakage current with trickle charge the super-
cap voltage to V
CC
. If V
BATT
= 5.25V and the power is
suddenly removed and then reapplied with V
CC
= 4.75V,
V
BATT
- V
CC
does not exceed the allowable 0.6V differ-
ence voltage.
Figure 5’s circuit uses the MAX704 with a ±10% tolerance
voltage supply. Note that if V
CC
= 5.5V and V
BATT
≤ 5.1V,
the power can be suddenly removed and reapplied with
V
CC
= 4.5V, and V
BATT
- V
CC
will not exceed the allow-
able 0.6V voltage difference.
Batteries and Power Supplies as
Backup Power Sources
Lithium batteries work well as backup batteries because
they have very low self-discharge rates and high-energy
density. Single lithium batteries with opencircuit volt-
ages of 3.0V to 3.6V are ideal for use with the MAX703/
MAX704. Batteries with an open-circuit voltage less than
the minimum reset threshold plus 0.3V can be directly
connected to the MAX703/MAX704 V
BATT
input with no
additional circuitry (see the Typical Operating Circuit).
However, batteries with open-circuit voltages greater than
the reset threshold plus 0.3V cannot be used as backup
batteries, since they source current into the substrate
through diode D1 (Figure 3) when V
CC
is close to the
reset threshold.
Using the MAX703/MAX704 without a
Backup Power Source
If a backup power source is not used, ground V
BATT
and connect V
CC
to V
OUT
. A direct connection to V
CC
eliminates any voltage drop across the internal switch,
which would otherwise appear at V
OUT
. Alternatively, use
the MAX705–MAX708, which do not have batterybackup
capabilities.
Figure 4. Using a Supercap as a Backup Power Source with a
MAX703 and a +5V ±5% Supply
Figure 5. Using a Supercap as a Backup Power Source with
the MAX704 and a +5V ±10% Supply
Table 2. Allowable Backup-Battery
Voltages
PART MAXIMUM BACKUP-BATTERY VOLTAGE (V)
MAX703 4.80
MAX704 4.55
+5V
V
CC
0.1F
V
OUT
V
BATT
GND
RESET
TOP
TO STATIC RAM
MAX703
+5V
V
CC
0.1F
V
OUT
V
BATT
GND
RESET
TOP
TO STATIC RAM
100kΩ
MAX704
MAX703/MAX704 Low-Cost Microprocessor Supervisory
Circuits with Battery Backup
www.maximintegrated.com
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