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MAX703

Low-Cost Microprocessor Supervisory Circuits with Battery Backup

Maxim Integrated 

MAX703/MAX704

Low-Cost Microprocessor Supervisory

Circuits with Battery Backup

drops to more than 1V below VBATT, 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 VCC exceeds the MAX703/MAX704 reset

thresholds (4.65V and 4.40V, respectively), VBATT may

not exceed VCC by more than 0.6V. Thus, with a 5%

tolerance on VCC, VBATT should not exceed VCC (min)

+ 0.6V = 5.35V. Similarly, with a 10% tolerance on VCC,

VBATT 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 VCC. However,

the diode leakage current with trickle charge the super-

cap voltage to VCC. If VBATT = 5.25V and the power is

suddenly removed and then reapplied with VCC = 4.75V,

VBATT - VCC 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 VCC = 5.5V and VBATT ≤ 5.1V,

the power can be suddenly removed and reapplied with

VCC = 4.5V, and VBATT - VCC will not exceed the allow-

able 0.6V voltage difference.

+5V

0.1F

VCC

VOUT

MAX703

VBATT

RESET

GND

TO STATIC RAM

TOP

Figure 4. Using a Supercap as a Backup Power Source with a

MAX703 and a +5V ±5% Supply

+5V

100kΩ

VCC

VOUT

MAX704

VBATT

RESET

GND

0.1F

TO STATIC RAM

TOP

Figure 5. Using a Supercap as a Backup Power Source with

the MAX704 and a +5V ±10% Supply

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 VBATT 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 VCC is close to the

reset threshold.

Table 2. Allowable Backup-Battery

Voltages

PART

MAX703

MAX704

MAXIMUM BACKUP-BATTERY VOLTAGE (V)

4.80

4.55

Using the MAX703/MAX704 without a

Backup Power Source

If a backup power source is not used, ground VBATT

and connect VCC to VOUT. A direct connection to VCC

eliminates any voltage drop across the internal switch,

which would otherwise appear at VOUT. Alternatively, use

the MAX705–MAX708, which do not have batterybackup

capabilities.

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