ed and the internal circuitry is powered from the battery

parator shuts off and the supply current falls to less

than 1µA.

__________Applications Information

Using a SuperCap™ as a Backup

Power Source

SuperCaps are capacitors with extremely high capaci-

tance values, on the order of 0.1F. Figure 4 shows a

SuperCap used as a backup power source. Do not allow

the SuperCap’s voltage to exceed the maximum reset

threshold by more than 0.6V. In Figure 4’s circuit, the

However, after a long time, the diode leakage current will

may not exceed 4.75V + 0.6V = 5.35V.

Use the SuperCap circuit of Figure 5 with a MAX692A

or MAX802M and a ±10% supply.

This circuit

charges up to 5.0V, only 0.5V above the maximum

reset threshold—well within the requisite 0.6V.

Microprocessor Supervisory Circuits

8

_______________________________________________________________________________________

RESET

GND

TO STATIC RAM

TO

µP

0.1F

MAX692A

MAX802M

+5V

+

100k

Figure 5. Using a SuperCap™ as a Backup Power Source with

the MAX692A/MAX802M and a +5V ±10% Supply

PFO

GND

TO

µP

MAX690

MAX692A

MAX802L

MAX802M

MAX805L

+5V

PFI

*OPTIONAL

+5V

0V

PFO

0V

C1*

Figure 6. Adding Hysteresis to the Power-Fail Comparator

Allowable Backup Power-Source

Batteries

Lithium batteries work very well as backup batteries due to

very low self-discharge rates and high energy density.

Single lithium batteries with open-circuit voltages of 3.0V to

3.6V are ideal. Any battery with an open-circuit voltage

less than the minimum reset threshold plus 0.3V can be

Table 2. Allowable Backup-Battery Voltages

(see

Using a SuperCap as a Backup Power Source

section for use with a SuperCap)

PART NO.

MAXIMUM BACKUP-BATTERY

VOLTAGE (V)

MAX690A/

MAX802L/MAX805L

4.80

MAX692A/

MAX802M

4.55

VTRIP

1.25/

R2

R2

R2

VIH

1.25/

R2 || R3

R1 R2 || R3

VL 1.25

R1

5

1.25

R3

1.25

R2

=

+

=

+

−

+

−

=

















™SuperCap is a trademark of Baknor Industries.