MAX690 datasheet(8/12 Pages) MAXIM

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VBATT is 0.6V or greater than VCC.

When VBATT connects to VOUT, backup mode is activat-

ed and the internal circuitry is powered from the battery

(Table 1). When VCC is just below VBATT, the current

drawn from VBATT is typically 30µA. When VCC drops to

more than 1V below VBATT, the internal switchover com-

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

SuperCap rapidly charges to within a diode drop of VCC.

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

pull the SuperCap voltage up to VCC. When using a

SuperCap with the MAX690A/MAX802L/MAX805L, VCC

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

ensures that the SuperCap only charges to VCC -

0.5V. At the maximum VCC of 5.5V, the SuperCap

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

reset threshold—well within the requisite 0.6V.

Microprocessor Supervisory Circuits

_______________________________________________________________________________________

VBATT

VCC

VOUT

RESET

GND

TO STATIC RAM

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

VCC

GND

µP

MAX690

MAX692A

MAX802L

MAX802M

MAX805L

+5V

PFI

*OPTIONAL

+5V

PFO

VTRIP

VIN

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/

VIH

1.25/

R2 || R3

R1 R2 || R3

VL 1.25

1.25

−

















™SuperCap is a trademark of Baknor Industries.

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