MAX690ACSLASHD datasheet(8/13 Pages) MAXIM

8 / 13 page

When VBATT connects to VOUT, backup mode is

activated 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 comparator 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

capacitance 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.

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 volt-

ages 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 connected directly to the VBATT input of

the MAX690A/MAX692A/MAX802L/MAX802M/MAX805L

with no additional circuitry (see the Typical Operating

Circuit). However, batteries with open-circuit voltages

that are greater

cannot be used for backup, as current is

sourced into the substrate through the diode (D1 in Figure 3)

when VCC is close to the reset threshold.

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

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

Figure 6. Adding Hysteresis to the Power-Fail Comparator

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

VBATT

VCC

VOUT

RESET

GND

TO STATIC RAM

TO µP

0.1F

MAX692A

MAX802M

+5V

100kΩ

PFO

VCC

GND

TO µP

MAX690

MAX692A

MAX802L

MAX802M

MAX805L

+5V

PFI

*OPTIONAL

+5V

PFO

VTRIP

VIN

C1*

VTRIP = 1.25/

R2 + R2

(

VIH = 1.25/

R2 || R3

R1 + R2 || R3

(

VL - 1.25

5 - 1.25

1.25

MAX690A/MAX692A/

MAX802L/MAX802M/

MAX805L

Microprocessor Supervisory Circuits

www.maximintegrated.com

Maxim Integrated │ 8

MAX690AC/D Arkusz danych(PDF) 8 Page - Maxim Integrated Products

MAX690AC/D Arkusz danych(HTML) 8 Page - Maxim Integrated Products

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13

Arkusz danych Pobierz

Link URL