MAX690ACSLASHD datasheet(7/13 Pages) MAXIM

7 / 13 page

dedicated to this function though, as it is completely

separate from the rest of the circuitry. The external volt-

age divider drives PFI to sense the unregulated DC input

to the +5V regulator (see Typical Operating Circuit). The

voltage-divider ratio can be chosen such that the voltage

at PFI falls below 1.25V just before the +5V regulator

drops out. PFO then triggers an interrupt which signals

the μP to prepare for power-down.

To conserve backup-battery power, the power-fail

detector comparator is turned off and PFO is forced low

when VBATT connects to VOUT.

Backup-Battery Switchover

In the event of a brownout or power failure, it may be

necessary to preserve the contents of RAM. With a back-

up battery installed at VBATT, the devices automatically

switch RAM to backup power when VCC fails.

As long as VCC exceeds the reset threshold, VOUT

connects to VCC through a 5Ω PMOS power switch.

Once VCC falls below the reset threshold, VCC or VBATT

(whichever is higher) switches to VOUT. Unlike the

MAX690/MAX692, the MAX690A/MAX692A/MAX802L/

MAX802M/MAX805L don’t always connect VBATT to

VOUT when VBATT is greater than VCC. VBATT connects

to VOUT (through an 80Ω switch) only when VCC is below

the reset threshold

and VBATT is greater than VCC.

When VCC exceeds the reset threshold, it is connected to

the MAX690A/MAX692A/MAX802L/MAX802M/MAX805L

substrate, regardless of the voltage applied to VBATT

(Figure 3). During this time, the diode (D1) between

VBATT and the substrate will conduct current from VBATT

to VCC if VBATT is 0.6V or greater than VCC.

Figure 3. Backup-Battery Switchover Block Diagram

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

MAX690A/MAX802L/MAX805L and a +5V ±5% Supply

Table 1. Wiper Position and Attenuation

SIGNAL

STATUS

VCC

Disconnected from VOUT

VOUT

Connected to VBATT through an internal

80Ω PMOS switch

VBATT

Connected to VOUT. Current drawn from

the battery is less than 1μA, as long as

VCC < VBATT - 1V.

PFI

Power-fail comparator is disabled.

PFO

Logic low

RESET

Logic low

RESET

Logic high (MAX805L only)

WDI

Watchdog timer is disabled

SW1/SW2 SW3/SW4

CONDITION

VCC > Reset Threshold

Open

Closed

Open

Closed

VCC < Reset Threshold and

VCC > VBATT

VCC < Reset Threshold and

VCC < VBATT

RESET THRESHOLD = 4.65V IN MAX690A/MAX802L/MAX805L.

RESET THRESHOLD = 4.4V IN MAX692A/MAX802M

VOUT

SUBSTRATE

SW2

SW1

SW4

SW3

VBATT

VCC

MAX690A

MAX692A

MAX802L

MAX802M

MAX805L

VBATT

VCC

VOUT

RESET

(RESET)

GND

TO STATIC RAM

TO µP

0.1F

MAX690A

MAX802L

MAX805L

+5V

( ) ARE FOR MAX805L ONLY.

MAX690A/MAX692A/

MAX802L/MAX802M/

MAX805L

Microprocessor Supervisory Circuits

www.maximintegrated.com

Maxim Integrated │ 7

MAX690AC/D Datasheet(PDF) 7 Page - Maxim Integrated Products

MAX690AC/D Datasheet(HTML) 7 Page - Maxim Integrated Products

Html Pages

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

Datasheet Download

Link URL