Microprocessor Supervisory Circuits

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7

SW1/SW2

SW3/SW4

CONDITION

Open

Closed

Closed

Open

Open

Closed

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

RESET THRESHOLD = 4.4V IN MAX692A/MAX802M

D3

SUBSTRATE

D1

D2

SW2

SW1

SW4

SW3

MAX690A

MAX692A

MAX802L

MAX802M

MAX805L

Figure 3. Backup-Battery Switchover Block Diagram

RESET

(RESET)

GND

TO STATIC RAM

TO

µP

0.1F

MAX690A

MAX802L

MAX805L

+5V

+

( ) ARE FOR MAX805L ONLY.

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

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

need not be dedicated to this function though, as it is

completely separate from the rest of the circuitry. The

external voltage divider drives PFI to sense the unregu-

lated 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 trig-

gers 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

Backup-Battery Switchover

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

necessary to preserve the contents of RAM. With a

MAX690A/MAX692A/MAX802L/MAX802M/MAX805L don’t

the MAX690A/MAX692A/MAX802L/MAX802M/MAX805L

Table 1. Input and Output Status in Battery-Backup Mode

SIGNAL

STATUS

PFI

Power-fail comparator is disabled.

PFO

Logic low

RESET

Logic low

RESET

Logic high (MAX805L only)

WDI

Watchdog timer is disabled

PMOS switch

battery is less than 1µA, as long as