Microprocessor Supervisory Circuits
MAX690A/MAX692A/MAX802L/MAX802M/MAX805L
V
BATT
V
CC
+5V
SW1
SW2
D1
D2
SW3
SW4
V
CC
V
OUT
TO STATIC RAM
SUBSTRATE
MAX690A MAX802L MAX805L
MAX692A MAX802M
V
OUT
CONDITION
V
CC
> Reset Threshold
V
CC
< Reset Threshold and
V
CC
> V
BATT
V
CC
< Reset Threshold and
V
CC
< V
BATT
D3
+
0.1F
MAX690A
MAX802L
V
BATT
MAX805L
RESET
(RESET)
TO
µP
GND
SW3/SW4
Closed
( ) ARE FOR MAX805L ONLY.
Open
Closed
Closed
Open
SW1/SW2
Open
RESET THRESHOLD = 4.65V IN MAX690A/MAX802L/MAX805L.
RESET THRESHOLD = 4.4V IN MAX692A/MAX802M
Figure 4. Using a SuperCap as a Backup Power Source with a
MAX690A/MAX802L/MAX805L and a +5V ±5% Supply
Figure 3. Backup-Battery Switchover Block Diagram
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
low when V
BATT
connects to V
OUT
.
than V
CC
.
When V
CC
exceeds the reset threshold, it is connected to
the MAX690A/MAX692A/MAX802L/MAX802M/MAX805L
substrate, regardless of the voltage applied to V
BATT
(Figure 3). During this time, the diode (D1) between V
BATT
and the substrate will conduct current from V
BATT
to V
CC
if
Table 1. Input and Output Status in Battery-Backup Mode
SIGNAL
V
CC
V
OUT
STATUS
Disconnected from V
OUT
Connected to V
BATT
through an internal 80Ω
PMOS switch
Connected to V
OUT
. Current drawn from the
battery is less than 1µA, as long as
V
CC
< V
BATT
- 1V.
Power-fail comparator is disabled.
Logic low
Logic low
Logic high (MAX805L only)
Watchdog timer is disabled
Backup-Battery Switchover
In the event of a brownout or power failure, it may be
necessary to preserve the contents of RAM. With a
backup battery installed at V
BATT
, the devices automati-
cally switch RAM to backup power when V
CC
fails.
As long as V
CC
exceeds the reset threshold, V
OUT
connects
to V
CC
through a 5Ω PMOS power switch. Once V
CC
falls
below the reset threshold, V
CC
or V
BATT
(whichever is high-
er) switches to V
OUT
. Unlike the MAX690/MAX692, the
MAX690A/MAX692A/MAX802L/MAX802M/MAX805L don’t
always connect V
BATT
to V
OUT
when V
BATT
is greater than
V
CC
. V
BATT
connects to V
OUT
(through an 80Ω switch) only
when V
CC
is below the reset threshold
and
V
BATT
is greater
V
BATT
PFI
PFO
RESET
RESET
WDI
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7
MAXIM [ MAXIM INTEGRATED PRODUCTS ]
