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
SW1/SW2
Open
Open
Closed
SW3/SW4
Closed
D3
+
0.1F
MAX690A
MAX802L
V
BATT
MAX805L
RESET
(RESET)
TO
µP
GND
( ) ARE FOR MAX805L ONLY.
Closed
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
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
.
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
V
BATT
is 0.6V or greater than V
CC
.
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
than V
CC
.
V
BATT
PFI
PFO
RESET
RESET
WDI
_______________________________________________________________________________________
7
MAXIM [ MAXIM INTEGRATED PRODUCTS ]
