MAX690AMJA-883B Maxim Integrated Products, Inc., MAX690AMJA-883B Datasheet - Page 7

• Manufacturer Part Number: MAX690AMJA-883B
• Description: Max690a, Max692a, Max802l, Max802m, Max805l Microprocessor Supervisory Circuits
• Manufacturer: Maxim Integrated Products, Inc.

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
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
cally switch RAM to backup power when V
As long as V
to V
below the reset threshold, V
er) switches to V
MAX690A/MAX692A/MAX802L/MAX802M/MAX805L don’t
always connect V
V
when V
than V
CC
. V
CC
V
BATT
CC
MAX690A
MAX692A
through a 5Ω PMOS power switch. Once V
BATT
CC
SW1
.
RESET THRESHOLD = 4.65V IN MAX690A/MAX802L/MAX805L.
RESET THRESHOLD = 4.4V IN MAX692A/MAX802M
V
V
is below the reset threshold and V
V
V
V
CC
CC
CC
CC
CC
connects to V
BATT
CC
CONDITION
> Reset Threshold
< Reset Threshold and
> V
< Reset Threshold and
< V
MAX802M
MAX802L
exceeds the reset threshold, V
BATT
BATT
SW2
connects to V
OUT
BATT
_______________________________________________________________________________________
Backup-Battery Switchover
. Unlike the MAX690/MAX692, the
MAX805L
to V
SUBSTRATE
OUT
D1
OUT
CC
V
OUT
Microprocessor Supervisory Circuits
SW1/SW2
(through an 80Ω switch) only
D2
BATT
or V
when V
D3
Closed
OUT
Open
Open
BATT
.
, the devices automati-
SW3
BATT
(whichever is high-
SW3/SW4
Closed
Closed
Open
CC
is greater than
BATT
OUT
fails.
SW4
is greater
connects
V
CC
CC
falls
Figure 4. Using a SuperCap as a Backup Power Source with a
MAX690A/MAX802L/MAX805L and a +5V ±5% Supply
When V
the MAX690A/MAX692A/MAX802L/MAX802M/MAX805L
substrate, regardless of the voltage applied to V
(Figure 3). During this time, the diode (D1) between V
and the substrate will conduct current from V
V
Table 1. Input and Output Status in Battery-Backup Mode
BATT
0.1F
SIGNAL
RESET
RESET
( ) ARE FOR MAX805L ONLY.
V
V
PFO
WDI
is 0.6V or greater than V
V
PFI
BATT
+5V
OUT
CC
CC
+
exceeds the reset threshold, it is connected to
Disconnected from V
Connected to V
PMOS switch
Connected to V
battery is less than 1µA, as long as
Power-fail comparator is disabled.
Logic low
Logic low
Logic high (MAX805L only)
Watchdog timer is disabled
V
V
V
CC
CC
BATT
< V
MAX690A
MAX802L
MAX805L
BATT
GND
- 1V.
CC
(RESET)
RESET
BATT
OUT
V
.
OUT
STATUS
. Current drawn from the
through an internal 80Ω
OUT
TO µP
TO STATIC RAM
BATT
to V
BATT
CC
BATT
7
if