MAX805LESA Maxim, MAX805LESA Datasheet - Page 7

• Manufacturer Part Number: MAX805LESA
• Description: Microprocessor Supervisory Circuits
• Manufacturer: Maxim

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
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
CC
. V
CC
V
BATT
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
OUT
D3
Closed
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
than V
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
Table 1. Input and Output Status in Battery-Backup Mode
0.1F
SIGNAL
RESET
RESET
( ) ARE FOR MAX805L ONLY.
V
V
PFO
WDI
CC
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.
(RESET)
RESET
BATT
OUT
V
OUT
STATUS
. Current drawn from the
through an internal 80Ω
OUT
TO P
TO STATIC RAM
BATT
to V
BATT
CC
BATT
7
if