Datasheet

Watchdog Input
(MAX690_/802_/804_/805_)
The watchdog circuit monitors the µP’s activity. If the µP
does not toggle the watchdog input (WDI) within 1.6sec,
a reset pulse is triggered. The internal 1.6sec timer is
cleared by either a reset pulse or by a transition (low-to-
high or high-to-low) at WDI. If WDI is tied high or low, a
–
R
—
E
—
S
—
E
—
T
–
pulse is triggered every 1.8sec (t
WD
plus t
RS
).
As long as reset is asserted, the timer remains cleared
and does not count. As soon as reset is deasserted,
the timer starts counting. Unlike the 5V MAX690 family,
the watchdog function cannot be disabled.
Power-Fail Comparator
The PFI input is compared to an internal reference. If
PFI is less than V
PFT
,
–
P
—
F
—
O
–
goes low. The power-fail
comparator is intended for use as an undervoltage
detector to signal a failing power supply. However, the
comparator does not need to be dedicated to this
function because it is completely separate from the rest
of the circuitry.
The power-fail comparator turns off and
–
P
—
F
—
O
–
goes low
when V
CC
falls below V
SW
on power-down. The power-
fail comparator turns on as V
CC
crosses V
SW
on
power-up. If the comparator is not used, connect PFI to
ground and leave
–
P
—
F
—
O
–
unconnected.
–
P
—
F
—
O
–
may be
connected to
–
M
—
R
–
on the MAX704_/MAX806_ so that a
low voltage on PFI will generate a reset (Figure 5b).
MAX690T/S/R, 704T/S/R, 802T/S/R, 804–806T/S/R
3.0V/3.3V Microprocessor Supervisory Circuits
_______________________________________________________________________________________ 7
VBATT
RESET
(RESET)
WDI
V
CC
MR
BATTERY
SWITCHOVER
COMPARATOR
RESET
COMPARATOR
RESET
GENERATOR
WATCHDOG
TIMER
BATTERY
SWITCHOVER
CIRCUITRY
MAX690T/S/R
MAX704T/S/R
MAX802T/S/R
MAX804T/S/R
MAX805T/S/R
MAX806T/S/R
PFO
V
OUT
PFI
V
PFT
1.237V
1.237V
**
*
*
POWER-FAIL
COMPARATOR
* MAX690T/S/R, MAX802T/S/R, MAX804T/S/R, MAX805T/S/R ONLY
** MAX704T/S/R, MAX806T/S/R ONLY
( ) MAX804T/S/R, MAX805T/S/R ONLY
VBATT = 3.6V
3.0V OR 3.3V
V
SW
t
WP
RESET
PFO
V
CC
( ) MAX804T/S/R, MAX805T/S/R ONLY, RESET EXTERNALLY PULLED UP TO V
CC
V
OUT
V
SW
(RESET)
3.0V OR 3.3V
3.0V OR 3.3V
0V
V
RST
VBATT = PFI = 3.6V
I
OUT
= 0mA
Figure 1. Block Diagram Figure 2. Timing Diagram