MAX704R Maxim Integrated Products, MAX704R Datasheet - Page 7

• Manufacturer Part Number: MAX704R
• Description: P Reset ic With Watchdog And Battery Switchover
• Manufacturer: Maxim Integrated Products

Figure 1. Block Diagram
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
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.
—
V
BATT
E
WDI
—
V
MR
3.0V/3.3V Microprocessor Supervisory Circuits
PFI
CC
S
—
*
**
E
—
T
–
* 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
pulse is triggered every 1.8sec (t
WATCHDOG
BATTERY
SWITCHOVER
COMPARATOR
TIMER
*
1.237V
1.237V
_______________________________________________________________________________________
RESET
COMPARATOR
(MAX690_/802_/804_/805_)
V
PFT
SWITCHOVER
CIRCUITRY
BATTERY
POWER-FAIL
COMPARATOR
MAX690T/S/R
MAX704T/S/R
MAX802T/S/R
MAX804T/S/R
MAX805T/S/R
MAX806T/S/R
Watchdog Input
GENERATOR
RESET
WD
plus t
RS
V
RESET
(RESET)
PFO
OUT
).
The PFI input is compared to an internal reference. If
PFI is less than V
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
when V
fail comparator turns on as V
power-up. If the comparator is not used, connect PFI to
ground and leave
connected to
low voltage on PFI will generate a reset (Figure 5b).
Figure 2. Timing Diagram
3.0V OR 3.3V
3.0V OR 3.3V
3.0V OR 3.3V
V
RESET
(RESET)
PFO
V
CC
OUT
( ) MAX804T/S/R, MAX805T/S/R ONLY, RESET EXTERNALLY PULLED UP TO V
CC
0V
falls below V
V
SW
–
M
—
R
–
on the MAX704_/MAX806_ so that a
PFT
–
P
—
F
,
—
VBATT = 3.6V
O
–
P
SW
—
–
F
—
Power-Fail Comparator
unconnected.
O
on power-down. The power-
–
goes low. The power-fail
CC
crosses V
–
P
—
–
P
t
—
WP
V
F
I
—
OUT
BATT = PFI = 3.6V
F
—
O
–
O
= 0mA
–
goes low
may be
CC
SW
V
V
RST
SW
on
7