Datasheet

MAX793/MAX794/MAX795
Reset Threshold
The MAX793T/MAX795T are intended for 3.3V systems
with a ±5% power-supply tolerance and a 10% systems
tolerance. Except when MR is asserted, reset does not
assert as long as the power supply remains above
3.15V (3.3V - 5%). Reset is guaranteed to assert before
the power supply falls below 3.0V (3.3V - 10%).
The MAX793S/MAX795S are designed for 3.3V ±10%
power supplies. Except when MR is asserted, they are
guaranteed not to assert reset as long as the supply
remains above 3.0V (3.0V is just above 3.3V - 10%).
Reset is guaranteed to assert before the power supply
falls below 2.85V (3.3V - 14%).
The MAX793R/MAX795R are optimized to monitor 3.0V
±10% power supplies. Reset does not occur until V
CC
falls below 2.7V (3.0V - 10%), but is guaranteed to
occur before the supply falls below 2.55V (3.0V - 15%).
Program the MAX794’s reset threshold with an external
voltage divider to RESET IN. The reset-threshold toler-
ance is a combination of the RESET IN tolerance and
the tolerance of the resistors used to make the external
voltage divider. Calculate the reset threshold as follows:
V
RST
= V
RST IN
(R1 / R2 + 1)
Using the standard application circuit (Figure 3), the
reset threshold can be programmed anywhere in the
range of V
SW
(the battery switch threshold) to 5.5V.
Reset is asserted when V
CC
falls below V
SW
.
Battery Freshness Seal
The MAX793/MAX794’s battery freshness seal discon-
nects the backup battery from internal circuitry until it is
needed. This allows an OEM to ensure that the backup
battery connected to BATT is fresh when the final prod-
uct is put to use. To enable the freshness seal, connect
a battery to BATT, ground PFO, bring V
CC
above the
reset threshold, and hold it there until reset is deassert-
ed following the reset timeout period, then bring V
CC
back down again (Figure 4). Once the battery fresh-
ness seal is enabled (disconnecting the backup battery
from the internal circuitry and anything connected to
OUT), it remains enabled until V
CC
is brought above
V
RST
. Note that connecting PFO to MR does not inter-
fere with battery freshness seal operation.
BATT OK Output (MAX793)
BATT OK indicates the status of the backup battery.
When reset is not asserted, the MAX793 checks the
battery voltage continuously. If V
BATT
is below V
BOK
(2.0V min), BATT OK goes low; otherwise, it remains
pulled up to V
CC
. BATT OK also goes low when V
CC
goes below V
SW
.
Watchdog Input (MAX793/MAX794)
In the MAX793/MAX794, the watchdog circuit monitors
the µP’s activity. If the µP does not toggle the watchdog
input (WDI) within 1.6s, WDO goes low. The internal
1.6s timer is cleared and WDO returns high either when
3.0V/3.3V Adjustable Microprocessor
Supervisory Circuits
10 ______________________________________________________________________________________
MAX794
RESET
LOWLINE
WDI
CE IN
CE OUT
3.3V
+5V
BATT
RESET IN
A0-A15
MR
+5V SUPPLY
FAILURE
BATT ON
PFI
4.7kΩ
WDO
OUT
CMOS
RAM
ADDRESS
DECODER
0.1µF
PMOS
0.1µF
V
CC
PFO
V
RST
= V
RST IN (
R1
+ 1
)
GND
I/O
NMI
RESET
V
CC
V
CC
0.1µF
3.6V
R1
D
S
R2
(OPTIONAL)
Si9433DY
SILICONIX
R2
Figure 3. MAX794 Standard Application Circuit
V
CC
V
RST V
RST
RESET
PFO
(EXTERNALLY HELD AT 0V)
RESET PULLED UP TO V
CC
PFO STATE LATCHED,
FRESHNESS SEAL ENABLED.
t
RP
Figure 4. Battery Freshness Seal Enable Timing