MAX795 MAXIM [Maxim Integrated Products], MAX795 Datasheet - Page 10

• Manufacturer Part Number: MAX795
• Description: 3.0V/3.3V Adjustable Microprocessor Supervisory Circuits
• Manufacturer: MAXIM [Maxim Integrated Products]

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 will 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 will not occur until V
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 will be 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:
3.0V/3.3V Adjustable Microprocessor
Supervisory Circuits
Figure 3. MAX794 Standard Application Circuit
10
3.6V
R1
R2
+5V
______________________________________________________________________________________
3.3V
+5V SUPPLY
FAILURE
0.1 F
0.1 F
V
RST
= V
V
BATT
RESET IN
PFO
PFI
MR
WDO
CC
RST IN
(OPTIONAL)
D
SILICONIX
Si9433DY
BATT ON
MAX794
GND
PMOS
LOWLINE
CE OUT
S
RESET
CE IN
(R1 / R2 + 1)
OUT
WDI
DECODER
ADDRESS
V
Reset Threshold
CC
CMOS
4.7k
0.1 F
RAM
V
RST
= V
RST IN
(
R1 + 1
R2
I/O
NMI
A0-A15
RESET
V
CC
)
CC
Using the standard application circuit (Figure 3), the
reset threshold may be programmed anywhere in the
range of V
Reset is asserted when V
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 will be fresh when the final
product is put to use. To enable the freshness seal,
connect a battery to BATT, ground PFO, bring V
above the reset threshold and hold it there until reset is
deasserted following the reset timeout period, then
bring V
tery freshness seal is enabled (disconnecting the back-
up battery from the internal circuitry and anything
connected to OUT), it remains enabled until V
brought above V
will not interfere with battery freshness seal operation.
BATT OK indicates the status of the backup battery.
When reset is not asserted, the MAX793 checks the
battery voltage continuously. If V
(2.0V min), BATT OK goes low; otherwise, it remains
pulled up to V
goes below V
In the MAX793/MAX794, the watchdog circuit monitors
the µP’s activity. If the µP does not toggle the watch-
dog input (WDI) within 1.6sec, WDO goes low. The
internal 1.6sec timer is cleared and WDO returns high
Figure 4. Battery Freshness Seal Enable Timing
V
PFO
(EXTERNALLY HELD AT 0V)
RESET
RESET PULLED UP TO V
CC
CC
Watchdog Input (MAX793/MAX794)
back down again (Figure 4). Once the bat-
SW
SW
V
CC
RST
(the battery switch threshold) to 5.5V.
CC
.
RST
. BATT OK also goes low when V
. Note that connecting PFO to MR
BATT OK Output (MAX793)
CC
Battery Freshness Seal
falls below V
t
RP
PFO STATE LATCHED,
FRESHNESS SEAL ENABLED.
BATT
is below V
SW
.
V
RST
CC
BOK
CC
CC
is