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MAX793T Datasheet

  • MAX793T

  • 3.0V/3.3V可调、微处理器监控电路, 微处理器监控电路, 提供备用...

  • 604.40KB

  • 20页

  • MAXIM

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3.0V/3.3V Adjustable Microprocessor
Supervisory Circuits
MAX793/MAX794/MAX795
3.3V
0.1碌F
R1
(OPTIONAL)
Si9433DY
SILICONIX
D
S
0.1碌F
CMOS
RAM
t
RP
V
CC
MAX794
V
RST
V
CC
V
RST
PMOS
V
CC
BATT ON OUT
RESET IN
CE OUT
R2
CE IN
WDI
WDO
MR
+5V SUPPLY
+5V FAILURE
PFO
RESET
PFI
GND
LOWLINE
V
CC
4.7k
ADDRESS
DECODER
RESET
3.6V
0.1碌F
BATT
A0-A15
I/O
NMI
PFO
(EXTERNALLY HELD AT 0V)
RESET PULLED UP TO V
CC
RESET
PFO STATE LATCHED,
FRESHNESS SEAL ENABLED.
Figure 4. Battery Freshness Seal Enable Timing
V
RST
= V
RST IN
R1 + 1
R2
(
)
Using the standard application circuit (Figure 3), the
reset threshold may 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
.
Figure 3. MAX794 Standard Application Circuit
Battery Freshness Seal
The MAX793/MAX794鈥檚 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
CC
above the reset threshold and hold it there until reset is
deasserted following the reset timeout period, then
bring V
CC
back down again (Figure 4). Once the bat-
tery freshness seal is enabled (disconnecting the back-
up 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
will not interfere with battery freshness seal operation.
Reset Threshold
The MAX793T/MAX795T are intended for 3.3V systems
with a 卤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 卤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
卤10% power supplies. Reset will 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鈥檚 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:
V
RST
= V
RST IN
(R1 / R2 + 1)
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 碌P鈥檚 activity. If the 碌P 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
10
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