MAX818MCSA Maxim Integrated, MAX818MCSA Datasheet - Page 13

• Manufacturer Part Number: MAX818MCSA
• Description: Supervisory Circuits
• Manufacturer: Maxim Integrated
• Series: MAX817, MAX817L, MAX817M, MAX818, MAX818L, MAX818M, MAX819, MAX819L, MAX819Mr

Specifications of MAX818MCSA

• Number Of Voltages Monitored: 1
• Monitored Voltage: 4.4 V
• Undervoltage Threshold: 4.25 V
• Overvoltage Threshold: 4.5 V
• Output Type: Active Low, Open Drain
• Manual Reset: Not Resettable
• Watchdog: Watchdog
• Battery Backup Switching: Backup
• Power-up Reset Delay (typ): 280 ms
• Supply Voltage - Max: 5.5 V
• Maximum Operating Temperature: + 70 C
• Mounting Style: SMD/SMT
• Package / Case: SOIC-8 Narrow
• Chip Enable Signals: Yes
• Maximum Power Dissipation: 471 mW
• Minimum Operating Temperature: 0 C
• Power Fail Detection: Yes
• Supply Current (typ): 11 uA
• Supply Voltage - Min: 0 V

The MAX817/MAX818 WDI inputs are internally driven
through a buffer and series resistor from the watchdog
counter (Figure 1). When WDI is left unconnected, the
watchdog timer is serviced within the watchdog timeout
period by a low-high-low pulse from the counter chain.
For minimum watchdog input current (minimum overall
power consumption), leave WDI low for the majority of the
watchdog timeout period, pulsing it low-high-low once
within
watchdog timer. If instead WDI is externally driven high for
the majority of the timeout period, up to 150µA can flow
into WDI.
SuperCaps are capacitors with extremely high capaci-
tance values (on the order of 0.47F) for their size. Since
BATT has the same operating voltage range as V
the battery switchover threshold voltages are typically
±30mV centered at V
charging circuit can be used as a backup power source.
Figure 11 shows a SuperCap used as a backup source.
If V
above V
until the voltage at BATT is less than 0.5V above V
For example, if a SuperCap is connected to BATT
through a diode to V
5.4V to 4.9V, the capacitor discharges through OUT
and V
rent through the SuperCap charging diode and the
internal power diode eventually discharges the
SuperCap to V
0.1V above the reset threshold and power is lost at
Figure 11. Using a SuperCap™ as a Backup Power Source
with a +5V ±10% Supply
SuperCap is a trademark of Baknor Industries.
CC
100k
CC
7
+5V
is above the reset threshold and V
/
8
CC
until V
0.1F
of the watchdog timeout period to reset the
, current flows to OUT and V
BATT
CC
+5V Microprocessor Supervisory Circuits
V
BATT
. Also, if V
______________________________________________________________________________________
CC
CC
reaches 5.1V typical. Leakage cur-
BATT
, and V
MAX817
MAX818
MAX819
Using a SuperCap™ as a
Watchdog Input Current
GND
Backup Power Source
, a SuperCap and simple
CC
RESET
CC
OUT
and V
quickly changes from
BATT
CC
TO P
TO STATIC RAM
BATT
from BATT
start from
CC
is 0.5V
, and
CC
.
V
until V
mode is then initiated and the current through V
goes to zero.
The MAX817/MAX818/MAX819 were designed for bat-
tery-backed applications. If a backup battery is not
used, connect V
ground.
The backup power source can be removed while V
remains valid, without danger of triggering a reset
pulse, if BATT is decoupled with a 0.1µF capacitor to
ground. As long as V
old, battery-backup mode cannot be entered.
The power-fail comparator has a typical input hystere-
sis of 4mV. This is sufficient for most applications where
a power-supply line is being monitored through an
external voltage divider (see Monitoring an Additional
Supply).
For additional noise margin, connect a resistor between
PFO and PFI, as shown in Figure 12. Select the ratio of
R1 and R2 such that PFI sees V
Figure 12. Adding Hysteresis to the Power-Fail Comparator
CC
, the SuperCap on BATT discharges through V
BATT
PFO
V
V
V
H
TRIP
L
+5V
Adding Hysteresis to the Power-Fail
= 1.25V
- 1.25
R1
R1
R2
0V
= 1.25V
reaches the reset threshold. Battery-backup
V
0V
IN
+
(
R1
Comparator (MAX817/MAX819)
5
(
Replacing the Backup Battery
R1 + R2
- 1.25
R2
R3
+
CC
R2
C1*
R2
||
R3
TO P
)
||
=
CC
to OUT, and connect BATT to
R3
R3
)
1.25
R2
stays above the reset thresh-
V
Backup Power Source
L
PFI
PFO
Operation Without a
V
V
TRIP
PFT
IN
MAX817
MAX819
GND
+5V
V
V
when V
CC
H
IN
*OPTIONAL
falls to the
CC
CC
CC
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