MAX6909 MAXIM [Maxim Integrated Products], MAX6909 Datasheet - Page 30

• Manufacturer Part Number: MAX6909
• Description: I2C-Compatible Real-Time Clocks with uP Supervisor and NV RAM Controller
• Manufacturer: MAXIM [Maxim Integrated Products]

Connecting an ordinary signal diode in series with R3
(Figure 27(b)) causes the lower trip point (V
cide with the trip point without hysteresis (V
that the entire hysteresis window occurs above V
This method provides additional noise margin without
compromising the accuracy of the power-fail threshold
when the monitored voltage is falling. It is useful for
accurately detecting when a voltage falls past a thresh-
old. The current through R1 and R2 should be at least
1µA to ensure that the 100nA (max over temperature)
PFI input current does not shift the trip point. R3 should
be larger than 82kΩ so it does not load down the PFO
pin. Capacitor C1 is optional and adds noise rejection.
I
Supervisor and NV RAM Controller
Figure 27. Adding Hysteresis to the Power-Fail Comparator
30
2
C-Compatible Real-Time Clocks with µP
______________________________________________________________________________________
V
H
PFO
= (V
V
PFT
CC
R1
R2
+ V
V
V
TRIP
L
V
PFH
IN
= R1 x V
= V
) (R1)
PFT
a) SYMMETRICAL HYSTERESIS
(
PFT
x R1 + R2
R1
(
1
C1
x
TO µP
(
+
R2
R1
1
R2
1
V
R3
L
+
)
+
R2
1
R3
1
PFI
PFO
+
)
R3
1
V
MAX6909/
MAX6910
TRIP
)
GND
V
-
CC
V
R3
CC
V
H
L
) to coin-
TRIP
), so
TRIP
V
IN
.
Critical systems often require an early warning indicat-
ing that power is failing. This warning provides time for
the µP to store vital data and take care of any additional
“housekeeping” functions before the power supply gets
too far out of tolerance for the µP to operate reliably. If
access to the unregulated supply is feasible, the
power-fail comparator input (PFI) can be connected to
the unregulated supply through a voltage-divider, with
the power-fail comparator output (PFO) providing the
nonmaskable interrupt (NMI) to the µP (Figure 28).
PFO
R1
R2
V
0V
CC
V
IN
V
V
V
V
TRIP
H
D
L
= V
= R1 x (V
= DIODE FORWARD-VOLTAGE DROP
= V
TRIP
b) R3 HYSTERESIS ONLY ON RISING V
PFT
C1
x R1 + R2
PFT
TO µP
(
+ V
R2
R3
PFH
Early Power-Fail Warning
V
)
)
TRIP
PFI
PFO
x
(
R1
1
MAX6909/
MAX6910
+
GND
V
Using the PFI Input
R2
1
CC
+
R3
1
)
-
V
V
R3
H
IN
D
V
IN