MAX697EWE Просмотр технического описания (PDF) - Maxim Integrated
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MAX697EWE Datasheet PDF : 16 Pages
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MAX696/MAX697
Microprocessor Supervisory Circuits
Applications Information
Adding Hysteresis to the
Power-Fail Comparator
Since the power-fail comparator circuit is noninvert-
ing, hysteresis can be added by connecting a resistor
between the PFO output and the PFI input as shown in
Figure 7. When PFO is low, resistor R3 sinks current from
the summing junction at the PFI pin. When PFO is high,
the series combination of R3 and R4 source current into
the PFI summing junction.
Alternate Watchdog Input Drive Circuits
The watchdog feature can be enabled and disabled
under program control by driving WDI with a three-state
buffer (Figure 8). The drawback to this circuit is that a
software fault may erroneously three-state the buffer,
thereby preventing the MAX696/MAX697 from detecting
that the microprocessor is no longer working. In most
cases, a better method is to extend the watchdog period
rather than disabling the watchdog. See Figure 9. When
the control input is high, the OSC SEL pin is low and
the watchdog timeout is set by the external capacitor.
A 0.01μF capacitor sets a watchdog timeout delay of
100s. When the control input is low, the OSC SEL pin is
driven high, selecting the internal oscillator. The 100ms or
the 1.6s period is chosen, depending on which diode in
Figure 9 is used.
7V - 15V
7805
R1
75kΩ
+5V
VCC
R4
10kΩ
PFO
MAX696
MAX697
R2
13kΩ
R3
300kΩ
PFI
GND
VH = 9.125V
VL = 7.9V
HYSTERESIS = 1.23V
TOP
{ } VH = 1.3V
1+
R1
R2
+
R1
R3
{ } VL = 1.3V
1
+
R1
R2
+1(5.3VV-(1R.33V+)
R1
R4)
HYSTERESIS 5V x R1
R3
ASSUMING R4 << R3
Figure 7. Adding Hysteresis to the Power-Fail Voltage
Comparator
WATCHDOG
STROBE
EN
WATCHDOG
DISABLE
+5V
VCC
MAX696
MAX697
WDI
GND
Figure 8. Disabling the Watchdog Under Program Control
LOW = INTERNAL
WATCHDOG TIMEOUT
HI = EXTERNAL
WATCHDOG
TIMEOUT
CONNECT FOR
100ms TIMEOUT
WHEN INTERNAL
TIMEOUT IS
SELECTED
Figure 9. Selecting Internal or External Watchdog Timeout
+5V
VCC
OSC SEL
MAX696
MAX697
OSC IN
GND
CONNECT FOR
1.6s INTERNAL
TIMEOUT
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