MAX791CSE 查看數據表（PDF） - Maxim Integrated

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MAX791CSE

Microprocessor Supervisory Circuit

Maxim Integrated 

Microprocessor Supervisory Circuit

+5V

R1

VCC

PFI

PFO

R2

V-

+5V

PFO

0V

5

- 1.25

R1

=

1.25

- VTRIP

R2

NOTE: VTRIP IS NEGATIVE

MAX791

GND

VTRIP

0V

V-

100

VCC = +5V

TA = +25°C

80

0.1µF CAPACITOR

FROM VOUT TO GND

60

40

20

0

10

100

1000

10,000

RESET COMPARATOR OVERDRIVE (mV)

(Reset Threshold Voltage - VCC)

Figure 15. Monitoring a Negative Voltage

Figure 16. Maximum Transient Duration Without Causing a

Reset Pulse vs. Reset Comparator Overdrive

close to SWT as possible. The sum of PC board leak-

age + SWT capacitor leakage must be small compared

to ±100nA.

Watchdog Software Considerations

A way to help the watchdog timer keep a closer watch

on software execution involves setting and resetting the

watchdog input at different points in the program,

rather than “pulsing” the watchdog input high-low-high

or low-high-low. This technique avoids a “stuck” loop

where the watchdog timer continues to be reset within

the loop, keeping the watchdog from timing out.

Figure 17 shows an example flow diagram where the

I/O driving the watchdog input is set high at the begin-

ning of the program, set low at the beginning of every

subroutine or loop, then set high again when the pro-

gram returns to the beginning. If the program should

“hang” in any subroutine, the I/O is continually set low

and the watchdog timer is allowed to time out, causing

a reset or interrupt to be issued.

Maximum VCC Fall Time

The VCC fall time is limited by the propagation delay of

the battery switchover comparator and should not

exceed 0.03V/µs. A standard rule of thumb for filter

capacitance on most regulators is on the order of

100µF per amp of current. When the power supply is

shut off or the main battery is disconnected, the associ-

ated initial VCC fall rate is just the inverse or 1A / 100µF

= 0.01V/µs. The VCC fall rate decreases with time as

VCC falls exponentially, which more than satisfies the

maximum fall-time requirement.

16 ______________________________________________________________________________________

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