MAX794(1996) 查看數據表（PDF） - Maxim Integrated

零件编号

产品描述 (功能)

生产厂家

MAX794
(Rev.:1996)

3.0V/3.3V Adjustable Microprocessor Supervisory Circuits

Maxim Integrated 

3.0V/3.3V Adjustable Microprocessor

Supervisory Circuits

Adding Hysteresis to the Power-Fail

Comparator (MAX793/MAX794)

The power-fail comparator has a typical input hystere-

sis of 10mV. This is sufficient for most applications

where a power-supply line is being monitored through

an external voltage divider (see the section Monitoring

an Additional Power Supply).

If additional noise margin is desired, connect a resistor

between PFO and PFI as shown in Figure 16a. Select

the ratio of R1 and R2 such that PFI sees VPFT when

VIN falls to its trip point (VTRIP). R3 adds the additional

hysteresis and should typically be more than 10 times

the value of R1 or R2. The hysteresis window extends

both above (VH) and below (VL) the original trip point

(VTRIP).

Connecting an ordinary signal diode in series with R3,

as shown in Figure 16b, causes the lower trip point (VL)

to coincide with the trip point without hysteresis (VTRIP),

so the entire hysteresis window occurs above VTRIP.

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 25nA (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.

Monitoring an Additional Power Supply

These µP supervisors can monitor either positive or

negative supplies using a resistor voltage divider to

PFI. PFO can be used to generate an interrupt to the

µP or to cause reset to assert (Figure 12).

Interfacing to µPs with

Bidirectional Reset Pins

Since the RESET output is open drain, the MAX793/

MAX794/MAX795 interface easily with µPs that have

bidirectional reset pins, such as the Motorola 68HC11.

Connecting the RESET output of the µP supervisor

directly to the RESET input of the microcontroller with a

single pull-up resistor allows either device to assert

reset (Figure 17).

Negative-Going VCC Transients

These supervisors are relatively immune to short-dura-

tion negative-going VCC transients (glitches) while issu-

ing resets to the µP during power-up, power-down, and

brownout conditions. Therefore, resetting the µP when

VCC experiences only small glitches is usually not rec-

ommended.

Figure 18 shows maximum transient duration vs. reset-

comparator overdrive, for which reset pulses are not

generated. The graph was produced using negative-

going VCC pulses, starting at 3.3V and ending below

the reset threshold by the magnitude indicated (reset

comparator overdrive). The graph shows the maximum

pulse width a negative-going VCC transient can typically

START

100

90

80

70

60

50

40

30

20

10

0

10 20 30 40 50 60 70 80 90 100

RESET COMPARATOR OVERDRIVE, VRST - VCC (mV)

SET WDI

HIGH

PROGRAM

CODE

Subroutine or

Program Loop

SET WDI LOW

RETURN

Figure 18. Maximum Transient Duration without Causing a

Reset Pulse vs. Reset Comparator Overdrive

Figure 19. Watchdog Flow Diagram

18 ______________________________________________________________________________________

Share Link: