MAX793RCSE データシートの表示（PDF） - Maxim Integrated

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MAX793RCSE

3.0V/3.3V Adjustable Microprocessor Supervisory Circuits

Maxim Integrated 

3.0V/3.3V Adjustable Microprocessor

Supervisory Circuits

3.0V OR 3.3V

VCC BATT ON OUT

TO CMOS RAM

PMOS FET

BODY DIODE

D

S

G

VCC BATT ON OUT

MAX793

MAX794

MAX795

MAX793

MAX794

MAX795

GND

(a)

Figure 14. Driving an External Transistor with BATT ON

3.0V OR 3.3V

VCC

1N4148

BATT

0.47F

MAX793

MAX794

OUT

TO STATIC

VCC RAM

RESET

TO µP

GND

GND

(b)

+5V

3.0V OR

3.3V

1N4148

VCC

BATT

0.47F

MAX793

MAX794

OUT

TO STATIC

VCC RAM

RESET

TO µP

GND

(a)

Figure 15. Using a Super Cap as a Backup Source

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

(b)

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.

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