Low-Power Dual-/Triple-Voltage SC70 µP

Supervisory Circuits

12

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Adding Hysteresis to the

Power-Fail Comparator

The power-fail comparator has a typical input hystere-

sis of 2.5mV. This is sufficient for most applications in

which a power-supply line is being monitored through

an external voltage-divider. If additional noise margin is

desired, connect a resistor between

PFO and PFI, as

shown in Figure 6. Select the values of R1, R2, and R3

thresholds. R3 adds the additional hysteresis by sink-

ing current from the R1/R2 divider network when the

PFO output is logic low and sourcing current into the

network when

PFO is logic high. R3 is typically an order

of magnitude greater than R1 or R2.

The current through R2 should be at least 1µA to ensure

that the 10nA (max) PFI input current does not significant-

ly shift the trip points. Therefore, for most applications:

Ω

PFO is an open-drain output requiring an external

pullup resistor, R4. Select R4 to be less than 1% of R3.

R2 = 488k

Ω (FOR >1µA R2 CURRENT)

R4

≤ 0.01 x R3

Power Sequencing Applications

Many dual-voltage processors/ASICs require specific

power-up/power-down sequences for the I/O and core

supplies.

RR

VV

VV

V

V

V

GOOD

TH PFI

TH PFI

GOOD

FAIL

PU

TH

PFI

12

=

() ()

−

−

-

-

-

-

(

MAX6742

MAX6745

PFI

GND

R1

R2

PFO

R1 + R2

R2

()

MAX6742

MAX6745

PFI

GND

R1

R2

PFO

1

1

+

(

()

)

-

Figure 4. Using Power-Fail Input to Monitor an Additional

Power Supply

MAX6737

MAX6739

MAX6743

MAX6744

MAX6745

GND

GND

RESET

RESET TO OTHER SYSTEM COMPONENTS

RESET

µP

4.7k

Ω

Figure 5. Interfacing to µPs with Bidirectional Reset I/O

MAX6742

MAX6745

R1

R4

R3

R2

PFI

GND

PFO

R2 = 200k

Ω (FOR > 2.5µA R2 CURRENT)

PFO

Figure 6. Adding Hysteresis to Power Fail for Push-Pull

PFO