3.0V/3.3V Adjustable Microprocessor

Supervisory Circuits

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13

through a voltage divider, with the power-fail compara-

tor output (PFO) providing the NMI to the µP (Figure

10). If there is no easy access to the unregulated sup-

ply, the LOWLINE output can be used to generate an

NMI to the µP (see

LOWLINE Output section).

LOWLINE Output (MAX793/MAX794)

voltage typically 45mV above the reset threshold (10mV

of hysteresis) for the MAX793, and 15mV above RESET

IN (4mV of hysteresis) for the MAX794. For normal

µP when power begins to fall.

In most battery-operated portable systems, reserve

energy in the battery provides ample time to complete

the shutdown routine once the low-line warning is

encountered and before reset asserts. If the system

as when the main battery is disconnected or a high-

side switch is opened during normal operation, use

the shutdown routine (Figure 11).

First, calculate the worst-case time required for the sys-

tem to perform its shutdown routine. Then, with the worst-

case shutdown time, the worst-case load current, and the

the amount of capacitance required to allow the shut-

down routine to complete before reset is asserted:

the system to complete an orderly shutdown routine.

Power-Fail Comparator (MAX793/MAX794)

The MAX793/MAX794’s PFI input is compared to an

internal reference. If PFI is less than the power-fail

parator is intended for use as an undervoltage detector

to signal a failing power supply (Figure 12). However,

the comparator does not need to be dedicated to this

function because it is completely separate from the rest

of the circuitry.

GND

50pF

CE IN

50

Ω

3.6V

25

Ω EQUIVALENT

SOURCE IMPEDANCE

50

Ω

50

Ω CABLE

BATT

CE OUT

MAX793

MAX794

MAX795

Figure 9. CE Propagation Delay Test Circuit

GND

PFI

TO

µP NMI

R1

UNREGULATED

SUPPLY

3.0V OR 3.3V

R2

PFO

MAX793

MAX794

REGULATOR

Figure 10. Using the Power-Fail Comparator to Generate

Power-Fail Warning

GND

TO

µP NMI

3.0V OR 3.3V

LOWLINE

MAX793

MAX794

REGULATOR

Figure 11. Using LOWLINE to Provide Power-Fail Warning

to the µP