Full-Featured µP Supervisory Circuit with

±1.5% Reset Accuracy

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11

(see the Chip-Enable Propagation Delay vs. CE OUT

Load Capacitance graph in the Typical Operating

Characteristics). The CE propagation delay is produc-

tion tested from the 50% point on CE IN to the 50%

point on CE OUT using a 50

Ω driver and 50pF of load

capacitance (Figure 8). For minimum propagation

delay, minimize the capacitive load at CE OUT and use

a low output-impedance driver.

Chip-Enable Output

In the enabled mode, the impedance of CE OUT is equiv-

alent to 75

Ω in series with the source driving CE IN. In the

disabled mode, the 75

Ω transmission gate is off and CE

source turns off when the transmission gate is enabled.

Low-Line Comparator

voltage typically 52mV above the reset threshold, with

13mV of hysteresis. Use LOW LINE to provide a non-

maskable interrupt (NMI) to the µP when power begins

to fall to initiate an orderly software shutdown routine.

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 must

the main battery is disconnected, a DC-DC converter

shuts down, or a high-side switch is opened during

provide time to execute the shutdown routine (Figure 9).

First calculate the worst-case time required for the

system to perform its shutdown routine. Then, with the

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

calculate the amount of capacitance required to allow the

shutdown routine to complete before reset is asserted:

line to reset threshold.

CE IN

RESET

THRESHOLD

CE OUT

RESET

RESET

26

µs

28

µs

26

µs

MAX807

CE IN

50pF

CE OUT

GND

50

Ω DRIVER

Figure 7. Reset and Chip-Enable Timing

Figure 8. CE Propagation Delay Test Circuit

GND

TO

µP NMI

4.5V to 5.5V

LOW LINE

MAX807

REGULATOR

Figure 9. Using LOW LINE to Provide a Power-Fail Warning to

the µP