Detailed Description

The MAX807 µP supervisory circuit provides power-

supply monitoring, backup-battery switchover, and pro-

gram execution watchdog functions in µP systems

(Figure 1). Use of BiCMOS technology results in an

improved 1.5% reset-threshold precision, while keeping

supply currents typically below 70µA. The MAX807 is

intended for battery-powered applications that require

high reset-threshold precision, allowing a wide power-

supply operating range while preventing the system

from operating below its specified voltage range.

RESET and RESET Outputs

The MAX807’s RESET output ensures that the µP pow-

ers up in a known state, and prevents code execution

errors during power-down and brownout conditions. It

accomplishes this by resetting the µP, terminating pro-

old or MR is pulled low. Each time RESET is asserted it

stays low for the 200ms reset timeout period, which is

set by an internal timer to ensure the µP has adequate

below the reset threshold before the reset-timeout peri-

od is completed, the internal timer restarts. The watch-

dog timer can also initiate a reset if WDO is connected

to MR (see the Watchdog Input section).

Full-Featured µP Supervisory Circuit with

±1.5% Reset Accuracy

8

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MAX807

CE IN

PFI

GND

BATT

THE HIGHER

BATTERY-BACKUP

COMPARATOR

RESET

COMPARATOR

LOW-LINE

COMPARATOR

BATTERY-OK

COMPARATOR

POWER-FAIL

COMPARATOR

2.275V

P

OUT

BATT ON

LOW LINE

BATT OK

PFO

WDI

MR

50k

Ω

RESET

RESET

WDO

CE OUT

N

P

P

N

WATCHDOG

TRANSITION

DETECTOR

STATE

MACHINE

OSCILLATOR

Figure 1. Block Diagram