MAX801L/M/N, MAX808L/M/N
8-Pin µP Supervisory Circuits
with ±1.5% Reset Accuracy
8 _______________________________________________________________________________________
_______________Detailed Description
The MAX801/MAX808 microprocessor (µP) supervisory
circuits provide power-supply monitoring and backup-
battery switchover in µP systems. The MAX801 also
provides program-execution watchdog functions
(Figure 1). Use of BiCMOS technology results in an
improved, 1.5% reset-threshold precision while keeping
supply currents typically at 68µA (48µA for the
MAX808). The MAX801/MAX808 are intended for bat-
tery-powered applications that require high reset-
threshold precision, allowing a wide power-supply
operating range while preventing the system from oper-
ating below its specified voltage range.
RESET
and RESET Outputs
The MAX801/MAX808’s RESET output ensures that the
µP powers up in a known state, and prevents code-
execution errors during power-down and brownout
conditions. It does this by resetting the µP, terminating
program execution when V
CC
dips below the reset
threshold. Each time RESET is asserted, it stays low for
at least the 200ms reset timeout period (set by an inter-
nal timer) to ensure the µP has adequate time to return
to an initial state. The internal timer restarts any time
V
CC
goes below the reset threshold (V
RST
) before the
reset timeout period is completed. The watchdog timer
on the MAX801 can also initiate a reset (see the
MAX801 Watchdog Timer section).
The RESET output is active low, and is implemented with
a strong pull-down/relatively weak pull-up structure. It is
guaranteed to be a logic low for 0V < V
CC
< V
RST
, pro-
vided V
BATT
is greater than 2V. Without a backup bat-
tery, RESET is guaranteed valid for V
CC
≥ 1V.
The RESET output is the inverse of the RESET output; it
both sources and sinks current and cannot be wire-OR
connected.
Low-Line Comparator
The low-line comparator monitors V
CC
with a threshold
voltage typically 52mV above the reset threshold, with
13mV of hysteresis. Use LOWLINE to provide a non-
maskable interrupt (NMI) to the µP when power begins
to fall, initiating an orderly software shutdown routine. In
most battery-operated portable systems, reserve ener-
gy in the battery provides ample time to complete the
shutdown routine once the low-line warning is encoun-
tered and before reset asserts. If the system must con-
tend with a more rapid V
CC
fall time (such as when the
main battery is disconnected, when a DC-DC converter
shuts down, or when a high-side switch is opened dur-
ing normal operation), use capacitance on the V
CC
line
to provide time to execute the shutdown routine (Figure
3). First calculate the worst-case time required for the
system to perform its shutdown routine. Then, with
worst-case shutdown time, worst-case load current,
and minimum low-line to reset threshold (V
LR(min)
),
