Datasheet

MAX803L/M/R/S/T/Z, MAX809J/L/M/R/S/T/Z, MAX810L/M/R/S/T/Z
ground, holding RESET low (Figure 2). R1’s value is not
critical; 100kΩ is large enough not to load RESET and
small enough to pull RESET to ground.
A 100kΩ pull-up resistor to V
CC
is also recommended
for the MAX810 if RESET is required to remain valid for
V
CC
< 1V.
Interfacing to µPs with
Bidirectional Reset Pins
Since the RESET output on the MAX803 is open drain,
this device interfaces easily with µPs that have bidirec-
tional reset pins, such as the Motorola 68HC11.
Connecting the µP supervisor’s RESET output directly
to the microcontroller’s (µC’s) RESET pin with a single
pull-up resistor allows either device to assert reset
(Figure 3).
MAX803 Open-Drain RESET Output
Allows Use with Multiple Supplies
Generally, the pull-up connected to the MAX803 will
connect to the supply voltage that is being monitored at
the IC’s V
CC
pin. However, some systems may use the
open-drain output to level-shift from the monitored sup-
ply to reset circuitry powered by some other supply
(Figure 4). Note that as the MAX803’s V
CC
decreases
below 1V, so does the IC’s ability to sink current at
RESET. Also, with any pull-up, RESET will be pulled
high as V
CC
decays toward 0. The voltage where this
occurs depends on the pull-up resistor value and the
voltage to which it is connected.
Benefits of Highly Accurate
Reset Threshold
Most µP supervisor ICs have reset threshold voltages
between 5% and 10% below the value of nominal sup-
ply voltages. This ensures a reset will not occur within
5% of the nominal supply, but will occur when the sup-
ply is 10% below nominal.
When using ICs rated at only the nominal supply ±5%,
this leaves a zone of uncertainty where the supply is
between 5% and 10% low, and where the reset may or
may not be asserted.
The MAX8__L/T/Z use highly accurate circuitry to
ensure that reset is asserted close to the 5% limit, and
long before the supply has declined to 10% below
nominal.
3-Pin Microprocessor Reset Circuits
6 _______________________________________________________________________________________
RESET
GND
RESET
INPUT
R
PULLUP
MAX803
5V SYSTEM
GND
V
CC
+3.3V
+5.0V
V
CC
Figure 4. MAX803 Open-Drain RESET Output Allows Use with
Multiple Supplies
RESET
GND
RESET
INPUT
MAX803
MOTOROLA
68HCXX
µP
GND
V
CC
V
CC
V
CC
Figure 3. Interfacing to µPs with Bidirectional Reset I/O
TRANSISTOR COUNT: 275 (SOT23)
380 (SC70)
___________________Chip Information