Datasheet

maximum allowable pulse width decreases. Typically,
for the MAX671_L and MAX671_M, a V
CC
transient that
goes 100mV below the reset threshold and lasts 20µs or
less will not cause a reset pulse. A 0.1µF bypass capaci-
tor mounted as close as possible to the V
CC
pin provides
additional transient immunity.
Ensuring a Valid Reset Output
Down to V
CC
= 0
When V
CC
falls below 1V, the MAX6711 RESET output
no longer sinks current—it becomes an open circuit.
Therefore, high-impedance CMOS-logic inputs connect-
ed to RESET can drift to undetermined voltages. This
presents no problem in most applications since most
µP and other circuitry is inoperative with V
CC
below 1V.
However, in applications where RESET must be valid
down to 0, adding a pull-down resistor to RESET caus-
es any stray leakage currents to flow to ground, holding
RESET low (Figure 2). R1's value is not critical; 100kW is
large enough not to load RESET and small enough to pull
RESET to ground.
A 100kW pull-up resistor to V
CC
is also recommended
for the MAX6712 if RESET is required to remain valid for
V
CC
< 1V.66
Interfacing to µPs with
Bidirectional Reset Pins
Since the RESET output on the MAX6713 is open-
drain, this device interfaces easily with µPs that have
bidirectional reset pins, such as the Motorola 68HC11.
Connecting the µP supervisor's RESET output directly to
the µP's RESET pin with a single pull-up resistor allows
either device to assert reset (Figure 3).
MAX6713 Open-Drain RESET Output
Allows Use with Multiple Supplies
Generally, the pull-up connected to the MAX6713 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 MAX6713's V
CC
decreases
below 1V, so does the IC's ability to sink current at
RESET. Also, with any pullup, 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.
Benets of Highly Accurate
Reset Threshold
Most µP supervisor ICs have reset threshold voltages
between 5% and 10% below the value of nominal supply
voltages. This ensures a reset will not occur within 5% of
the nominal supply, but will occur when the supply is 10%
below nominal.
When using ICs rated at only the nominal supply ±5%,
a zone of uncertainty where the supply is between 5%
and 10% low, and where the reset may or may not be
asserted is left.
The MAX671_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.
Figure 4. MAX6713 Open-Drain RESET Output Allows Use
with Multiple Supplies
Figure 3. Interfacing to μPs with Bidirectional Reset I/O
RESET
GND
RESET
INPUT
R
PULLUP
MAX6713
5V SYSTEM
GND
V
CC
+3.3V
+5.0V
V
CC
RESET
GND
MR
RESET
INPUT
MAX6713
MOTOROLA
68HCXX
µP
GND
V
CC
V
CC
V
CC
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Maxim Integrated │ 6
MAX6711L/M/R/S/T/Z,
MAX6712L/M/R/S/T/Z,
MAX6713L/M/R/S/T/Z
4-Pin SC70 Microprocessor
Reset Circuits with Manual Reset Input
Chip Information
TRANSISTOR COUNT: 380