MAX6711S Просмотр технического описания (PDF) - Maxim Integrated
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MAX6711S Datasheet PDF : 8 Pages
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4-Pin SC70 Microprocessor Reset Circuits
with Manual Reset Input
VCC
VCC
MAX6713
RESET
MR
GND
VCC
µP
MOTOROLA
RESET 68HCXX
INPUT
GND
+3.3V
+5.0V
VCC
MAX6713
RESET
RPULLUP
VCC
5V SYSTEM
RESET
INPUT
GND
GND
Figure 3. Interfacing to µPs with Bidirectional Reset I/O
maximum allowable pulse width decreases. Typically, for
the MAX671_L and MAX671_M, a VCC 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 VCC pin provides
additional transient immunity.
Ensuring a Valid Reset Output
Down to VCC = 0
When VCC falls below 1V, the MAX6711 RESET output
no longer sinks current—it becomes an open circuit.
Therefore, high-impedance CMOS-logic inputs con-
nected to RESET can drift to undetermined voltages.
This presents no problem in most applications since
most µP and other circuitry is inoperative with VCC
below 1V. However, in applications where RESET must
be valid down to 0, adding a pull-down resistor to
RESET causes any stray leakage currents to flow to
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 VCC is also recommended
for the MAX6712 if RESET is required to remain valid for
VCC < 1V.
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 bidirec-
tional 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).
Figure 4. MAX6713 Open-Drain RESET Output Allows Use with
Multiple Supplies
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 VCC 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 VCC decreases
below 1V, so does the IC’s ability to sink current at
RESET. Also, with any pullup, RESET will be pulled high
as VCC 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%,
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.
___________________Chip Information
TRANSISTOR COUNT: 380
6 _______________________________________________________________________________________
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