AAT3520/2/4
MicroPower™ Microprocessor Reset Circuit
Applications Information
Reset Output Options
The AAT3524 has an active low open drain output
and the AAT3520/2 have a push-pull output. The
AAT3524 may be used in multiple voltage systems
with an external pull-up resistor. This allows the
AAT3524 to interface to microprocessors with high-
er supply voltages up to 5.5V (Figure 3). In the
event of a power down or brown-out condition, the
reset signal remains valid until the V
CC
drops below
1.2V. To ensure validity down to ground, an exter-
nal 100kΩ resistor should be connected between
the RESET output and GND.
Supply Voltage Transient Behavior
In some cases, fast negative transients of short
duration can appear on the V
CC
power supply. The
AAT3520 series devices provide some immunity to
line transients which can generate invalid reset puls-
es. Figure 1 shows typical behavior to short duration
pulses versus RESET comparator overdrive. As
shown in the Maximum Transient Duration vs. Reset
Threshold Overdrive graph, when the transient volt-
age becomes larger, the time allowed before assert-
ing a reset becomes shorter (i.e., typically a transient
of 100mV below the reset threshold would have to
present for more than 50µs to cause a reset).
Immunity can be increased by the addition of a small
bypass capacitor of 0.1µF connected as closely to
the V
CC
pin as possible.
V
CC
VCC
*
100kΩ
(nominal)
VCC
0.1µF
AAT3520
AAT3522
RESET
AAT3524
(RESET)
RESET
µP
INPUT
GND
GND
*AAT3524 Only
Figure 2: Using Recommended Bypass Capacitor.
+3.3V
+5.0V
VCC
100kΩ
RESET
INPUT
VCC
AAT3524
RESET
5V
System
GND
GND
Figure 3: Using AAT3524 Open Drain Output with Multiple Supplies.
3520.2006.03.1.2
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