lm3722im5x-4.63 National Semiconductor Corporation, lm3722im5x-4.63 Datasheet - Page 8

• Manufacturer Part Number: lm3722im5x-4.63
• Description: 5-pin Microprocessor Reset Circuits
• Manufacturer: National Semiconductor Corporation

www.national.com
ENSURING A VALID RESET OUTPUT DOWN TO V
When V
able to sink the rated current. A high-impedance CMOS logic
input connected to RESET can therefore drift to undetermined
voltages. To prevent this situation, a 100kΩ resistor should
be connected from the RESET output to ground, as shown in
Figure 1.
A 100kΩ pull-up resistor to V
LM3723, if RESET is required to remain valid for V
OPEN DRAIN OUTPUT (LM3724)
An open drain output allows easy paralleling of multiple mi-
croprocessor reset circuits without requiring additional logic
gates. Open drain outputs also allow interfacing devices of
differing logic levels or families, since the output pull-up re-
sistor may be connected to any supply voltage up to 5.5V,
regardless of LM3724 V
The pull up resistor is calculated so that maximum current flow
into RESET is less than 10 mA when activated. The resistor
must be small enough so that the leakage current of all con-
FIGURE 1. Circuit for RESET Valid from V
Threshold
4.63 ± %
3.08 ± %
2.32 ± %
Reset
CC
TABLE 1. Monitored Tolerance Table
falls below 1V, the LM3722 RESET output is un-
90.8-94.8
2.5V
CC
%
.
CC
Supply Voltage
is also recommended for the
91.8-95.2
3.3V
%
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90.8-94.4
CC
5.0V
%
CC
= 0V
CC
< 1V.
= 0V
8
nected devices does not create an excessive voltage drop
when the output is not activated. A resistor value of 100 kΩ
will generally suffice.
NEGATIVE-GOING V
The LM3722/LM3723/LM3724 are relatively immune to short
negative-going transients or glitches on V
the maximum pulse width a negative-going V
have without causing a reset pulse. In general, as the mag-
nitude of the transient increases, going further below the
threshold, the maximum allowable pulse width decreases.
Typically, a V
threshold and lasts 40 µs or less will not cause a reset pulse.
A 0.1 µF bypass capacitor mounted as close as possible to
the V
FIGURE 2. Maximum Transient Duration without Causing
INTERFACING TO µPS WITH BIDIRECTIONAL RESET
PINS
Microprocessors with bidirectional reset pins, such as the
Motorola 68HC11 series, can be connected to the LM3722
RESET output. To ensure a correct output on the LM3722
even when the microprocessor reset pin is in the opposite
state, connect a 4.7kΩ resistor between the LM3722 RE-
SET output and the µP reset pin, as shown in Figure 3. Buffer
the LM3722 RESET output to other system components.
a Reset Pulse vs. Reset Comparator Overdrive
CC
pin will provide additional transient rejection.
CC
transient that goes 125 mV below the reset
CC
TRANSIENTS
CC
. Figure 2 shows
CC
transient can
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