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參數(shù)資料
| 型號： | LM3722 |
| 廠商： | National Semiconductor Corporation |
| 元件分類： | 復(fù)位半導(dǎo)體 |
| 英文描述： | 5-Pin Microprocessor Reset Circuits |
| 中文描述： | 5引腳微處理器復(fù)位電路 |
| 文件頁數(shù)： | 9/12頁 |
| 文件大?。?/td> | 219K |
| 代理商： | LM3722 |
Applications Information
(Continued)
TABLE 1. Monitored Tolerance Table
Reset
Threshold
4.63
±
%
3.08
±
%
2.32
±
%
Supply Voltage
3.3V
2.5V
5.0V
90.8-94.4%
91.8-95.2%
90.8-94.8%
Ensuring a Valid Reset Output Down to V
CC
= 0V
When V
falls below 1V, the LM3722 RESET output is
unable to sink the rated current. A high-impedance CMOS
logic input connected to RESET can therefore drift to unde-
termined voltages. To prevent this situation, a 100k
resistor
should be connected from the RESET output to ground, as
shown in Figure 1
A 100k
pull-up resistor to V
is also recommended for the
LM3723, if RESET is required to remain valid for V
CC
<
1V.
Open Drain Output (LM3724)
An open drain output allows easy paralleling of multiple
microprocessor reset circuits without requiring additional
logic gates. Open drain outputs also allow interfacing de-
vices of differing logic levels or families, since the output
pull-up resistor may be connected to any supply voltage up
to 5.5V, regardless of LM3724 V
CC
.
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 connected devices does not create an excessive voltage
drop when the output is not activated. A resistor value of 100
k
will generally suffice.
Negative-Going V
CC
Transients
The LM3722/LM3723/LM3724 are relatively immune to short
negative-going transients or glitches on V
. Figure 2shows
the maximum pulse width a negative-going V
transient
can have without causing a reset pulse. In general, as the
magnitude of the transient increases, going further below the
threshold, the maximum allowable pulse width decreases.
Typically, a V
transient that goes 125 mV below the reset
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
CC
pin will provide additional transient rejection.
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 4.7k
resistor between the LM3722 RESET
output and the μP reset pin, as shown in Figure 3 Buffer the
LM3722 RESET output to other system components.
20014410
FIGURE 1. Circuit for RESET Valid from V
CC
= 0V
20014414
FIGURE 2. Maximum Transient Duration without
Causing a Reset Pulse vs. Reset Comparator
Overdrive
L
www.national.com
9
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