LM3722 Datasheet, PDF (9/12 Pages) National Semiconductor (TI) – 5-Pin Microprocessor Reset Circuits

Applications Information (Continued)
TABLE 1. Monitored Tolerance Table
Reset
Threshold
4.63 ± %
3.08 ± %
2.32 ± %
Supply Voltage
2.5V
3.3V
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_CC 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 100kΩ resistor
should be connected from the RESET output to ground, as
shown in Figure 1.
A 100kΩ pull-up resistor to V_CC is also recommended for the
LM3723, if RESET is required to remain valid for V_CC < 1V.
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
kΩ will generally suffice.
Negative-Going V_CC Transients
The LM3722/LM3723/LM3724 are relatively immune to short
negative-going transients or glitches on V_CC. Figure 2 shows
the maximum pulse width a negative-going V_CC 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_CC 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.
20014410
FIGURE 1. Circuit for RESET Valid from V_CC = 0V
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
20014414
FIGURE 2. Maximum Transient Duration without
Causing a Reset Pulse vs. Reset Comparator
Overdrive
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 RESET
output and the µP reset pin, as shown in Figure 3. Buffer the
LM3722 RESET output to other system components.
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