LM810M3-2.93 National Semiconductor, LM810M3-2.93 Datasheet - Page 5

• Manufacturer Part Number: LM810M3-2.93
• Description: 3-Pin Microprocessor Reset Circuits
• Manufacturer: National Semiconductor

Applications Information
Benefits of Precision Reset Thresholds
A microprocessor supply supervisor must provide a reset
output within a predictable range of the supply voltage. A
common threshold range is between 5% and 10% below the
nominal supply voltage. The 4.63V and 3.08V options of the
LM809/810 use highly accurate circuitry to ensure that the
reset threshold occurs only within this range (for 5V and 3.3V
supplies). The other voltage options have the same tight tol-
erance to ensure a reset signal for other narrow monitor
ranges. See Table 1 for examples of how the standard reset
thresholds apply to 3V, 3.3V, and 5V nominal supply volt-
ages.
Ensuring a Valid Reset Output Down to V
When V
longer sinks current. A high-impedance CMOS logic input
connected to RESET can therefore drift to undetermined
voltages. To prevent this situation, a 100k
be connected from the RESET output to ground, as shown in
Figure 1 .
A 100k pull-up resistor to V
LM810, if RESET is required to remain valid for V
Threshold
4.63
4.38
4.00
3.08
2.93
2.63
TABLE 1. Reset Thresholds Related to Common
FIGURE 1. RESET Valid to V
Reset
±
±
±
±
±
±
CC
3%
3%
3%
3%
3%
3%
falls below 1V, the LM809 RESET output no
85 - 90%
3.0V
Supply Voltages
CC
90 - 95%
86 - 90%
77 - 81%
is also recommended for the
3.3V
DS101057-10
CC
= Ground Circuit
90 - 95%
85 - 90%
78 - 82%
CC
resistor should
5.0V
= 0V
CC
<
1V.
5
Negative-Going V
The
negative-going transients or glitches on V
the maximum pulse width a negative-going V
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, for the 4.63V and 4.38V version of the LM809/810,
a V
and lasts 20µs or less will not cause a reset pulse. A 0.1 µF
bypass capacitor mounted as close as possible to the V
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 LM809
RESET output. To ensure a correct output on the LM809
even when the microprocessor reset pin is in the opposite
state, connect a 4.7k
output and the µP reset pin, as shown in Figure 3 . Buffer the
LM809 RESET output to other system components.
CC
FIGURE 2. Maximum Transient Duration without
FIGURE 3. Interfacing to Microprocessors with
Causing a Reset Pulse vs. Reset Comparator
LM809/810
transient that goes 100mV below the reset threshold
Bidirectional Reset I/O
CC
are
Transients
resistor between the LM809 RESET
Overdrive
relatively
immune
DS101057-9
CC
. Figure 2 shows
CC
www.national.com
to
transient
DS101057-11
short
CC