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PDF LM809M3-2.93 Data sheet ( Hoja de datos )
| Número de pieza | LM809M3-2.93 | |
| Descripción | 3-Pin Microprocessor Reset Circuits | |
| Fabricantes | National Semiconductor | |
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September 1999
LM809/LM810
3-Pin Microprocessor Reset Circuits
General Description
The LM809/810 microprocessor supervisory circuits can be
used to monitor the power supplies in microprocessor and
digital systems. They provide a reset to the microprocessor
during power-up, power-down and brown-out conditions.
The function of the LM809/810 is to monitor the VCC supply
voltage, and assert a reset signal whenever this voltage de-
clines below the factory-programmed reset threshold. The
reset signal remains asserted for 240ms after VCC rises
above the threshold. The LM809 has an active-low RESET
output, while the LM810 has an active-high RESET output.
Six standard reset voltage options are available, suitable for
monitoring 5V, 3.3V, and 3V supply voltages.
With a low supply current of only 15µA, the LM809/810 are
ideal for use in portable equipment. The LM809/LM810 are
available in the 3-pin SOT23 package.
Features
n Precise monitoring of 3V, 3.3V, and 5V supply voltages
n Superior upgrade to MAX809/810
n Fully specified over temperature
n 140ms min. Power-On Reset pulse width, 240ms typical
Active-low RESET Output (LM809)
Active-high RESET Output (LM810)
n Guaranteed RESET Output valid for VCC≥1V
n Low Supply Current, 15µA typ.
n Power supply transient immunity
Applications
n Microprocessor Systems
n Computers
n Controllers
n Intelligent Instruments
n Portable/Battery-Powered Equipment
n Automotive
Typical Application Circuit
Connection Diagram
DS101057-1
DS101057-2
( ) are for LM810
© 1999 National Semiconductor Corporation DS101057
www.national.com
1 page  
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.
TABLE 1. Reset Thresholds Related to Common
Supply Voltages
Reset
Threshold
4.63 ± 3%
4.38 ± 3%
4.00 ± 3%
3.08 ± 3%
2.93 ± 3%
2.63 ± 3%
3.0V
85 - 90%
3.3V
90 - 95%
86 - 90%
77 - 81%
5.0V
90 - 95%
85 - 90%
78 - 82%
Ensuring a Valid Reset Output Down to VCC = 0V
When VCC falls below 1V, the LM809 RESET output no
longer sinks 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 VCC is also recommended for the
LM810, if RESET is required to remain valid for VCC < 1V.
Negative-Going VCC Transients
The LM809/810 are relatively immune to short
negative-going transients or glitches on VCC. Figure 2 shows
the maximum pulse width a negative-going VCC 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, for the 4.63V and 4.38V version of the LM809/810,
a VCC transient that goes 100mV below the reset threshold
and lasts 20µs or less will not cause a reset pulse. A 0.1 µF
bypass capacitor mounted as close as possible to the VCC
pin will provide additional transient rejection.
DS101057-9
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 LM809
RESET output. To ensure a correct output on the LM809
even when the microprocessor reset pin is in the opposite
state, connect a 4.7kΩ resistor between the LM809 RESET
output and the µP reset pin, as shown in Figure 3. Buffer the
LM809 RESET output to other system components.
DS101057-10
FIGURE 1. RESET Valid to VCC = Ground Circuit
DS101057-11
FIGURE 3. Interfacing to Microprocessors with
Bidirectional Reset I/O
5 www.national.com
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