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

tolerance 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

3.0V

3.3V

5.0V

90 - 95%

85 - 90%

78 - 82%

90 - 95%

86 - 90%

85 - 90%

77 - 81%

79 - 84%

72 - 76%

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

Ω resistor should

be connected from the RESET output to ground, as shown in

Figure 1.

A 100k

Ω pull-up resistor to V

LM810, if RESET is required to remain valid for V

Negative-Going V

The

LM809/810

are

relatively

immune

to

short

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,

aV

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

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 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

Ω 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.

LLP Mounting

The LLP package requires special mounting techniques

which are detailed in National Semiconductor Application

Note AN-1187. Referring to the section PCB Design Recom-

mendations, it should be noted that the pad style which

should be used with the LLP package is the NSMD (non-

solder mask defined) type.

10105710

FIGURE 1. RESET Valid to V

10105709

FIGURE 2. Maximum Transient Duration without

Causing a Reset Pulse vs. Reset Comparator

Overdrive

10105711

FIGURE 3. Interfacing to Microprocessors with

Bidirectional Reset I/O

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