RESET

INPUT

R

D

GND

V

REF

-

+

7

LM809, LM810

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SNVS052E – SEPTEMBER 1999 – REVISED APRIL 2016

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Copyright © 1999–2016, Texas Instruments Incorporated

7 Detailed Description

7.1 Overview

The LM809 and LM810 microprocessor supervisory circuits provide a simple solution to monitor the power

supplies in microprocessor and digital systems and provide a reset during power-up, power-down, and brown-out

conditions. The reset signal is controlled by the factory-programmed reset threshold on the VCC supply voltage

pin. When the voltage declines below the reset threshold, the reset signal is asserted and remains asserted for

240 ms after VCC rises above the threshold. The LM809 has an active-low RESET output, while the LM810 has

an active-high RESET output. The available threshold options are 2.63 V, 2.93 V, 3.08 V, 4.38 V, and 4.63 V to

provide precision monitoring of supply voltages.

7.2 Functional Block Diagram

7.3 Feature Description

7.3.1 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 4.63-V and 3.08-V

options of the LM809 and LM810 use highly accurate circuitry to ensure that the reset threshold occurs only

within this range (for 5-V and 3.3-V 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 3-V, 3.3-V, and 5-V nominal supply voltages.

Table 1. Reset Thresholds Related to Common Supply Voltages

Reset Threshold

3 V

3.3 V

5 V

4.63 ± 3%

90 – 95%

4.38 ± 3%

85 – 90%

4.00 ± 3%

78 – 82%

3.08 ± 3%

90 – 95%

2.93 ± 3%

86 – 90%

2.63 ± 3%

85 – 90%

77 – 81%

2.45 ± 3%

79 – 84%

72 – 76%