Datasheet
LM809, LM810
SNVS052D –SEPTEMBER 1999–REVISED MAY 2013
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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 voltages.
Table 1. Reset Thresholds Related to Common Supply Voltages
Reset Threshold 3.0V 3.3V 5.0V
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%
Ensuring a Valid Reset Output Down to V
CC
= 0V
When V
CC
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 9.
A 100kΩ pull-up resistor to V
CC
is also recommended for the LM810, if RESET is required to remain valid for V
CC
< 1V.
Figure 9. RESET Valid to V
CC
= Ground Circuit
Negative-Going V
CC
Transients
The LM809/810 are relatively immune to short negative-going transients or glitches on V
CC
. Figure 10 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, for the 4.63V and 4.38V version of the LM809/810, a V
CC
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 V
CC
pin will provide additional transient rejection.
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