1

10

100

1000

240

160

80

0

Reset Comparator Overdrive (ms)

320

400

560

480

V

CC

RESET

GND

LM809

R1

100 k

8

LM809, LM810

SNVS052E – SEPTEMBER 1999 – REVISED APRIL 2016

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Copyright © 1999–2016, Texas Instruments Incorporated

7.3.1.1 Ensuring a Valid Reset Output Down to VCC = 0 V

When VCC falls below 1 V, 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 100-k

Ω

resistor should be connected from the RESET output to ground, as shown in Figure 7.

A 100-k

Ω pullup resistor to VCC is also recommended for the LM810, if RESET is required to remain valid for VCC

< 1 V.

Figure 7. RESET Valid to VCC = Ground Circuit

7.3.1.2 Negative-Going VCC Transients

The LM809 and LM810 are relatively immune to short negative-going transients or glitches on VCC. Figure 8

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 4.63-V and 4.38-V version of the LM809 or LM810, a VCC transient that

goes 100 mV below the reset threshold and lasts 20 µs 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.

Figure 8. Maximum Transient Duration without Causing a Reset Pulse vs Reset Comparator Overdrive