Datasheet

MAX811/MAX812
__________ Applications Information
Negative-Going V
CC
Transients
In addition to issuing a reset to the µP during power-up,
power-down, and brownout conditions, the MAX811/
MAX812 are relatively immune to short duration nega-
tive-going V
CC
transients (glitches).
Figure 1 shows typical transient durations vs. reset
comparator overdrive, for which the MAX811/MAX812
do not generate a reset pulse. This graph was generat-
ed using a negative-going pulse applied to V
CC
, start-
ing above the actual reset threshold and ending below
it by the magnitude indicated (reset comparator over-
drive). The graph indicates the typical maximum pulse
width a negative-going V
CC
transient may have without
causing a reset pulse to be issued. As the magnitude
of the transient increases (goes farther below the reset
threshold), the maximum allowable pulse width
decreases. Typically, a V
CC
transient that goes 125mV
below the reset threshold and lasts 40µs or less
(MAX81_L/M) or 20µs or less (MAX81_T/S/R) will not
cause a reset pulse to be issued. A 0.1µF capacitor
mounted as close as possible to V
CC
provides addi-
tional transient immunity.
Ensuring a Valid RESET Output
Down to V
CC
= 0V
When V
CC
falls below 1V, the MAX811 RESET output
no longer sinks current—it becomes an open circuit.
Therefore, high-impedance CMOS-logic inputs con-
nected to the RESET output can drift to undetermined
voltages. This presents no problem in most applica-
tions, since most µP and other circuitry is inoperative
with V
CC
below 1V. However, in applications where the
RESET output must be valid down to 0V, adding a pull-
down resistor to the RESET pin will cause any stray
leakage currents to flow to ground, holding RESET low
(Figure 2). R1’s value is not critical; 100kΩ is large
enough not to load RESET and small enough to pull
RESET to ground.
A 100kΩ pull-up resistor to V
CC
is also recommended
for the MAX812 if RESET is required to remain valid for
V
CC
< 1V.
4-Pin µP Voltage Monitors
with Manual Reset Input
6 _______________________________________________________________________________________
5
6
7
8
4
0
1 100 1000
2
1
3
RESET COMPARATOR OVERDRIVE , V
TH
- V
CC
(mV)
MAXIMUM TRANSIENT DURATION (ms)
10
T
A
= +25°C
MAX81 _L/M
MAX81 _R/S/T
R1
V
CC
GND
MAX811
RESET
Figure 1. Maximum Transient Duration without Causing a
Reset Pulse vs. Comparator Overdrive
Figure 2. RESET Valid to V
CC
= Ground Circuit