AMS811/812
8
MAXIMUM TRANSIENT DURATION (ms)
T
A
= +25°C
7
6
A
MS81
_L/M
5
4
3
2
1
0
1
10
100
1000
RESET COMPARATOR OVERDRIVE , V
TH
- V
CC
(mV)
GND
A
MS81
_R/S/T
V
CC
AMS811
RESET
R1
Figure 1. Maximum Transient Duration without Causing a
Reset Pulse vs. Comparator Overdrive
Figure 2.
RESET
Valid to V
CC
= Ground Circuit
__________ 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
AMS811/
AMS812
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
AMS811/AMS812
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
(AMS81_L/M) or 20µs or less (AMS81_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.
When V
CC
falls below 1V, the
AMS811
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
AMS812
if RESET is required to remain valid for
V
CC
< 1V.
Ensuring a Valid
RESET
Output
Down to V
CC
= 0V
6
Advanced Monolithic Systems
http://www.ams-semitech.com
AMS [ Advanced Monolithic Systems Ltd ]
