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MAX812LEUS Datasheet(PDF) 6 Page - Maxim Integrated Products

Part No. MAX812LEUS
Description  4-Pin uP Voltage Monitors with Manual Reset Input
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Maker  MAXIM [Maxim Integrated Products]
Homepage  http://www.maxim-ic.com
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MAX812LEUS Datasheet(HTML) 6 Page - Maxim Integrated Products

   
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__________ Applications Information
Negative-Going VCC 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 VCC 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 VCC, 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 VCC 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 VCC 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 VCC provides addi-
tional transient immunity.
Ensuring a Valid RESET Output
Down to VCC = 0V
When VCC 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 VCC 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; 100k
Ω is large
enough not to load RESET and small enough to pull
RESET to ground.
A 100k
Ω pull-up resistor to VCC is also recommended
for the MAX812 if RESET is required to remain valid for
VCC < 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 , VTH - VCC (mV)
10
TA = +25
°C
MAX81 _L/M
MAX81 _R/S/T
R1
VCC
GND
MAX811
RESET
Figure 1. Maximum Transient Duration without Causing a
Reset Pulse vs. Comparator Overdrive
Figure 2. RESET Valid to VCC = Ground Circuit


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