MAX812(2018) 查看數據表（PDF） - Maxim Integrated
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MAX812 Datasheet PDF : 9 Pages
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MAX811/MAX812
4-Pin μP Voltage Monitors
with Manual Reset Input
8
TA = +25°C
7
6
MAX81 _L/M
5
4
MAX81 _R/S/T
3
2
1
0
1
10
100
1000
RESET COMPARATOR OVERDRIVE, VTH - VCC (mV)
Figure 1. Maximum Transient Duration without Causing a Reset
Pulse vs. Comparator Overdrive
VCC
MAX811
RESET
R1
GND
Figure 2. RESET Valid to VCC = Ground Circuit
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 negative-
going VCC transients (glitches).
Figure 1 shows typical transient durations vs. reset com-
parator overdrive, for which the MAX811/MAX812 do
not generate a reset pulse. This graph was generated
using a negative-going pulse applied to VCC, starting
above the actual reset threshold and ending below it by
the magnitude indicated (reset comparator overdrive).
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 tran-
sient increases (goes farther below the reset threshold),
the maximum allowable pulse width decreases. Typically,
a VCC transient that goes 125mV below the reset thresh-
old 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 additional 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 connected
to the RESET output can drift to undetermined voltages.
This presents no problem in most applications, 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 pulldown 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 VCC is also recommended
for the MAX812 if RESET is required to remain valid for
VCC < 1V.
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