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MAX6841 Datasheet(PDF) 6 Page - Maxim Integrated Products

Part No. MAX6841
Description  Ultra-Low-Voltage μP Reset Circuits and Voltage Detectors
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Maker  MAXIM [Maxim Integrated Products]
Homepage  http://www.maxim-ic.com
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MAX6841 Datasheet(HTML) 6 Page - Maxim Integrated Products

   
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RESET IN decreases. The MAX6843/MAX6844/
MAX6845 assert a reset when the voltage at RESET IN
falls below the RESET IN threshold (VRSTIN). The low-
leakage current at RESET IN allows for relatively large-
value resistors to be used, which reduces power
consumption. For example, for VCC-TH = 0.9V, if R2 =
100k
Ω, then R1 = 26.3kΩ.
Applications Information
VCC Falling Transients
The MAX6841–MAX6845 are relatively immune to short-
duration falling VCC transients (glitches). Figure 2
shows typical transient duration vs. reset comparator
overdrive, for which the MAX6841–MAX6845 do not
generate a reset pulse. The graph was generated using
a falling pulse applied to VCC, starting 0.1V above the
actual reset threshold and ending below it by the mag-
nitude indicated (reset comparator overdrive). The
graph indicates the maximum pulse width that a falling
VCC transient can have without causing a reset pulse.
As the magnitude of the transient increases (goes fur-
ther below the reset threshold), the maximum allowable
pulse width decreases. A 0.1µF bypass capacitor
mounted as close as possible to the VCC pin provides
additional transient immunity.
Ensuring a Valid Reset Output
Down to VCC = 0
When VCC falls below 0.55V, the MAX6841/MAX6843
push-pull RESET output no longer sinks current; it
becomes an open circuit. Therefore, high-impedance
CMOS-logic inputs connected to RESET can drift to
undetermined voltages. This presents no problem in
most applications, because most µP and other circuitry
are inoperative with VCC lower than 0.55V. However, in
applications in which RESET must be valid down to 0,
adding a pulldown resistor to RESET causes any stray
leakage currents to flow to ground, holding RESET low
(Figure 3). R3’s value is not critical; 200k
Ω is large
enough not to load RESET and small enough to pull
RESET to ground.
A 200k
Ω pullup resistor to VCC is also recommended
for the MAX6841/MAX6842/MAX6844 if push-pull
RESET is required to remain valid for VCC
≤ 0.75V.
Interfacing to µPs with Bidirectional
Reset Pins
Because the RESET output on the MAX6842/MAX6845
is open drain, these devices interface easily with µPs
that have bidirectional reset pins. Connecting the µP
supervisor’s RESET output directly to the µP’s RESET
pin with a single pullup resistor allows either device to
assert reset (Figure 4).
Ultra-Low-Voltage µP Reset Circuits and
Voltage Detectors
6
_______________________________________________________________________________________
MAXIMUM TRANSIENT DURATION
vs. OVERDRIVE
OVERDRIVE VTH - VCC (mV)
10
150
200
250
300
350
400
450
500
550
600
100
1
100
RESET OCCURS ABOVE THIS LINE
Figure 2. Maximum Transient Duration vs. Overdrive
MAX6841
MAX6843
VCC
RESET
GND
R3
Figure 3. RESET Valid to VCC = 0
MAX6842
MAX6845
VCC
VCC
RESET
GND
VCC
RESET
INPUT
GND
µP
Figure 4. Interfacing to Microprocessors with Bidirectional
Reset I/0


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