Datasheet

MAX6841–MAX6845
RESET IN decreases. The MAX6843/MAX6844/
MAX6845 assert a reset when the voltage at RESET IN
falls below the RESET IN threshold (V
RSTIN
). The low-
leakage current at RESET IN allows for relatively large-
value resistors to be used, which reduces power
consumption. For example, for V
CC-TH
= 0.9V, if R2 =
100kΩ, then R1 = 26.3kΩ.
Applications Information
V
CC
Falling Transients
The MAX6841–MAX6845 are relatively immune to short-
duration falling V
CC
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 V
CC
, 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
V
CC
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 V
CC
pin provides
additional transient immunity.
Ensuring a Valid Reset Output
Down to V
CC
= 0
When V
CC
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 V
CC
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; 200kΩ is large
enough not to load RESET and small enough to pull
RESET to ground.
A 200kΩ pullup resistor to V
CC
is also recommended
for the MAX6841/MAX6842/MAX6844 if push-pull
RESET is required to remain valid for V
CC
≤ 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 V
TH
- V
CC
(mV)
MAXIMUM TRANSIENT DURATION (µs)
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
V
CC
RESET
GND
R3
Figure 3. RESET Valid to V
CC
= 0
MAX6842
MAX6845
V
CC
V
CC
RESET
GND
V
CC
RESET
INPUT
GND
µP
Figure 4. Interfacing to Microprocessors with Bidirectional
Reset I/0