at RESET-IN allows for relatively large-value resistors to
be used, which reduce power consumption. For exam-
ple, for a 0.6V monitored trip level, if R2 = 200kΩ, then
R1 = 70.3kΩ. Note that the minimum V
CC
of 1.1V is
required to guarantee the RESET-IN threshold accura-
cy (see
Electrical Characteristics
table).
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 MAX6832–
MAX6840 are relatively immune to short-duration nega-
tive-going V
CC
transients (glitches).
Figure 2 shows typical transient duration vs. reset com-
parator overdrive, for which the MAX6832–MAX6840 do
not generate a reset pulse. The graph was generated
using a negative-going pulse applied to V
CC
, starting
0.1V above the actual reset threshold and ending
below it by the magnitude indicated (reset comparator
overdrive). The graph indicates the maximum pulse
width a negative-going V
CC
transient can have without
causing a reset pulse. As the magnitude of the tran-
sient increases (goes farther 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 MAX6832/MAX6835/
MAX6838 push-pull RESET output no longer sinks cur-
rent—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 since most µP and other
circuitry are inoperative with V
CC
lower than 0.55V.
However, in applications where 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; 100kΩ
is large enough not to load RESET and small enough to
pull RESET to ground.
A 100kΩ pullup resistor to V
CC
is also recommended
for the MAX6833/MAX6836/MAX6839 if RESET is
required to remain valid for V
CC
< 0.85V.
Interfacing to µPs with Bidirectional
Reset Pins
Since the RESET output on the MAX6834/MAX6837/
MAX6840 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 a reset (Figure 4).
MAX6832–MAX6840
Ultra-Low-Voltage SC70 Voltage Detectors
and µP Reset Circuits
7
Maxim Integrated
vs. OVERDRIVE
