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 = 200k

Ω, then

R1 = 70.3k

required to guarantee the RESET-IN threshold accura-

cy (see Electrical Characteristics table).

Applications Information

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-

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

0.1V above the actual reset threshold and ending

below it by the magnitude indicated (reset comparator

overdrive). The graph indicates the maximum pulse

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

Ensuring a Valid Reset Output

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

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; 100k

Ω

is large enough not to load RESET and small enough to

pull RESET to ground.

A 100k

for the MAX6833/MAX6836/MAX6839 if RESET is

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).

Ultra-Low-Voltage SC70 Voltage Detectors

and µP Reset Circuits

_______________________________________________________________________________________

7

0

200

300

400

500

600

700

800

900

1

10

100

MAXIMUM TRANSIENT DURATION

vs. OVERDRIVE

100

RESET OCCURS

ABOVE THIS

LINE

Figure 2. Maximum Transient Duration Without Causing a

Reset Pulse vs. Reset Comparator Overdrive

R3

100k

Ω

GND

MAX6832

MAX6835

MAX6838

RESET

RESET

GND

RESET

INPUT

MAX6834

MAX6837

MAX6840

BIDIRECTIONAL

µP

GND

Figure 4. Interfacing to µPs with Bidirectional Reset I/O