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MAX809SEUR
3-Pin Microprocessor Reset Circuits
General DescriptionThe MAX803/MAX809/MAX810 are microprocessor (µP)
supervisory circuits used to monitor the power supplies
in µP and digital systems. They provide excellent circuit
reliability and low cost by eliminating external compo-
nents and adjustments when used with +5V, +3.3V,
+3.0V, or +2.5V-powered circuits.
These circuits perform a single function: they assert a
reset signal whenever the VCCsupply voltage declines
below a preset threshold, keeping it asserted for at
least 140ms after VCChas risen above the reset thresh-
old. Reset thresholds suitable for operation with a vari-
ety of supply voltages are available.
The MAX803 has an open-drain output stage, while the
MAX809/MAX810 have push-pull outputs. The
MAX803’s open-drain RESEToutput requires a pull-up
resistor that can be connected to a voltage higher than
VCC. The MAX803/MAX809 have an active-low RESET
output, while the MAX810 has an active-high RESET
output. The reset comparator is designed to ignore fast
transients on VCC, and the outputs are guaranteed to
be in the correct logic state for VCCdown to 1V.
Low supply current makes the MAX803/MAX809/
MAX810 ideal for use in portable equipment. The
MAX803 is available in a 3-pin SC70 package, and the
MAX809/MAX810 are available in 3-pin SC70 or SOT23
packages.
ApplicationsComputers
Controllers
Intelligent Instruments
Critical µP and µC Power Monitoring
Portable/Battery-Powered Equipment
Automotive
____________________________FeaturesPrecision Monitoring of +2.5V, +3V, +3.3V, and
+5V Power-Supply VoltagesFully Specified Over TemperatureAvailable in Three Output Configurations
Open-Drain RESETOutput (MAX803)
Push-Pull RESET Output (MAX809)
Push-Pull RESETOutput (MAX810)140ms min Power-On Reset Pulse Width12µA Supply CurrentGuaranteed Reset Valid to VCC= +1VPower Supply Transient ImmunityNo External Components3-Pin SC70 and SOT23 Packages
3-Pin Microprocessor Reset Circuits
Pin ConfigurationTypical Operating Circuit19-0344; Rev 4; 12/99
Note: These parts are offered in 2.5k or 10k reels, and must beordered in 2.5k or 10k increments. Order MAX803_EXR-T for
2.5k reels and MAX803_EXR-T10 for 10k reels. Insert the
desired suffix letter from the Selector Guide into the blank to
complete the part number. All versions of these products may
not be available at the time of announcement. Contact factory for
availability.
Ordering Information
3-Pin Microprocessor Reset Circuits
ABSOLUTE MAXIMUM RATINGS
ELECTRICAL CHARACTERISTICS(VCC= full range, TA= -40°C to +105°C, unless otherwise noted. Typical values are at TA= +25°C, VCC= 5V for L/M/J versions, VCC
= 3.3V for T/S versions, VCC= 3V for R version, and VCC= 2.5V for Z version.) (Note 1)
Stresses beyond those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. These are stress ratings only, and functional
operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to
absolute maximum rating conditions for extended periods may affect device reliability.
Terminal Voltage (with respect to GND)
VCC....................................................................-0.3V to +6.0V
RESET, RESET(push-pull).....................-0.3V to (VCC+ 0.3V)
RESET(open drain)...........................................-0.3V to +6.0V
Input Current, VCC..............................................................20mA
Output Current, RESET, RESET..........................................20mA
Rate of Rise, VCC............................................................100V/µs
Continuous Power Dissipation (TA= +70°C)
3-Pin SC70 (derate 2.17mW/°C above +70°C)............174mW
3-Pin SOT23 (derate 4mW/°C above +70°C)...............320mW
Operating Temperature Range.........................-40°C to +105°C
Storage Temperature Range.............................-65°C to +150°C
Lead Temperature (soldering, 10s).................................+300°C
3-Pin Microprocessor Reset Circuits
Note 1:Production testing done at TA= +25°C; limits over temperature guaranteed by design only.
Note 2:RESEToutput for MAX803/MAX809; RESET output for MAX810.
ypical Operating Characteristics(VCC= full range, TA= -40°C to +105°C, unless otherwise noted. Typical values are at TA= +25°C, VCC= +5V for L/M/J versions,
VCC= +3.3V for T/S versions, VCC= +3V for R version, and VCC= +2.5V for Z version.)
ELECTRICAL CHARACTERISTICS (continued)(VCC= full range, TA= -40°C to +105°C, unless otherwise noted. Typical values are at TA= +25°C, VCC= 5V for L/M/J versions, VCC
= 3.3V for T/S versions, VCC= 3V for R version, and VCC= 2.5V for Z version.) (Note 1)
3-Pin Microprocessor Reset Circuits
Typical Operating Characteristics (continued)(VCC= full range, TA= -40°C to +105°C, unless otherwise noted. Typical values are at TA= +25°C, VCC= +5V for L/M/J versions,
VCC= +3.3V for T/S versions, VCC= +3V for R version, and VCC= +2.5V for Z version.)
Selector Guide
Detailed DescriptionA microprocessor’s (µP’s) reset input starts the µP in a
known state. The MAX803/MAX809/MAX810 assert
reset to prevent code-execution errors during power-
up, power-down, or brownout conditions. They assert a
reset signal whenever the VCCsupply voltage declines
below a preset threshold, keeping it asserted for at
least 140ms after VCChas risen above the reset thresh-
old. The MAX803 uses an open-drain output, and the
MAX809/MAX810 have a push-pull output stage.
Connect a pull-up resistor on the MAX803’s RESETout-
put to any supply between 0 and 6V.
Applications Information
Negative-Going VCCTransientsIn addition to issuing a reset to the µP during power-up,
power-down, and brownout conditions, the MAX803/
MAX809/MAX810 are relatively immune to short-duration
negative-going VCCtransients (glitches).
Figure 1 shows typical transient duration vs. reset com-
parator overdrive, for which the MAX803/MAX809/
MAX810 do notgenerate a reset pulse. The graph was
generated using a negative-going pulse applied to VCC,
starting 0.5V above the actual reset threshold and end-
ing below it by the magnitude indicated (reset compara-
tor overdrive). The graph indicates the maximum pulse
width a negative-going VCCtransient can have without
causing a reset pulse. As the magnitude of the transient
increases (goes farther below the reset threshold), the
maximum allowable pulse width decreases. Typically, for
the MAX8__L and MAX8__M, a VCCtransient that goes
100mV below the reset threshold and lasts 20µs or less
will not cause a reset pulse. A 0.1µF bypass capacitor
mounted as close as possible to the VCCpin provides
additional transient immunity.
Ensuring a Valid Reset Output
Down to VCC= 0 When VCCfalls below 1V, the MAX809 RESEToutput
no longer sinks current—it becomes an open circuit.
Therefore, high-impedance CMOS logic inputs con-
nected to RESETcan 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 RESETmust
be valid down to 0V, adding a pull-down resistor to
RESETcauses any stray leakage currents to flow to
3-Pin Microprocessor Reset Circuits
ground, holding RESETlow (Figure 2). R1’s value is not
critical; 100kΩis large enough not to load RESETand
small enough to pull RESETto ground.
A 100kΩpull-up resistor to VCCis also recommended
for the MAX810 if RESET is required to remain valid for
VCC< 1V.
Interfacing to µPs with
Bidirectional Reset PinsSince the RESEToutput on the MAX803 is open drain,
this device interfaces easily with µPs that have bidirec-
tional reset pins, such as the Motorola 68HC11.
Connecting the µP supervisor’s RESEToutput directly
to the microcontroller’s (µC’s) RESETpin with a single
pull-up resistor allows either device to assert reset
(Figure 3).
MAX803 Open-Drain RESET Output
Allows Use with Multiple SuppliesGenerally, the pull-up connected to the MAX803 will
connect to the supply voltage that is being monitored at
the IC’s VCCpin. However, some systems may use the
open-drain output to level-shift from the monitored sup-
ply to reset circuitry powered by some other supply
(Figure 4). Note that as the MAX803’s VCCdecreases
below 1V, so does the IC’s ability to sink current at
RESET. Also, with any pull-up, RESETwill be pulled
high as VCCdecays toward 0. The voltage where this
occurs depends on the pull-up resistor value and the
voltage to which it is connected.
Benefits of Highly Accurate
Reset ThresholdMost µP supervisor ICs have reset threshold voltages
between 5% and 10% below the value of nominal sup-
ply voltages. This ensures a reset will notoccur within
5% of the nominal supply, but will occur when the sup-
ply is 10% below nominal.
When using ICs rated at only the nominal supply ±5%,
this leaves a zone of uncertainty where the supply is
between 5% and 10% low, and where the reset may or
may not be asserted.
The MAX8__L/T/Z use highly accurate circuitry to
ensure that reset is asserted close to the 5% limit, and
long before the supply has declined to 10% below
nominal.
3-Pin Microprocessor Reset CircuitsTRANSISTOR COUNT:275 (SOT23)
380 (SC70)
___________________Chip Information
