MAX822LUS-T ,4-Pin P Voltage Monitors with Pin-Selectable Power-On Reset Timeout DelayApplicationsMAX822LUS-T 4.63 BF_ _Bar-Code ScannersMAX822MUS-T 4.38 BG_ _ComputersMAX822TUS-T 3.08 ..
MAX822RUS+T ,4-Pin, µP Voltage Monitors with Pin-Selectable Power-On Reset Timeout DelayMAX821/MAX82219-1122; Rev 3; 12/054-Pin µP Voltage Monitors with Pin-SelectablePower-On Reset Timeo ..
MAX822RUS-T ,4-Pin P Voltage Monitors with Pin-Selectable Power-On Reset Timeout DelayFeaturesThe MAX821/MAX822 microprocessor (µP) supervisory
MAX822LUS-T-MAX822RUS-T-MAX822TUS-T
Microprocessor Voltage Monitors with Programmable Voltage Detection
General DescriptionThe MAX821/MAX822 microprocessor (µP) supervisory
circuits monitor power supplies in µP and digital sys-
tems. They provide excellent circuit reliability and low
cost by eliminating external components and adjust-
ments when used with 5V-powered or 3V-powered cir-
cuits. The MAX821/MAX822 also provide a power-on
reset timeout delay that is pin selectable to 1ms (max),
20ms (min), or 100ms (min).
These devices perform a single function: they assert a
reset signal whenever the VCCsupply voltage falls
below a preset threshold, and they keep reset asserted
for the pin-selected reset timeout delay after VCChas
risen above the reset threshold. The only difference
between the two devices is that the MAX821 has an
active-low RESEToutput (which is guaranteed to be in
the correct state for VCCdown to 1V), while the
MAX822 has an active-high RESET output. The reset
comparator is designed to ignore fast transients on
VCC. Reset thresholds suitable for operation with a vari-
ety of supply voltages are available.
Low supply current makes the MAX821/MAX822 ideal
for use in portable equipment. These devices come in a
4-pin SOT143 package.
________________________ApplicationsBar-Code Scanners
Computers
Controllers
Intelligent Instruments
Critical μP and μC Power Monitoring
Portable/Battery-Powered Equipment
FeaturesPin-Selectable, Precision Power-On Reset Delay:
1ms (max), 20ms (min), or 100ms (min)Precision Monitoring of +3V to +5V Power-Supply
Voltages2.5µA Supply CurrentGuaranteed Over Temperature (-40°C to +125°C)Guaranteed RESETValid to VCC= 1V (MAX821)Power-Supply Transient ImmunityNo External ComponentsSOT143 Package
MAX821/MAX822
4-Pin µP Voltage Monitors with Pin-Selectable
Power-On Reset Timeout Delay
Pin Configuration
__________Typical Operating Circuit19-1122; Rev 1; 12/99
Note: These parts are offered in the SOT143-4 package, in theextended temperature range (-40°C to +85°C).
MAX821/MAX822
4-Pin µP Voltage Monitors with Pin-Selectable
Power-On Reset Timeout Delay
ABSOLUTE MAXIMUM RATINGS
ELECTRICAL CHARACTERISTICS(VCC= full range, TA= -40°C to +125°C, unless otherwise noted. Typical values are at TA= +25°C.)
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
All Other Inputs.......................................-0.3V to (VCC+ 0.3V)
Input Current, VCC, SRT......................................................20mA
Output Current, RESET or RESET.......................................20mA
Continuous Power Dissipation (TA= +70°C)
SOT143-4 (derate 4mW/°C above +70°C)...................320mW
Operating Temperature Range.........................-40°C to +125°C
Storage Temperature Range.............................-65°C to +160°C
Lead Temperature (soldering, 10sec).............................+300°C
MAX821/MAX822
4-Pin µP Voltage Monitors with Pin-Selectable
Power-On Reset Timeout Delay
Note 1:RESEToutput for MAX821; RESET output for MAX822.
Note 2:During reset active timeout period only.
Note 3:Guaranteed by design.
ELECTRICAL CHARACTERISTICS (continued)(VCC= full range, TA= -40°C to +125°C, unless otherwise noted. Typical values are at TA= +25°C.)
__________________________________________Typical Operating Characteristics(TA = +25°C, unless otherwise noted.)
MAX821/MAX822
4-Pin µP Voltage Monitors with Pin-Selectable
Power-On Reset Timeout Delay
____________________________Typical Operating Characteristics (continued)(TA = +25°C, unless otherwise noted.)
______________________________________________________________Pin DescriptionMAX82_R/S/T POWER-DOWN RESET DELAY
vs. TEMPERATURE
MAX 821/822 TOC-02
TEMPERATURE (°C)
POWER-DOWN RESET DELAY (6080
MAX82_L/M POWER-DOWN RESET DELAY
vs. TEMPERATURE
MAX 821/822 TOC-03
TEMPERATURE (°C)
POWER-DOWN RESET DELAY (6080
NORMALIZED SUPPLY
CURRENT vs. TEMPERATURE
MAX 821/822 TOC-01
TEMPERATURE (°C)
NORMALIZED SUPPLY CURRENT (6080
1.10
_______________Detailed Description
Reset OutputA microprocessor’s (µP’s) reset input starts the µP in a
known state. These μP supervisory circuits assert reset
to prevent code-execution errors during power-up,
power-down, or brownout conditions. They also provide
a reset timeout delay that is pin programmable to 1ms
(max), 20ms (min), or 100ms (min). This feature allows
flexibility in designing bar-code scanners, hand-held
devices, and other applications that require quick or
nonstandard power-up times.
The MAX821’s RESEToutput is guaranteed to be a
logic low for VCC> 1V. Once VCCexceeds the reset
threshold, an internal timer keeps RESETlow for the
reset timeout period, as determined by the Set Reset
Timeout (SRT) input. See the Setting the Reset Timeout
Delaysection.
If a brownout condition occurs (VCCdips below the
reset threshold), RESETgoes low. Any time VCCgoes
below the reset threshold, the internal timer resets to
zero, and RESETgoes low. The internal timer begins
counting after VCCreturns above the reset threshold,
and RESETremains low for the reset timeout period.
The MAX822 has an active-high RESET output that is
the inverse of the MAX821’s RESEToutput.
Setting the Reset Timeout DelayUse the three-level Set Reset Timeout (SRT) input to set
the reset timeout delay. Connect SRT to GND for a 1ms
(max) delay; connect it to VCCfor a 20ms (min) delay;
or leave it unconnected for a 100ms (min) delay.
If you choose to drive the SRT pin with an external sig-
nal, make sure the signal source can charge/discharge
the capacitance on SRT quickly enough (<500µs) to
avert an unintended reset timeout delay.
To ensure proper operation when selecting the 100ms
timeout (SRT = unconnected), minimize capacitive
loading on the SRT pin (< 200pF). Excessive capacitive
loading can select an unintended faster timeout mode.
Reset Threshold AccuracyThe MAX821/MAX822 are designed to meet their worst-
case specifications over their entire operating tempera-
ture range. Choose a reset threshold guaranteed to
assert at a voltage below the power supply’s regulation
range and above the minimum specified operating volt-
age range for the system’s ICs.
__________Applications Information
Negative-Going VCCTransientsWhile designed to issue a reset to the microprocessor
(µP) during power-up, power-down, and brownout con-
ditions, the MAX821/MAX822 are relatively immune to
short-duration, negative-going VCCtransients (glitches).
Figure 1 shows the maximum transient duration vs. reset
comparator overdrive for which the MAX821/MAX822
typically do not generate a reset pulse. This graph was
generated using a negative-going pulse applied to VCC,
starting above the actual reset threshold and ending
below it by the magnitude indicated (reset comparator
overdrive). The graph indicates the typical maximum
pulse width a negative-going VCCtransient may have
without causing a reset pulse to be issued. As the mag-
nitude of the transient increases (goes farther below the
reset threshold), the maximum allowable pulse width
decreases. Typically, for the MAX821/MAX822, a VCC
transient that goes 100mV below the reset threshold and
lasts 12µs or less will not cause a reset pulse to be
issued. A 0.1µF capacitor mounted as close as possible
to VCCcan provide additional transient immunity, if
desired.
MAX821/MAX822
4-Pin µP Voltage Monitors with Pin-Selectable
Power-On Reset Timeout Delay
MAX821/MAX822
Ensuring a Valid RESETOutput
Down to VCC= 0VWhen VCCfalls below 1V, the MAX821 RESEToutput
no longer sinks current—it becomes an open circuit.
Therefore, high-impedance CMOS logic inputs con-
nected to the RESEToutput can drift to undetermined
voltages. This presents no problem in most applica-
tions, since most μP and other circuitry is inoperative
with VCCbelow 1V. However, in applications where theRESEToutput must be valid down to 0V, adding a pull-
down resistor to the RESETpin will cause any stray
leakage currents to flow to ground, holding RESETlow
(Figure 2a). R1’s value is not critical; 100kΩis large
enough not to load RESET, and small enough to pull
RESETto ground.
A 100kΩpull-up resistor to VCCis also recommended
for the MAX822 if RESET is required to remain valid for
VCC< 1V (Figure 2b).
Interfacing to µPs with
Bidirectional Reset PinsµPs with bidirectional reset pins (such as the Motorola
68HC11 series) can contend with the MAX821 reset
output. For example, if the MAX821 RESEToutput is
asserted high and the µP wants to pull it low, indetermi-
nate logic levels may result. To correct such cases,
connect a 4.7kΩresistor between the MAX821 RESET
output and the µP reset I/O (Figure 3). Buffer the reset
output to other system components.
___________________Chip InformationTRANSISTOR COUNT: 492
4-Pin µP Voltage Monitors with Pin-Selectable
Power-On Reset Timeout DelayFigure 3. Interfacing to µPs with Bidirectional Reset I/O
Figure 2. RESET/RESET Valid to VCC= Ground Circuit
