MAX6821/MAX6825
Low-Voltage SOT23 µP Supervisors
with Manual Reset and Watchdog Timer
General Description
The MAX6821–MAX6825 are low-voltage microprocessor (µP) supervisory circuits that combine voltage monitoring, watchdog timer, and manual reset input functions in a 5-pin SOT23 package. Microprocessor supervisory circuits significantly improve system reliability and
accuracy compared to separate ICs or discrete components. These devices assert a reset signal whenever the
monitored voltage drops below its preset threshold, keeping it asserted for a minimum timeout period after VCC rises
above the threshold. In addition, a watchdog timer monitors
against code execution errors. A debounced manual reset
is also available. The MAX6821–MAX6825 monitor voltages from +1.8V to +5.0V. These outputs are guaranteed to
be in the correct state for VCC down to +1.0V.
Nine preprogrammed reset threshold voltages are
available (see the Threshold Suffix Guide). The
MAX6821, MAX6822, and MAX6823 all have a manual
reset input and a watchdog timer. The MAX6821 has
push-pull RESET, the MAX6822 has open-drain RESET,
and the MAX6823 has push-pull RESET. The MAX6824
has a watchdog timer and both push-pull RESET and
push-pull RESET. The MAX6825 has a manual reset
input and both push-pull RESET and push-pull RESET.
The Selector Guide explains the functions offered in
this series of parts.
Applications
●● Set-Top Boxes
●● Computers and Con trollers
●● Embedded Controllers
●●
●●
●●
Intelligent Instruments
Critical µP Monitoring
Portable/BatteryPowered Equipment
Pin Configurations
TOP VIEW
RESET(RESET) 1
GND 2
VCC
4
WDI
MAX6821
MAX6822
MAX6823
MR 3
(RESET) MAX6821 ONLY
5
SOT23-5
Pin Configurations continued at end of data sheet.
Typical Operating Circuit appears at end of data sheet.
19-1868; Rev 3; 7/14
Features
●● Monitors +1.8V, +2.5V, +3.0V, +3.3V, +5.0V Supplies
●● 140ms (min) Reset Timeout Delay
●● 1.6s Watchdog Timeout Period (MAX6821/MAX6822/
MAX6823/MAX6824)
●● Manual Reset Input (MAX6821/MAX6822/MAX6823/
MAX6825)
●● Three Reset Output Options
Push-Pull RESET
Push-Pull RESET
Open-Drain RESET
●● Guaranteed Reset Valid to VCC = +1.0V
●● Immune to Short Negative VCC Transients
●● No External Components
●● Small 5-Pin SOT23 Packages
Ordering Information
TEMP RANGE
PIN-PACKAGE
MAX6821_UK-T
PART*
-40°C to +125°C
5 SOT23-5
MAX6822_UK-T
-40°C to +125°C
5 SOT23-5
MAX6823_UK-T
-40°C to +125°C
5 SOT23-5
MAX6824_UK-T
-40°C to +125°C
5 SOT23-5
MAX6825_UK-T
-40°C to +125°C
5 SOT23-5
*Insert the desired suffix letter (from the table below) into the
blank to complete the part number
Devices are available in both leaded and lead(Pb)-free packaging.Specify lead-free by replacing “-T” with “+T” when ordering.
Threshold Suffix Guide
RESET
THRESHOLD (V)
L
4.63
M
4.38
T
3.08
S
2.93
R
2.63
Z
2.32
Y
2.19
W
1.67
V
1.58
Note: Bold indicates standard versions. Samples are typically
available for standard versions only. All parts require a 2.5k
minimum order increment. Contact factory for availability.
Selector Guide appears at end of data sheet.
SUFFIX
MAX6821/MAX6825
Low-Voltage SOT23 µP Supervisors
with Manual Reset and Watchdog Timer
Absolute Maximum Ratings
VCC to GND..........................................................-0.3V to +6.0V
Open-Drain RESET...............................................-0.3V to +6.0V
Push-Pull RESET, RESET, MR, WDI........ -0.3V to (VCC + 0.3V)
Input Current (VCC).............................................................20mA
Output Current (RESET, RESET).......................................20mA
Continuous Power Dissipation (TA = +70°C)
5-Pin SOT23 (derate 7.1mW/°C above +70°C)...........571mW
Operating Temperature Range.......................... -40°C to +125°C
Junction Temperature.......................................................+150°C
Storage Temperature Range............................. -65°C to +150°C
Lead Temperature (soldering, 10s).................................. +300°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.
Electrical Characteristics
(VCC = +4.5V to +5.5V for MAX682_L/M, VCC = +2.7V to +3.6V for MAX682_T/S/R, VCC = +2.1V to +2.75V for MAX682_Z/Y, VCC =
+1.53V to +2.0V for MAX682_W/V, TA = -40°C to +125°C, unless otherwise specified. Typical values are at TA = +25°C.) (Note 1)
PARAMETER
Operating Voltage Range
SYMBOL
VCC
CONDITIONS
TA = 0°C to +85°C
TA = -40°C to +125°C
VCC = +5.5V, no load
VCC Supply Current
(MR and WDI Unconnected)
ICC
VCC = +3.6V, no load
VCC = +3.6V, no load
(MAX6825 only)
MAX682_L
MAX682_M
MAX682_T
MAX682_S
VCC Reset Threshold
(VCC Falling)
VTH
MAX682_R
MAX682_Z
MAX682_Y
MAX682_W
MAX682_V
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MAX
5.5
1.2
5.5
10
V
30
TA = -40°C to +85°C
7
16
TA = -40°C to +125°C
25
TA = -40°C to +85°C
5
µA
12
TA = -40°C to +125°C
20
TA = -40°C to +85°C
4.50
4.63
4.75
TA = -40°C to +125°C
4.47
4.63
4.78
TA = -40°C to +85°C
4.25
4.38
4.50
TA = -40°C to +125°C
4.22
4.38
4.53
TA = -40°C to +85°C
3.00
3.08
3.15
TA = -40°C to +125°C
2.97
3.08
3.17
TA = -40°C to +85°C
2.85
2.93
3.00
TA = -40°C to +125°C
2.83
2.93
3.02
TA = -40°C to +85°C
2.55
2.63
2.70
TA = -40°C to +125°C
2.53
2.63
2.72
TA = -40°C to +85°C
2.25
2.32
2.38
TA = -40°C to +125°C
2.24
2.32
2.40
TA = -40°C to +85°C
2.12
2.19
2.25
TA = -40°C to +125°C
2.11
2.19
2.27
TA = -40°C to +85°C
1.62
1.67
1.71
TA = -40°C to +125°C
1.61
1.67
1.72
TA = -40°C to +85°C
1.52
1.58
1.62
VCC = VTH to (VTH - 100mV)
UNITS
20
TA = -40°C to +125°C
Reset Threshold Hysteresis
tRD
TYP
1.0
TA = -40°C to +85°C
Reset Threshold Temperature
Coefficient
VCC to Reset Output Delay
MIN
V
60
ppm/°C
2 × VTH
mV
20
µs
Maxim Integrated │ 2
MAX6821/MAX6825
Low-Voltage SOT23 µP Supervisors
with Manual Reset and Watchdog Timer
Electrical Characteristics (continued)
(VCC = +4.5V to +5.5V for MAX682_L/M, VCC = +2.7V to +3.6V for MAX682_T/S/R, VCC = +2.1V to +2.75V for MAX682_Z/Y, VCC =
+1.53V to +2.0V for MAX682_W/V, TA = -40°C to +125°C, unless otherwise specified. Typical values are at TA = +25°C.) (Note 1)
PARAMETER
Reset Timeout Period
RESET Output LOW
(Push-Pull or Open-Drain)
RESET Output HIGH
(Push-Pull Only)
Open-Drain RESET Output
Leakage Current (Note 1)
RESET Output HIGH
(Push-Pull Only)
SYMBOL
tRP
VOL
VOH
ILKG
CONDITIONS
TA = -40°C to +85°C
TA = -40°C to +125°C
TYP
140
200
MAX
280
100
320
VCC ≥ 1.0V, ISINK = 50µA, reset asserted,
TA = 0°C to +85°C
0.3
VCC ≥ 1.2V, ISINK = 100µA, reset asserted
0.3
VCC ≥ 2.55V, ISINK = 1.2mA, reset asserted
0.3
VCC ≥ 4.25V, ISINK = 3.2mA, reset asserted
0.4
VCC ≥ 1.8V, ISOURCE = 200µA, reset not
asserted
0.8 × VCC
VCC ≥ 3.15V, ISOURCE = 500µA, reset not
asserted
0.8 × VCC
VCC ≥ 4.75V, ISOURCE = 800µA, reset not
asserted
0.8 × VCC
1.0
VOL
ms
V
µA
0.8 × VCC
VCC ≥ 1.50V, ISOURCE = 100µA, reset asserted 0.8 × VCC
V
VCC ≥ 2.55V, ISOURCE = 500µA, reset asserted 0.8 × VCC
VCC ≥ 4.25V, ISOURCE = 800µA, reset asserted 0.8 × VCC
RESET Output LOW
(Push-Pull Only)
UNITS
V
VCC > VTH, RESET not asserted
VCC ≥ 1.0V, ISOURCE = 1µA, reset asserted,
TA = 0°C to +85°C
VOH
MIN
VCC ≥ 1.8V, ISINK = 500µA, reset not asserted
0.3
VCC ≥ 3.15V, ISINK = 1.2mA, reset not asserted
0.3
VCC ≥ 4.75V, ISINK = 3.2mA, reset not asserted
V
0.4
MANUAL RESET INPUT (MAX6821/MAX6822/MAX6823/MAX6825)
MR Input Voltage
MR Minimum Input Pulse
VIL
VIH
0.3 × VCC
0.7 × VCC
1
MR Glitch Rejection
µs
100
MR to Reset Delay
ns
200
MR Pullup Resistance
V
ns
25
50
75
1.12
1.60
2.40
kΩ
WATCHDOG INPUT (MAX6821/MAX6822/MAX6823/MAX6824)
Watchdog Timeout Period
tWD
WDI Pulse Width (Note 2)
tWDI
WDI Input Voltage
WDI Input Current
TA = -40°C to +85°C
TA = -40°C to +125°C
VIL
VIH
IWDI
WDI = VCC, time average
0.80
2.60
50
0.7 × VCC
ns
0.3 × VCC
120
160
WDI = 0, time average
-20
-15
Note 1: Over-temperature limits are guaranteed by design and not production tested. Devices tested at TA = +25°C.
Note 2: Guaranteed by design and not production tested.
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s
V
µA
Maxim Integrated │ 3
MAX6821/MAX6825
Low-Voltage SOT23 µP Supervisors
with Manual Reset and Watchdog Timer
Typical Operating Characteristics
(TA = +25°C, unless otherwise noted.)
10
9
5
VCC = +1.8V, V VERSION
-40 -20
0
20
40
60
180
170
20
15
10
5
-40 -20
0
20
40
60
0
80 100 120
-40 -20
0
20
40
60
80 100 120
TEMPERATURE (°C)
TEMPERATURE (°C)
WATCHDOG TIMEOUT PERIOD
vs. TEMPERATURE
NORMALIZED RESET THRESHOLD
DELAY vs. TEMPERATURE
MAXIMUM VCC TRANSIENT DURATION
vs. RESET THRESHOLD OVERDRIVE
1.06
1.04
RESET THRESHOLD (V)
1.8
1.7
1.6
1.5
1.4
1.3
1.2
1.02
1.00
0.98
0.96
1.1
-40 -20
0
20
40
60
0.94
-40 -20
80 100 120
0
20
40
60
80 100 120
VOLTAGE OUTPUT LOW
vs. SINK CURRENT
2.92
MAX6821 toc07
VCC = +2.9V
0.30
RESET OCCURS
ABOVE CURVE
60
40
20
0
0
20 40 60 80 100 120 140 160 180 200
VOLTAGE OUTPUT HIGH
vs. SOURCE CURRENT
VCC = +2.9V
2.88
VOUT (V)
VOUT (V)
80
2.86
0.20
0.15
2.84
2.82
2.80
0.10
2.78
0.05
2.76
0
1
2
3
4
ISINK (mA)
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100
2.90
0.25
0
120
RESET THRESHOLD OVERDRIVE (mV), VTH - VCC
TEMPERATURE (°C)
TEMPERATURE (°C)
0.35
MAX6821 toc03
25
TEMPERATURE (°C)
1.9
1.0
190
150
80 100 120
MAX6821 toc04
WATCHDOG TIMEOUT PERIOD (s)
2.0
200
160
4
3
210
MAX6821 toc06
6
220
VCC FALLING, VCC = VTH - 100mV
30
MAX6821 toc08
7
VCC = +3.3V, S VERSION
230
VCC TO RESET OUTPUT DELAY
vs. TEMPERATURE
35
TRANSIENT DURATION (µs)
8
MAX6821 toc02
VCC = +5V, L VERSION
MAX6821 toc05
11
240
RESET TIMEOUT PERIOD (ms)
SUPPLY CURRENT (µA)
12
250
MAX6821 toc01
13
RESET TIMEOUT PERIOD
vs. TEMPERATURE
PROPAGATION DELAY (µs)
SUPPLY CURRENT
vs. TEMPERATURE
5
6
7
2.74
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
ISOURCE (mA)
Maxim Integrated │ 4
MAX6821/MAX6825
Low-Voltage SOT23 µP Supervisors
with Manual Reset and Watchdog Timer
Pin Description
PIN NUMBERS
PIN
MAX6821 MAX6822 MAX6823 MAX6824 MAX6825 NAME
1
1
1
2
3
2
3
2
1
1
RESET
Active-Low Open-Drain or Push-Pull Reset Output. RESET
changes from high to low when the VCC input drops below the
selected reset threshold, MR is pulled low, or the watchdog triggers
a reset. RESET remains low for the reset timeout period after VCC
exceeds the device reset threshold, MR goes low to high, or the
watchdog triggers a reset.
Active-High Push-Pull Reset Output. RESET changes from low to
high when the VCC input drops below the selected reset threshold,
MR is pulled low, or the watchdog triggers a reset. RESET remains
high for the reset timeout period after VCC exceeds the device
reset threshold, MR goes low to high, or the watchdog triggers a
reset.
3
3
RESET
2
2
GND
3
4
4
4
4
4
5
5
5
5
5
MR
WDI
Watchdog Input. If WDI remains high or low for longer than the
watchdog timeout period, the internal watchdog timer runs out
and a reset is triggered for the reset timeout period. The internal
watchdog timer clears whenever reset is asserted, the manual
reset is asserted, or WDI sees a rising or falling edge. If WDI is left
unconnected or is connected to a three-stated buffer output, the
watchdog feature is disabled.
VCC
Supply Voltage and Input for Reset Threshold Monitor
RESET/RESET Output
A µP’s reset input starts the µP in a known state. The
MAX6821–MAX6825 µP supervisory circuits assert a
reset to prevent code-execution errors during power-up,
power-down, and brownout conditions. Whenever VCC
falls below the reset threshold, the reset output asserts
low for RESET and high for RESET. Once VCC exceeds
the reset threshold, an internal timer keeps the reset output asserted for the specified reset timeout period (tRP);
after this interval, reset output returns to its original state
(see Figure 2).
Many µP-based products require manual reset capability, allowing the operator, a test technician, or external
logic circuitry to initiate a reset. On the MAX6821/
MAX6822/MAX6823/MAX6825, a logic low on MR
asserts a reset. Reset remains asserted while MR is
low, and for the timeout period (140ms min) after it
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Ground
Active-Low Manual Reset Input. Internal 50kW pullup to V CC.
Pull low to force a reset. Reset remains active as long as MR is
low and for the reset timeout period after MR goes high. Leave
unconnected or connect to VCC if unused.
Detailed Description
Manual Reset Input
FUNCTION
returns high. MR has an internal 50kΩ pullup resistor,
so it can be left open if not used. This input can be driven with CMOS logic levels or with open-drain/collector
outputs. Connect a normally open momentary switch
from MR to GND to create a manual reset function;
external debounce circuitry is not required. If MR is driven from long cables or the device is used in a noisy
environment, connect a 0.1µF capacitor from MR to
GND to provide additional noise immunity.
Watchdog Input
In the MAX6821–MAX6824, the watchdog circuit monitors the µP’s activity. If the µP does not toggle (low to
high or high to low) the watchdog input (WDI) within the
watchdog timeout period (1.6s nominal), reset asserts
for the reset timoeout period. The internal 1.6s timer
can be cleared by either a reset pulse or by toggling
WDI. The WDI can detect pulses as short as 50ns.
While reset is asserted, the timer remains cleared and
does not count. As soon as reset is released, the timer
starts counting (see Figure 3).
Maxim Integrated │ 5
MAX6821/MAX6825
Low-Voltage SOT23 µP Supervisors
with Manual Reset and Watchdog Timer
VCC
MAX6821–
MAX6825
RESET (MAX6822–MAX6825)
RESET
GENERATOR
RESET
(MAX6821/MAX6824/ MAX6825)
VCC
1.26V
MR
(MAX6821–MAX6823,
MAX6825)
WDI
(MAX6821–MAX6824)
WATCHDOG
TRANSITION
DETECTOR
WATCHDOG
TIMER
GND
Figure 1. Functional Diagram
Disable the watchdog function by leaving WDI unconnected or by three-stating the driver connected to WDI.
The watchdog input is internally driven low during the
first 7/8 of the watchdog timeout period and high for the
last 1/8 of the watchdog timeout period. When WDI is
left unconnected, this internal driver clears the 1.6s timer
every 1.4s. When WDI is three-stated or unconnected,
the maximum allowable leakage current is 10µA and the
maximum allowable load capacitance is 200pF.
VCC
1V
VTH
tRP
tRD
RESET
tRP
tRD
GND
Figure 2. Reset Timing Diagram
Watchdog Input Current
Interfacing to µPs with
Bidirectional Reset Pins
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1V
RESET
Applications Information
The MAX6821/MAX6822/MAX6823/MAX6824 WDI inputs
are internally driven through a buffer and series resistor
from the watchdog timer (Figure 1). When WDI is left
unconnected, the watchdog timer is serviced within the
watchdog timeout period by a low-high-low pulse from
the counter chain. For minimum watchdog input current
(minimum overall power consumption), leave WDI low for
the majority of the watchdog timeout period, pulsing it lowhigh-low once within the first 7/8 of the watchdog timeout
period to reset the watchdog timer. If WDI is externally
driven high for the majority of the timeout period, up to
160µA can flow into WDI.
VTH
Since the RESET output on the MAX6822 is open drain,
it interfaces easily with µPs that have bidirectional reset
pins, such as the Motorola 68HC11. Connecting the µP
supervisor’s RESET output directly to the microcontroller’s RESET pin with a single pullup resistor allows
either device to assert reset (see Figure 4).
Negative-Going VCC Transients
These supervisors are relatively immune to short-duration, negative-going VCC transients (glitches), which
usually do not require the entire system to shut down.
Maxim Integrated │ 6
MAX6821/MAX6825
VCC
RESET*
Low-Voltage SOT23 µP Supervisors
with Manual Reset and Watchdog Timer
tRST
VCC
VCC
MAX6822
tRP
tWD
RESET
tRP
WDI
RESET
µP
RESET
GENERATOR
GND
GND
*RESET IS THE INVERSE OF RESET.
Figure 3. Watchdog Timing Relationship
Figure 4. Interfacing open-Drain RESET to µPs with
Bidirectional Reset I/O
Resets are issued to the µP during power-up, power-down, and brownout conditions. The Typical Operating
Characteristics show a graph of the MAX6821–MAX6825’s
Maximum VCC Transient Duration vs. Reset Threshold
Overdrive, for which reset pulses are not generated. The
graph was produced using negative-going VCC pulses,
starting at the standard monitored voltage and ending
below the reset threshold by the magnitude indicated
(reset threshold overdrive). The graph shows the maximum pulse width that a negative-going VCC transient
can typically have without triggering a reset pulse. As the
amplitude of the transient increases (i.e., goes farther
below the reset threshold), the maximum allowable pulse
width decreases. Typically, a VCC transient that goes
100mV below the reset threshold and lasts for 20µs or
less will not trigger a reset pulse.
Watchdog Software Considerations
One way to help the watchdog timer monitor software
execution more closely is to set and reset the watchdog
input at different points in the program, rather than pulsing
the watchdog input high-low-high or low-high-low. This
technique avoids a stuck loop, in which the watchdog
timer would continue to be reset inside the loop, keeping
the watchdog from timing out.
Figure 5 shows an example of a flow diagram where the
I/O driving the watchdog input is set high at the beginning of the program, set low at the beginning of every
subroutine or loop, then set high again when the program
returns to the beginning. If the program should hang in
any subroutine, the problem would quickly be corrected,
since the I/O is continually set low and the watchdog
timer is allowed to time out, causing a reset or interrupt
to be issued. As described in the Watchdog Input Current
section, this scheme results in higher time average WDI
input current than does leaving WDI low for the majority of
the timeout period and periodically pulsing it low-high-low.
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START
SET WDI
HIGH
PROGRAM
CODE
SUBROUTINE OR
PROGRAM LOOP
SET WDI LOW
RETURN
Figure 5. Watchdog Flow Diagram
Typical Operating Circuit
VCC
VCC
VCC
RESET
RESET
MAX6823
MANUAL
RESET
MR
µP
WDI
I/O
GND
Maxim Integrated │ 7
MAX6821/MAX6825
Low-Voltage SOT23 µP Supervisors
with Manual Reset and Watchdog Timer
Selector Guide
FUNCTION
ACTIVE-LOW
RESET
ACTIVE-HIGH
RESET
OPEN-DRAIN
RESET
WATCHDOG
INPUT
MANUAL
RESET INPUT
MAX6821
—
✔
—
✔
✔
MAX6822
—
—
✔
✔
✔
MAX6823
✔
—
—
✔
✔
MAX6824
✔
✔
—
✔
—
MAX6825
✔
✔
—
—
✔
Pin Configurations (continued)
TOP VIEW
RESET 1
GND 2
5
VCC
MAX6824
RESET 3
RESET 1
GND 2
4
WDI
TRANSISTOR COUNT: 750
PROCESS: BiCMOS
4
MR
SOT23-5
Package Information
For the latest package outline information and land patterns
(footprints), go to www.maximintegrated.com/packages. Note
that a “+”, “#”, or “-” in the package code indicates RoHS status
only. Package drawings may show a different suffix character, but
the drawing pertains to the package regardless of RoHS status.
PACKAGE
TYPE
5 SOT23
www.maximintegrated.com
VCC
MAX6825
RESET 3
SOT23-5
Chip Information
5
PACKAGE
OUTLINE NO.
CODE
U5-1
21-0057
LAND PATTERN
NO.
—
Maxim Integrated │ 8
MAX6821/MAX6825
Low-Voltage SOT23 µP Supervisors
with Manual Reset and Watchdog Timer
Revision History
REVISION
NUMBER
REVISION
DATE
3
7/14
PAGES
CHANGED
DESCRIPTION
No /V OPNs; removed automotive reference from Applications section
1
For pricing, delivery, and ordering information, please contact Maxim Direct at 1-888-629-4642, or visit Maxim Integrated’s website at www.maximintegrated.com.
Maxim Integrated cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim Integrated product. No circuit patent licenses
are implied. Maxim Integrated reserves the right to change the circuitry and specifications without notice at any time. The parametric values (min and max limits)
shown in the Electrical Characteristics table are guaranteed. Other parametric values quoted in this data sheet are provided for guidance.
Maxim Integrated and the Maxim Integrated logo are trademarks of Maxim Integrated Products, Inc.
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