General Description
The MAX6821–MAX6825 are low-voltage microprocessor
(µP) supervisory circuits that combine voltage monitor-
ing, watchdog timer, and manual reset input functions in
a 5-pin SOT23 package. Microprocessor supervisory cir-
cuits significantly improve system reliability and accura-
cy compared to separate ICs or discrete components.
These devices assert a reset signal whenever the moni-
tored 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 moni-
tors 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 guaran-
teed to be in the correct state for VCC down to +1.0V.
Nine preprogrammed reset threshold voltages are
available (see 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
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
MAX6821–MAX6825
Low-Voltage SOT23 µP Supervisors with
Manual Reset and Watchdog Timer
________________________________________________________________ Maxim Integrated Products 1
GND
WDIMR
15VCC
RESET(RESET)
(RESET) MAX6821 ONLY
MAX6821
MAX6822
MAX6823
SOT23-5
TOP VIEW
2
34
Pin Configurations
19-1868; Rev 1; 1/01
Ordering Information
Threshold Suffix Guide
*Insert the desired suffix letter (from the table below) into the blank
to complete the part number.
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.
PART* TEMP. RANGE PIN-PACKAGE
MAX6821_UK-T -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
SUFFIX 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
Typical Operating Circuit appears at end of data sheet.
Pin Configurations continued at end of data sheet.
Selector Guide appears at end of data sheet.
Set-Top Boxes
Computers and
Controllers
Embedded Controllers
Intelligent Instruments
Automotive Systems
Critical µP Monitoring
Portable/Battery-
Powered Equipment
For price, delivery, and to place orders, please contact Maxim Distribution at 1-888-629-4642,
or visit Maxim’s website at www.maxim-ic.com.
MAX6821–MAX6825
Low-Voltage SOT23 µP Supervisors with
Manual Reset and Watchdog Timer
2 _______________________________________________________________________________________
ABSOLUTE MAXIMUM RATINGS
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)
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.
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
PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS
TA = 0°C to +85°C 1.0 5.5
Operating Voltage Range VCC TA = -40°C to +125°C 1.2 5.5 V
TA = -40°C to +85°C1020
VCC = +5.5V, no load TA = -40°C to +125°C30
TA = -40°C to +85°C716
VCC = +3.6V, no load TA = -40°C to +125°C25
TA = -40°C to +85°C512
VCC Supply Current
(MR and WDI Unconnected) ICC
VCC = +3.6V, no load
(MAX6825 only) TA = -40°C to +125°C20
µA
TA = -40°C to +85°C 4.50 4.63 4.75
MAX682_L TA = -40°C to +125°C 4.47 4.63 4.78
TA = -40°C to +85°C 4.25 4.38 4.50
MAX682_M TA = -40°C to +125°C 4.22 4.38 4.53
TA = -40°C to +85°C 3.00 3.08 3.15
MAX682_T TA = -40°C to +125°C 2.97 3.08 3.17
TA = -40°C to +85°C 2.85 2.93 3.00
MAX682_S TA = -40°C to +125°C 2.83 2.93 3.02
TA = -40°C to +85°C 2.55 2.63 2.70
MAX682_R TA = -40°C to +125°C 2.53 2.63 2.72
TA = -40°C to +85°C 2.25 2.32 2.38
MAX682_Z TA = -40°C to +125°C 2.24 2.32 2.40
TA = -40°C to +85°C 2.12 2.19 2.25
MAX682_Y TA = -40°C to +125°C 2.11 2.19 2.27
TA = -40°C to +85°C 1.62 1.67 1.71
MAX682_W TA = -40°C to +125°C 1.61 1.67 1.72
VCC Reset Threshold
(VCC Falling) VTH
MAX682_V TA = -40°C to +85°C 1.52 1.58 1.62
V
Reset Threshold Temperature
Coefficient 60 p p m/°C
Reset Threshold Hysteresis 2 ✕ VTH mV
VCC to Reset Output Delay tRD VCC = VTH to (VTH - 100mV) 20 µs
MAX6821–MAX6825
Low-Voltage SOT23 µP Supervisors with
Manual Reset and Watchdog Timer
_______________________________________________________________________________________ 3
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)
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.
PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS
TA = -40°C to +85°C 140 200 280
Reset Timeout Period tRP TA = -40°C to +125°C 100 320 ms
VCC ≥ 1.0V, ISINK = 50µA, reset asserted,
TA = 0°C to +85°C0.3
VCC ≥ 1.2V, ISINK = 100µA, reset asserted 0.3
VCC ≥ 2.55V, ISINK = 1.2mA, reset asserted 0.3
RESET Output LOW
(Push-Pull or Open-Drain) VOL
VCC ≥ 4.25V, ISINK = 3.2mA, reset asserted 0.4
V
VCC ≥ 1.8V, ISOURCE = 200µA, reset not
asserted 0.8 ✕ VCC
VCC ≥ 3.15V, ISOURCE = 500µA, reset not
asserted 0.8 ✕ VCC
RESET Output HIGH
(Push-Pull Only) VOH
VCC ≥ 4.75V, ISOURCE = 800µA, reset not
asserted 0.8 ✕ VCC
V
Open-Drain RESET Output
Leakage Current (Note 1) ILKG VCC > VTH, RESET not asserted 1.0 µA
VCC ≥ 1.0V, ISOURCE = 1µA, reset asserted,
TA = 0°C to +85°C 0.8 ✕ VCC
V
C C ≥ 1.50V , ISOURCE = 100µA, r eset asser ted 0.8 ✕ V C C
V
CC ≥ 2.55V , ISOURCE = 500µA, r eset asser ted 0.8 ✕ V C C
RESET Output HIGH
(Push-Pull Only) VOH
V
CC ≥ 4.25V , ISOURCE = 800µA, r eset asser ted 0.8 ✕ V C C
V
V
CC ≥ 1.8V , ISINK = 500µA, r eset not asser ted 0.3
V
CC ≥ 3.15V , ISINK = 1.2m A, r eset not asser ted 0.3
RE S E T Outp ut LO W
( P ush- P ul l Onl y) V OL
V
CC ≥ 4.75V , IS IN K = 3.2m A, r eset not asser ted 0.4
V
MANUAL RESET INPUT (MAX6821/MAX6822/MAX6823/MAX6825)
VIL 0.3 ✕ VCC
MR Input Voltage VIH 0.7 ✕ VCC
V
MR Minimum Input Pulse 1µs
MR Glitch Rejection 100 ns
MR to Reset Delay 200 ns
MR Pullup Resistance 25 50 75 kΩ
WATCHDOG INPUT (MAX6821/MAX6822/MAX6823/MAX6824)
TA = -40°C to +85°C 1.12 1.60 2.40
Watchdog Timeout Period tWD TA = -40°C to +125°C 0.80 2.60 s
WDI Pulse Width (Note 2) tWDI 50 ns
VIL 0.3 ✕ VCC
WDI Input Voltage VIH 0.7 ✕ VCC
V
WDI = VCC, time average 120 160
WDI Input Current IWDI WDI = 0, time average -20 -15 µA
MAX6821–MAX6825
Low-Voltage SOT23 µP Supervisors with
Manual Reset and Watchdog Timer
4 _______________________________________________________________________________________
Typical Operating Characteristics
(TA = +25°C, unless otherwise noted.)
3
5
4
7
6
9
8
10
12
11
13
-40 0 20 40-20 60 80 100 120
SUPPLY CURRENT vs.
TEMPERATURE
MAX6821 toc01
TEMPERATURE (°C)
SUPPLY CURRENT (µA)
VCC = +5V, L VERSION
VCC = +3.3V, S VERSION
VCC = +1.8V, V VERSION
150
170
160
190
180
210
200
220
240
230
250
-40 0 20 40-20 60 80 100 120
RESET TIMEOUT PERIOD vs.
TEMPERATURE
MAX6821 toc02
TEMPERATURE (°C)
RESET TIMEOUT PERIOD (ms)
0
10
5
20
15
30
25
35
-40 0 20-20 40 60 80 100 120
VCC TO RESET OUTPUT DELAY vs.
TEMPERATURE
MAX6821 toc03
TEMPERATURE (°C)
PROPAGATION DELAY (µs)
VCC FALLING, VCC = VTH - 100mV
1.0
1.2
1.1
1.4
1.3
1.6
1.5
1.7
1.9
1.8
2.0
-40 0 20 40-20 60 80 100 120
WATCHDOG TIMEOUT PERIOD vs.
TEMPERATURE
MAX6821 toc04
TEMPERATURE (°C)
WATCHDOG TIMEOUT PERIOD (s)
0.94
0.96
1.00
0.98
1.04
1.02
1.06
-40 0 20-20 40 60 80 100 120
NORMALIZED RESET THRESHOLD
DELAY vs. TEMPERATURE
MAX6821 toc05
TEMPERATURE (°C)
RESET THRESHOLD (V)
0
60
40
20
80
100
120
0806020 40 100 120 140 160 180 200
MAXIMUM VCC TRANSIENT DURATION
vs. RESET THRESHOLD OVERDRIVE
MAX6821 toc06
RESET THRESHOLD OVERDRIVE (mV), VTH - VCC
TRANSIENT DURATION (µs)
RESET OCCURS
ABOVE CURVE
0
0.10
0.05
0.20
0.15
0.30
0.25
0.35
0231 4567
VOLTAGE OUTPUT LOW vs. ISINK
MAX6821 toc07
ISINK (mA)
VOUT (V)
VCC = +2.9V
2.74
2.78
2.76
2.82
2.80
2.84
2.86
2.90
2.88
2.92
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
VOLTAGE OUTPUT HIGH vs. ISOURCE
MAX6821 toc08
ISOURCE (mA)
VOUT (V)
VCC = +2.9V
MAX6821–MAX6825
Low-Voltage SOT23 µP Supervisors with
Manual Reset and Watchdog Timer
_______________________________________________________________________________________ 5
Pin Description
PIN NUMBERS
MAX6821 MAX6822 MAX6823 MAX6824 MAX6825
PIN
NAME FUNCTION
1 1 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.
1 3 3 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.
2 2 2 2 2 GND Ground
3 3 3 4 MR
Active-Low Manual Reset Input. Internal 50kΩ pullup to VCC. 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.
4 4 4 4 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.
5 5 5 5 5 VCC Supply Voltage and Input for Reset Threshold Monitor
Detailed Description
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).
Manual Reset Input
Many µP-based products require manual reset capabil-
ity, 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
returns high. MR has an internal 50kΩpullup resistor,
so it can be left open if not used. This input can be dri-
ven 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 dri-
ven 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 moni-
tors 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).
Disable the watchdog function by leaving WDI uncon-
nected 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.
Applications Information
Watchdog Input Current
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
low-high-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 peri-
od, up to 160µA can flow into WDI.
Interfacing to µPs with
Bidirectional Reset Pins
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 microcon-
troller’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-dura-
tion, negative-going VCC transients (glitches), which
usually do not require the entire system to shut down.
MAX6821–MAX6825
Low-Voltage SOT23 µP Supervisors with
Manual Reset and Watchdog Timer
6 _______________________________________________________________________________________
Figure 1. Functional Diagram
Figure 2. Reset Timing Diagram
VCC
1V
1V
tRD
VTH VTH
tRD
RESET
GND
RESET
tRP
tRP
MAX6821–
MAX6825
VCC
VCC
WDI
(MAX6821– MAX6824)
1.26V
GND
RESET
(MAX6821/MAX6824/MAX6825)
RESET (MAX6822– MAX6825)
MR
(MAX6821/MAX6822/MAX6823,
MAX6825)
RESET
GENERATOR
WATCHDOG
TIMER
WATCHDOG
TRANSITION
DETECTOR
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 trigger-
ing 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 watch-
dog 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.
MAX6821–MAX6825
Low-Voltage SOT23 µP Supervisors with
Manual Reset and Watchdog Timer
_______________________________________________________________________________________ 7
Figure 3. Watchdog Timing Relationship Figure 4. Interfacing Open-Drain
RESET
to µPs with
Bidirectional Reset I/O
VCC
tWD tRP
tRP
tRST
WDI
RESET*
*RESET IS THE INVERSE OF RESET.
MAX6822
RESET
GENERATOR
GND GND
VCC VCC
RESET RESET
µP
Figure 5. Watchdog Flow Diagram
START
SET WDI
HIGH
PROGRAM
CODE
SUBROUTINE OR
PROGRAM LOOP
SET WDI LOW
RETURN
MAX6823 µP
VCC
VCC
VCC
GND
MANUAL
RESET
RESET
I/O
RESET
MR
WDI
Typical Operating Circuit
Chip Information
TRANSISTOR COUNT: 750
PROCESS: BiCMOS
Selector Guide
FUNCTION ACTIVE-LOW
RESET
ACTIVE-HIGH
RESET
OPEN-DRAIN
RESET
WATCHDOG
INPUT
MANUAL
RESET INPUT
MAX6821 — ✔ — ✔ ✔
MAX6822 — — ✔ ✔ ✔
MAX6823 ✔ — — ✔ ✔
MAX6824 ✔ ✔ — ✔ —
MAX6825 ✔ ✔ — — ✔
GND
WDIRESET
15VCC
RESET
MAX6824
SOT23-5
TOP VIEW
2
34
GND
MRRESET
15VCC
RESET
MAX6825
SOT23-5
2
34
Pin Configurations (continued)
MAX6821–MAX6825
Low-Voltage SOT23 µP Supervisors with
Manual Reset and Watchdog Timer
8 _______________________________________________________________________________________
Low-Voltage SOT23 µP Supervisors with
Manual Reset and Watchdog Timer
Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses are
implied. Maxim reserves the right to change the circuitry and specifications without notice at any time.
Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600 _____________________ 9
© 2001 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.
MAX6821–MAX6825
Package Information
SOT5L.EPS
