19-2440; Rev 4; 12/05
Low-Power, SC70/SOT µP Reset Circuits with
Capacitor-Adjustable Reset Timeout Delay
General Description
The MAX6340/MAX6421–MAX6426 low-power micro-
processor supervisor circuits monitor system voltages
from 1.6V to 5V. These devices perform a single function:
they assert a reset signal whenever the V
CC
supply volt-
age falls below its reset threshold. The reset output
remains asserted for the reset timeout period after V
CC
rises above the reset threshold. The reset timeout is exter-
nally set by a capacitor to provide more flexibility.
The MAX6421/MAX6424 have an active-low, push-
pull reset output. The MAX6422 has an active-high,
push-pull reset output and the MAX6340/MAX6423/
MAX6425/MAX6426 have an active-low, open-drain
reset output. The MAX6421/MAX6422/MAX6423 are
offered in 4-pin SC70 or SOT143 packages. The
MAX6340/MAX6424/MAX6425/MAX6426 are available
in 5-pin SOT23-5 packages.
Features
♦
Monitor System Voltages from 1.6V to 5V
♦
Capacitor-Adjustable Reset Timeout Period
♦
Low Quiescent Current (1.6µA typ)
♦
Three RESET Output Options
Push-Pull
RESET
Push-Pull RESET
Open-Drain
RESET
♦
Guaranteed Reset Valid to V
CC
= 1V
♦
Immune to Short V
CC
Transients
♦
Small 4-Pin SC70, 4-Pin SOT143, and 5-Pin SOT23
Packages
♦
MAX6340 Pin Compatible with LP3470
♦
MAX6424/MAX6425 Pin Compatible with
NCP300–NCP303, MC33464/MC33465,
S807/S808/S809, and RN5VD
♦
MAX6426 Pin Compatible with PST92XX
MAX6340/MAX6421–MAX6426
Applications
Portable Equipment
Battery-Powered Computers/Controllers
Automotive
Medical Equipment
Intelligent Instruments
Embedded Controllers
Critical µP Monitoring
Set-Top Boxes
Computers
Typical Operating Circuit appears at end of data sheet.
Ordering Information
PART
MAX6340UK_
_-T
MAX6421XS_
_-T
MAX6421US_ _-T
MAX6422XS_
_-T
MAX6422US_ _-T
MAX6423XS_
_-T
MAX6423US_ _-T
MAX6424UK_
_-T
MAX6425UK_
_-T
MAX6426UK_
_-T
TEMP RANGE
-40°C to +125°C
-40°C to +125°C
-40°C to +125°C
-40°C to +125°C
-40°C to +125°C
-40°C to +125°C
-40°C to +125°C
-40°C to +125°C
-40°C to +125°C
-40°C to +125°C
PIN-PACKAGE
5 SOT23-5
4 SC70-4
4 SOT143-4
4 SC70-4
4 SOT143-4
4 SC70-4
4 SOT143-4
5 SOT23-5
5 SOT23-5
5 SOT23-5
Pin Configurations
TOP VIEW
V
CC
1
4
RESET
(RESET)
MAX6421X
MAX6422X
MAX6423X
GND
2
3
SRT
Note:
The MAX6340/MAX6421–MAX6426 are available with fac-
tory-trimmed reset thresholds from 1.575V to 5.0V in approxi-
mately 0.1V increments. Insert the desired nominal reset
threshold suffix (from Table 1) into the blanks. There are 50 stan-
dard versions with a required order increment of 2500 pieces.
Sample stock is generally held on standard versions only (see
Standard Versions Table). Required order increment is 10,000
pieces for nonstandard versions. Contact factory for availability.
All devices are available in tape-and-reel only.
Devices are available in both leaded and lead-free packaging.
Specify lead-free by replacing "-T" with "+T" when ordering.
Selector Guide appears at end of data sheet.
1
SC70
( ) ARE FOR THE MAX6422
Pin Configurations continued at end of data sheet.
________________________________________________________________
Maxim Integrated Products
For pricing, delivery, and ordering information, please contact Maxim/Dallas Direct! at
1-888-629-4642, or visit Maxim’s website at www.maxim-ic.com.
Low-Power, SC70/SOT µP Reset Circuits with
Capacitor-Adjustable Reset Timeout Delay
MAX6340/MAX6421–MAX6426
ABSOLUTE MAXIMUM RATINGS
All Voltages Referenced to GND
V
CC
........................................................................-0.3V to +6.0V
SRT,
RESET,
RESET (push-pull).................-0.3V to (V
CC
+ 0.3V)
RESET
(open drain)...............................................-0.3V to +6.0V
Input Current (all pins)......................................................±20mA
Output Current (RESET, RESET) ......................................±20mA
Continuous Power Dissipation (T
A
= +70°C)
4-Pin SC70 (derate 3.1mW/°C above +70°C) ..............245mW
4-Pin SOT143 (derate 4mW/°C above +70°C).............320mW
5-Pin SOT23 (derate 7.1mW/°C above +70°C)............571mW
Operating Temperature Range .........................-40°C to +125°C
Storage Temperature Range .............................-65°C to +150°C
Junction Temperature ......................................................+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
(V
CC
= 1V to 5.5V, T
A
= T
MIN
to T
MAX
, unless otherwise specified. Typical values are at V
CC
= 5V and T
A
= +25°C.) (Note 1)
PARAMETER
Supply Voltage Range
Supply Current
SYMBOL
V
CC
V
CC
≤
5.0V
I
CC
V
CC
≤
3.3V
V
CC
≤
2.0V
V
CC
Reset Threshold Accuracy
Hysteresis
V
CC
to Reset Delay
Reset Timeout Period
V
SRT
Ramp Current
V
SRT
Ramp Threshold
RAMP Threshold Hysteresis
RESET
Output Voltage Low
V
OL
V
TH
V
HYST
t
RD
t
RP
I
RAMP
V
TH-RAMP
V
CC
falling at 1mV/µs
C
SRT
= 1500pF
C
SRT
= 0
V
SRT
= 0 to 0.65V; V
CC
= 1.6V to 5V
V
CC
= 1.6V to 5V (V
RAMP
rising)
V
RAMP
falling threshold
V
CC
≥
1.0V, I
SINK
= 50µA
V
CC
≥
2.7V, I
SINK
= 1.2mA
V
CC
≥
4.5V, I
SINK
= 3.2mA
RESET
Output Voltage High,
Push-Pull
RESET
Output Leakage Current,
Open-Drain
V
CC
≥
1.8V, I
SOURCE
= 200µA
V
OH
V
CC
≥
2.25V, I
SOURCE
= 500µA
V
CC
≥
4.5V, I
SOURCE
= 800µA
I
LKG
V
CC
> V
TH
, reset not asserted
V
CC
≥
1.0V, I
SOURCE
= 1µA
RESET Output Voltage High
V
OH
V
CC
≥
1.8V, I
SOURCE
= 150µA
V
CC
≥
2.7V, I
SOURCE
= 500µA
V
CC
≥
4.5V, I
SOURCE
= 800µA
V
CC
≥
1.8V, I
SINK
= 500µA
RESET Output Voltage Low
V
OL
V
CC
≥
2.7V, I
SINK
= 1.2mA
V
CC
≥
4.5V, I
SINK
= 3.2mA
0.8 x V
CC
0.8 x V
CC
0.8 x V
CC
0.8 x V
CC
0.3
0.3
0.4
V
V
0.8 x V
CC
0.8 x V
CC
0.8 x V
CC
1.0
µA
V
3.00
T
A
= +25°C
T
A
= -40°C to +125°C
V
TH
- 1.5%
V
TH
- 2.5%
80
4.375
0.275
240
0.65
33
0.3
0.3
0.4
V
5.75
CONDITIONS
MIN
1.0
2.5
1.9
1.6
TYP
MAX
5.5
4.2
3.4
2.5
V
TH
+ 1.5%
V
TH
+ 2.5%
V
mV
µs
ms
nA
V
mV
µA
UNITS
V
4 x V
TH
Note 1:
Devices production tested at +25°C. Overtemperature limits are guaranteed by design.
2
_______________________________________________________________________________________
Low-Power, SC70/SOT µP Reset Circuits with
Capacitor-Adjustable Reset Timeout Delay
Typical Operating Characteristics
(V
CC
= 5V, C
SRT
= 1500pF, T
A
= +25°C, unless otherwise noted.)
MAX6340/MAX6421–MAX6426
SUPPLY CURRENT
vs. SUPPLY VOLTAGE
MAX6421/26 toc01
RESET TIMEOUT PERIOD vs. C
SRT
MAX6421/26 toc02
RESET TIMEOUT PERIOD
vs. TEMPERATURE
C
SRT
= 1500pF
RESET TIMEOUT PERIOD (ms)
4.25
MAX6421/26 toc03
MAX6421/26 toc06
4.0
3.5
SUPPLY CURRENT (µA)
3.0
2.5
2.0
1.5
1.0
0.5
0
0
1
2
3
4
5
6
SUPPLY VOLTAGE (V)
T
A
= -40°C
T
A
= +25°C
T
A
= +125°C
10,000
4.30
RESET TIMEOUT PERIOD (ms)
1000
100
4.20
10
4.15
1
0.1
0.001
4.10
0.01
0.1
1
C
SRT
(nF)
10
100
1000
-50
-25
0
25
50
75
100
125
TEMPERATURE (°C)
RESET TIMEOUT PERIOD
vs. TEMPERATURE
MAX6421/26 toc04
MAXIMUM TRANSIENT DURATION
vs. RESET THRESHOLD OVERDRIVE
MAX6421/26 toc05
V
CC
TO RESET DELAY
vs. TEMPERATURE (V
CC
FALLING)
160
V
CC
FALLING AT 1mVµs
150
V
CC
TO RESET DELAY (µs)
140
130
120
110
100
90
80
1000
-50
-25
0
75
50
TEMPERATURE (°C)
25
100
125
600
550
RESET TIMEOUT PERIOD (µs)
500
450
400
350
300
250
200
-50
-25
0
25
50
75
100
C
SRT
= 0
175
150
TRANSIENT DURATION (µs)
125
100
75
50
25
V
TH
= 2.95V
0
RESET OCCURS
ABOVE THE CURVE
125
0
200
400
600
800
TEMPERATURE (°C)
RESET THRESHOLD OVERDRIVE (mV)
POWER-UP/POWER-DOWN
CHARACTERISTIC
MAX6421/26 toc07
NORMALIZED RESET THRESHOLD
vs. TEMPERATURE
MAX6421/26 toc08
NORMALIZED RESET THRESHOLD
V
CC
V
TH
= 1.6V
1.006
1.004
1.002
1.000
0.998
0.996
0.994
1V/div
RESET
1V/div
400µs/div
-50
-25
0
25
50
75
100
125
TEMPERATURE (°C)
_______________________________________________________________________________________
3
Low-Power, SC70/SOT µP Reset Circuits with
Capacitor-Adjustable Reset Timeout Delay
MAX6340/MAX6421–MAX6426
Pin Description
PIN
MAX6340
SOT23
MAX6421
MAX6422
MAX6423
SOT143
SC70
MAX6424
MAX6425
SOT23
MAX6426
SOT23
Set Reset Timeout Input. Connect a capacitor between SRT
and ground to set the timeout period. Determine the period as
follows: t
RP
= 2.73
✕
10
6
✕
C
SRT
+ 275µs with t
RP
in seconds
and C
SRT
in farads.
Ground
Not Internally Connected. Can be connected to GND.
Supply Voltage and Reset Threshold Monitor Input
RESET
changes from high to low whenever V
CC
drops below
the selected reset threshold voltage.
RESET
remains low for
the reset timeout period after V
CC
exceeds the reset
threshold.
RESET changes from low to high whenever V
CC
drops below
the selected reset threshold voltage. RESET remains high for
the reset timeout period after V
CC
exceeds the reset
threshold.
3.3V
NAME
FUNCTION
1
3
3
5
1
SRT
2
3
4
1
—
2
2
—
1
3
4
2
2, 3
—
5
GND
N.C.
V
CC
5
4
—
4
1
4
RESET
—
—
RESET
Detailed Description
Reset Output
The reset output is typically connected to the reset input
of a µP. A µP’s reset input starts or restarts the µP in a
known state. The MAX6340/MAX6421–MAX6426 µP
supervisory circuits provide the reset logic to prevent
code-execution errors during power-up, power-down,
and brownout conditions (see
Typical Operating
Characteristics).
RESET
changes from high to low whenever V
CC
drops
below the threshold voltage. Once V
CC
exceeds the
threshold voltage,
RESET
remains low for the capacitor-
adjustable reset timeout period.
The MAX6422 active-high RESET output is the inverse
logic of the active-low
RESET
output. All device outputs
are guaranteed valid for V
CC
> 1V.
The MAX6340/MAX6423/MAX6425/MAX6426 are open-
drain
RESET
outputs. Connect an external pullup resis-
tor to any supply from 0 to 5.5V. Select a resistor value
large enough to register a logic low when
RESET
is
asserted and small enough to register a logic high
while supplying all input current and leakage paths
connected to the
RESET
line. A 10kΩ to 100kΩ pullup
is sufficient in most applications.
V
CC
LASER-TRIMMED
RESISTORS
5.0V
V
REF
RESET
10kΩ
5V
SYSTEM
SRT
C
SRT
RESET
TIMEOUT
N
MAX6340
MAX6423
MAX6425
MAX6426
GND
Figure 1. MAX6340/MAX6423/MAX6425/MAX6426 Open-Drain
RESET
Output Allows Use with Multiple Supplies
Selecting a Reset Capacitor
The reset timeout period is adjustable to accommodate
a variety of µP applications. Adjust the reset timeout
period (t
RP
) by connecting a capacitor (C
SRT
) between
SRT and ground. Calculate the reset timeout capacitor
as follows:
4
_______________________________________________________________________________________
Low-Power, SC70/SOT µP Reset Circuits with
Capacitor-Adjustable Reset Timeout Delay
C
SRT
= (t
RP
- 275µs) / (2.73
✕
10
6
)
where t
RP
is in seconds and C
SRT
is in farads.
The reset delay time is set by a current/capacitor-con-
trolled ramp compared to an internal 0.65V reference.
An internal 240nA ramp current source charges the
external capacitor. The charge to the capacitor is
cleared when a reset condition is detected. Once the
reset condition is removed, the voltage on the capacitor
ramps according to the formula: dV/dt = I/C. The C
SRT
capacitor must ramp to 0.65V to deassert the reset.
C
SRT
must be a low-leakage (<10nA) type capacitor;
ceramic is recommended.
Applications Information
Interfacing to Other Voltages for Logic
Compatibility
The open-drain outputs of the MAX6340/MAX6423/
MAX6425/MAX6426 can be used to interface to µPs with
other logic levels. As shown in Figure 1, the open-drain
output can be connected to voltages from 0 to 5.5V. This
allows for easy logic compatibility to various µPs.
MAX6340/MAX6421–MAX6426
Wired-OR Reset
To allow auxiliary circuitry to hold the system in reset,
an external open-drain logic signal can be connected
to the open-drain
RESET
of the MAX6340/MAX6423/
MAX6425/MAX6426, as shown in Figure 2. This config-
uration can reset the µP, but does not provide the reset
timeout when the external logic signal is released.
Operating as a Voltage Detector
The MAX6340/MAX6421–MAX6426 can be operated in a
voltage detector mode by floating the SRT pin. The reset
delay times for V
CC
rising above or falling below the
threshold are not significantly different. The reset output is
deasserted smoothly without false pulses.
V
CC
V
DD
10kΩ
MAX6340
MAX6423
MAX6425
MAX6426
RESET
N
GND
µP
Negative-Going V
CC
Transients
In addition to issuing a reset to the µP during power-up,
power-down, and brownout conditions, these supervisors
are relatively immune to short-duration negative-going
transients (glitches). The graph Maximum Transient
Duration vs. Reset Threshold Overdrive in the
Typical
Operating Characteristics
shows this relationship.
The area below the curve of the graph is the region in
which these devices typically do not generate a reset
pulse. This graph was generated using a negative-
going pulse applied to V
CC
, starting above the actual
reset threshold (V
TH
) and ending below it by the magni-
tude indicated (reset-threshold overdrive). As the mag-
nitude of the transient decreases (farther below the
reset threshold), the maximum allowable pulse width
decreases. Typically, a V
CC
transient that goes 100mV
below the reset threshold and lasts 50µs or less does
not cause a reset pulse to be issued.
Ensuring a Valid RESET or
RESET
Down to V
CC
= 0
RESET
OPEN-DRAIN
LOGIC
N
Figure 2. Wired-OR Reset Circuit
When V
CC
falls below 1V,
RESET/RESET
current-sink-
ing (sourcing) capabilities decline drastically. In the
case of the MAX6421/MAX6424, high-impedance
CMOS-logic inputs connected to
RESET
can drift to
undetermined voltages. This presents no problems in
most applications, since most µPs and other circuitry
do not operate with V
CC
below 1V.
In those applications where
RESET
must be valid down
to zero, adding a pulldown resistor between
RESET
and ground sinks any stray leakage currents, holding
RESET
low (Figure 3). The value of the pulldown resis-
tor is not critical; 100kΩ is large enough not to load
RESET
and small enough to pull
RESET
to ground. For
applications using the MAX6422, a 100kΩ pullup resis-
5
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