_______________General Description
The MAX921–MAX924 single, dual, and quad micro-
power, low-voltage comparators feature the lowest
power consumption available. These comparators draw
less than 4µA supply current over temperature
(MAX921/MAX922), and include an internal 1.182V
±1% voltage reference, programmable hysteresis, and
TTL/CMOS outputs that sink and source current.
Ideal for 3V or 5V single-supply applications, the
MAX921–MAX924 operate from a single +2.5V to +11V
supply (or a ±1.25V to ±5V dual supply), and each
comparator’s input voltage range swings from the
negative supply rail to within 1.3V of the positive
supply.
The MAX921–MAX924’s unique output stage con-
tinuously sources as much as 40mA. And by eliminating
power-supply glitches that commonly occur when com-
parators change logic states, the MAX921–MAX924
minimize parasitic feedback, which makes them easier to
use.
The single MAX921 and dual MAX923 provide a unique
and simple method for adding hysteresis without
feedback and complicated equations, simply by using
the HYST pin and two resistors.
________________________Applications
Battery-Powered Systems
Threshold Detectors
Window Comparators
Oscillator Circuits
____________________________Features
♦µMAX®Package—Smallest 8-Pin SO
(MAX921/MAX922/MAX923)
♦Ultra-Low 4µA Max Quiescent Current
Over Extended Temp. Range (MAX921)
♦Power Supplies:
Single +2.5V to +11V
Dual ±1.25V to ±5.5V
♦Input Voltage Range Includes Negative Supply
♦Internal 1.182V ±1% Bandgap Reference
♦Adjustable Hysteresis
♦TTL/CMOS-Compatible Outputs
♦12µs Propagation Delay (10mV Overdrive)
♦No Switching Crowbar Current
♦40mA Continuous Source Current
Ordering Information continued at end of data sheet.
*Dice are tested at TA= +25°C, DC parameters only.
**Contact factory for availability.
MAX921–MAX924
Ultra Low-Power,
Single/Dual-Supply Comparators
________________________________________________________________ Maxim Integrated Products 1
MAX921
OUT
IN+
HYST
REF
V- GND
V+
21
4
5
6
8
3
7
IN-
THRESHOLD DETECTOR
VIN
__________Typical Operating Circuit
19-0115; Rev 6; 4/09
PART TEMP RANGE PIN-PACKAGE
MAX921CPA 0°C to +70°C 8 Plastic DIP
MAX921CSA 0°C to +70°C 8 SO
MAX921C/D 0°C to +70°C Dice*
PART
INTERNAL 1%
PRECISION
REFERENCE
COMPARATORS
PER
PACKAGE
INTERNAL
HYSTERESIS
PACKAGE
MAX921 Yes 1 Yes 8-Pin
DIP/SO/µMAX
MAX922 No 2 No 8-Pin
DIP/SO/µMAX
MAX923 Yes 2 Yes 8-Pin
DIP/SO/µMAX
MAX924 Yes 4 No 16-Pin
DIP/SO/µMAX
MAX921EPA -40°C to +85°C 8 Plastic DIP
MAX921ESA -40°C to +85°C 8 SO
MAX921MJA -55°C to +125°C 8 CERDIP
MAX921CUA 0°C to +70°C 8 µMAX
For pricing, delivery, and ordering information, please contact Maxim Direct at 1-888-629-4642,
or visit Maxim's website at www.maxim-ic.com.
Ordering Information
µMAX is a registered trademark of Maxim Integrated Products, Inc.
MAX921–MAX924
Ultra Low-Power,
Single/Dual-Supply Comparators
2 _______________________________________________________________________________________
ABSOLUTE MAXIMUM RATINGS
V+ to V-, V+ to GND, GND to V-................................-0.3V, +12V
Inputs
Current, IN_+, IN_-, HYST...............................................20mA
Voltage, IN_+, IN_-, HYST................(V+ + 0.3V) to (V- – 0.3V)
Outputs
Current, REF....................................................................20mA
Current, OUT_ .................................................................50mA
Voltage, REF ....................................(V+ + 0.3V) to (V- – 0.3V)
Voltage, OUT_ (MAX921/924) .....(V+ + 0.3V) to (GND – 0.3V)
Voltage OUT_ (MAX922/923)...........(V+ + 0.3V) to (V- – 0.3V)
OUT_ Short-Circuit Duration (V+ ≤5.5V) ...............Continuous
Continuous Power Dissipation (TA= +70°C)
8-Pin Plastic DIP (derate 9.09mW/°C above +70°C) ...727mW
8-Pin SO (derate 5.88mW/°C above +70°C)................471mW
8-Pin µMAX (derate 4.1mW/°C above +70°C) .............330mW
8-Pin CERDIP (derate 8.00mW/°C above +70°C)........640mW
16-Pin Plastic DIP (derate 10.53mW/°C above +70°C)..842mW
16-Pin SO (derate 8.70mW/°C above +70°C) ................696mW
16-Pin CERDIP (derate 10.00mW/°C above +70°C) ......800mW
Operating Temperature Ranges:
MAX92_C_ _ .......................................................0°C to +70°C
MAX92_E_ _.....................................................-40°C to +85°C
MAX92_MJ_ ..................................................-55°C to +125°C
Storage Temperature Range .............................-65°C to +150°C
Lead Temperature (soldering, 10s) .................................+300°C
ELECTRICAL CHARACTERISTICS: 5V OPERATION
(V+ = 5V, V- = GND = 0V, TA= TMIN to TMAX, unless otherwise noted.)
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.
PARAMETER
MAX923,
HYST = REF
MIN TYP MAX UNITS
MAX924
7.5
VV- V+ – 1.3
5.5 6.5
MAX921,
HYST = REF
8.5
Supply Current IN+ = IN- + 100mV
IN+ = IN- = 2.5V
11
µA
Supply Voltage Range
TA= +25°C, 100pF load
2.5 11 V
2.5 3.2
MAX922
4
5
2.5 3.2
Input Offset Voltage
CONDITIONS
M temp. range
±10 mV
TA= +25°C
C/E temp. ranges
M temp. range
±0.01 ±5 nAInput Leakage Current (IN-, IN+)
(Note 1)
±40
Input Leakage Current (HYST)
TA= +25°C
C/E temp. ranges
M temp. range
±0.02 nA
Input Common-Mode Voltage Range
TA= +25°C
VCM = 2.5V
C/E temp. ranges
Common-Mode Rejection Ratio
M temp. range
MAX921, MAX923
0.1 1.0 mV/V
Power-Supply Rejection Ratio
V- to (V+ – 1.3V)
V+ = 2.5V to 11V
100Hz to 100kHz
0.1 1.0 mV/V
Voltage Noise
MAX921, MAX923
Overdrive = 10mV
Overdrive = 100mV
20 µVRMS
Hysteresis Input Voltage Range REF- 0.05V REF V
C/E temp. ranges
M temp. range
TA= +25°C
12 µsResponse Time
C/E temp. ranges
4
4
5
3.1 4.5
6
POWER REQUIREMENTS
COMPARATOR
MAX921–MAX924
Ultra Low-Power,
Single/Dual-Supply Comparators
_______________________________________________________________________________________ 3
PARAMETER
Voltage Noise
C/E temp. ranges: IOUT = 17mA;
M temp. range: IOUT = 10mA
MIN TYP MAX UNITS
100Hz to 100kHz 100 µVRMS
MAX922/
MAX923
C/E temp. ranges: IOUT = 1.8mA;
M temp. range: IOUT = 1.2mA
C/E temp. ranges: IOUT = 1.8mA;
M temp. range: IOUT = 1.2mA
CONDITIONS
C temp. range
VOutput High Voltage
E temp. range
MAX92_
M temp. range
V+ – 0.4
TA= +25°C
V- + 0.4
V
15 25
µAC/E temp. ranges 6Source Current
M temp. range 4
TA= +25°C
GND + 0.4
Output Low Voltage
815
MAX921/
MAX924
µA
X
C/E temp. ranges 4
Sink Current
1.170 1.182 1.194
V
M temp. range
1.158 1.206Reference Voltage
2
1.147 1.217
ELECTRICAL CHARACTERISTICS: 5V OPERATION (continued)
(V+ = 5V, V- = GND = 0V, TA= TMIN to TMAX, unless otherwise noted.)
ELECTRICAL CHARACTERISTICS: 3V OPERATION
(V+ = 3V, V- = GND = 0V, TA= TMIN to TMAX, unless otherwise noted.)
PARAMETER
MAX923
MIN TYP MAX UNITS
MAX924
7.2
5.2 6.2
MAX921
8.0
Supply Current HYST = REF,
IN+ = (IN- + 100mV)
IN+ = IN- = 1.5V
10.5
µA
2.4 3.0
MAX922
3.8
4.8
2.4 3.0
Input Offset Voltage
CONDITIONS
M temp. range
±10 mV
TA= +25°C
C/E temp. ranges
M temp. range
±0.01 ±5 nAInput Leakage Current (IN-, IN+) ±40
Input Leakage Current (HYST)
TA= +25°C
C/E temp. ranges
M temp. range
±0.02 nA
TA= +25°C
VCM = 1.5V
C/E temp. ranges
M temp. range
MAX921, MAX923
C/E temp. ranges
M temp. range
TA= +25°C
C/E temp. ranges
3.8
4.8
3.4 4.3
5.8
REFERENCE (MAX921/MAX923/MAX924 ONLY)
POWER REQUIREMENTS
COMPARATOR
Note 1: MAX924 comparators work below 2.5V, see Low-Voltage Operation section for more details.
MAX921–MAX924
Ultra Low-Power,
Single/Dual-Supply Comparators
4 _______________________________________________________________________________________
Typical Operating Characteristics
(V+ = 5V, V- = GND, TA = +25°C, unless otherwise noted).
0.0
020
OUTPUT VOLTAGE LOW
vs. LOAD CURRENT
0.5
2.5
MAX921/4-TOC1
LOAD CURRENT (mA)
VOL (V)
12
1.5
1.0
48 16
2.0
V+ = 5V
V+ = 3V
1.5
010 30 50
OUTPUT VOLTAGE HIGH vs.
LOAD CURRENT
2.5
4.5
MAX921/924-TOC2
LOAD CURRENT (mA)
VOH (V)
20 40
3.5
5.0
2.0
4.0
3.0
V+ = 5V
V+ = 3V
1.155
0 5 15 25
REFERENCE OUTPUT VOLTAGE vs.
OUTPUT LOAD CURRENT
1.165
1.185
MAX921/924-TOC3
OUTPUT LOAD CURRENT (μA)
REFERENCE OUTPUT VOLTAGE (V)
10 20
1.175
1.190
1.160
1.180
1.170
V+ = 5V
OR
V+ = 3V
30
SINK
SOURCE
ELECTRICAL CHARACTERISTICS: 3V OPERATION (continued)
(V+ = 3V, V- = GND = 0V, TA= TMIN to TMAX, unless otherwise noted.)
PARAMETER
Voltage Noise
C/E temp. ranges: IOUT = 10mA;
M temp. range: IOUT = 6mA
MIN TYP MAX UNITS
100Hz to 100kHz 100 µVRMS
Input Common-Mode Voltage Range VV- V+ – 1.3
MAX922/
MAX923
TA= +25°C, 100pF load
C/E temp. ranges: IOUT = 0.8mA;
M temp. range: IOUT = 0.6mA
C/E temp. ranges: IOUT = 0.8mA;
M temp. range: IOUT = 0.6mA
CONDITIONS
Common-Mode Rejection Ratio 0.2 1 mV/V
Power-Supply Rejection Ratio
V- to (V+ – 1.3V)
V+ = 2.5V to 11V
100Hz to 100kHz
0.1 1 mV/V
Voltage Noise
MAX921, MAX923
Overdrive = 10mV
Overdrive = 100mV
20 µVRMS
Hysteresis Input Voltage Range REF- 0.05V REF V
14 µsResponse Time 5
C temp. range
VOutput High Voltage
E temp. range
MAX92_
M temp. range
V+ – 0.4
TA= +25°C
V- + 0.4
V
15 25
µA
C/E temp. ranges 6
Source Current
M temp. range 4
TA= +25°C
GND + 0.4
Output Low Voltage
815
MAX921/
MAX924
µA
C/E temp. ranges 4
Sink Current
1.170 1.182 1.194
V
M temp. range
1.158 1.206Reference Voltage
2
1.147 1.217
REFERENCE
MAX921-MAX924
Ultra Low-Power,
Single/Dual-Supply Comparators
_______________________________________________________________________________________ 5
1.22
1.14
-60 -20 60 140
REFERENCE VOLTAGE
vs. TEMPERATURE
1.16
1.20
MAX921/924-TOC4
TEMPERATURE (°C)
REFERENCE VOLTAGE (V)
20 100
1.18
1.21
1.15
1.19
1.17
-40 0 8040 120
MILITARY TEMP. RANGE
EXTENDED TEMP. RANGE
COMMERCIAL
TEMP. RANGE
2.0
-60 140
MAX921
SUPPLY CURRENT vs.
TEMPERATURE
2.5
4.5
MAX921/924-TOC5
TEMPERATURE (°C)
SUPPLY CURRENT (μA)
60
3.5
3.0
-20 20 100
4.0
V+ = 5V, V- = - 5V
V+ = 3V, V- = 0V
V+ = 5V, V- = 0V
IN+ = (IN- + 100mV)
4.5
1.5
-60 140
MAX922
SUPPLY CURRENT vs. TEMPERATURE
2.0
4.0
MAX921/924-TOC6
TEMPERATURE (°C)
SUPPLY CURRENT (μA)
60
3.0
2.5
-20 20 100
3.5
IN+ = (IN- + 100mV)
V+ = 5V, V- = 0V
V+ = 3V, V- = 0V
V+ = 10V, V- = 0V
4.5
5.0
-60 140
MAX923
SUPPLY CURRENT vs. TEMPERATURE
2.0
4.0
MAX921/924-TOC7
TEMPERATURE (°C)
SUPPLY CURRENT (μA)
60
3.0
2.5
-20 20 100
3.5
V+ = 5V, V- = 0V
V+ = 3V, V- = 0V
80
-80
010 30 50
HYSTERESIS CONTROL
-40
40
MAX921/924 TOC10
VREF -VHYST (mV)
IN+ – IN- (mV)
20 40
0
60
-60
20
-20
NO CHANGE
OUTPUT LOW
OUTPUT HIGH
10
3
-60 140
MAX924
SUPPLY CURRENT vs. TEMPERATURE
4
8
MAX921/924-TOC8
TEMPERATURE (°C)
SUPPLY CURRENT (μA)
60
6
5
-20 20 100
7
9
IN+ = (IN- + 100mV)
V+ = 5V, V- = 0V
V+ = 3V, V- = 0V
V+ = 5V, V- = -5V
10
0.01
1.0 2.0 2.5
MAX924
SUPPLY CURRENT vs.
LOW SUPPLY VOLTAGES
0.1
1
MAX921/924-TOC9
SINGLE-SUPPLY VOLTAGE (V)
SUPPLY CURRENT (μA)
1.5
5.0
0
-0.3 -0.1 0.3
TRANSFER FUNCTION
1.0
4.0
MAX921/924-TOC11
IN+ INPUT VOLTAGE (mV)
OUTPUT VOLTAGE (V)
0.1
3.0
2.0
4.5
0.5
3.5
2.5
1.5
-0.2 0 0.2
100k V0
10μF
18
2
0 20 60 100
RESPONSE TIME vs.
LOAD CAPACITANCE
6
14
MAX921/924 TOC12
LOAD CAPACITANCE (nF)
RESPONSE TIME (μs)
40 80
10
16
4
12
8
VOLH
V- = 0V
VOHL
Typical Operating Characteristics (continued)
(V+ = 5V, V- = GND, TA = +25°C, unless otherwise noted).
MAX921–MAX924
Ultra Low-Power,
Single/Dual-Supply Comparators
6 _______________________________________________________________________________________
Typical Operating Characteristics (continued)
(V+ = 5V, V- = GND, TA = +25°C, unless otherwise noted).
-2 2 10 18
RESPONSE TIME FOR VARIOUS
INPUT OVERDRIVES
0
4
MAX921/924-TOC13
RESPONSE TIME (μs)
OUTPUT VOLTAGE (V)
614
2
0
5
100
3
150mV
100mV
20mV
10mV
INPUT VOLTAGE (mV)
-2 2 10 18
RESPONSE TIME FOR VARIOUS
INPUT OVERDRIVES
0
4
MAX921/924-TOC14
RESPONSE TIME (μs)
OUTPUT VOLTAGE (V)
614
2
0
5
100
3
1
50mV
100mV
20mV
10mV
INPUT VOLTAGE (mV)
10
0.01
1.0 2.0 2.5
MAX924 RESPONSE TIME
AT LOW SUPPLY VOLTAGES
0.1
1
MAX921/924-TOC15
SINGLE-SUPPLY VOLTAGE (V)
RESPONSE TIME (ms)
1.5
±20mV OVERDRIVE
±100mV
OVERDRIVE
100
0.1
1.0 2.0 2.5
MAX924 RESPONSE TIME
AT LOW SUPPLY VOLTAGES
1
10
MAX921/924-TOC16
SINGLE-SUPPLY VOLTAGE (V)
CURRENT (mA)
1.5
SINK CURRENT AT VOUT = 0.4V
SOURCE CURRENT INTO 0.75V LOAD
200
0
0 1.0 3.0 5.0
SHORT-CIRCUIT SOURCE CURRENT
vs. SUPPLY VOLTAGE
40
160
MAX121/124-TOC17
TOTAL SUPPLY VOLTAGE (V)
SOURCE CURRENT (mA)
2.0 4.0
120
80
20
60
180
140
100
OUT CONNECTED TO V-
20
0
010
SHORT-CIRCUIT SINK CURRENT
vs. SUPPLY VOLTAGE
MAX121/124-TOC18
TOTAL SUPPLY VOLTAGE (V)
SINK CURRENT (mA)
5
10
OUT CONNECTED TO V+
GND CONNECTED TO V-
PIN
MAX921 MAX922 MAX923 NAME FUNCTION
1 – – GND Ground. Connect to V- for single-supply operation. Output swings from V+ to GND.
– 1 OUTA Comparator A output. Sinks and sources current. Swings from V+ to V-.
2 2 2 V- Negative supply. Connect to ground for single-supply operation (MAX921).
1
3 – – IN+ Noninverting comparator input
– 3 3 INA+ Noninverting input of comparator A
4 – – IN- Inverting comparator input
____________________________________________________________Pin Descriptions
– 4 – INA- Inverting input of comparator A
MAX921–MAX924
Ultra Low-Power,
Single/Dual-Supply Comparators
_______________________________________________________________________________________ 7
PIN
–
– 6 –
MAX921 MAX922 MAX923 NAME FUNCTION
– 5 4 INB- Inverting input of comparator B
INB+
5 5 HYST Hysteresis input. Connect to REF if not used. Input voltage range is from
VREF to VREF - 50mV.
6 – 6 REF Reference output. 1.182V with respect to V-.
Noninverting input of comparator B
7 7 7 V+ Positive supply
8 – – OUT Comparator output. Sinks and sources current. Swings from V+ to GND.
– 8 8 OUTB Comparator B output. Sinks and sources current. Swings from V+ to V-.
_______________________________________________Pin Descriptions (continued)
PIN
MAX924 NAME FUNCTION
1 OUTB Comparator B output. Sinks and sources current. Swings from V+ to GND.
2 OUTA Comparator A output. Sinks and sources current. Swings from V+ to GND.
3 V+ Positive supply
4 INA- Inverting input of comparator A
5 INA+ Noninverting input of comparator A
6 INB- Inverting input of comparator B
7 INB+ Noninverting input of comparator B
8 REF Reference output. 1.182V with respect to V-.
9 V- Negative supply. Connect to ground for single-supply operation.
10 INC- Inverting input of comparator C
11 INC+ Noninverting input of comparator C
12 IND- Inverting input of comparator D
13 IND+ Noninverting input of comparator D
14 GND Ground. Connect to V- for single-supply operation.
15 OUTD Comparator D output. Sinks and sources current. Swings from V+ to GND.
16 OUTC Comparator C output. Sinks and sources current. Swings from V+ to GND.
MAX921–MAX924
Ultra Low-Power,
Single/Dual-Supply Comparators
8 _______________________________________________________________________________________
_______________Detailed Description
The MAX921–MAX924 comprise various combinations
of a micropower 1.182V reference and a micropower
comparator. The Typical Operating Circuit shows the
MAX921 configuration, and Figures 1a-1c show the
MAX922–MAX924 configurations.
Each comparator continuously sources up to 40mA,
and the unique output stage eliminates crowbar
glitches during output transitions. This makes them
immune to parasitic feedback (which can cause
instability) and provides excellent performance, even
when circuit-board layout is not optimal.
Internal hysteresis in the MAX921 and MAX923 provides
the easiest method for implementing hysteresis. It also
produces faster hysteresis action and consumes much
less current than circuits using external positive feedback.
Power-Supply and Input Signal Ranges
This family of devices operates from a single +2.5V to
+11V power supply. The MAX921 and MAX924 have a
separate ground for the output driver, allowing
operation with dual supplies ranging from ±1.25V to
±5.5V. Connect V- to GND when operating the
MAX921 and the MAX924 from a single supply. The
maximum supply voltage in this case is still 11V.
For proper comparator operation, the input signal can
swing from the negative supply (V-) to within one volt of
the positive supply (V+ – 1V). The guaranteed
common-mode input voltage range extends from V- to
(V+ - 1.3V). The inputs can be taken above and below
the supply rails by up to 300mV without damage.
Operating the MAX921 and MAX924 at ±5V provides
TTL/CMOS compatibility when monitoring bipolar input
signals. TTL compatibility for the MAX922 and MAX923
is achieved by operation from a single +5V supply.
Low-Voltage Operation: V+ = 1V (MAX924 Only)
The guaranteed minimum operating voltage is 2.5V (or
±1.25V). As the total supply voltage is reduced below
2.5V, the performance degrades and the supply
current falls. The reference will not function below
OUTA
V-
INA+
INA-
OUTB
V+
INB+
INB-
MAX922
1
2
3
4
8
7
6
5
Figure 1a. MAX922 Functional Diagram
OUTA
V-
INA+
INB-
OUTB
V+
REF
HYST
MAX923
1
2
3
4
8
7
6
5
V-
Figure 1b. MAX923 Functional Diagram
OUTA
V+
INA-
INA+
OUTD
GND
IND+
IND-
MAX924
2
3
4
5
15
14
13
12
16
11
10
9
1
6
7
8
OUTB
INB-
INB+
REF
OUTC
INC+
INC-
V-
Figure 1c. MAX924 Functional Diagram
about 2.2V, although the comparators will continue to
operate with a total supply voltage as low as 1V. While
the MAX924 has comparators that may be used at
supply voltages below 2V, the MAX921, MAX922, and
MAX923 may not be used with supply voltages sig-
nificantly below 2.5V.
At low supply voltages, the comparators’ output drive is
reduced and the propagation delay increases (see
Typical Operating Characteristics). The useful input
voltage range extends from the negative supply to a
little under 1V below the positive supply, which is
slightly closer to the positive rail than the device
operating from higher supply voltages. Test your
prototype over the full temperature and supply-voltage
range if operation below 2.5V is anticipated.
Comparator Output
With 100mV of overdrive, propagation delay is typically
3µs. The Typical Operating Characteristics show the
propagation delay for various overdrive levels.
The MAX921 and MAX924 output swings from V+ to
GND, so TTL compatibility is assured by using a +5V
±10% supply. The negative supply does not affect the
output swing, and can range from 0V to -5V ±10%.
The MAX922 and MAX923 have no GND pin, and their
outputs swing from V+ to V-. Connect V- to ground and
V+ to a +5V supply to achieve TTL compatibility.
The MAX921–MAX924’s unique design achieves an
output source current of more than 40mA and a sink
current of over 5mA, while keeping quiescent currents in
the microampere range. The output can source 100mA
(at V+ = 5V) for short pulses, as long as the package's
maximum power dissipation is not exceeded. The
output stage does not generate crowbar switching
currents during transitions, which minimizes feedback
through the supplies and helps ensure stability without
bypassing.
Voltage Reference
The internal bandgap voltage reference has an output
of 1.182V above V-. Note that the REF voltage is
referenced to V-, not to GND. Its accuracy is ±1% in
the range 0°C to +70°C. The REF output is typically
capable of sourcing 15µA and sinking 8µA. Do not
bypass the REF output.
Noise Considerations
Although the comparators have a very high gain, useful
gain is limited by noise. This is shown in the Transfer
Function graph (see Typical Operating Characteristics).
As the input voltage approaches the comparator's
offset, the output begins to bounce back and forth; this
peaks when VIN = VOS. (The lowpass filter shown on
the graph averages out the bouncing, making the
transfer function easy to observe.) Consequently, the
comparator has an effective wideband peak-to-peak
noise of around 0.3mV. The voltage reference has
peak-to peak noise approaching 1mV. Thus, when a
MAX921–MAX924
Ultra Low-Power,
Single/Dual-Supply Comparators
_______________________________________________________________________________________ 9
THRESHOLDS
OUT
IN-
IN+
VHB
HYSTERESIS
BAND
VREF - VHYST
Figure 2. Threshold Hysteresis Band
7
2
5
6
HYST
REF
V-
V+
R1
R2
MAX921
MAX923
2.5V TO 11V
IREF
Figure 3. Programming the HYST Pin
MAX921–MAX924
comparator is used with the reference, the combined
peak-to-peak noise is about 1mV. This, of course, is
much higher than the RMS noise of the individual
components. Care should be taken in the layout to
avoid capacitive coupling from any output to the
reference pin. Crosstalk can significantly increase the
actual noise of the reference.
__________Applications Information
Hysteresis
Hysteresis increases the comparators’ noise margin by
increasing the upper threshold and decreasing the
lower threshold (see Figure 2).
Hysteresis (MAX921/MAX923)
To add hysteresis to the MAX921 or MAX923, connect
resistor R1 between REF and HYST, and connect
resistor R2 between HYST and V- (Figure 3). If no
hysteresis is required, connect HYST to REF. When
hysteresis is added, the upper threshold increases by
the same amount that the lower threshold decreases.
The hysteresis band (the difference between the upper
and lower thresholds, VHB) is approximately equal to
twice the voltage between REF and HYST. The HYST
input can be adjusted to a maximum voltage of REF
and to a minimum voltage of (REF – 50mV). The
maximum difference between REF and HYST (50mV)
will therefore produce a 100mV max hysteresis band.
Use the following equations to determine R1 and R2:
Where IREF (the current sourced by the reference)
should not exceed the REF source capability, and
should be significantly larger than the HYST input
current. IREF values between 0.1µA and 4µA are
usually appropriate. If 2.4MΩis chosen for R2 (IREF =
0.5µA), the equation for R1 and VHB can be
approximated as:
When hysteresis is obtained in this manner for the
MAX923, the same hysteresis applies to both comparators.
Hysteresis (MAX922/MAX924)
Hysteresis can be set with two resistors using positive
feedback, as shown in Figure 4. This circuit generally
draws more current than the circuits using the HYST
pin on the MAX921 and MAX923, and the high
feedback impedance slows hysteresis. The design
procedure is as follows:
1. Choose R3. The leakage current of IN+ is under
1nA (up to +85°C), so the current through R3 can be
around 100nA and still maintain good accuracy.
The current through R3 at the trip point is VREF/R3,
or 100nA for R3 = 11.8MΩ. 10MΩis a good
practical value.
2. Choose the hysteresis voltage (VHB), the voltage
between the upper and lower thresholds. In this
example, choose VHB = 50mV.
3. Calculate R1.
4. Choose the threshold voltage for VIN rising (VTHR).
In this example, choose VTHR = 3V.
5. Calculate R2.
A 1% preferred value is 64.9kΩ.
R1 = R3 V
V
10M
0.05
5
100k
HB
×+
=×
=Ω
R1 (k ) = V (mV)
HB
Ω
R1 = V
2 I
R2 =
1.182 – V
2
I
HB
REF
HB
REF
×
()
⎛
⎝
⎜⎞
⎠
⎟
Ultra Low-Power,
Single/Dual-Supply Comparators
10 ______________________________________________________________________________________
GND
V-
V+
MAX924 OUT
R3
R1
R2
VREF
VIN
V+
Figure 4. External Hysteresis
6. Verify the threshold voltages with these formulas:
Board Layout and Bypassing
Power-supply bypass capacitors are not needed if the
supply impedance is low, but 100nF bypass capacitors
should be used when the supply impedance is high or
when the supply leads are long. Minimize signal lead
lengths to reduce stray capacitance between the input
and output that might cause instability. Do not bypass
the reference output.
_______________Typical Applications
Auto-Off Power Source
Figure 5 shows the schematic for a 40mA power supply
that has a timed auto power-off function. The
comparator output is the switched power-supply
output. With a 10mA load, it typically provides a
voltage of (VBATT – 0.12V), but draws only 3.5µA
quiescent current. This circuit takes advantage of the
four key features of the MAX921: 2.5µA supply current,
an internal reference, hysteresis, and high current
output. Using the component values shown, the three-
resistor voltage divider programs the maximum ±50mV
of hysteresis and sets the IN- voltage at 100mV. This
gives an IN+ trip threshold of approximately 50mV for
IN+ falling.
The RC time constant determines the maximum power-
on time of the OUT pin before power-down occurs.
This period can be approximated by:
R x C x 4.6sec
For example: 2MΩx 10µF x 4.6 = 92sec. The actual
time will vary with both the leakage current of the
capacitor and the voltage applied to the circuit.
Window Detector
The MAX923 is ideal for making window detectors
(undervoltage/overvoltage detectors). The schematic
is shown in Figure 6, with component values selected
for an 4.5V undervoltage threshold, and a 5.5V
overvoltage threshold. Choose different thresholds by
changing the values of R1, R2, and R3. To prevent
chatter at the output when the supply voltage is close
to a threshold, hysteresis has been added using R4
and R5. OUTA provides an active-low undervoltage
indication, and OUTB gives an active-low overvoltage
indication. ANDing the two outputs provides an active-
high, power-good signal.
The design procedure is as follows:
1. Choose the required hysteresis level and calculate
values for R4 and R5 according to the formulas in
the Hysteresis (MAX921/MAX923) section. In this
example, ±5mV of hysteresis has been added at the
comparator input (VH= VHB/2). This means that the
hysteresis apparent at VIN will be larger because of
the input resistor divider.
V rising :
V V R1 1
R1
1
R2
1
R3
V falling :
V V R1 V
R3
IN
THR REF
IN
THF THR
=××++
⎛
⎝
⎜⎞
⎠
⎟
=−
×+
()
MAX921–MAX924
Ultra Low-Power,
Single/Dual-Supply Comparators
______________________________________________________________________________________ 11
MAX921
OUT
IN-
HYST
REF
V- GND
IN+
V+
VBATT -0.15V
10mA
21
4
5
6
8
3
100k
1.1M
47k
4.5V TO 6.0V
MOMENTARY
SWITCH
7
R
C
Figure 5. Auto-off power switch operates on 2.5µA quiescent
current.
MAX921–MAX924
Ultra Low-Power,
Single/Dual-Supply Comparators
12 ______________________________________________________________________________________
2. Select R1. The leakage current into INB- is normally
under 1nA, so the current through R1 should
exceed 100nA for the thresholds to be accurate. R1
values up to about 10MΩcan be used, but values in
the 100kΩto 1MΩrange are usually easier to deal
with. In this example, choose R1 = 294kΩ.
3. Calculate R2 + R3. The overvoltage threshold
should be 5.5V when VIN is rising. The design
equation is as follows:
4. Calculate R2. The undervoltage threshold should
be 4.5V when VIN is falling. The design equation is
as follows:
5. Calculate R3.
Choose R3 = 1MΩ(1% standard value)
6. Verify the resistor values. The equations are as
follows, evaluated for the above example.
Bar-Graph Level Gauge
The high output source capability of the MAX921 series
is useful for driving LEDs. An example of this is the
simple four-stage level detector shown in Figure 7.
The full-scale threshold (all LEDs on) is given by VIN =
(R1 + R2)/R1 volts. The other thresholds are at 3/4 full
scale, 1/2 full scale, and 1/4 full scale. The output
resistors limit the current into the LEDs.
Level Shifter
Figure 8 shows a circuit to shift from bipolar ±5V inputs
to TTL signals. The 10kΩresistors protect the
comparator inputs, and do not materially affect the
operation of the circuit.
Overvoltage threshold :
V (V V ) (R1 R2 R3)
R1
5.474V.
Undervoltage threshold :
V (V V ) (R1 R2 R3)
(R1 + R2)
4.484V,
where the hysteresis voltage V V R5
R4
OTH REF H
UTH REF H
H REF
=+×
++
=
=−×
++
=
=×
.
R3 (R2 + R3) R2
.068M 6 k
1.006M
=−
=−
=
119 .
Ω
R2 (R1 + R2 + R3) (V V )
V R1
(294k + 1.068M) (1.182 0.005)
4.5 294k
62.2k
Choose R2 61.9k (1% standard value).
REF H
UTH
=×
−−
=×
−−
=
=
Ω
Ω
R2 R3 R1 V
V V
1
294k 5.5
(1.182 0.005) 1
1.068M
OTH
REF H
+=× +−
⎛
⎝
⎜⎞
⎠
⎟
=× +−
⎛
⎝
⎜⎞
⎠
⎟
=Ω
MAX923
INB-
REF
HYST
INA+
V-
V+
OUTA
OUTB
10k
R5
2.4M
R1
R2
R3
UNDERVOLTAGE
POWER GOOD
OVERVOLTAGE
VIN VOTH = 5.5V
VUTH = 4.5V
R4
+5V
Figure 6. Window Detector
MAX921–MAX924
Ultra Low-Power,
Single/Dual-Supply Comparators
______________________________________________________________________________________ 13
INB+
INB-
INC+
INC-
IND+
IND-
INA+
INA-
OUTA
OUTB
OUTC
OUTD
5
4
7
6
11
10
13
12
750mV
1V
500mV
250mV
250k
250k
250k
250k
182k
9
2
1
16
15
REF
8
V-
V+
GND
14
MAX924
3
+5V
VIN
R1 R2
1.182V
330Ω
330Ω
330Ω
330Ω
INB+
INB-
INC+
INC-
IND+
IND-
INA+
INA-
OUTA
OUTB
OUTC
OUTD
10k
V+
GND
MAX924
+5V
10k
10k
10k
REF
V-
N.C.
-5V
0 FOR VINA < 0V
1 FOR VINB > 0V
VINA
VINB
VINC
VIND
Figure 7. Bar-Graph Level Gauge Figure 8. Level Shifter: ±5V Input to CMOS Output
MAX921–MAX924
Ultra Low-Power,
Single/Dual-Supply Comparators
14 ______________________________________________________________________________________
1
2
3
4
8
7
6
5
OUT
V+
REF
HYST
IN-
IN+
V-
GND
MAX921
DIP/SO/μMAX
TOP VIEW
1
2
3
4
8
7
6
5
OUTB
V+
INB+
INB-
INA-
INA+
V-
OUTA
MAX922
DIP/SO/μMAX
1
2
3
4
8
7
6
5
OUTB
V+
REF
HYST
INB-
INA+
V-
OUTA
MAX923
DIP/SO/μMAX
16
15
14
13
12
11
10
9
1
2
3
4
5
6
7
8
OUTC
OUTD
GND
IND+
INA-
V+
OUTA
OUTB
MAX924
IND-
INC+
INC-
V-
REF
INB+
INB-
INA+
DIP/Narrow SO
_________________Pin Configurations _Ordering Information (continued)
*Dice are tested at TA= +25°C, DC parameters only.
**Contact factory for availability.
PART
TEMP RANGE
PIN-PACKAGE
MAX921MSA/PR
-55°C to +125°C
8 SO**
MAX921MSA/PR-T
-55°C to +125°C
8 SO**
MAX922CPA 0°C to +70°C 8 Plastic DIP
MAX922CSA 0°C to +70°C 8 SO
MAX922CUA 0°C to +70°C 8 µMAX
MAX922C/D 0°C to +70°C Dice*
MAX922EPA
-40°C to +85°C
8 Plastic DIP
MAX922ESA
-40°C to +85°C
8 SO
MAX922MJA
-55°C to +125°C
8 CERDIP**
MAX922MSA/PR
-55°C to +125°C
8 SO**
MAX922MSA/PR-T
-55°C to +125°C
8 SO**
MAX923CPA 0°C to +70°C 8 Plastic DIP
MAX923CSA 0°C to +70°C 8 SO
MAX923CUA 0°C to +70°C 8 µMAX
MAX923C/D 0°C to +70°C Dice*
MAX923EPA
-40°C to +85°C
8 Plastic DIP
MAX923ESA
-40°C to +85°C
8 SO
MAX923MJA
-55°C to +125°C
8 CERDIP**
MAX923MSA/PR
-55°C to +125°C
8 SO**
MAX923MSA/PR-T
-55°C to +125°C
8 SO**
MAX924CPE 0°C to +70°C 16 Plastic DIP
MAX924CSE 0°C to +70°C 16 Narrow SO
MAX924C/D 0°C to +70°C Dice*
MAX924EPE
-40°C to +85°C
16 Plastic DIP
MAX924ESE
-40°C to +85°C
16 Narrow SO
MAX924MJE
-55°C to +125°C
16 CERDIP**
MAX924MSE/PR
-55°C to +125°C
16 Narrow SO**
MAX924MSE/PR-T
-55°C to +125°C
16 Narrow SO**
MAX921–MAX924
Ultra Low-Power,
Single/Dual-Supply Comparators
______________________________________________________________________________________ 15
__________________________________________________________Chip Topographies
9
8
6
7
1
2
3
4
0.075"
(1.91mm)
0.059"
(1.50mm)
5
10
GND
IND+
INC+
IND-
V+
INA+
INA-
INB-
OUTA OUTB OUTC OUTD
INC-REFINB+ V-
0.108"
(2.74mm)
0.069"
(1.75mm)
TRANSISTOR COUNT: 164
SUBSTRATE CONNECTED TO V+
TRANSISTOR COUNT: 267
SUBSTRATE CONNECTED TO V+
DIE PAD MAX921 MAX922 MAX923
1 GND OUTA OUTA
2 V-V-V-
3 V-V-V-
4 IN+ INA+ INA+
5 IN- INA- INB-
6 HYST INB- HYST
7 REF INB+ REF
8 V+V+V+
9 V+V+V+
10 OUT OUTB OUTB
MAX921/MAX922/MAX923 MAX924
MAX921–MAX924
Ultra Low-Power,
Single/Dual-Supply Comparators
16 ______________________________________________________________________________________
Package Information
For the latest package outline information and land patterns, go to www.maxim-ic.com/packages.
PACKAGE TYPE PACKAGE CODE DOCUMENT NO.
8 Plastic DIP P8-1 21-0043
16 Plastic DIP P16-1 21-0043
8 SO S8-2 21-0041
16 SO S16-3 21-0041
8 µMAX U8-1 21-0036
8 CERDIP J8-1 21-0045
16 CERDIP J16-3 21-0045
MAX921/MAX924
Ultra Low-Power,
Single/Dual-Supply Comparators
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 ____________________ 17
© 2009 Maxim Integrated Products Maxim is a registered trademark of Maxim Integrated Products, Inc.
Revision History
REVISION
NUMBER
REVISION
DATE DESCRIPTION PAGES
CHANGED
4 8/08 Updated TOCs 5 and 10 5
5 8/08 Adding information for rugged plastic product 1, 14
6 4/09 Updated Ordering Information 1, 14, 16