Texas Instruments | Inverting comparator with hysteresis circuit (Rev. A) | Application notes | Texas Instruments Inverting comparator with hysteresis circuit (Rev. A) Application notes

Texas Instruments Inverting comparator with hysteresis circuit (Rev. A) Application notes
Analog Engineer's Circuit: Amplifiers
SNOA997A – July 2018 – Revised August 2019
Inverting Comparator With Hysteresis Circuit
Design Goals
Output
Thresholds
Supply
V0 = HIGH
Vo = LOW
VH
VL
VHYS
Vcc
VPU
Vref
Vi < VL
Vi > VH
2.5V
2.2V
300 mV
3V
3V
3V
Design Description
Comparators are used to differentiate between two different signal levels. With noise, signal variation, or
slow-moving signals, undesirable transitions at the output can be observed with a constant threshold.
Setting upper and lower hysteresis thresholds eliminates these undesirable output transitions. This circuit
example will focus on the steps required to design the positive feedback resistor network necessary to
obtain the desired hysteresis for an inverting comparator application.
Vpu
Vref
Vcc
Vpu
R3 100k
R1 112k
Vref
Vcc
U1 TLV7041
-
+
Vo
+ +
VTH
R2 500k
Vi
R4 809k
Design Notes
1. The accuracy of the hysteresis threshold voltages are related to the tolerance of the resistors used in
the circuit, the selected comparator’s input offset voltage specification, and any internal hysteresis of
the device.
2. The TLV7041 has an open-drain output stage, so a pull-up resistor is needed.
SNOA997A – July 2018 – Revised August 2019
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Design Steps
1. Select the lower biasing resistor, R2. This resistor can be modified for any design. In this case, it is
assumed that power conservation is necessary, therefore, R2 is selected to be large.
2. Select the switching thresholds for when the comparator will transition from high to low (VL) and low to
high (VH). VL is the necessary input voltage for the comparator output to transition low and VH is the
required input voltage for the comparator to output high.
VL
VH
3. Analyze the circuit when the input voltage is VH. At this point, Vo=3V=VPU and the transition to a logic
low is initiated in the comparator output. Using Kirchhoff's Current Law, solve for an equation for R1.
R3
Vpu
R4
R1
Vref
R2
VH
4. Analyze the circuit when the input voltage is VL. At this point, Vo=0V and the transition to a logic high is
initiated in the comparator output. Using Kirchhoff's Current Law, solve for an equation for R1.
R1
Vcc
2
Inverting Comparator With Hysteresis Circuit
R4
R2
VL
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5. After defining some constants, set the two equations for R1 equal to obtain a quadratic equation for R4.
a. If the output stage is push-pull, then make the following modifications to the above equations:
6. Solve the quadratic equation for R4 and pick the most logical result.
7. Calculate R1 by substituting the value for the "A" constant into the equation for R1 found in step 4.
SNOA997A – July 2018 – Revised August 2019
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DC Transfer Simulation Results
Transient Simulation Results
4
Inverting Comparator With Hysteresis Circuit
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Design References
See Analog Engineer's Circuit Cookbooks for TI's comprehensive circuit library.
See Comparator with Hysteresis Reference Design TIPD144, www.ti.com/tipd144.
See Circuit SPICE Simulation File SLVMCQ0, http://www.ti.com/lit/zip/slvmcq0.
For more information on many comparator topics including hysteresis, propagation delay and input
common mode range please see training.ti.com/ti-precision-labs-op-amps.
Design Featured Comparator
TLV7031 / TLV7041
Output Type
PP (7031) / OD (7041)
Vcc
1.6V to 6.5V
VinCM
Rail-to-rail
Vos
±100µV
VHYS
7mV
Iq
335nA/Ch
tpd
3µs
#Channels
1, 2
www.ti.com/product/tlv7041
Design Alternate Comparator
TLV1701
TLV7011 / TLV7021
Output Type
Open Collector
PP (7011) / OD (7021)
Vcc
2.2V to 36V
1.6V to 5.5V
VinCM
Rail-to-rail
Rail-to-rail
VHYS
N/A
4.2mV
Vos
±500µV
±500µV
Iq
55µA/Ch
5uA
tpd
560ns
260ns
#Channels
1, 2, 4
1, 2
www.ti.com/product/tlv
1701
http://www.ti.com/prod
uct/TLV7011
SNOA997A – July 2018 – Revised August 2019
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Inverting Comparator With Hysteresis Circuit
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