Texas Instruments | Window comparator with integrated reference circuit | Application notes | Texas Instruments Window comparator with integrated reference circuit Application notes

Texas Instruments Window comparator with integrated reference circuit Application notes
Analog Engineer's Circuit: Amplifiers
SNVA832 – December 2018
Window comparator with integrated reference circuit
Design Goals
Input
VMON
Output
VMON
Min
0V
VOUT
Max
6V
Supply
VOUT
Min
0V
Max
3.3V
VDD
VREF
3.3V
400mV
Lower Threshold (VL)
Upper Threshold (VH )
Divider Load Current (IMAX) at VH
3.2V
4.1V
10uA
Design Description
This circuit utilizes the TLV6710, which contains two comparators and a precision internal reference of
400mV. The monitored voltage (VMON) is divided down by R1, R2, and R3. The voltage across R2 and R3 is
compared to the 400mV internal reference voltage (VREF). If the input signal (VMON) is within the window,
the output is high. If the signal level is outside of the window, the output is low.
The TLV6710 will be utilized for this example, which conveniently contains two comparators and a
common precision internal reference trimmed to a 400mV threshold. Two discrete comparators and an
external reference may also be used.
VMON
VDD
R1
VPULLUP
VDD
INA
+
RPULLUP
50 NŸ
OUTA
±
R2
+
INB
OUTB
OUT
±
+
±
VREF
400 mV
R3
GND
TLV6710
Design Notes
1. Make sure the comparator input voltage range is not violated at the highest expected VMON voltage.
2. If the outputs are to be combined together (ORed), open collector or open drain output devices must
be used.
3. It is also recommended to repeat the following calculations using the minimum and maximum resistor
tolerance values and comparator positive and negative offset voltages.
4. The TLV6710 has built-in asymmetrical hysteresis, resulting in the rising edge VL and falling edge VH
being slightly shifted. Comparators without hysteresis will meet the calculated thresholds.
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Design Steps
The resistor divider will be calculated in separate VH and VL segments to create 400mV at the appropriate
comparator input at the desired threshold voltage.
1. The total divider resistance RTOTAL is calculated from the upper threshold voltage and divider current:
2. The upper threshold voltage is set by the "bottom" divider resistor R3 going into the INB pin. From the
reference voltage and the divider current, the value of R3 is calculated from:
3. The "middle" resistor R2 is found by looking at R2 and R1 as one resistor, and calculating the value for
that total resistance for VREF at VL, then subtracting out the known R3:
4. R1 is found by taking the total resistance and subtracting the sum of R2 and R3:
Because these are calculated ideal resistor values, the next closest 0.1% standard resistor values will
be used. The following table summarizes the changes due to the resistor value changes and the
resulting trip point voltage change.
Nearest 0.1% Resistor Values
Resistor
Calculated Ideal Value
Nearest Standard 0.1% (E192) Value
R1
358.750 kΩ
361 kΩ
R2
11.25 kΩ
11.3 kΩ
R3
40 kΩ
40.2 kΩ
Because the values of the divider string resistors were changed, the resulting new threshold voltages must
be calculated. The thresholds are found by multiplying the divider ratio by the reference voltage:
Ideal and Standard Resistor Thresholds
Threshold
Using Ideal Resistors
Using Standard Resistors
Percent Change
VH
4.1V
4.1045V
+0.109%
VL
3.2V
3.2039V
+0.121%
To ensure that the maximum 6V VMON voltage does not violate the TLV6710 1.7V maximum input voltage
rating, the VMON_MAX and the VL division ratio found in step 4 above are used to calculate the maximum
voltage at the TLV6710 input:
The value 749mV is less than 1.7V, so the input voltage is well below the input maximum. If using discrete
comparators, make sure the votlage is within the specified input common mode range (VICR) of the device
used.
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Window comparator with integrated reference circuit
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Design Simulations
Transient Simulation Results
Note: The Rising edge VL and falling edge VH thresholds are slightly shifted due to the built-in asymmetrical hysteresis
of the TLV6710. Comparators without hysteresis will meet the calculated thresholds.
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Window comparator with integrated reference circuit
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Design References
For more information on many comparator topics including input votlage range, output types and
propagation delay, please visit TI Precision Labs - Comparator Applications.
See Analog Engineer's Circuit Cookbooks for TI's comprehensive circuit library.
See TINA-TI™ TLV6710 Reference Design circuit simulation file, Literature Number SNVMB09.
Design Featured Comparator
TLV6710
Vss
2V to 36 V
VinCM
0V to 1.7V
Vout
0V to 25V
Vref
400 mV ±0.25%
Iq
11 µA
Ib
1 nA
Prop Delay
10 µs
#Channels
2
www.ti.com/product/tlv6710
Design Alternate Comparator
TLV6700
Vss
1.8V to 18 V
VinCM
0V to 6.5V
Vout
0V to 18V
Vref
400 mV ±0.5%
Iq
5.5 µA
Ib
1 nA
Prop Delay
29 µs
#Channels
2
www.ti.com/product/tlv6700
Design Alternate Comparator
TLV1702
Vss
2.7 to 36 V
VinCM
Rail to Rail
Vout
Open Drain to 36 V
Vos
±3.5 mV
Iq
75 µA
Ib
15 nA
Prop Delay
0.4 µs
#Channels
2
www.ti.com/product/tlv1702
4
Window comparator with integrated reference circuit
Copyright © 2018, Texas Instruments Incorporated
SNVA832 – December 2018
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