SNOAA74 April   2021

Design Goals

Input SupplyComparator Output Status (OUT)

Operating Range

26V ≤ Vin ≤ 30V

Vin < 26V or Vin > 30V

Total Ionizing Dose (TID)

SEL Immunity to LET

20 V to 36 V

Vout = Vpu

Vout = GND

85 MeV·cm2/mg

Design Description

This application brief shows how to implement a voltage window comparator circuit, targeted to monitor a 28-V power rail, a spacecraft bus voltage commonly found in smaller aircraft. This wide single-supply window comparator circuit utilizes a dual open-collector comparator and 3 resistors to set the window voltage. A shunt regulator, TL1431-SP, is used to provide a reference voltage from the input voltage. Therefore, only a single power supply is utilized for the input portion of the circuit. The LM193AQML-SP was used for its open collector output, radiation specifications, and two channel count. Whenever the input voltage, Vin, is within the window of comparison (26 V to 30 V), the output of the circuit, VOUT, is high. Whenever Vin is outside of the window of comparison, the VOUT is pulled down to GND.

Design Notes

1. Select a high-voltage comparator with an open collector output stage.
2. Select a comparator with low input offset voltage to optimize accuracy.
3. Calculate values for the resistor divider so that VOUT goes high whenever V1 crosses VREF and goes low whenever V2 crosses VREF.
4. Calculate R5 such that shunt regulator is within sink current specification for entire operating range.

Design Steps

1. Select a high-voltage comparator with an open collector output stage that can operate at the highest possible supply voltage. In this design, the highest input/supply voltage is 36 V.
2. Determine an appropriate reference level, VREF, for the window comparator. The TL1431-SP internal reference voltage, 2.5 V, was used for ease of calculations. If another reference voltage were to be used with the TL1431-SP, a voltage divider would be needed between the cathode and anode of the shunt regulator, with VREF between the resistors.
3. Calculate the value of R5, the resistor across VIN and VREF, by relating VREF to the operating voltage range. Ensure that R5 is at a level where the shunt regulator is sufficiently biased for the entire operating range. The current needed to bias the TL1431-SP, IBias, has to be between 1 mA and 100 mA. A 4.7-kΩ resistor was chosen as it kept the bias current within this range for the entire voltage operating range.
Equation 1.
Equation 1.
Values between 350 Ω and 16 kΩ could be used in this design. Consideration was made to minimize the bias current, yet give some buffer from the 1 mA minimum specification. If VREF is seen to be noisy, a decoupling capacitor can be placed between the node and GND to filter out the noise.
4. The positive input to the top comparator, V1, and the negative input to the bottom comparator, V2, can be related to Vin through voltage division:
Equation 1.

The window comparator trips when V1 passes VREF to output high, and again when V2 passes VREF to output low. The comparator is low if V1 is less than VREF. In this design, the window comparator will trip high when Vin equals 26 V and trip low when Vin equals 30 V; both while VREF equals 2.5 V.

Equation 1.
Equation 1.

5. Solve both equations from step 4 for (R1+R2+R3) and substitute one equation for the other.
Equation 1.
Equation 1.
Equation 1.
Equation 1.
6. Using the relationship obtained in step 5, solve for a relationship between R1 and R2.
Equation 1.
Equation 1.
7. Using the equations derived in steps 5 and 6, size resistors R1, R2, and R3 accordingly. For this design, R2 was set to be 2.55 kΩ, which meant R1 and R3 would be 179.775 kΩ and 16.575 kΩ, respectively. The magnitude of these resistors were chosen based off of the current consumption across the voltage divider (around 100 to 180 μA across the operating condition).
8. Select a 5% tolerant resistor to act as the pullup resistor, R4, from the output of the window comparator to VPU. Size this component large enough to ensure the current sinked by the comparator is not large, but small enough that the leakage current drawn by the comparator output when high is not causing too large of a voltage drop.
9. The values obtained in step 7 were adjusted for 1% resistor tolerances to be 178 kΩ, 2.55 kΩ, and 16.5 kΩ for R1, R2, and R3, respectively. Due to these changes, the window of comparison was shifted to trip earlier for overvoltage conditions and later for undervoltage conditions. In the DC Simulation Results, the window of comparison is between 25.8595 V and 29.856 V.

Design Simulations

DC Simulation Results

Transient Simulation Results

References:

1. SPICE Simulation File: http://www.ti.com/lit/zip/snom708.

Design Featured Comparator

LM193QML-SP

VS2 V to 36 V
VinCM

0 V to 34.5 V

VOUT

Open-Collector

VOS5 mV
IQ

200 μA/channel

tPD(HL)2.50 μs

TID Radiation Lot Acceptance Test (RLAT) / RHA

TID Characterization (ELDRS-Free)

SEL Immune to LET

SEL Immune (Bipolar process)

http://www.ti.com/product/LM193QML-SP

Design Featured Shunt Reference

TL1431-SP

VKA

2.5 V to 36 V

IKA

1 mA to 100 mA

VI(ref)

2.5 V

Initial Accuracy

0.4%

TID

SEL Immune to LET

SEL Immune (Bipolar process)

www.ti.com/product/TL1431-SP

Design Alternate Comparator

TLV1704-SEP

LM139AQML-SP

VS 2.2 V to 36 V 2 V to 36 V
VinCM Rail-to-rail

0 V to 34 V

VOUT

Open-Collector, Rail-to-rail

Open-Collector
VOS 500 µV

2 mV

IQ 55 µA/channel

200 μA/channel

tPD(HL) 460 ns 2.50 μs

TID Characterization (ELDRS-Free)