Design Goals

Design Description

Comparators are used to compare two different signal levels and create an output based on the input with the higher input voltage. Noise or signal variation at the comparison threshold will cause the comparator output to have multiple output transitions. Hysteresis sets upper- and lower-threshold voltages to eliminate the multiple transitions caused by noise.

Design Notes

1. Use a comparator with low quiescent current to reduce power consumption.
2. The accuracy of the hysteresis threshold voltages are related to the tolerance of the resistors used in the circuit.
3. The propagation delay is based on the specifications of the selected comparator.

Design Steps

1. Select components for the comparator with hysteresis.
   1. Select V_L, V_H, and R₁.
      ${\mathrm{V}}_{\mathrm{L}}=2.3\mathrm{V}$
      ${\mathrm{V}}_{\mathrm{H}}=2.7\mathrm{V}$
      ${\mathrm{R}}_1=100\mathrm{k\Omega }\text{ (Standard Value)}$
   2. Calculate R₂.
      ${\mathrm{R}}_2=\frac{{\mathrm{V}}_{\mathrm{L}}}{{\mathrm{V}}_{\mathrm{cc}}-{\mathrm{V}}_{\mathrm{H}}}\times {\mathrm{R}}_1=\frac{2.3\mathrm{V}}{5\mathrm{V}-2.7\mathrm{V}}\times 100\mathrm{k\Omega }=100\mathrm{k\Omega }\text{ (Standard Value)}$
   3. Calculate R₃.
      ${\mathrm{R}}_3=\frac{{\mathrm{V}}_{\mathrm{L}}}{{\mathrm{V}}_{\mathrm{H}}-{\mathrm{V}}_{\mathrm{L}}}\times {\mathrm{R}}_1=\frac{2.3\mathrm{V}}{2.7\mathrm{V}-2.3\mathrm{V}}\times 100\mathrm{k\Omega }=575\mathrm{k\Omega }\approx 576\mathrm{k\Omega }\text{ (Standard Value)}$
   4. Verify hysteresis width.
      ${\mathrm{V}}_{\mathrm{H}}-{\mathrm{V}}_{\mathrm{L}}=\frac{{\mathrm{R}}_1\times {\mathrm{R}}_2}{\left({\mathrm{R}}_3\times {\mathrm{R}}_1\right)+\left({\mathrm{R}}_3\times {\mathrm{R}}_2\right)+\left({\mathrm{R}}_1\times {\mathrm{R}}_2\right)}\times {\mathrm{V}}_{\mathrm{cc}}$
      $=\frac{100\mathrm{k\Omega }\times 100\mathrm{k\Omega }}{\left(576\mathrm{k\Omega }\times 100\mathrm{k\Omega }\right)+\left(576\mathrm{k\Omega }\times 100\mathrm{k\Omega }\right)+\left(100\mathrm{k\Omega }\times 100\mathrm{k\Omega }\right)}\times 5\mathrm{V}=0.399\mathrm{V}$
2. Select components for comparator without hysteresis.
   1. Select V_th and R₄.
      ${\mathrm{V}}_{\mathrm{th}}=2.5\mathrm{V}$
      ${\mathrm{R}}_4=100\mathrm{k\Omega }\text{ (Standard Value)}$
   2. Calculate R₅.
      ${\mathrm{R}}_5=\frac{{\mathrm{V}}_{\mathrm{th}}}{{\mathrm{V}}_{\mathrm{cc}}-{\mathrm{V}}_{\mathrm{th}}}\times {\mathrm{R}}_4=\frac{2.5\mathrm{V}}{5\mathrm{V}-2.5\mathrm{V}}\times 100\mathrm{k\Omega }=100\mathrm{k\Omega }\text{ (Standard Value)}$

Design Simulations

Transient Simulation Results

Noise Only Present From 0s to 120µs

Zoomed in From 40µs to 110µs

Design References

Texas Instruments, SBOC515 circuit SPICE simulation file, software download

Texas Instruments, Comparator with Hysteresis, reference design

Design Featured Comparator