Schmitt Trigger Circuit For a Push-Pull Output

Quote of the Day

If you were replaced, what would your successors do?

— Andy Grove, former CEO of Intel. This is a question he would ask himself in difficult management situations.

Introduction

Figure 1: Basic Schmitt Trigger Circuit For
a Push-Pull Output Comparator Drawn in
LTSpice.

I previously wrote a blog post about how to select components for a Schmitt trigger circuit using a comparator with an open-collector output. An engineer stopped by my cube yesterday and asked if I could write-up the same analysis for a Schmitt trigger circuit using a comparator with a push-pull output. This post will provide that analysis. The only thing unusual about the circuit is the use of a Zener diode as a voltage reference instead of the more commonly seen resistor divider network.

This a very common electronic circuit. I think I have had some form of Schmitt ­­trigger comparator circuit in every large analog design I have ever done.

The analysis is very similar to my previous presentation and I will let the mathematics speak for itself – this means a minimum of gloss.

Background

Objective

Figure 1 is referred to as an inverting Schmitt-trigger circuit. For a rising input voltage, we want the output of the circuit to transition from a high voltage (V_CC) to a low voltage (~0 V) when the input level reaches V_TH­↑. For a falling input voltage, we want the output of the circuit to transition from a low voltage (~0 V) to high voltage (~V_CC) when the input level reaches V_TH↓.

Definitions

V_TH­↑

Comparator threshold voltage for positive-going signals.

V_TH↓

Comparator threshold voltage for negative-going signals.

V_CC

Supply voltage for the circuit. This will be a single-supply Schmitt trigger.

V_Z

The Zener diode breakdown voltage.

Analysis

Setup Circuit Equations

Figure 2 shows how I apply Kirchoff's nodal equations to the circuit of Figure 1 and I determine equations for V_TH↓ and V_TH­↑. V_Plus and V_Minus refer to the comparator inputs. I often solve circuit equations in terms of normalized component values. Normalized values have an "n" appended to their symbol.

Figure 2: Equation Setup for the Push-Pull Schmitt Trigger.

Given equations for V_TH↓ and V_TH­↑, I can solve them for normalized R₃ and R₅ values.

Solve Equations for R₃ and R₅

Figure 3 shows how I solved for R₃ and R₅ in terms of the hysteresis voltages (V_TH↓, V_TH­↑) and Zener diode breakdown voltage (V_Z).

Figure 3: Solve for the normalized values of R3 and R5.

Denormalization

While not required, you can denormalize R₃ by multiplying by R₄. Similarly, R₅ is denormalized by multiplying by R₁. Figure 4 illustrates the process.

Figure 4: Denormalize the Solution.

Example

To illustrate how to use the equations for R₃ and R₅, I will work an example with the following parameters.

• V_CC = 3.3 V, which is the system supply voltage.
• R₄ = 10 kΩ, arbitrary chosen value
• R₁ = 1.33 kΩ, arbitrarily chosen value
• V_Z = 2.5 V
• V_TH↓ = 11.75 V
• V_TH­↑= 12.25 V

Given these design parameters, I will now use the formulas for R₃ and R₅ to complete the circuit design.

Determine Component Values

Figure 5 shows how we can compute value for R₃ and R₅.

Figure 5: A Worked Example.

Simulation Results

I used LTSpice to simulate the circuit of Figure 1 populated with the circuit values shown in Figure 6.

Figure 6: Circuit of Figure 1 with Component Values.

Figure 7 shows the simulation results, which show that V_TH↓ = 11.75 V,  V_TH­↑ = 12.25 V, which are our desired hysteresis voltages. Here is the color code used in this plot.

• Yellow is my annotation color (i.e. I added them).
• Green is the output voltage (vOUT) from the circuit of Figure 7.
• Blue is the input voltage (vIN), which has a trapezoid.
• Red is the Zener voltage.

Figure 7: Simulation Results.

Conclusion

Just a quick note to demonstrate how to solve a common circuit design problem using a computer algebra system.