Page 170 - Op Amps Design, Applications, and Troubleshooting

Voltage Comparator with Hysteresis 153

For purposes of op amp selection, we can assume that the output swings between
the two power supply limits. That is, assume that ±V SAT - ±V CC. The required slew
rate can then be computed with Equation (3.13) as

These calculations assume a 50-percent duty cycle on the output waveform. If the
output is asymmetrical, then the shortest allowable alternation is given as

Select on Op Amp. Appendix 1 indicates that the slew rate for a 741 is 0.5
volts per microsecond, which exceeds our requirement of at least 0.04 volts per
microsecond. The power supply requirements are also compatible with our stated
design requirements. We will build our design around the 741 op amp.

Determine R F and RI . The ratio of R F and R a is dependent on the ratio of V SAT
voltage to hysteresis voltage. Appendix 1 indicates that the lightly loaded (R L >
10 fcO) output swing will be typically ±14 volts. We will use this value for our
computations. If the circuit is expected to drive a greater load (smaller load resis-
tor and/or feedback network), then the output will be correspondingly smaller.
The ratio of R F to RI is computed with Equation (3.14).

We select RI and calculate the value of R f. For purposes of this example, we select
RI to be 33 kilohms. Having done this, we can now compute R F by applying Equa-
tion (3.15) where the ratio (R F/Ri) is known.