Page 163 - Op Amps Design, Applications, and Troubleshooting

146 VOLTAGE COMPARATORS

3. Hysteresis 1.0 volts
4. Highest operating frequency ISkilohertz
5. Maximum ratio (p) of switch- 0.2
ing time to stable time
6. Power supply voltages ±15 volts

Determine the Required Slew Rate. In this type of application, slew rate is
probably the most critical parameter with regard to op amp selection. The slew
rate must be high enough to allow the output to switch between saturation levels
within the allowed switching time (t s). The switching time is computed by using a
transposed version of the/(max) equation. That is,

For the present circuits, we have

For purposes of op amp selection, we can assume that the output swing
between the two power supply limits. That is, assume that ±V SAT = ±V Cc- Th
required slew rate can then be computed as

For the present circuit, we have

Select an Op Amp. Appendix 1 indicates that the slew rate for a 741 is only
0.5 volts per microsecond, which clearly eliminates this device as an option
because we will require a slew rate of at least 5.4 volts per microsecond. Appendix
4, however, shows that the MC1741SC op amp has a minimum slew rate of 10
volts per microsecond. This will satisfy our present requirements nicely, and it is
compatible with our power supply requirements. We will build our design
around the MC1741SC op amp.

Determine RF and R), The ratio of R F and jR a is dependent on the ratio of
VSAT voltage to hysteresis voltage. Appendix 4 indicates that the unloaded out-