Page 217 - Op Amps Design, Applications, and Troubleshooting

Variable-Duty Cycle 199

Let us utilize this practice and determine the required voltage for the reference
diodes.

We choose to use 1N5226 zeners for our application. As Appendix 5 shows, these
diodes have a rated voltage of 3.3 volts; however, with less than 20 milliamperes
of zener current, the actual voltage will be somewhat lower.

Compute the Value for R 5. Resistor R 5 is computed in the same way as resis-
tor K 6. We use the same guideline, Equation (4.32), that sets the zener current to 20
percent of l os, which was previously computed to be 4 milliamperes. The value for
R 5 can be found by applying the principle represented in Equation (4.24),

We will choose the standard value of 2.2 kilohms.

Compute €) and the Timing Resistors. The first step in determining values
for Ci and RI - K 4 is to determine the required RC time constant for the shorter
period in the design requirement. For this, we utilize a transposed version of Equa-
tions (4.27) or (4.28). If the positive alternation is the shorter, use a transposed ver-
sion of Equation (4.27). Equation (4.28) should be utilized if the negative
alternation is shorter. For our case, the 1-millisecond positive output time is clearly
the shorter, so we will apply Equation (4.27) to determine the required RC product.

To ensure that we come up with practical values, it is generally best to select
R 4 at this point and compute Q. Additionally, we are working with the shortest
time period, so we should select a fairly small value for R 4 as long as we don't go