Electronic Design

# Two Triggers Have Flexible Time Intervals And Pulse Widths

Generating two independent pulse triggers separated by a certain time delay can be handy in half-bridge drivers and thyristor-commutated lamp flashers. Time delay is set to avoid overlapping of the power switches' ON states or to minimize the stress on the thyristor's gate. A simple implementation is shown in Figure 1.

Augmenting an earlier published design,1 the circuit for CMP1 is a free-running multivibrator whose period is found by T = 1.39R1C. Voltage across C (VC) alternately charges and discharges between VCC/3 and 2VCC/3. Whenever VC exceeds R3VCC/(R2 + R3), CMP2's output (Q1) goes low while CMP3's output (Q2) is high. Similarly, whenever VC falls below R4VCC/(R4 + R5), Q2 goes low while Q1 is high. Then when VC is found between these two trip points, both Q1 and Q2 are low.

Figure 2 shows the circuit's timing signals. Letting k = R3/(R2 + R3) and m = R4/(R4 + R5), where 2/3 > k > m >1 /3, it can be shown by using the RC charge-discharge equations2 that the pulse widths and time delays are:

tW1 = R1CLN\[2(1 − k)/k\]
tD1 = R1CLN(k/m)
tW2 = R1CLN\[2m/(1 − m)\]
tD2 = R1CLN\[(1 − m)/(1 − k)\]
T = tW1 + tD1 + tW2 + tD2

You will observe that VCC, which in this case could be anywhere between 2 and 28 V, doesn't affect the timing characteristics. The three 1-kΩ resistors are for pull-ups.

References:

1. Edang, Arthur E., "Independently Vary A Multivibrator's Duty Cycle and Frequency," electronic design, Oct. 28, 2002, p. 74.
2. Franco, Sergio, Design with Operational Amplifiers and Analog Integrated Circuits 3e. McGraw-Hill, 2002, p. 450.