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Delayed-pulse generator uses dual flip-flop

30 Mar 2016  | Luca Bruno

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IC1A is a one-shot circuit (Reference 1). The output pulse's width, TW, is R4C2×ln(VDD/VTH), where VTH is the threshold voltage of the digital CMOS. Because VTH≈VDD/2, then TW≈R4C2×0.69. Diode D1 reduces recovery time. After power-up, Q3 is in saturation, absorbing the current source's output, and, as soon as an input pulse triggers the circuit, IC1B's Q output goes low, switching off Q3, starting a ramp. When the ramp exceeds the control voltage, then the IC2A comparator's output goes high, and the rising edge triggers one-shot IC1A and switches on Q3 through IC1B, allowing the discharge of the capacitor C1. When an input pulse triggers the circuit, any other trigger pulse that occurs before the falling edge of the delayed output pulse does not produce an output pulse; in other words, the circuit is not retriggerable. This feature permits you, at the same time, to divide and delay an input-trigger clock.

Although IC1 and IC2 can operate from a 3 to 16V supply, the minimum supply voltage of the circuit is 5V; otherwise, Q1 and Q2 approach saturation, generating to a less linear ramp voltage. Voltage comparator IC2A, an STMicroelectronics TS3702, has an input-common-mode-voltage range that includes ground, permitting you to monitor input voltages as low as 0V.

However, for correct operation of the circuit, the minimum control voltage must be greater than the saturation voltage of Q3. For the components in figure, the measured value is 12 mV. If you want to reduce this voltage, you can use a digital N-channel MOSFET with low on-resistance. The optional input lowpass filter, comprising R6 and C4, helps to clean noise from the dc-control voltage.

If a DAC drives the control input, you can build a digitally programmable delay generator. A suitable low-cost, 8bit DAC is the AD558 from Analog Devices, which features an internal precision bandgap reference to provide an output voltage of 0 to 2.56V, making 1 LSB equal to 1µsec. It operates from 5 to 16V, with a 1-µsec settling time. The circuit's quiescent current, IDD, is less than 300µA because all ICs are micropower.


Reference
Bhandarkar, Santosh, "Single-IC-based electronic circuit replaces mechanical switch" EDN, March 15, 2007, pg 76.


About the author
Luca Bruno contributed this article.


This article is a Design Idea selected for re-publication by the editors. It was first published on April 17, 2008 in EDN.com.


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