Linear PWM Using a 555

[mediatechnology]

Hello all. I haven't dropped in for awhile but I've been going through my filing cabinet and have become a scanning maniac.

Here's a little circuit I did in the early 1980s that linearizes PWM operation of a (7)555 timer. Something like this may have been posted here already but some of you may find it useful. I was originally going to submit it to EDN but never did.



Pulse width modulation of the 555 is possible by using the control voltage input, pin 5. This method has several limitations and exhibits poor linearity over wide ranges of input. Trigger and threshold levels in the 555 vary as a function of control voltage. While it is desirable to vary the threshold level for modulation, the trigger input level should remain constant for reliable operation. For a linear transfer characteristic a current source should be used for capacitor charging.

For improved operation, an external comparator is used in conjunction with a three terminal current source, the LM334. When the 555 is triggered at pin 2, the discharge transistor at pin 7 opens allowing C1 to charge linearly until it reaches the control voltage level at the non inverting comparator input. The comparator output then changes state resetting the 555 and capacitor. The reset function of the 555 is used rather than the threshold input due to the internal logic of the 555. Without this measure, the 555 output would be enabled for the trigger pulse width irrespective of the comparator output state. For 0 volt input, the output pulse is thus disabled. Selection of C2 and R2 depends upon the trigger input pulse width. Diode D1 is essential to limit signal excursion at the trigger pin, particularly with unclamped bipolar 555's. Although bipolar parts work fine in this application, the 7555 CMOS part is recomended due to its many advantages. For single supply operation, a comparator with an input common mode range including ground should be used. The LM393 dual or LM339 quad general purpose devices suit this requirement. (Note that an input bias resistor for the comparator input is not shown.)

Although only the 555 logic and output stages are used, the addition of the comparator and current source represent little circuit overhead yet greatly improve operation.