Electronic – TL494CN and error amplifier

power supplypwmswitch-mode-power-supply

I'm designing a PWM output for a DC motor and I'm using
TL494CN chip.

It got two error amplifiers and one feedback. I can't understand why there
are error amplifiers and a feedback there? What does that error amplifier do
in there? I can't figure out.

enter image description here

And I have reviewed some motor driver circuits, like this one
enter image description here

I can't figure out why he connected reference voltage to error amplifier IN- pin?

Is that error amplifier gives a unity gain? Is that what it mean?

Best Answer

The intended use of the TL494 is a closed loop SMPS controller. Error amps are used in the feedback loop to make the output voltage regulated.

enter image description here (source: fig.35 here)

On the other hand, you are re-purposing TL494 as a PWM motor controller. You only want the PWM function without closed loop feedback. You don't need the error amps, and they shouldn't interfere. Notice that:

  • Error amps are connected to the feedback node through diodes
  • Feedback node is accessible on pin.3.

If you can have the output of the error amps always below the feedback (pin.3), then the diodes will always be reverse-biased, and the error amps will not affect the operation of TL494.

Look at the 2nd circuit in the O.P. Both error amps are wired as comparators such that V(IN+) < V(IN-) always. The output of the error amps is always low and lower than the feedback.
That is what is needed.

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