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.
And I have reviewed some motor driver circuits, like this one 
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?
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.
(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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