I still can't figure out what is function of the operational amplifier?
Electronic – There is an op amp in switching power supply
amplifierswitch-mode-power-supply
Related Solutions
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.
The PNP who's collector node you have highlighted - let's call it Q1 (as well as the NPN directly above it on the schematic) are for current limiting at the output. The output current develops a voltage over the two resistors connected to OUT - IOUT*RSENSE. When this voltage exceeds a diode voltage, Q1 starts to turn on. The positive and negative current limits work differently, but the negative current limit which you have asked about specifically operates by sending a current back to the input and reducing the differential current generated by the input pair!
Best Answer
It is an error amplifier. The amplifier provides an analog feedback signal through the LED/phototransistor to control the PWM across the isolation barrier.
Here is what a real circuit typically looks like (schematic from this TI document):
Since the circuit is powered by the isolated voltage, the LED is initially 'off' and the PWM runs at maximum output. As the voltage approaches the target voltage the LED current increases, causing the PWM to back off and (if tuned correctly) keeps the voltage from overshooting the target, whilst minimizing the rise time. I agree that your block diagram appears to be reversed from what is normally possible.
The three-pin TL431 contains both an op-amp-like amplifier and a fairly accurate and stable band-gap reference, all for a penny or two!