LED resistor in the TL431

Question

I read the Christophe Basso APEC 2011 seminar, about this figure I have one question.

1.) Why RLED,max need to below this equation(Red line), what happen if RLED above this equation(Red line)

Answer 1

If the resistor is too large, there will be not enough current available to light up the LED brightly enough, so the feedback does not work properly.

Answer 2

The circuit stabilizes V_out. And, depending on the response speed of the feedback block, V_out fluctuates around an average value. Look at the dark blue lines on the oscillograms below:

Img Src

The opto's LED senses this fluctuation: If the output voltage increases then the opto's LED current increases (basic Ohm's law). If opto's LED current increases then the opto's collector current increases (I_C = CTR x I_LED). If the opto's collector current increases then the feedback voltage (V_FB) decreases (There's a pull-up). So, if the output voltage exceeds (or hits) a certain value then the feedback voltage decreases to a threshold value. And the controller adjusts its PWM depending on this feedback voltage. You can guess what will happen if the output voltage decreases and hits a threshold value.

what happen if RLED above this equation

If R_LED is too high then the amount of LED current variation will be too low (For example, for I_LED=0.2mA you may want to detect 0.21mA or 0.19mA). This very little variation may not be sufficient to affect the feedback voltage (Think about the effect of CTR). To affect the feedback voltage, the LED current variation should be higher. This higher variation drops higher voltage across R_LED. This means the output voltage will fluctuate more. This results in poor or even no regulation of the output voltage.

Likewise, if R_LED is too low then the LED current will be too high. With too high LED current, it may not be possible to detect the LED current variation (For example, for I_LED=10mA you may want to detect 10.05mA or 9.95mA). The same as above applies.