LM5022: FB pin when using TL431+optocoupler

Ye Zhao

Part Number: LM5022
Other Parts Discussed in Thread: TL431

hi TI engineers

I would like to know the reason to ground the FB pin of LM5022 when using the compensation circuits of TL431 + opto

Opto output is in the common-emitter configuration, which has the correct signal polarity in terms of the control feedback.

In this case, internal op amp is kind of "disabled" by grounding the FB pin. Could you explain it in details?

Ref: www.ti.com/.../tidrf08.pdf

Thanks

over 4 years ago

0 Brigitte over 4 years ago

TI__Guru 70785 points

Hello Ye,

The function of the error amplifier is at the end already present on the secondary side (compare output voltage with reference voltage). Using the error amplifier in the IC is possible, but why adding a second error amplifier stage? So the signal that is coming out of the opto coupler can be used to directly drive the COMP pin to increase the reaction speed of the system somewhat.

Best regards,
Brigitte

0 Ye Zhao over 4 years ago in reply to Brigitte

Prodigy 180 points

hi Brigitte

It seems that the 5kohm resistor inside LM5022 works as the pull-up resistor, working with the optocoupler's output, Rcm and Ccm as the compensation network on the primary side of the optocoupler.

I am not sure how the op amp will work inside the LM5022. Since the FB is 0V, and Ref=1.25V. The op amp's output will saturate at positive voltage output. How would this affect the COMP pin voltage? Will op amp just give an offset voltage at COMP pin. Therefore, in small-signal domain, only TL431 and opto work together at COMP pin?

Please advise.

Thanks