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Subject: SMPS feedback

Date: 03/31/03 at 4:00 AM
Posted by: Joe Cocker
E-mail: u_r_so_ugly@hotmail.com
Message Posted:

I'm working on an SMPS feedback circuit and I found this document:

http://www-s.ti.com/sc/psheets/sluu109a/sluu109a.pdf

which has such a circuit (page 7). I have 3 questions:

1. My problem is that I dont understand how the TL431 keeps the voltage constant at 2,5V yet still the voltage across the LED (optocoupler) can change. The LED-voltage must change, otherwise feedback cannot occur.

2. There is D12 and D13 which are two parallel Zener diodes with different voltages, what is the purpose of that?

3. R33, C10, C11 are for pole/zero compensation. But the feedback signal changes slowly because it's DC, so does it make sense to remove them?

Thanx in advance!


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