I several hours of researching and examining, there are still questions I have about the feedback of this SMPS.

The LT4430 has a FB regulated voltage of 0.60 V. The way I understand this is.. if the voltage presented at the LT4430 FB pin from the voltage divider at the output is less than 0.60 V, then the following happens...

  • The LT 4430 will reduce its output voltage at the OPTO pin.
  • The current through the U1 optocoupler LED will drop
  • The current through the transistor will drop
  • The voltage across R7 will drop
  • The LT3751 will adjust to increase the output voltage

And the opposite happens when the voltage at the LT4430 FB pin is above 0.60 V. Is my analysis so far correct?

Then here is what I still struggle to understand..

The LT3751 SMPS-Controller has a regulated voltage of 1.22 V on the FB pin. How does the LT4430 Opto-Driver output the proper voltage to provide feedback about this 1.22 V regulated voltage on the LT3751.

As I wrote the remainder of my question, I think the concept clicked. Still it would help to see someone else confirm or correct it.

For example, if the output voltage is too low, leading to the LT4430 FB reading below 0.60 V, then the feedback voltages and current increase, including the voltage on the LT3751 FB pin. If the voltage on the LT3751 is still below 1.22 V and the output voltage is still below the desired level, then the LT4430 will again increase the output voltage on the OPTO pin. This will repeat until the LT3751 FB is above 1.22 V, at which time the output voltage will increase. Once the output voltage increases past the desired level, the opposite happens. The LT4430 will reduce the voltage on the OPTO pin, decreasing the current through the optocoupler, and reducing the voltage across R7 and the LT3751 FB pin. This is a constant back and forth regulating the output voltage?

Voltage Regulator

  • 2
    \$\begingroup\$ Blimey, 10 questions in a week. You're a curious one. \$\endgroup\$ Mar 29, 2018 at 9:48

1 Answer 1


Your description of the working of the circuit is correct.

The circuit does not maintain 1.22V on the feedback pin of the LT3751, the voltage there will fluctuate as the signal from the LT4430 changes,

When the voltage on FB is high the LT4430 stops feeding power into the transformer, when it is low it resumes feeding power into the transformer, in this way the output voltage is maintained.

There's probably a simulation of this circuit available for the ltspice simulator. (free download from linear technology, runs on Windows, OS/X, or Wine)


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