The larger design is for a 170 V input, 500 V 2 mA output using a flyback converter. I will refer to the schematic on page 29 of the LT3751 switching controller, also found below.

The feedback pin of the switch controller controls the mode of operation: CHARGE MODE, REGULATION, NO LOAD (Fig 1 page 10 LT3751), also included below. I wish to avoid exceeding 1.34 V on this pin as that pushes it into NO LOAD, and the output voltage can increase up to 10% above nominal (page 14 LT3751).

Using the Isolated 282 V Voltage Regulator on page 29 LT3751, wouldn't this result in the voltage on the LT3751 switching between 0 V and Vcc?

If this is the case, how can I avoid this? I wish to operate in REGULATION mode. Is there a analog optocoupler type device, like a isolated operational amplifier what would pass the voltage sensed between R17 and R18 to the LT3751 FB?

Note the FB pin is internally regulated to 1.22 V (page 8 LT3751), so it would be acceptable to apply a voltage between 1.16 V and 1.22 V when the output is low, and between 1.22 and 1.34 V when the output is high. How could I achieve this using the optocoupler? Also, would the optocoupler present any significant resistance itself into a voltage divider circuit (ON/OFF).

Isolated Regulator enter image description here

  • \$\begingroup\$ How accurately must your output voltage be controlled? Can you analyze the LTC datasheet to determine what is possible with OPIso feedback? \$\endgroup\$ Mar 28, 2018 at 3:08
  • \$\begingroup\$ @analogsystemsrf A 10% increase on the 500 V max would be unacceptable for the design, and greater than the transformer rating. A 5 V peak to peak would be acceptable. \$\endgroup\$
    – A.S.
    Mar 28, 2018 at 3:14
  • \$\begingroup\$ The figure which you've posted from the datasheet mentions NEC PS2801-1 for optocoupler. Do you have reasons why that would not work? \$\endgroup\$ Mar 28, 2018 at 3:45
  • \$\begingroup\$ @NickAlexeev wouldn't that optocoupler diver the LT3751 FB pin between Vcc and ground constantly? I want to avoid the 10% increase in output current when the FB pin goes above 1.34 V. \$\endgroup\$
    – A.S.
    Mar 28, 2018 at 4:03
  • \$\begingroup\$ @A.S. It operates as an analog optocoupler. The output transistor acts as an analog current sink. (The output of the optocoupler isn't linear in this case, but you can still use it to close the feedback loop.) \$\endgroup\$ Mar 28, 2018 at 4:13

1 Answer 1


"Using the Isolated 282 V Voltage Regulator on page 29 LT3751, wouldn't this result in the voltage on the LT3751 switching between 0 V and Vcc?"

No, the optocoupler does not act as a switch. The LT4430 compares the feedback to a reference, and drives the optocoupler to whatever photocurrent it needs to maintain the feedback = reference.

So it will regulate the output voltage to the desired setpoint. The linearity of the optocoupler is only important to the extent that it affects the loop gain of the control loop. If the control loop is properly designed it's not a factor at all.

This is similar (but more expensive) than the optocoupler/TL431 scheme that has been used in hundreds of millions of AC adapters for many years:

TL431 secondary side feedback and isolation

  • 1
    \$\begingroup\$ Would you recommend using the exact feedback schematic on the 282 V example for my 500 V application, with the R16 R17 R18 resistors adjusted for the 500 V desired output? \$\endgroup\$
    – A.S.
    Mar 29, 2018 at 5:37
  • \$\begingroup\$ Yes, though your control loop may need tweaking, meaning the values on the Comp pin of U2 may not be correct for your application. Watch the voltage across your feedback resistors vs. their rating and split into 2 if necessary for good derating. \$\endgroup\$
    – John D
    Mar 29, 2018 at 19:23
  • \$\begingroup\$ Thanks John D, I posted a follow up question regarding the feedback behavior. I think I got it but could use a second opinion. electronics.stackexchange.com/questions/365280/… \$\endgroup\$
    – A.S.
    Mar 29, 2018 at 19:29

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