Opto feedback for smps

[eem2am]

hi

do you know what the pull-up resistor is for at the collector of the opto-transistor in the diagram on page 9 of the UCC38C43 datasheet?

UCC38C43 DATASHEET
https://focus.ti.com/lit/ds/symlink/ucc38c43.pdf

would the circuit work without this resistor?

 

[keith1200rs]

If you mean the resistor on the far left - unlabelled but next to R6 - then it is the load resistor. If you replaced it with a short then the opto wouldn't be able to pull down pin 1 of the IC. If you omitted it altogether then the transistor would have no load and the output on the collector would be zero all the time.

Keith.

 

[FvM]

would the circuit work without this resistor?

According to the datasheet, the comparator output would source 0.5 to 1 mA, so the pull-up isn't absolutely required.

 

[keith1200rs]

According to the datasheet, the comparator output would source 0.5 to 1 mA, so the pull-up isn't absolutely required.

My mistake. I didn't read enough of the data sheet. I guess the resistor would only be required if you had a high opto collector current.

Keith.