UC1845/UCC38C45 comp pin characteristic

Nassir Abdalla

Other Parts Discussed in Thread: UC1845, UCC38C45

I am working in a flyback PS running on DCM using UC1845 or UCC38C45 the compensation is done by directly modulating the comp pin source current FB pin is at 0V.

What is the slope of the linear part of Vcomp vs Icomp (the source current of the comp pin) for both chips.

over 12 years ago

0 John Bottrill over 12 years ago

TI__Mastermind 25130 points

Nassir,

The output of the voltage error amplifier is a voltage controlled current source current when the FB pin is held low. As such the effective "IMPEDANCE" is very high. Though many people do operate with the FB pin grounded and a opto-coupler pulling down on the COMP pin we do not design or measure the output impedance of the error amplifier in this configuration. Therefore Vcomp vs Icomp is undefined and will vary from part to part.

I do not like this method because, in my opinion, you are dealing with two many uncontrolled variables to get reliable operation.

I would recommend that you review http://www.designworldonline.com/articles/6503/5/Feedback-Loops-Using-Integrated-Circuit-Error-Amplifiers.aspx and see if this makes more sense for your application.

Regards,

John

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UC1845/UCC38C45 comp pin characteristic