• State
  • Locked Locked
  • Replies 2 replies
  • Subscribers 63 subscribers
  • Views 2682 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

UC3843 Slope Compensation

Bob Krysiak
Prodigy 80 points
Other Parts Discussed in Thread: UC3843

Good afternoon,

I'm using this  part, UC3843 for a project I'm working on and I have a couple of questions.  Please refer to the Unitrode U-111 document. I'm interested with calculating the value for the R2 resistor.  Please see figure 6. Emitter Follower Circuit on page 3-108. 

1.  The Turns Ratio: 15/1, (Np/Ns) value is used in the calculation.  My application is a transformer less boost converter.  Can I assume a value of '1' in equation 2?

2.  d V osc calls for a value of 1.8.  When I check the date sheet for this part I see a typical value of 1.7 for the oscillator voltage.  Which value is correct?

3.  The Slope Compensation (M) value is set at 0.75.  How is the value determined?

Thanks!

  • 0
    TI__Mastermind 25570 points

    Hello Bob

    1/ If the system is transformerless then assume a transformer with a 1:1 turns ratio as you note.

    2/ I'd use the value in the datasheet. The DS is the document we all run to in case of questions and is very closely tied to the test program limits. The value in the other document may have come from an earlier version of the DS or from a value which seemed sensible while the part was being designed but which got altered slightly as the device went into production or may simply have been a typo.

    3/ The M = 0.75 value is suggested in the paragraph 'Determining the Ramp Slope' on page P3-106 of U-111  ( http://www.ti.com/lit/an/slua110/slua110.pdf )  Anyhow, the whole thing is to calculate the inductor downslope as it appears at teh CS pin and to a compensating ramp with 0.75 times that slope. The app note describes the whole process better than I can here but do feel free to ask questions if something doesn't make sense.

    Regards

    Colin

  • 0 in reply to Colin Gillmor
    Prodigy 80 points

    Colin,

    Thanks for the info!