Datasheet of LM5117

York Chung

Other Parts Discussed in Thread: LM5117

An equation is shown in page19 of the datasheet. I have no idea how this equstion is derived. Please tell me the process of derivation if you know. It's really helpful for me.

regards,

York

over 6 years ago

0 Piotr F. over 6 years ago

TI__Intellectual 2360 points

Hi York,

If you are asking how slope compensation is operating in a peak current mode control buck topology, please have a look at following articles:

www.eetimes.com/document.asp - Part 1
www.eetimes.com/document.asp - Part 2

The sub-harmonic oscillation issue, the purpose of slope compensation are nicely explained there.

Equation 19 from LM5117 datasheet describes the characteristics of LM5117 internal slope compensation circuitry. As you can see K factor is introduced there, that is in relation to dl1/dl0.

The K factor value is defined by the circuit components. You can find the K factor equation in Table 1 in LM5117 datasheet (page 31 in English version).

If you have more question I will be happy to answer,
Piotr

0 York Chung over 6 years ago in reply to Piotr F.

Prodigy 100 points

Hi Piotr,

Really thank you for your reply. Actually I am not asking how slope compensation is operating in PCMC. I have read a few articles about it already and know a little about it.

The LM5117 uses an emulated control ramp produced by a resistor and a capacitor to emulate the waveform of the current in inductance. The K factor value is defined by Rramp, Cramp and Rs, so the K factor just makes sense in the emulated situation. But the dl1/dl0 represents the actual current variation. How to get the relation between the K factor and the dl1/dl0? In other words, how to link the emulated ramp to the actual one? That's the point that I can't understand all the time. I am looking forward to your reply.

regards,

York

0 Timothy Hegarty over 6 years ago in reply to York Chung

TI__Mastermind 49207 points

Hi York,

Here is additional material on current-mode control for your review:

http://www.how2power.com/pdf_view.php?url=/newsletters/1406/articles/H2PToday1406_design_TexasInstruments.pdf

http://www.how2power.com/pdf_view.php?url=/newsletters/1407/articles/H2PToday1407_design_TexasInstruments.pdf

Regards,

Tim

0 Timothy Hegarty over 6 years ago in reply to York Chung

TI__Mastermind 49207 points

0 York Chung over 6 years ago in reply to Timothy Hegarty

Prodigy 100 points

Hi Tim,

Thanks for your reply. I have read the similar articles you sent me before and know that there is a matter of current-mode control stability and it can be solved by a compensator. I don't really think you get the point that I want to express.

I just want to know how to relate the emulated situation with the actual situation. Why could the emulated ramp waveform represent the actual current waveform in inductance. The datasheet just tells us the result(dI1/dI0=1-1/K) to link the emulated situation to the actual situation directly but does't tell us the procedure. Maybe there are more details that not mentioned in the datasheet, aren't there? I will be grateful if you tell me more about it! Thank you!

regards,

York

0 Timothy Hegarty over 6 years ago in reply to York Chung

TI__Mastermind 49207 points

Hi York,

Please review the following links for further insight, particularly as it pertains to the emulated current-mode control technique:

http://www.ti.com/lit/an/snva542/snva542.pdf

http://www.ti.com/lit/an/snva537/snva537.pdf

Regards,

Tim

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Datasheet of LM5117