# Need help on Current Mode Control

#### BlackMelon

Aug 7, 2012
188
I've read the Texas Instruments' application note provided in the attached file. But, the app note doesn't clarify the waveform of Vslope on page 3. Is it a triangular wave or a sawtooth wave?

Thank you
BlackMelon

#### Attachments

• Understanding and Applying Current-Mode Control Theory.pdf
391.9 KB · Views: 132

#### Arouse1973

Dec 18, 2013
5,178
If it's for slope compensation, which I presume it is then it will be a sawtooth.
Thanks

#### BlackMelon

Aug 7, 2012
188
Thank you very much, this will help me do a math of this equation:

Vslope+(Gi*Vs) = Vramp

Where Vs is voltage across current-sensing resistor(Rs). And the other two parameters are explained on the app note.

BlackMelon

Last edited:

#### BlackMelon

Aug 7, 2012
188
The app note says that voltage across Rs is amplified by the gain Gi and then summed with Vslope to produce Vramp.

Here I made a graphical plot, but my Vramp doesn't look like that on the app note. Is there anything that I misunderstood?

BlackMelon

#### Attachments

• P_20160614_105053.jpg
224.9 KB · Views: 81

#### Arouse1973

Dec 18, 2013
5,178
What Vramp are you looking at in the app note? Why does your Vramp not start at zero volts?
Thanks

#### BlackMelon

Aug 7, 2012
188
What Vramp are you looking at in the app note?

It is the Vramp on Page 2 in Figure1. Yes, it should start at zero. But please let me explain why I need to do all of these plots.

Figure 2 shows that the Vramp is not just a sawtooth generator. But it's the summation of "amplified Vs (Gi*Vs)" and "Vslope", for it states that:

" For current-mode control, the ramp is created by monitoring the inductor current. This signal is comprised of two parts: the ac ripple current, and the dc or average value of the inductor current. The output of the current-sense amplifier Gi is summed with an external ramp VSLOPE, to produce VRAMP at the inverting input of the comparator"

So what I have done is just to plot the waveform of "amplified Vs" and Vslope to see if their summation is exactly the same as Vramp on Page 2. But it doesn't. So did I make any mistakes?

Thanks
BlackMelon

#### Arouse1973

Dec 18, 2013
5,178
From my view point I think Fig 1 is just showing the classic PWM achieved by a basic comparator and giving you an example waveform. It shows a D.C control line changing the pulse with over many cycles. It would have been nice for them to show a waveform of the different inputs and outputs of Fig 2.
Thanks

Last edited:

#### BlackMelon

Aug 7, 2012
188
Thank you for pointing out what is missing. I think I will mail this to Texas Instruments and will update you when they reply ASAP.

Thank you
BlackMelon

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