current mode buck compensation

[benemorius]

I'm looking at the datasheet for TLE6389 trying to figure out the loop compensation. The datasheet isn't very verbose about it, and I lack sufficient experience to wing it.

It has a COMP pin but says only to hang a 640R resistor and 2.2nF cap off it. It provides no equation or explanation for how these values were chosen.

I'm trying to adjust the compensation for a 15uH inductor rather than 47uH as in the application circuit.

[amspire]

They don't give away too much in the data sheet, do they.

They use the same compensation in the circuits regardless of the voltage divider ratio, so I assume that the compensation circuit, by adding a zero at 100KHz, is stable under all loop gains. Only a guess though - there is no information about the compensation at all. The block diagram doesn't show how the compensation circuit is connected to the voltage regulator circuit.

[rbola35618]

I would recomment that you put what I call a Veneable resistor of 10 ohms between the Vout and pin 3 of the PWM. You can then may be able to measure the crossover frequency (loop bandwidth) by injecting a signal (sinewave) from a function generator and then measuring both Vout (channel 1)  and Pin 3 (channel 2). You increase the frequency of the generator until both channels are equal. At the frequency where they are equal is then the loop bandwidth. If you take measuremet (gain and phase) at 10hz, 100hz, etc at every decade, you can then make a plot of the total loop response.

I will be making a video demostrating this poor's man Frequency Response Analyzer (FRA) in a week or so and then compare this technque with real FRA (Ridley and Venable FRA)

Robert 

[amspire]

I will be making a video demostrating this poor's man Frequency Response Analyzer (FRA) in a week or so and then compare this technque with real FRA (Ridley and Venable FRA)

Robert, that sounds like it will be a great post. Looking forward to it.

[benemorius]

Thank you both. That has helped considerably.

I measured the loop response both with and without the compensation connected. I've attached an image of both. The instability is now clearly visible, as is the effect of the external compensation, but I'm still not sure how to correct it. Actually it's even unstable when built as shown in the datasheet. Go figure.

Adding some series resistance to the buck capacitor (I'm using ceramic, compared to tantalum in the datasheet) is the only way I've been able to significantly improve the stability, which is interesting but not a solution. Playing with the compensation alone I was only able to get the phase margin up to 30 degrees at most. That's a lot better than 0 but it still oscillates.

[amspire]

Beautiful graphs - love it!

It should be stable with a 30 deg margin, so I suspect there is some more phase shift somewhere. Not sure how you did your measurements, but did it take into account the phase shift caused by the filter cap on the rectified output? If it did, no load will be the worse case scenario, and full load will be the most stable scenario.

Some switching regulators need a minimum load for just this reason, but I do not like that as a solution. What is the oscillation frequency of the instability? Was t 28KHz? Can't quite read the graph. You could try adding some lead compensation in the voltage divider to the regulator - like a cap in parallel to the top resistor in the divider.

What you normally want to see is a 6db/octave roll-off in the loop gain above the zero gain point  - much more then that and you will almost certainly have stability issues to deal with. It looks like that even with compensation, you are getting more like a 12db/octave roll-off between 10kH and 20kHz. See what some lead compensation at 15Khz does.