Hello,
I'm using a monolithic current-controlled buck regulator IC (i.e. LTC3633A) which integrates an OTA error amplifier and expects an external type II compensation network to stabilize the supply. I don't have any feed-forward compensation stuffed. A curious characteristic of the phase response for this chip is that it dips quite close (~16 degrees) to zero for a 5kHz wide shelf well below the unit gain crossover (fc) frequency. The phase margin at fc is good at ~60 degrees, but I'm wondering if the low frequency dip is indicative of borderline conditional stability even though it hasn't dipped all the way to zero. I can't seem to find technical address of this online.
The gain-phase plot (attached) was obtained at room temperature with a single sample. I've used the Venable method to design the compensation pole/zero and the measured results are as expected. I designed for 60 degree margin and I hypothesize that I can readily lift the 16 degree shelf by increasing to 70 or 80 degrees, which would widen the pole-zero pair, but I'm trying to figure out if I have a stability problem in the first place. At 40kHz, the crossover is not aggressive at all, so I don't think that's the problem. It just seems like the plant phase lag is dominating earlier than expected.
fs=1MHz, K=3.732, fc=40kHz, fz=10.7kHz, fp=149kHz.
Any thoughts?
Thank you in advance,
GuruInTraining
I'm using a monolithic current-controlled buck regulator IC (i.e. LTC3633A) which integrates an OTA error amplifier and expects an external type II compensation network to stabilize the supply. I don't have any feed-forward compensation stuffed. A curious characteristic of the phase response for this chip is that it dips quite close (~16 degrees) to zero for a 5kHz wide shelf well below the unit gain crossover (fc) frequency. The phase margin at fc is good at ~60 degrees, but I'm wondering if the low frequency dip is indicative of borderline conditional stability even though it hasn't dipped all the way to zero. I can't seem to find technical address of this online.
The gain-phase plot (attached) was obtained at room temperature with a single sample. I've used the Venable method to design the compensation pole/zero and the measured results are as expected. I designed for 60 degree margin and I hypothesize that I can readily lift the 16 degree shelf by increasing to 70 or 80 degrees, which would widen the pole-zero pair, but I'm trying to figure out if I have a stability problem in the first place. At 40kHz, the crossover is not aggressive at all, so I don't think that's the problem. It just seems like the plant phase lag is dominating earlier than expected.
fs=1MHz, K=3.732, fc=40kHz, fz=10.7kHz, fp=149kHz.
Any thoughts?
Thank you in advance,
GuruInTraining
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