Part Number: LM5116
I would like some assistance from an FAE regarding a design with an LM5116 buck controller.
My problem is that when the duty cycle goes above 70 %, the controller goes into an odd mode where the square wave suddenly becomes very broken. If I start out at a lower output voltage, the switching voltage looks very nice. A good, steady square wave with no visible ringing. However, when reaching a certain point somewhere above 70 % duty cycle, the pulse width suddenly varies wildly from pulse to pulse.
I don't think that it has anything to do with the compensation network, because when it is not in the "odd" mode, it handles load steps and other changes nicely. Again, no ringing or significant overshoots. Also, lowering the loop gain considerably had no effect.
I have attached a snapshot of the switching voltage in the "odd" mode. As you can see, some large spikes appear, some pulses are almost missing, and some are nearly 100 % in width. To me, this doesn't look like an untrimmed compensation network, because it happens way too fast.
Would someone please have a look at this, and come up with some good ideas? Snubber circuit? Small resistors in series with the FET gates?
Best regards, Erik
P.S. I expect to receive a collection of small resistors today, that I intend to insert in series with the top FET gate, and see if it makes a difference.
