I have a design that needs to work in buck-boost regulating the output to 21 V with VIN varying from 19V to 37V. I am seeing the buck work just fine, the boost works with lower loads on the output. When I start loading the output significantly the LM5118 sets the duty cycle of the Q2 transistor to 100%, basically making the circuit into a linear regulator, droping the output voltage to 15V. I have simulated and found that WEBENCH shows the same approximate behavior. My application is around 4 Amps, which works fine in buck mode, however in boost mode the LM5118 doesn't seem to regulate the Q2 duty cycle correctly. I have tried lowering the RAMP capacitance. Currently my RSENSE is 8 milliohms, RAMP capacitance is set to 330 pF. Any help is appreciated.
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