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LM2841 Duty Cycle

We design a Buck converter for LM2841X. The input voltage is 16V ~32V, output voltage is 3V , load current is 0.15A. The schematic is the same as that of Webench, L is 120uH, C is 22uF*2, Cboot is 150nF. We probe duty cycle by voltage of SW pin. 

We found that the duty cycle of the circuit is not constant. Why? The Buck should be in continuous conduction mode for the working condition, and the duty cycle should be constant. And for higher input voltage,  it is more obvious.

The following is the captured oscillator image. http://e2e.ti.com/cfs-file.ashx/__key/communityserver-discussions-components-files/858/3652.duty.tif