SNIU028D February 2016 – September 2020 UCD3138 , UCD3138064 , UCD3138064A , UCD3138128 , UCD3138A , UCD3138A64
The RESON_MODE_FIXED_DUTY_EN bit only controls the duty cycle width in the resonant modes. With the default (0) value, the duty cycle comes from the filter directly. This is for use above the lower resonant frequency, Fmin, and provides a duty cycle that fills half the period minus a fixed dead time.
Setting this bit causes the pulse width to be derived from the Auto Switch High Upper Threshold Register.
This bit is generally only set for LLC for the Sync FETs in the mode where the output frequency is below the lowest resonant frequency of the circuit. At this point any increases in pulse width are not beneficial, so they are stopped. There is also a waveform showing the modes in Section 2.11.1.
This bit is duplicated in the AMS registers.