To support higher duty cycle operation, the TPS56837 includes an On-Time Extension function. This function operates by increasing the High-side FET On-Time beyond that during normal operation, thus lowering the operating frequency and allowing large duty cycles to be maintained.

From application note Large Duty Cycle Operation With the TPS568230, we can know that TPS568230 extends its on-time through twice or trice one shot trigger signal. However, different with the OTE of TPS568230, TPS56837 is using an advanced one shot circuit to expand its on-timer.

The On-Time Extension function is implemented following Figure 3-3 flow chart.

Figure 3-3 Flow Chart of On-Time Extension Function

Large duty cycle operation of TPS56837 only happens during on-time. When the duty cycle is larger than 62%, the device starts to detect its loop response signal and decides whether extends the on-time or not cycle by cycle. However, the on-time still has a maximum value and can't be extended once it gets the max. Figure 3-4 shows the performance under OTE. The on-timer is extended per loop response signal and the Feedback voltage could keep near the reference point.

Figure 3-4 On-Time Extension of TPS56837