SNVSB79B November 2018 – May 2020 LM25180
When the internal power MOSFET is turned off, the leakage inductance of the transformer resonates with the SW node parasitic capacitance. The resultant ringing behavior can be excessive with large transformer leakage inductance and may corrupt the secondary zero-current detection. In order to prevent such a situation, a minimum switch off-time, designated as tOFF-MIN, of maximum 450 ns is set internally to ensure proper functionality. This sets a lower limit for the transformer magnetizing inductance as discussed in the Detailed Design Procedure.
Furthermore, noise effects as a result of power MOSFET turn-on can impact the internal current sense circuit measurement. To mitigate this effect, the LM25180 provides a blanking time after the MOSFET turns on. This blanking time forces a minimum on-time, tON-MIN, of 140 ns.