8.3.8 Minimum On-Time and Off-Time

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 can corrupt the secondary zero-current detection. To prevent such a situation, a minimum switch off-time, designated as t_OFF-MIN, of a maximum of 360 ns is set internally to ensure proper functionality. This sets a lower limit for the transformer magnetizing inductance as discussed in Detailed Design Procedure.

Furthermore, noise effects as a result of power MOSFET turnon can impact the internal current sense circuit measurement. To mitigate this effect, the LM5181-Q1 provides a blanking time after the MOSFET turns on. This blanking time forces a minimum on-time, t_ON-MIN, of 140 ns.