I have designed a power supply based on the LM21305. I haven't yet a prototype, but I have performed Orcad spice simulations using the encrypted model.
The model and project can be downloaded from here: www.ti.com/.../snvm705
The simulation results gives rise to concern. It appear that the load required for CCM operation is much high than indicated in the datasheet. My simulations shows the correct switching frequency during startup. But after startup the frequency is reduced and the ripple current increases significantly.
The datasheet section 8.3.4 describes the current at the critical conduction boundary. The simulations indicates that a much high current is required. Can you help explain what is going on?
I have simulated with my own design, and also with the example design included with the encrypted model snvm705.zip
Upon further investigation, it appears that this problem is related to the minimum on-time of the controller. If I reduce the input voltage, performance is as expected. This presents another question:
Only the typical Ton-min is specified in the datasheet. What is the limits (maximum) of this parameter?
The simulation appears to have a Ton-min value much larger than the specified typical 70 ns. What value do the simulation module use as Ton-min?
The typical value is specified in the datasheet at 1.5 MHz. Is this value dependent on the switching frequency? What is the typical/maximum value at e.g. 500 kHz?