Hi,
I have a strange issue that I cannot solve We are using the LT3748 to convert 25-52VDC to 12VDC together with this transformer POE71F-12 which has a winding factor of 2 and a primary inductance of 15uH and a saturation current of 6.6A. When powering up th LT3748 I am using an external IO to the enable pin, providing 3.3V when the converter shall go active. This setup is working fine with no load applied. If I enable the converter it starts to regulate ok, with a small load. If I then add a lower value resistor it continues to regulate fine as expected. But now if i change and have the load connected from start, then the conevrter does not activate at all. The voltage never increase as expected, it is stuck around 1V. I have tried to bypass the current sensor, but no difference really. I have tried to adjust the soft start, slowing things down to no help either.
What could be the reason for this problem? I am out of ideas. This is the second design on this, but we cannot crack it. The layout is identical to you layout on the evaluation board. Something is preventing the converter to start. We have elaborated with input and oputput capacitance and gate resistor values to the MOS, to see exactly the same behaviour. This is a project going to market Q3 so it is very urgent to get input here for me. I have read the datasheet over and over and calculated all the necessary values many times, double checked the layout and assembled pcb. Nothing seems to be an obvious culprit. The only question I have from my standpoint is the enable-pin, which are actively driven by us. Maybe the en/uv pin is needed in the regulation loop somehow? Do we actually need to do a vin divider to make this work? There are no primary side power dips measurable.
Br,
Mikael
