I have an application that requires Vout = 28V/28A 300mS single pulse at Vin= 16-50VDC. I think the LM5118 is a good candidate for this buck-boost regulation application. I am currently working on the LM5118 demo board and be able to meet the requirement at Vin < 20V. My problem is that when Vin<20V, my output voltage drop about 2-3V or the output voltage collapsed depending on the compensation and Rsense settings. When the output collapsed, it was running at a stop-start oscillation mode. What wiould be the cause to shut off the LM5118 output in the case. Another question is that will the LM5118 be capable for this output voltage and current? Has anyone done similar design?
Usually when you can't get full current, especially at lower voltages can be traced to the input supply not supplying the necessary current. As the input decreases, the input increases and if not careful, the input supply will current limit. Check that out. The input supply should not drop much under load. Also, the input supply will need to supply the peak current needed by the regulator unless you add a lot of input capacitance. Use the quick start which will calculate the peak current necessary.
Regarding you design, I see a couple of problems using the LM5118. First, to switch the very large currents you will need to add a gate driver. The LM5118 will not be able to effectively charge and discharge the large gate capacitance you will have with many MOSFETs in parallel, or with very large current MOSFETs. Also, this is a two switch buck-boost which results in two diodes in the power path. You efficiency will not be very good with the rather large power loss at the current levels you will require. The LM5118 was not really designed for such an application as yours. You probable could make it work, but it will not be optimum.
Thanks for the advice. In this design, I am taking the advantage of fixed load, short power pulse to use the small size TDSON-8 package FETs. So the LM5118 is able to drive them well. When I played with the modified demo board, I found the LM5118 Emulated Current Curve (Fig-9 on page 14 of the LM5118 data sheet) plays a big role of the operating mode. Is there any trick or rule of thumbs of optimizing the Pedestal Level (Rsense) and Ramping Rate (Rramp/Cramp) of the curve?
There isn't any trick. Just set Rsense low enough not to trigges current limit, and then calculate the ramp capacitor based on tthe values of Rsense and the inductor.
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