LM5156H: 12V/24Vin, 48V Vout Boost design Review Needed

Hello Pradeep,

Thank you for implementing the circuit with out device. For the UVLO circuit, you need to stay below the lowest input voltage, as long as you do not have any idea when which voltage is applied.

AGND and PGND shall be separated and only connected at one point close to the IC, Powerpad best. Please review the datasheet for the layout hints at the end of the document. You might as well review the boost EVM layout of LM5156 (without H).

For the MOSFET, it depends on the load if you create a heavy load transient with it or not. In general I do not like MOSFETs that suddenly turn on as they can as well suddenly turn off. So I would slow down the turn on with some capacitance from gate to drain or source.

Best regards,
Brigitte

Hi Brigitte,

Thank you so much for your detailed comments..

And, I also have concern on the inductor part for L1 (SER2915H-153KL) which was suggested in the webench schematic attached in my above post.

This is very huge in size. Since we need max. output current of only 2.5A, do we still require this big inductor?? Could you please suggest any smaller size alternate for this part..

Thanks

Pradeep. S

Hello Pradeep,

The most important parameter for the inductor is the saturation current. When you have an output power of 48V * 2.5A = 120W and an estimated efficiency of 85% (this is a bit conservative, but I like margin), the input current at the minimum input voltage of 10V is about 14A. (The highest current in a boost converter happens at full load and lowest input voltage).

So the inductor needs to have a saturation current that is higher than 14A. I would even want to have an inductor that is not going into saturation if the controller operates in current limit. The current limit threshold is 107mV max, which would translate to almost 18A with a 6 mohm current sense resistor.

So the inductor should be a soft saturating inductor with a saturation current of at least 14A, better 15A (soft saturation means that the inductance over current curve of the inductor is slowly decreasing and does not show a point where it suddenly decreases very fast).

To find an inductor that fits your needs, I would recommend to contact some inductor manufacturers and check out their available components. Webench has a limited amount of choices included.

Please keep in mind that a smaller inductor often means a higher series resistance and the series resistance will heat the inductor additionally to any switching activity. So please calculate the power dissipation at maximum current on the series resistance to make sure that the inductor really fulfills your requirements.

Best regards,
Brigitte

Hi Brigitte,

Thank you very much for your clear explanation of the facts here. Really helped us a lot!!

BTW, We have now chosen to go with 48V, 1.5A output limitations, hence I hvae updated the webench circuit accordingly and arrived at the new schematic having L1 inductor value changed to 16uH. When I checked for the L1 alternates in webench, I identified that It was showing inductors having rated DC Current of 9-10A range (size also big). So I optimized the DC current limit to 3-5A range, then webench listed 5A rated inductor alternates, among which I have chosen one (7443251600) which is now shown in the attached schematic. Could you please check that whether this 5A optimization in the inductor is good to go here..

Thanks,

Pradeep. S

WBDesign35.pdf

Hello Pradeep,

I would say with this inductor you are good to go. It looses inductance, but will not go easily into saturation. So I would recommend to do excessive tests at lowest input voltage and highest output load to check if the temperature is still ok, but I expect it is.

BTW, I recommend to review the layout of the LM5156 boost EVM. The IC is in a different package, but the power stage gives a good example how to lay it out. If you are using Altium as CAD software, you could even download the design files and only optimize the area around the IC.

Best regards,
Brigitte