LM5118 Higher input voltage

Also I have run the LM5118 Quick Start Component Calculator and it gives me values for the components which I have been able to find. However, when I used the WEBENCH it says that the Vmin is too low and will not give me a solution. This is why I am confused.

I'm not sure what happened in Webench but the part should work fine with the new input voltage requirements.

Ron Crews

Ok great! I am going to be swapping out the external components on the current power board to match the ones the calculator gave me so there shouldn't be any issues since it can take the voltage.

I just got a response from TI after they were looking into my question for 2 days. I do not understand why the current would be too high I am already successfully passing 5A through it at a lower voltage so increasing the input voltage by this much shouldn't push it over the edge right?

Hello Robert,

I apologize for the delay in my response.

I’ve reviewed our product selection and unfortunately we do not have a pin or package compatible device for the LM2677 with a higher input voltage range.

In regards to using the LM5118, the amount of current you are trying to pass far exceeds the specifications of the device.  As long as you were using the device in a manner in which the input voltage does not reach into the transition or boost boundaries of the output, the device will operate in buck mode consistently.  The LM5118 calculation spreadsheet will help in allowing you to visualize the input voltage threshold for operating in buck only mode.

http://www.ti.com/tool/lm5118quick-calc

If you have any questions, please feel free to contact us.

The LM5118 is a hard part to use at higher output current since the peak currents and input currents become very large.  Since the LM5118 is a controller, in principle the output current depends only on the external power componts.  However, because the peak currents become so large, and this is especially true in the buck-boost mode since the inductor (and MOSFET current) is no longer just the average output current as in the pure buck mode, but is the output current divided by 1-D where D is the duty cycle.  We have had user use this product at currents up to 10 amps output, but it requires special parts, a very good input supply and a lot of input capacitance.  Most people are not sucessful.  Look at your peak inductor current in the quick start and observe how high it is at minimum input voltage.  If you are prepared to deal with this amount of current and if your input supply will handle the peak currents and if you can get enough input capacitance to keep the input ripple voltage under control, then you can make it work.  And don't forget to size you inductor to handle the peak currents without saturation and also stay inside its temperature rating.  This also applies to the diode  and other power components.  This is why we don't recomment high output currents in most cases.

Regards,

Ron Crews

Ok thank you. I am sizing my inductor to handle 27A. I am also planning to only operate it in Buck mode since the input voltage will never go below the output voltage of 24V (it will stay around 36V).  Also the output current of 5A is worst case, it is more likely to be around 3A. So all in all I think it will be ok. We shall see. As long as the duty cycle is below 75% I should be fine.

Thanks again for the help.