LM5116: Emulation diode

hi Simone,

yes, with Rdemb = 0 the device will be in DEM all the time (DEM = the fet is off and the body diode works as recirculating diode of a non-synchronous buck)...

this is good for efficiency at light load but, as load increases, it could be better for efficiency to switch to synchronous topology...

for details about how to calculate the RDEMB please refer to the spreadsheet or use WeBench  

KR

Vincenzo

Hi Vincenzo

Thank for your cooperation.

At the begin I thought how you have explained : when I set Rdemb=0, the low side mosfet works as a diode with light load. BUT,  I see the Gate of low side mosfet goes ON about 1A with output load.

It seems that the low side fet works ad a diode in light load condition, but not in every load condition. With high load output, the low side fet is piloted as full synchronous mode.

Is it possible what I see in my pcb ?

Thank in advance

simone

Ciao Vincenzo,

no problem !!

So, It's right what I'm watching in my pcb : the LM5116 works in asynchonous mode until the inductor current becomes negative(DCM), afterwards  the LM5116 works in synchronous mode when the buck is in Continous Current mode.

In this way I have the benefits during light load condition (less output ripple voltage) and I get the good performance during high load condition (high efficiency).

Now, I have a doubt : If with Rdemb=0 I get the good performance for both condition DCM and CCM,  when is right to use LM5116 in full synchonous rettification ?

I hope you can help me, because I don't find answer to this question.

Regards

Simone

hi Simone,

let's start from the back... rule of thumb in designing SM DC/DC is to target the pk2pk current ripple into the inductor as 30-50% of the max load...
as the max load goes low, this rule starts to fail as you would need huge inductor: in these application, it is acceptable to have higher ripple...
in these same application the light load efficiency feature helps, so you can have both small size and high efficiency; also in apps in which the load changes over time...
While in apps where the output current is almost constant and always greater than 0.5 the current ripple, then makes sense to use synchronous controllers...

hope this helped a bit
KR
Vincenzo

Ok, it's clear.
So, synchronous rectification is very usefull where the output load is typically high . In application, like mine, where the output load is low for long time and, only for few time I need the maximum output current (for example to charge a battery), it's more correct to use an asynchronous rectification.
I think LM5116 is very a good controller because it's able to manage both this situation.

Exactly!
V.