LM5051

Hi Mark,

The LM5051 is a regulation mode ORing controller. It is meant to regulate the forward voltage drop to 12mV (typ). Thus at light load, it's intended to have a smaller Vgs voltage than at higher load.

The gate-source voltage it drives is entirely dependent on the load current and MOSFET's Rds-on. The gate-source voltage will regulate until load current * Rds-on = 12mV (typ). So if you want to drive more Vgs at light load, then you need to pick a higher Rds-on MOSFET.

You can check out the "MOSFET SELECTION" section on page 15.

Thanks,
Alex

Thanks,

Mark

Hi  Alex，

Could you help to explain why LM5051 will work at regulation mode in light load? Any risk if the FET always work at switch mode?

If use one higher Rds-on MOSFET, we can drive more Vgs at light load, but the efficiency will be reduced...

Thanks.

Hi Chris,

The LM5051 is a regulation mode ORing controller. Therefore it will decrease the gate voltage until the drop across the external FET is 12mV.

So if the Rds-on of the FET is low, then the Vgs will be low until its Rds-on increases to satisfy Vds = 12mV.

If you cannot use a larger Rds-on MOSFET, then just keep in mind the Vgs will be low at light load.

For example, if the load current is 0.5A (light load) then the GATE will be reduced until Rds-on equals 24mohm, so that Vds = 12mV.

Then if load current increased to 10A quickly, that would cause a voltage drop of 10A * 24mohm = 240mV drop across the FET while the GATE continues to rise and Rds-on decreases.

Another example, if the load current is 10mA (very light load), then the GATE will be reduced until Rds-on equals 1.2ohm in order to get 12mV Vds. So if you stepped up to 10A quickly, that would cause a voltage drop of 12V across the FET while the gate continues to rise and Rds-on decreases.

So in general, regulation ORing controllers are not very good when performing large steps from a very light load to a high load. But if you are doing load steps from 0.5A to 10A, then the drop may be ok during the transition.

If you are just trying to prevent oscillations at light load (because minor variations in current will produce large Vds differences), then you could install an RC filter on the sense lines like this (except for LM5051 it will be on the low side):

A value of 10ohm and 0.1uF will provide 1us time constant of filtering to help prevent oscillations at light load.

Edit: To learn more about hysteretic ORing (ON/OFF) versus regulation (linear) you can check out the TPS2410 datasheet page 13 "TPS2410 vs TPS2411 - MOSFET Control Methods):

Also we have this application note for additional information

Achieving Stable VGS Using LM5050-1 with Low Current

and Noisy Input Supply

Thanks!

Alex