Question:
I use 4x FETs in my MAX20034 dual buck controller circuit. Since I need to operate down to 4.5V battery input, the FETs in the circuit may operate at 99% on time during the low drop out mode. How do I use the SOA of the FETs in this case? Is the SOA of the FETs the the best characteristic to use during this case or is there a different characteristic of the part that can be used if I need to do calculation on this part?
Answer:
The SOA curve doesn’t apply. The SOA curve is trying to set the max drain current vs. VDS with VDS being held constant which isn’t a power supply mode of operation. In a switching power supply the FET is always turned on fully so VDS is set by RDSON * ID and will always be as low as it can be.
You should simply look at the power dissipation when VIN=4.5V (just like you would under normal 9V to 16V VBATT conditions). The good news is there will be minimal switching losses so only the RDSON losses can be looked at. So you can take your IOUT * RDSON to get the HS FET power dissipation.
For accuracy you’d have to measure the thetaJA of the FETs on your PCB. The thetaJA in the datasheet isn’t useful as it doesn’t give you realistic application conditions. I would say that if you measured thetaJA on your PCB (assuming a good layout on a 6+ layer PCB) you're looking at <= 40°C/W. This would give you a ~2W of power dissipation.
So a quick napkin calculation would be sqrt(13mohm * 1.7 / 2W) = 9.5A would be roughly what to expect for the FET to be capable of, 1.7 is the RDSON at TJ=150°C.
This isn’t a precise calculation since there are a lot of estimations here.
