Hi Shawn!
Thanks for your prompt reply.
I double checked so that I had the same datasheets as on the web, SLOS808A –DECEMBER 2012–REVISED MARCH 2013
Typical charactersitics Figure 4 rDS(on) - Drain-Source On-State Resistance mΩ vs TA - Ambient Temperature - °C.
From your information I understand that this typical charactersitics is for Tj instead of ambient, Rdson~=77mOhm with Tj=135 degC.
This solvs the contradition that I mentioned earlier and the data sheet heading is wrong.
Then back to my original resoning applying the 135degC instead of 150 degC.
Based on the thermal resistance JA 22 deg/W, 85 degC environment and now the thermal trip point 135 degC I get a dissipation of 2.27W in the device and a total resistance of 0.14mohms based on the 4A.
From you figure 4( now Tj instead of Ta) rdson is 77mOhms at 135 degC gives as you stated above ~=1.2W. Then other dissipations are ~= 1W meaning and additional series resistance of 62mohms at 4A.
Now it all comes together since total resistance is about 0.14ohms.
In my case we will have 40 degC ambient and therefore with the trip point at 135degC, power consumption in the device~=4.3W in the device. This seams rather high.
This in turn leads to that the possible current now is sqrt(4.3/0.14)~= 5.5A
Does it seem resonable?
From the data sheet there is also the thermal characterisation parameters. Based on the temperature measurements that I have done so far and using the thermal characterisation figure PHIjt for calculations I get unresonable high power dissipations.
I think I will apply the approach that I increase the current until the thermal trip point. I assume that this then is 135degC and use that as an understanding on how much margin I will have in the design.
Is it possible for thermal characterisation purposes to get hold of devices which have a thermal trippoint of 135 degC?
Thanks for your prompt reply.
I double checked so that I had the same datasheets as on the web, SLOS808A –DECEMBER 2012–REVISED MARCH 2013
Typical charactersitics Figure 4 rDS(on) - Drain-Source On-State Resistance mΩ vs TA - Ambient Temperature - °C.
From your information I understand that this typical charactersitics is for Tj instead of ambient, Rdson~=77mOhm with Tj=135 degC.
This solvs the contradition that I mentioned earlier and the data sheet heading is wrong.
Then back to my original resoning applying the 135degC instead of 150 degC.
Based on the thermal resistance JA 22 deg/W, 85 degC environment and now the thermal trip point 135 degC I get a dissipation of 2.27W in the device and a total resistance of 0.14mohms based on the 4A.
From you figure 4( now Tj instead of Ta) rdson is 77mOhms at 135 degC gives as you stated above ~=1.2W. Then other dissipations are ~= 1W meaning and additional series resistance of 62mohms at 4A.
Now it all comes together since total resistance is about 0.14ohms.
In my case we will have 40 degC ambient and therefore with the trip point at 135degC, power consumption in the device~=4.3W in the device. This seams rather high.
This in turn leads to that the possible current now is sqrt(4.3/0.14)~= 5.5A
Does it seem resonable?
From the data sheet there is also the thermal characterisation parameters. Based on the temperature measurements that I have done so far and using the thermal characterisation figure PHIjt for calculations I get unresonable high power dissipations.
I think I will apply the approach that I increase the current until the thermal trip point. I assume that this then is 135degC and use that as an understanding on how much margin I will have in the design.
Is it possible for thermal characterisation purposes to get hold of devices which have a thermal trippoint of 135 degC?