Regarding LM1085

Raju,

You may have found LM1085 from WEBENCH, but I'm quite certain that it was not a WEBENCH recommended solution based on your stated criteria ( Vin=19.0V, Vout= 5.0V at 3A).

In WEBENCH I set : Vin(Min)= 18V, Vin(Max)=20V (i.e. Vin=19V+/-1V), Vout=5V, Iout=3A, and the default Amb.Temp= 30C. WEBENCH found 76 possible solutions, of which there were zero LDOs.

This conversion is not practical with LDO simply due to the heat generated (((19V-5V) x 3A) = 42.0W) from the LDO inefficiency, and the difficulty of removing this heat from the package to the surrounding air (Theta(j-a)).

You can use an SMPS from the WEBENCH recommendations to convert directly from 19V to 5.0V. This is the recommended solution.

Or, you could do a preliminary step-down from 19V to 6.5V with the SMPS, and the convert the 6.5V to 5.0V with the LM1085. The dissipation in the LM1085 is reduced to a more manageable 4.5W, but there will still be a challenge getting the heat out of the LM1085 package.

Or, you can go inefficient and use a series resistance on the input to move the power dissipation out of the LM1085 package. That would call for Rs= (19V-6.5V)/3A) = 4.16 ohms @ 37.5 Watts for the external drop, and there will still be the challenge of getting the remaining 4.5W of heat out of the LM1085 package. This is NOT a recommended solution.

Hi Donald Jones,

Thanks for the reply.

I want to go with SMPS as you suggested.

Could you please suggest me the best suitable SMPS for my requirement.

And also let me know, what are the things need to be addressed or taken care during design and implementation of that SMPS. Like Heat and layout issues.

If possible, Provide me schematic diagram also.

Thanks,

Raju.

I suggest that you use the WEBENCH web site as it will give you far better comparisons, along with schematics, BOM, etc, that what I can possibly convey via email.

http://www.ti.com/ww/en/analog/webench/power.shtml

In the WEBENCH tool, click on the 'Power' tab, enter the power supply requirements, then click on 'Start Design'

Hi Donald Jones,

I did that and found TPS54332 based SMPS.

Here I attached pdf file which contains design, schematic and BOM.

Please review it and let me know your opinion on it.

And also let me know, what are the things should be taken care during implementation of this design. Like Heat, PCB layout etc ...

Thanks,

Raju.