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LM350-N: LDO with large output current

Chase Jeong
Genius 4670 points
Part Number: LM350-N
Other Parts Discussed in Thread: LM350A, LM1085

Hello TI experts,

My customer considers LM350 for their new product, instead of DCDC buck converter.

The application is medical equipment, so they are very sensitive to switching noise based on DCDC converter, and now considers LDO.

here are my few questions ;

1. what is the difference between LM350-N and LM350A? in datasheet I can see LM350A has strong points for output voltage tolerance, line regulation, load regulation.

also I see that LM350A is rated for -40 degree to +125 degree. is there more difference except I mentioned?

2. I see the Lead Temperature (metal package, soldering 10 sec / plastic package soldering 4 sec) 300 degree and 260 degree. what is the exact meaning of this temperature?

3. Here is the most important thing. the customer spec. is 12V input and 8V output, maximum output current is 3A.

is it okay to use LM350A under this condition? (I think it is pretty negative, because of junction temperature and power dissipation.)

if it is not okay, could you recommend other way to reduce switching noise for this customer spec.? I think 2 stage LDO circuit is okay, just if not use DCDC.

please check these issues. Thanks.

Best regards,

Chase

  • 0
    TI__Genius 12040 points

    Hello Chase,

                       Please find below my responses to your queries: 

    (1) Differences between LM350-N and LM350A: I think you have caught all the differences. LM350A has superior performance in Operating Junction Temperature range, reference voltage variability, line regulation and load regulation.

    (2) Lead Temperature: It essentially provides the maximum operating temperature at which the device can be safely soldered and still be functional. Beyond this temperature, there is a risk of the package melting and/or damaging the package leads.

    (3) You are correct in thinking that the device will not be functional with 12V Vin, 8V Vout and 3A of Iout. The power dissipation will result in a junction temperature that exceeds the Abs Max operating temperature and damage the device. The most efficient setup would be to use a Buck converter followed by an LDO. The LDO would help filter the switching noise. The less efficient method, which will not have any switching noise, will be to use two LDO's in series that can share the power dissipation and still be functional. If you could let me know which approach would work best for you, I will provide appropriate recommendations. Thanks!

    Regards,

    Srikanth    

  • 0 in reply to Srikanth Lolla Venkata
    Genius 4670 points

    Dear Srikanth,

    Thank you for your reply.

    I discussed this matter with my customer, they wanted one-chip solution due to price and PCB area.

    so could you recommend 1 LDO solution for the customer's usage?

    (I think it is very difficult to find, but please check it again.)

    Best regards,

    Chase

  • 0 in reply to Chase Jeong
    TI__Genius 12040 points

    Hello Chase,

                        At this point we don't have a device that can dissipate this large power as well as carry 3A of current and hence, we cant offer a One-chip solution at this time. Sorry about this. 

    Regards,

    Srikanth

  • 0 in reply to Srikanth Lolla Venkata
    Genius 4670 points

    Dear Srikanth,

    Thank you for your apply.

    then could you inform me about 2-stage LDO solution under this situation?

    in fact sometimes I got a question like this from my customer. then I can tell them efficient solution.

    Best regards,

    Chase

  • +1 in reply to Chase Jeong
    TI__Genius 12040 points

    Hello Chase,

                        With 2 LDO's we could either put them in series (split the voltage 12V->10V & 10V->8V) and share the power dissipation or put them in parallel and make each LDO carry half the current (1.5A each) and reduce the power dissipation. Unfortunately, either way, we are still looking at 6W of power dissipation within each LDO which is still too high and will result in thermal shutdown. I apologize, because I had earlier said that a two stage LDO could work, but it cant. We would need 3 or 4 LDO's cascaded to make this condition work. Would a combination of Buck Converter and an LDO be an acceptable solution? It would be far more efficient when compared to cascading LDO's. 

    Regards,

    Srikanth

  • 0 in reply to Srikanth Lolla Venkata
    TI__Genius 12040 points

    Hello Chase,

                        With 2 LDO's we could either put them in series (split the voltage 12V->10V & 10V->8V) and share the power dissipation or put them in parallel and make each LDO carry half the current (1.5A each) and reduce the power dissipation. Unfortunately, either way, we are still looking at 6W of power dissipation within each LDO which is still too high and will result in thermal shutdown. I apologize, because I had earlier said that a two stage LDO could work, but it cant. We would need 3 or 4 LDO's cascaded to make this condition work. Would a combination of Buck Converter and an LDO be an acceptable solution? It would be far more efficient when compared to cascading LDO's. 

    Regards,

    Srikanth

  • 0 in reply to Srikanth Lolla Venkata
    Genius 4670 points

    Dear Srikanth,

    Actually they do not want to use LDO-buck converter combination.

    so, I have to cascade LDO considering power dissipation, more than 2 LDOs depending on the situation.

    Thank you for your support.

    Best regards,

    Chase

  • +1 in reply to Chase Jeong
    TI__Genius 12040 points

    Hello Chase,

                        Using 4 LDO's in a cascade would be a workable solution, with each LDO having 1V drop across IN and OUT while carrying 3A, hence dissipating 3W of power. LM1085 is probably best suited for this application. LM350 would not be suitable since its thermal resistance is large enough to take the junction temperature beyond 125C for 3W of dissipation. 

    Regards,

    Srikanth 

  • 0 in reply to Srikanth Lolla Venkata
    Genius 4670 points

    Dear Srikanth,

    Thank you for your support.

    I will suggest your solution to my customer. Thanks again.

    Best regards,

    Chase