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UA78M: Technical issues

Zhonghui Xu
Expert 5445 points
Part Number: UA78M
Other Parts Discussed in Thread: TLV767

Hi team.

I got a question from customer.

Recently, I've been using UA78M33IKVURG3 change 12V (customer supplied 12V±10% ) to 3.3V.The current is 60mA.Ipeak=200mA.(Tnormal=900ms，Tpeak=100ms.then repeat).
The actual circuit is as follows. Because LDO is very hot, I want to remove the front capacitor C1. I don't know Which will be affected if the capacitor c1 is not added? Need to do relevant tests?

Sincerely looking forward to your help~

Best regards,

  • 0
    TI__Mastermind 23361 points

    Hi Zhonghui,

    Removing the C1 will have little impact on the temperature rise for the LDO. It's a good design practice to have a small input cap close to the input pin of the LDO to reduce source impedance for better AC performance. The input capacitor is usually optional for LDO and if the source impedance is already very low, you don't have to have the input capacitor. I would suggest running load transients test w/ and w/o input capacitor if there is any concern.

    To improve the thermal, it's best to optimize the layout. Please see this app note for some ideas on how thermal can be improved. 

    https://www.ti.com/lit/an/slvae85/slvae85.pdf

    Regards, 
    Jason Song

  • 0 in reply to Jason Song
    TI__Expert 5445 points

    Hi Jason,

    You've answered so well.

    Thank you.

    Best regards,

  • 0 in reply to Zhonghui Xu
    TI__Mastermind 23361 points

    Thanks for the feedback, if you could also mark this as "resolved", that would be perfect. 

  • 0 in reply to Jason Song
    TI__Expert 5445 points

    Hi Jason,

    Before closing, I'll add a few more questions.
    1The reason I want to remove C1 is that C1 is too close to the heat source .I'm afraid the duration of C1 will be shorted by the high temperature.
    2From your reply:C1 can be removed in DC application system.Is it right?(The load transients test w/ and w/o input capacitor is ok）
    3Can you recommend other suitable LDO better than 78M33 for12V to 3.3, The current is 60mA.Ipeak=200mA.(Tnormal=900ms，Tpeak=100ms.then repeat).

    Thank you.

    Best regards,

  • 0 in reply to Zhonghui Xu
    TI__Mastermind 23361 points

    1The reason I want to remove C1 is that C1 is too close to the heat source .I'm afraid the duration of C1 will be shorted by the high temperature.

    The input cap is normally placed right at the input pin, and yes, it could get some heat from the LDO. You should be looking at the specification of the capacitor to understand if the reliability will be reduced when exposed to a heat source.

    2From your reply:C1 can be removed in DC application system.Is it right?(The load transients test w/ and w/o input capacitor is ok）

    It's a good design practice to have an input cap to lower the source impedance, but, yes, it's optional. The added input capacitor can normally improve AC performance, and please run load transients test to decide if you will need to have one.


    3Can you recommend other suitable LDO better than 78M33 for12V to 3.3, The current is 60mA.Ipeak=200mA.(Tnormal=900ms，Tpeak=100ms.then repeat).

    Let's do some investigation and we will get back to you on the recommendation soon. 

    Regards, 
    Jason Song

  • 0 in reply to Jason Song
    TI__Mastermind 23361 points

    LMS81117A maybe a better option. If you can use TLV767, that would be better.