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something about TPS65381 input voltage and overheating

I post a forum about TPS65381;Supply rails;typical circuit.And I did not get any informations what I really needed.I described my question again and try my best to explain it much clearly.

I only use TPS65381 to supply 3.3V and 1.2V.And in the typical 12V input voltage, external N-MOS which connects to VDD1 pin feels much too hot. When the input power of 12V battery is 2.4W, I make the input voltage come down to the 5V.And now, the input power of battery is just 1.7W.In this way, the external N-MOS does not feel too hot. This is why I need to use low input voltage. 
But in my project, I cannot supply above 5.8V to TPS65381 at initial power up.

And are there any methods that I can use to solve the problem of overheating? All of circuits and components which I used are all following the design of the datasheet.

  • Hello user4205241,

    Please excuse the delayed response due to the recent Thanksgiving Holiday. The appropriate Applications Engineer has been assigned to answer your question(s). You can expect a follow-up response within a few days.

    Regards,

  • The typical use case of VDD1 is to supply the input of the FET from the VDD6 pre-regulator output for up to about 600mA. 

    Questions:

    1. Are you using VBatt as input to the FET or VDD6?
    2. How much current are you converting on VDD1 to 1.2V via the external FET?  The device is configure for up to about 600mA using this set up. 
    3. Does the FET and system have a good thermal plane to spread the heat?

    Please refer to the typical application diagram in the datasheet, figure 6-1, for this typical use case or the set up in the TPS65381 EVM.  Under normal use cases the FET shouldn't get too hot.

    If you need more current on the 1.2V rail, then we would recommend an external DCDC used downstream of the VDD6 pre-regulator.  There is an E2E on this use case at: http://e2e.ti.com/support/applications/automotive/f/26/p/358491/1311191.aspx#1311191