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TPS79942 output 3.8V not 4.2V

Zhuo Peng
Intellectual 410 points

Hi,

  My schematic is as below.

http://e2e.ti.com/cfs-filesystemfile.ashx/__key/CommunityServer-Components-PostAttachments/00-00-18-31-02/tps79942_2D00_error.jpg

The output of TPS79942 should be 4.2V. 

 But  why my output is 3.7V and the ripple is about 120mV. Is anyone know where is the problem?

  Thank you!

  • 0
    TI__Expert 8890 points

     

    Hello,

    Assuming that there is no huge ripple on your input coupling to your output one possibility is that the circuit is unstable.  I see nothing wrong in your schematic but I suppose that the input filter might be contributing to the problem.  You might try powering this circuit from a lab supply and using an external load to isolate the circuit and  to see if it still performs poorly.  This part has been in production for several years with no known oscillation problems or offset problems.  Please double check all connections and solder joints.  Please let us know the results of your testing.

    Regards

    Bill

  • 0 in reply to Bill Stokes
    Intellectual 410 points

    Hi Bill,

      My problem has been solved when I replace the TPS79942 with a third one.  I think the reason lies on the chip I bought is inferior.

      Thank you for your help.