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UCC28910: Start up current tolerance

Lutz Naumann
Expert 6460 points
Part Number: UCC28910

Dear Team,

here is a question regarding the startup current tolerance of UCC28910. The datasheet shows the following:

 

As the start up current source is charging the Cap on the VCC pin it defines the start up time. The datasheet shows a quite big variation of 1:10 for ICH2 and ICH3. Does this really mean the start up time can vary by a factor of 1:10? This seems to be quite large.

Fig. 16 shows the temp dependency with a factor of 1:2. 
I guess the parameter in the above chart also includes process variation. However why is the variation so huge?

thanks

Lutz 

  • 0
    TI__Mastermind 31580 points
    Hi Lutz

    The UCC28910 is supported by our team in the USA and they are on thanksgiving holidays until next week. I have assigned your post to them and they will answer when they get back to the office on Monday.

    Regards

    Peter
  • 0
    TI__Mastermind 40520 points
    Hello Lutz,

    Thanks for your interest in the UCC28910 switcher. The variations for ICH2 and ICH3 are indeed quite wide because the high-voltage current source consists of a MOSFET and some un-trimmed control circuitry. The control accuracy and MOSFET gain parameters are subject to process variations, as you surmised, and temperature swings. Narrower deviation could be obtained with trimming, but that would increase the cost considerably; not only due to additional die-size increase for the trim circuitry but also with longer test time to do the trimming.

    Wide as they are, these parameters are guaranteed over the entire rated temperature range and over all process variations from lot to lot.
    Figure 16 is a good indication of where the typical current will be for each parameter. The spec-table extremes are unusual and, of course, at the extremes. Note: the units for each graph in Figure 16 seem to be mixed up. I believe that ICH1 should be (uA) and ICH2 should be (mA).

    Regards,
    Ulrich