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PMP7760: Under Voltage Circuit

Maor Matityahu
Intellectual 340 points

Part Number: PMP7760

Hello,

In the PMP7760 design, how the Under Voltage circuit works?

what i have to do to make the under voltage limit to 11V?

Thank you,

Maor M.

  • 0
    TI__Genius 17680 points

    Hi Maor,

    When input is turned on, the base of Q101 is clamped to 5.1V. This will be high enough to turn on Q101, making UVLO pin have a Vbe drop below the base of Q101, or 4.4V, assuming Vbe drop is 0.7V. To change UV limit to 11V, you will need to change the clamp of D110 to ~11V+0.7V, so ~11.7V. 

    However, it is important to note that UVLO pin of part cannot handle voltages beyond 7V. In that case, what you can do is add another NPN stage where you divide the desired 11.7V output to be a voltage within UVLO pin limits. 

    Instead, I will recommend you look at LM5155, which can be configured in a flyback configuration. Here is an application note that covers the design of a flyback using the LM5155: http://www.ti.com/lit/an/snva866/snva866.pdf

    Thanks,

    Richard

  • 0 in reply to Richard Fu22
    Intellectual 340 points

    Hello,

    Thank you for the answer!

    i have two things that i dont understand:

    1) According to your answer, what is the role of R121 resistor?

    1) Why i cant use just voltage divider and that is it?

  • 0 in reply to Maor Matityahu
    TI__Genius 17680 points

    Hi Maor,

    R121 can be used as a way to discharge the input capacitors and C121 to ground when the input falls.

    You can use a voltage divider, but there's a minimum UVLO shutdown limit, as well as a maximum voltage that the UVLO pin can have. A resistor divider would require very specific values to cover the big input range. Because of this, a solution that is more easily implementable is to use zener diode and BJTs. 

    Thanks,

    Richard