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TL783 Figure 20 on Datasheet explaination

Jonathon Randall
Prodigy 40 points
Other Parts Discussed in Thread: TL783

Could some one please explain how Figure 20 on the TL783 datasheet works? I don't understand how the extra third transistor limits the current compared with Figure 19.

  • 0
    TI__Guru**** 164726 points

    Jonathon,

    Assume that the turn on voltage for all the transistor is 0.7V for this analysis.

    To get new PNP to start stealing bias from other PNP transistor requires 0.7 Amp (0.7V/1ohm) current to flow through small resistors and TL783.
    At this operating point there is 7.7V across the 10+1 ohm resistors, so 7mA will flow through 1k resistor. The NPN emitter current will be 7mA * PNP beta * NPN beta.

    So the maximum out put current current  is 700mA + 7mA*Beta*Beta which could vary a lot due to variation in PNP and NPN transistor beta.
    I do not like the current limit method used in figure 20. I do recommend using a fuse to protect against over current or other fault.