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PMP9068: POE circuit question

Alvin Tsang
Prodigy 20 points

Part Number: PMP9068

Hello,

What is the function of the following circuit?

BR

Alvin

  • 0
    TI__Intellectual 2530 points

    This is the soft-start circuit.  At turn-on, C17 is discharged.  As the voltage increases on VFB, current flows through C17, turning on Q3.  This pulls the feedback signal on the collector of U3 low, limiting the duty cycle.  As VFB increases, C17 gets charged up and the current through C17 is less.  This allows the feedback voltage to increase, increasing the duty cycle.  D9 discharges C17 at turn-off.  The time constant is approximately C17 x R21.