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UCC28051: why are we pulling down ZCD pin pulse by pulse?

Part Number: UCC28051
Other Parts Discussed in Thread: PMP11155

Hi, Please let me know what is the purpose of Q3 and D10 in PMP11155 schematic. and also explain why are we using the below circuit.

  • Hi Sudhakar,

    If R21 and R22 directly connects Vout, the ripple of Vout flows through R21 and R22. R21 and R22 appears in the modulation channel and produces zero pole and polar pole, which influences system configuration. On the other hand, R21 and R22 dominates the middle frequency gain of the feedback circuit. The limitation of the two aspects makes selection of R21 and R22 complex and difficult. Therefore, regulator network containing Q3, D10, is introduced to set a constant bias, which releases R21 and R22 from the modulation channel.

    The attached picture is broken, could you upload it again?

    Thank you.
    Teng

  • Thank you Teng,

    the circuit is attached below. 

  • Hi Sudhakar,
    Sorry, it is still broken. could you upload it by attachment or email me at teng-feng@ti.com                                                   
    Regards,
    Teng                                  

  • Hi Sudhakar,

    I haven’t heard back from you for quite sometime, I’m assuming you were able to resolve your issue. If not, just post a reply below (or create a new thread if the thread has locked due to time-out) – thanks, Teng