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PMP21781: Cx1 of the schematic

Athos Zhao
Expert 7320 points
Part Number: PMP21781

Hi expert,

Our customer has a 150V application and they refer to our PMP21781 reference design.

They found the Cx1 in the schematic is not the same with our general use, and the C11 is also DNP.

What is Cx1 here used for and why we connect FB and COMP using a capacitor? Is that related to the high output voltage?

Thank you!

  • 0
    TI__Mastermind 40660 points

    Hi Athos, thanks for the comments! I'm assigning this to an engineer to review this as the original design is currently not available. Thanks!

  • +1
    TI__Intellectual 1235 points

    Hi Athos,

    This design uses a modified feedback compensation because of the low value ceramic output capacitors. The mid-band gain of the error amplifier is set by resistor at the COMP output, in this case R16, times gm, times the feedback divider ratio Kfb. For the gm amplifier, the COMP resistor value must by adjusted in proportion to the output capacitance. A low resistor value in the COMP network will overload the amplifier output and exceed its drive current capability during a transient event, leading to potential overshoot of the output voltage. To overcome this, a modified feedback is used similar to a standard operational amplifier. The mid-band gain is now set by the ratio of the COMP resistor R16 to the top feedback divider resistor R11. For the gm amp, the high frequency capacitor C10 is still referenced to ground for noise suppression. 

    Regards,

    Bob Sheehan

    Power Design Services

  • 0 in reply to Bob Sheehan
    TI__Expert 7320 points

    Hi Bob, 

    Thank you so much!

    I also want to know why the Cx1 is connected by a air-wired trace, is that necessary or we can just place it on PCB layout?

  • +1 in reply to Athos Zhao
    TI__Intellectual 1235 points

    Hi Athos,

    The air-wire was done since Cx1 was not on the original board. You can place it on the PCB layout.