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TPS53355: CIN consideration(Except for ripple)

Evan Wang
Genius 10230 points
Part Number: TPS53355
Other Parts Discussed in Thread: TPS53353,

Hi All,

I have a question about Cin limitation of TPS53353.

Usually we will directly use the Vin ripple to evaluate the Cin requirement.

But as I know, the D-CAP control method will also sense the Vin for PWM.

So I was wondering what is the minimum capacitance we need that will not affect the normal operation of D-CAP

Thank you. 

  • 0
    TI__Expert 5310 points

    Hello,

    The D-CAP control topology senses the output ripple for PWM, and there is a need for a minimum ESR or a ripple injection circuit at the output for D-CAP parts such as the TPS53355.

    That being said there are several considerations for the selection of the input capacitors. For one, the input capacitance should be selected to meet any Vin ripple requirements of the design. It is also recommended to have a high frequency bypass capacitor on the input to reduce ringing on the switch node. In addition, there is a need for bypassing on the controller voltage input when tied to the PVIN voltage.

    Please let me know if you have any additional questions.

    Regards,

    Alec Biesterfeld

  • +1
    TI__Guru 56985 points

     

    The TPS53355 actually uses the SW pin to sense the input voltage during the high-side FET ON-time, rather than the VIN or VDD pin.  This allows the D-CAP adaptive on-time generator to better compensate for losses in the high-side FET and maintain a more consistent switching frequency over load conditions, as such it is very tolerant of VIN ripple for the control loop.

    If using external RCC injection to allow the use of D-CAP control with ceramic output capacitors https://www.ti.com/lit/an/slva453/slva453.pdf the Rr/Cr RC observer / ripple generator will also sense VIN through the SW pin, also allowing the loop to be tolerant of AC ripple on VIN.

    In addition to the normal VIN ripple design requirements, the discontinuous input current creates a high RMS current, so it is necessary to consider the input RMS current when selecting input capacitors.

    Since the the D-CAP mode control provides voltage feed-forward into the loop at very high frequency, the impedance looking into the VIN of the TPS53355 converter will be negative at high-frequencies, as such, it can be useful to higher ESR input capacitors to dampen any input resonance, especially if using a low-loss inductor to contain the high-frequency switching noise at the input of the converter local to the converter.