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TPS78533BQWDRBRQ1 : LC filtering at the output

beesetty rajesh
Expert 1065 points
Other Parts Discussed in Thread: TMS320F28335

Dear experts,

I am working on a customized DSP board design where I am planning to use LDO to power up the micro controller(TMS320F28335).  The datasheet(link given below) of the above part number(TPS78533BQWDRBRQ1) mentioned  indicated only a capacitor at the output. However, I would like to know if an LC or CLC filter at the output of LDO.

I want to bring to your notice that, the part number field doesn't recognize any LDO part number. Kindly look into it.

www.ti.com/.../tps785-q1.pdf

Regards,

Rajesh BN.

  • 0
    TI__Genius 12365 points

    Hi Rajesh, 

    Thank you for reaching out. 

    What is it that you are trying to filter at the output of the LDO and/or, what is the requirement that is driving this? Is this going to power something else or is it only the VDDIO 

    There is nothing wrong in using a Pi filter (CLC), although this is typically not required since an LDO provides a clean rail. Using a CLC will improve the noise and PSRR performance of the LDO.

    Best, 

    Edgar Acosta

  • 0 in reply to Edgar Acosta1
    Expert 1065 points

    Hi Ecosta,

    There is a requirement of 1.9VDC the power supply pins despite of load disturbances. These  disturbances are basically digital loads like PWM, communication ICs. These Voltage may effect the ADC power supply pin which share the same 1.9Vdc. therefore I would like to add a seperate LCL filter exclusively for 1.9Vdc that is connected to ADC pins. 

    In summary, there shall be two no.s of 1.9vdc. One 1.9v dc shall be given to the microcontroller core supply pins and another one which is filtered (LCL)1.9vdc to the ADC supply voltage pins. This applies to 3.3v also.

    And another thing I want to is how to design LCL filter for anyr given requirement. Is there any material or TI document that talks about LDOs and design of output filters like LCL.

    Regards,

    Rajesh BN.

  • 0 in reply to beesetty rajesh
    TI__Genius 12365 points

    Hi Rajesh, 

    Thank you for the detailed explanation. 

    Im sharing some material on filter design.

    Filters.zipsnva538 - input filter design.pdf

    As mentioned previously, LDOs provide a cleaner rail which is why it is not common to see an LCL filter unless some ripple is expected at the output that is outside the bandwidth of the LDO. 

    Knowing what is needed to filter is essential i.e frequency and ripple as with any other filter design. The LDO itself filters some of the ripple seen at the input of the LDO + the capacitor combination and good pcb layout practices, which are normally sufficient to provide a clean rail. 

    The main concern with doing filters like LC or LCL is that, if not designed properly, they can cause unwanted ringing (oscillations) therefore some damping might be needed to avoid high Q peaks. 

    Some of the material shown is labeled as "input filters" but the concepts apply to any filter design. 

    Best, 

    Edgar Acosta

  • 0 in reply to Edgar Acosta1
    Expert 1065 points

    Hi Edgar,

    Thank you for sharing material. It helped me a lot. Lastly I want to know, I see few LDOs have got a sense pin which can be connected to the output. If a filter used at the output of the filter, can we connect the sense pin to the output of the filter?

  • +1 in reply to beesetty rajesh
    TI__Genius 12365 points

    Hi Rajesh, 

    The sense in some of those devices is due to the device being in a unity gain configuration. There are times where the SNS can be connected at the point of load, however, close attention must be paid to this configuration, specially if an LC filter is used, as this can make the device unstable if there is significant inductance, and with an LC this will be the case. 

    Therefore, it is recommended to connect SNS directly to OUT. 

    Best, 

    Edgar Acosta