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TPS62441-Q1: Regarding Rcf at COMP/FSET

Ryan S
Prodigy 10 points
Part Number: TPS62441-Q1

Tool/software:

If I were to set the switching frequency at 4 MHz and have the compensation set for smallest output capacitance with SSC enabled, I calculated Rcf using Equation 2 as seen in the image below:

The Rcf result is 14.4 kΩ


Basing from the second row of Table 9-1 in the datasheet:

14.4 kΩ falls outside of the range between 33 kΩ and 18 kΩ.

Is 14.4 kΩ still okay to use as Rcf?

and would the compensation (for smallest output capacitance with SSC enabled) still function as intended?

  • 0
    TI__Mastermind 25430 points

    Hi,

    Please choose Rcf values within the table recommendations.

    In this case, I suggest 18k.

    Thanks,

    Febin

  • 0 in reply to Febin
    Prodigy 10 points

    Hi Febin

    If we use 18K, does that mean that the switching frequency is actually set to 3.23 MHz? (doing reverse engineering on Equation 2)

    I'm a bit confused with the table, for example at 2nd row, third column, it indicates that if we use 33K, then it is set at 1.8 MHz and if we use 18K, then it is set at 4 MHz.  Using Equation 2, setting to 1.8 MHz would yield 32.7K which I guess would approximate to 33K.  However, setting it at 4 MHz would yield 14.4K if we use Equation 2 which is far from the expected 18K.

    Can you help clarify?

    Also what is the consequence of using values outside of the range that is indicated on the table?

  • 0 in reply to Ryan S
    TI__Mastermind 25430 points

    Hi,

    I will answer your question. But before that can you please provide Vin, Vout, Iout for your application?

    Thanks,

    Febin

  • 0 in reply to Febin
    Prodigy 10 points

    Vin is 3.3 V, Vout is 2.6 V and 1.2 V

    Iout for 2.6 V is 250 mA

    Iout for 1.2 V on the average is 300 mA but can peak up to 500 mA

  • 0 in reply to Ryan S
    TI__Mastermind 25430 points

    Hi,

    Thanks. I will check and get back to you tomorrow.

    Regards,

    Febin

  • 0 in reply to Febin
    TI__Mastermind 25430 points

    Hi Ryan,

    Thank you for your patience.

    Here's how I understand, 

    The calculated Rcf from equations was approximated to the closest standard resistor values and recommended in the table. However, for the SSC enabled comp setting-1, there is a difference from the calculated value of 14.4k vs the recommended 18k resistor.

    With recommended 18k Rcf, the fsw would be only 3.23MHz.

    With calculated 14.4k Rcf, the fsw would be 4MHz.

    Now if we add max 20% tolerance for SSC to both cases we get a max of ~3.9MHz (with 18k Rcf) & ~4.8MHz (with 14.4k Rcf). The table recommendations makes more sense to me to keep the frequency below 4MHz. 

    I am also discussing internally to understand if there is anything else I missed here... I hope to have some updates early next week.

    Regards,

    Febin

  • 0 in reply to Febin
    Prodigy 10 points

    I appreciate your explanation about the SSC having 20% tolerance.  For now, I'd interpret that it's safer to just operate within the 18K to 33K recommendation with the switching frequency being under 4 MHz.

    But I'm definitely curious about the situation going outside of this range.  Just let me know in this thread in case you have any new updates. Thanks.

  • 0 in reply to Ryan S
    TI__Mastermind 25430 points

    Hi Ryan,

    I hope to have some updates early next week.

    Regards,

    Febin