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TPS56C215: Question about Fsw Load Regulation

Hong Jieun
Prodigy 240 points
Part Number: TPS56C215

I have a question about TPS56C215.


According to the datasheet, it is mentioned that, under the same conditions,

the switching frequency increases as the output voltage(Vout) decreases for a same input voltage(Vin).


As I know, by this equation, fsw ≤ 1/t_on ×(Vout/Vin)

when the output voltage decreases, then the switching frequency also decreases.


I was wondering if I can get explanation for this?
Does the value of t_on change according to the Vout voltage?

  • 0
    Prodigy 240 points

    I have one more question about TPS56C215.

    According to the datasheet, a pullup resistor of 10k is recommended to pull it up to VREG5 for PGOOD.

    Even when external power is not supplied to VREG5, a pull-up should be applied to PGOOD on VREG5?

  • 0 in reply to Hong Jieun
    TI__Mastermind 26830 points

    If external voltage is not applied to VREG5, it will still drive around 4.7V since it is an output of an internally regulated LDO. So, you can use a pullup resistor of 10k to pull it up to VREG5.

    Thanks,

    Amod

  • 0 in reply to Amod Vaze
    Prodigy 240 points

    Thank you for reply, 

    Could you also answer the question at the top?

  • 0 in reply to Hong Jieun
    TI__Mastermind 26830 points

    The on-time is internally generated by a time constant and definitely there is some tolerance to that time constant. Also, the off-time also varies depending on the input/output conditions as it is dependent on Vout, Iout, DCR, LSFET Rdson variations and also inductor ripple. So this can lead to slight variations in the time period and hence the frequency. For a given Vin and Vout, the frequency variation is minimal so this should not be a big concern. Please let me know if there are any more questions.

    Thanks,

    Amod