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TPS548D22: FSEL-pin strap configuration

Yasushi Fujiwara
Genius 11755 points
Part Number: TPS548D22

Hello Experts,

I would like to know how to select RFSEL value from table 2 in datasheet. If a following condition is considering, what is better value of RFSEL?
- Vin=12V
- Vout=0.9V
- fsw: 650KHz
- FCCM

Which one is a better choice, 22.1Kohm or 16.5Kohm?

Best Regards,
Fujiwara

  • 0
    TI__Mastermind 48210 points

     

    1) It looks like you posed this question three times, so I will delete the other 2.

    2) There are 4 FSEL resistors that select 650kHz with Forced Continuous Conduction Mode (FCCM), each selects a different R-C ramp for internal ripple injection.

    16.5kOhms

    22.1kOhms

    29.4kOhms

    37.4kOhms

    Which one you should select will depend on the output capacitance and transient performance.

    Table 7 in section 8.2.3.1 of the datasheet lists the R-C time-constants of the internal ripple generator.

    For a 0.9V output,

    The R/2 (7us time constant - Rfsel = 16.5k) ramp will require less capacitance to stabilize the output, but will have slower transient response.

    The R (13.5us time constant - Rfsel = 22.1k) ramp will require more minimum output capacitance but will have faster transient response with less output voltage deflection during a rapid load change.

    Equation 7 in section 8.2.3.3.1 explains the minimum output capacitance and the influence tau has on it.