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TPS92515: Finding switching frequency Fsw

Tyler Reed
Prodigy 105 points
Part Number: TPS92515

Hey,

In figures 12 and 13 of the datasheet it is shown that Fsw varies with change in Vin and Vout. I have also observed it change from 367kHz to over 1 MHz over a min to max dim range on a test circuit. Should I take the median value and base my calculation off that?

Is Fsw an intrinsic value of the inductor, maybe its resonant frequency?

How do I find or calculate Fsw?

Thanks,

Tyler Reed

  • 0
    TI__Mastermind 28655 points

    Hello Tyler,

    The TPS92515 is a hysteretic topology that regulates peak current so it uses a constant off-time to keep the ripple constant. Constant off-time isn't really constant, it changes with the LED string voltage but if the LED string voltage is constant the off-time should be fairly constant. The on-time will vary with input voltage since the delta of the current is constant so if Vin doubles the on-time halves, V = L * (di/dt), L and di are constant, if V doubles dt halves. The switching frequency is a function of the LED stack voltage, the off-time component values and the input voltage. The inductor isn't part of that, it's value determines the current ripple and average current. So if you double the inductor value the ripple would half and the average current will go up (since the TPS92515 regulates peak current) but the switching frequency would remain constant.

    As for calculating things, what are you using the Fsw for? In the design the higher the input voltage the higher the switching frequency, this will affect switching loss so you want to use the high Vin and high switching frequency for calculating switching loss.

    Calculating Fsw is in the datasheet, Vled and the off-time components give toff, ton depends on the dutycycle and toff.  If Vout is 12V and Vin is 24V, duty cycle is 50% and toff = ton.  If Vin is 36V ton is 1/2 of toff, duty cycle will be 33% and the Fsw will go up 33%.  At Vin is 48V, ton is 1/3 of toff and the Fsw will go up 50%.

    Regards,