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TPS92515-Q1: How much Vf margin should we consider?

NY
Expert 3040 points
Part Number: TPS92515-Q1
Other Parts Discussed in Thread: TPS92515

Hi 

In case of 2* LEDs in series, the Vf will be 5.5V (2.75x2), because Vin_typ=7V in our use case, the difference between will be 7 - 5.5 =1.5V. 

In addtion, if Vin_min=6V, difference will be 0.5V, is this the problem? so the question is how much Vf margin should we consider? 

regards, ny

  • 0
    TI__Mastermind 28655 points
    Hello NY,

    The TPS92515 can regulate this however the average current may increase as the voltage is lowered. This is because the TPS92515 regulates peak current and as the inductor charging voltage drops, 6V-5.5V, the inductor/LED load starts looking more resistive so the current is changing from sawtooth to an RL charging. To reduce this effect you can add capacitance across the LEDs, has to be large enough to overcome the 'RL' effect. The other part of the design that helps is if the inductor current ripple is relatively small. If peak to peak ripple is 20% the highest the current can go is Iaverage + Ipeakpeak/2 which would be a maximum of 10% high.

    Best to try it and if there are issues try the above recommendations.

    Best Regards,
  • 0 in reply to Irwin Nederbragt
    Prodigy 30 points
    thank you very much.