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TPS92691: Calculation method for rD, string resister value

Satoshi
Guru 19645 points
Part Number: TPS92691

Please let me confirm about mean of rD(string resister).

And, Is rD value calculated below, correct?

rD = Vf × # of serial / ILED

ex) Vf: 6V, serial: 20pcs, ILED: 500mA

　⇒rD = 6 ×20 / 0.5

　　　 = 240Ω

※I feel that above value became large.

Best regards,

Satoshi

  • 0
    TI__Mastermind 28655 points

    Hello Satoshi,

    That is not correct.  rD is the dynamic impedance of the LED at an operating current.  In the LED datasheet there is a curve showing Vf versus If.  At 500 mA a tangent line at that point shows the slope of (delta V)/(delta I) which equals rD for one led.  If there are 20 leds you multiply the single LED rD at 500 mA by 20.

    As an example, I draw a line tangent to the 500 mA point and read the values of Vf at 400 mA and 600 mA (can be other values).  If they are 5.95V and 6.05V for example then rD becomes (6.05V - 5.95V)/(600 mA - 400 mA) = 0.5 ohms for one LED, or 10 ohms for 20 LEDs.

    Best Regards,