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TPS92518: I(pk) is changed due to Vin even though same PKTH_DAC?

KANJI SHIBATANI
Mastermind 6205 points
Part Number: TPS92518

Hello

Customer and  I also evaluated TPS92518 EVM to confirm whether ILED was constant or Not depend on  VIN=24V/48V.

Where: VLED=12V, Peak_threshold=30 ( to set ILED=200mA) ,TOFF=30, PWMx=100%.

 

Conclusion:Not constant.

Even though same TOFF=30/ PKTH=30, ILED (average)  were changed.

In Vin=48V Ipeak =263mA  and in case Vin=24V , Ipeak=220mA

I(av)=195mA ( Vin=48V), and I(av)=145mA( Vin=24V)

Ipeak-peak is controlled well by TPS92518 in the bot of Vin=48V/ Vin=24V, same . ( approx. 140mA)

 

Question

It may be fundamental question..., but  is this proper ( inherent) operation of TPS92518 ?

I know duty will be changed by Vin ( VLED=12V) ,  but Ipeak =ILEDx_PKTH_DAC/(1000 * Rsense) is independent for duty isn't it?

Why Ipeak  is  changed even though Peak threshold(PKTH)  is same? ( Customer and I want to know the mechanism )

Please advise.

Thank you for your support!

 

12V_LED_eval_english.pptx

 

 

 

  • 0
    TI__Mastermind 28655 points
    Hello Kanji,

    If you look at the on-time for the 48 volt test, 25% duty cycle and 675 KHz it is about 370 ns. The current ripple percent is fairly high on this design, close to 70% at 48 volts input and 93% at 24 volts input. It appears to be the turn-off delay of the MOSFET. 58 ns is the delay in the part and then the delay to actually turn off the MOSFET. There is another phenomena which is minimum on-time but that is 235 ns max so it shouldn't be hitting that.

    58 ns would change the peak by approximately 20 mA, if the MOSFET turn-off has about the same delay it would be close to this offset. You can check this by placing a large value inductor in the design and see if the change is much smaller.

    Regards,
  • 0 in reply to Irwin Nederbragt
    Mastermind 6205 points
    Dear Irwin san
    Thank you for your answer.