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TPS92518: Max off time optimization

KANJI SHIBATANI
Mastermind 6205 points
Part Number: TPS92518

Hello


My customer had chosen TPS92518 ( MP will be soon) but

they had changed their shunt FET ( Rdson became 0.25 ohm smaller ) and flicker issue at 0.1% duty found...

Customer  has optimized MAX_OFF from 70 to 90 and flicker fixed .


Q1

If MAX_OFF were not adequate such as  MAXOFF_DAC

is big difference ( 20~25 ) , what is the issue in parctically?

I think following will be issue if MAX_OFF was not

optimized, but do you think anything else?


MAXOFF_DAC is not optimized=>


. transition of inductive energy is not smooth.

. So, in-rush current cause LED current ringing by parastic

  inductance resonane .

. It will be one of the cause for flicker.

Q2

Would you have your comment about my guess of

flicker in case  MAXOFF_TIME was not optimized ?

Best Regards

Thank you for your help.

  • 0
    TI__Mastermind 28655 points

    Hello Kanji,

    Q1:  Maximum off-time needs to be set to keep the ripple current similar to when it is operating in normal mode.  This can be calculated or it can be checked with an oscilloscope.  If the maximum off-time is too short the current can increase due to minimum on-time and can present higher current when the shunt is opened until it starts regulating correct again.  Again, this can be seen with an oscilloscope looking at the inductor current.

    I don't think resonance is causing your issue.  What you may want to look at is the switching frequency versus the PWM on-time.  If the PWM on-time is short the current can vary because of where the inductor current is.  If it is at a low point the when the shunt fet is opened the average can be lower then when the current is at a high point.  Making sure there are multiple switch cycles when there is an on time or reducing the current ripple helps with this.

    Q2:  This can be check by calculation or simply looking at the inductor current.  If the off-time is too short the current can actually increase beyond the peak threshold due to minimum on-time.

    Best Regards,

  • 0 in reply to Irwin Nederbragt
    Mastermind 6205 points

    Dear Irwin san

    Thank you for your detail commet.

      

    Customer  keeps  tLEB>250ns ( does not violate).

    I think customer's setting LED_TOFF itself has no problem .

    Vin=48/VLED=12V,Io=1A, Ilpkpk=15%, TOFF=0.9us, Fsw=880KHz.

    LED_TOFF_DAC=21.

    LED_MAX_OFF= optimzed by checking the oscillo scope. (70~90)

    However, I understand Mamimum_off time will affect to inductor current ripple.

    I will talk with customer.