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LM3424: ITF current check when VDIF is 0V or negative

Ray Chang
Intellectual 1345 points
Part Number: LM3424

Hi, Team

My customer use LM3424-Q1 for automotive lughting, we have the question for ITF.

Could you please help to explain below parameters for ITF?

If the VTREF < VTENSE, what is the value of ITF?

Will it affect the ILED accuracy when VTREF<= VTSENSE?

  • 0
    TI__Mastermind 28655 points
    Hello Ray,

    It doesn't really matter since 2 uA will affect the set-point by less than 1% if using a 100 Kohm thermistor. At 90 degrees for a typical 100K thermistor the resistance is approximately 6.481 Kohm, at 95 degrees Celsius it is 5.437 Kohms which is 20% different or 4% per degree Celsius. 2uA * 6.481K is less than 1% difference, less than the resistor and NTC tolerance.

    Only slightly as stated above.

    Regards,
  • 0 in reply to Irwin Nederbragt
    TI__Intellectual 1345 points
    Hi, Irwin

    If the VTREF < VTENSE, what is the value of ITF?

    For example, we design the ILED current by ICSH (100uA) and calculated external resistor RCSH,
    Assume the ITF is 2uA when VTREF < VTENSE.
    In order to follow internal error amplifier reference 1.24V, the ICSH will be regulated 98uA and ITF provide 2uA.
    Then the ILED accuracy will lose 2%, because the ITF 2uA current through to ICSH.
    Is it correct?
  • 0 in reply to Ray Chang
    TI__Intellectual 1345 points
    Hi, Irwin

    Do you have any update for this question?
  • 0 in reply to Ray Chang
    TI__Mastermind 28655 points

    Hello Ray,

    Sorry, it didn't show up in my email, I think due to it being resolved.  Are you using this for something other than thermal foldback?  As the differential voltage gets further apart the current will go down.  There will always be some leakage but it will be less than 2 uA.

    Regards,