• State TI Thinks Resolved
  • Locked Locked
  • Replies 5 replies
  • Answers 3 answers
  • Subscribers 63 subscribers
  • Views 2738 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

LM3550: LED used as ambient light sensor

John Smith54
Prodigy 30 points
Part Number: LM3550
Other Parts Discussed in Thread: LM3535,

Hi team,

I am looking into circuits of LED driver ICs. I see the need of separate photo-diode which senses ambient light and provides output to ALD pin, which in turn controls brightness of the LEDs connected to the LED driver (for example, in LM3550, LM3535. It is known that LEDs are also capable of acting as light sensors in reverse bias, hence the LEDs which are being used as light emitters can themselves sense ambient light of surroundings, and control brightness.

Are there LED drivers available in TI which perform ambient light sensing using LED itself, and not requiring a photo-diode to sense ambient light.


Regards,

John

  • 0
    TI__Intellectual 1940 points

    Hi John,

    There is not a LED driver that I know of that can use the white LEDs themselves, reverse-biased, for use in ALS.  I looked into this a while ago and found that the part to part variation of a white LED in a "sensor" application was too variable to be reliable to be useful in most applications. 

    Greg

  • 0 in reply to Greg Lubarsky
    Prodigy 30 points

    Hi Greg,

    Thank you for your response. This is really useful.


    I believe photo-diodes are used for ALS, such as driving the input pin ALD/ TMP.  So is the ambient light sensed based on discharge time of junction capacitance (of the photodiode in reverse bias)? I have read that discharge time of reversed biased photodiode can be used to estimate the ambient light in photodiodes. Yet I did not identify any such explanation in datasheets.


    Thanks in advance!

    Regards,

    John

  • 0 in reply to John Smith54
    TI__Intellectual 1940 points

    John,

    We rely on the reverse current of the photodiodes to create a voltage across a known resistor.  As the light increases, so does the current and generated voltage.  We then use internal comparators to determine which level of brightness is present and act accordingly.

    Greg

  • 0 in reply to Greg Lubarsky
    Prodigy 30 points

    Hi Greg,

    If I understand correctly, this seems to be an alternative to measuring discharge time of the capacitor (reverse biased photodiode). Or does the technique explained by you also uses discharge time for determining ambient light?

    Thank you for your responses!

    Best regards,

    John

  • 0 in reply to John Smith54
    TI__Intellectual 1940 points
    Hello,
    The method I am mentioning is an alternative. I have not worked with discharge times of a photodiode in an ambient light detection circuit.

    Greg