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recommend part for short length air coupled light data transmission?!

Stefan Krueger
Intellectual 670 points
Other Parts Discussed in Thread: TLC5971, TLC5916, TPS54201Hello at all :-)

a little unusual request here in the forum i think...
i have an project idea for a Persistence of Vision Display where i want to use a short air-coupled light data transmission to get the raw LED-Driver data to the rotary part.
(currently thinking of drivers like TLC5971 or similar - so currently iam planing on two identical links - one for the clock signal and one for the data signal)

my distance between transmitter and receiver is about 5-10mm and my target data rate is about 20Mbit/s.
After some research i found that this should be easily possible as the datarates that are used in fiber system are much higher...
i would like to use a ring of fast IR Leds on the static side and on fast Photodiod on the rotation side.
As fare as i found a Transimpedance Amplifier seems to be the 'keyword' / technology for this to use..
the goal here is to have as low part count as possible on the receiver site and get a 'stable enough' 3.3 or 5V signal out of the receiver circuit.

Is it possible to use on of the TI parts - normally designed for fiber application for this kind of thing?!
the transmission though air can be isolated very good from ambient/other light - and on the led transmitter site i think i can have a little 'overpowered' sender..
(also at this site i will have to find a good driver to get this fast led-responds)

I hope some of you have a good idea what parts for sender (led) driver - and receiver side can be used for this?!

sunny greetings
stefan
  • 0
    TI__Genius 14295 points
    Hi, Stefan,

    From the sender side, I find that you're using a ring of IR LEDs.

    So how many IR LEDs are there in the ring? What's the Vf of the IR LED? The max current?
    Any specific features like PWM dimming, analog dimming, that you need for this application?

    Thanks.
    Regards,
    Kenneth
  • 0 in reply to Kenneth Du
    Intellectual 670 points
    Hi Kenneth,

    thanks for your reply.
    The LED i am currently favoring is VSMF4720. VF 1.45V @ 100mA (pulse)
    currently i think i need 5 leds in the ring. so we have a combined VF of 7,25V @ 100mA.
    i don't need any dimming. instead i need very fast on off switching response times.
    in the LED Datasheet it says that at 100mA the rise and fall time is 15ns. And the junction capacitance is 125pF.
    if i am correct in my calculations i need a maximum bit-time of 50ns.
    my input signal to this is a 3.3V logic signal from a uC.


    on the receiver side i currently favor TEMT7100X01.

    sunny greetings
    stefan

    PS: i hope that this time my post gets seen by a moderator and i got the formatting correct..
  • 0 in reply to Stefan Krueger
    TI__Genius 14295 points
    Hi, Stefan,

    Since TLC5971 can only support 75mA max for each channel, 100mA is a challenge and you may need to combine two or three channels together.

    Moreover, the shift register of TLC5971 is 224 bit, and everytime you need to send 224 bit data to the device. If using 10MHz SCKI, it will take ~22.4us, which maybe unable to meet your 50ns requirement.

    Even for devices like TLC5916 without dimming function, you need to send 8 bit data to the device everytime and this takes ~0.8us using 10MHz CLK.

    You can also check TPS54201, which should be more appropriate for your application.

    Please let me know if this solves your problem.
    Thanks.
    Regards,
    Kenneth
  • 0 in reply to Kenneth Du
    Intellectual 670 points
    Hi Kenneth,

    ok seems i have formulated my original question unclear :-)
    i have drawn a overview of the hole concept i am considering:
    pov concept overview
    for this question only the logic (data and clock) part is relevant.
    so i hope it is now clearer what i mean ;-)

    i had a quick look at the TPS54201. for other projects it looks promising ;-)
    but for this one i think the sentence in the datasheet at '7.4.4 PWM Dimming Mode Operation' (page 18)
    Due to the limited control loop response, to get a relatively linear dimming performance, suggested PWM signal frequency should be less than 1 kHz.
     seems to point out that the frequency i will need are not reachable..

    i don't know if i really need a specialised driver for this application -
    it was just an idea - as the fibre-things are related....

    sunny greetings
    stefan

  • 0 in reply to Stefan Krueger
    TI__Genius 14295 points
    Hi, Stefan,

    Understand your application.

    Since your application requires very fast on off switching response times, even using high speed serial interface, it still needs us level bit transfer time.

    After recieving the data, the rising and falling time are around several decades of ns.

    Please check if this works for your system.

    Thanks.
    Regards,
    Kenneth
  • 0 in reply to Stefan Krueger
    TI__Genius 14295 points
    Hi, Stefan,

    I'll close this thread due to inactivity and if you have more questions please reply to it directly and it will open again.

    Please help to click "Resolve" if the problem is solved.

    Thanks.
    Regards,
    Kenneth
  • 0 in reply to Kenneth Du
    Intellectual 670 points
    Hi Kenneth,

    iam fine with closing this for now.

    if i find a solution / have more information i will post theme here,
    so if others find this thread they will can benefit from it.


    sunny greetings
    stefan