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tidu370.pdf for tlc5958 - Enable circuit

Leonardo Garberoglio
Intellectual 420 points
Other Parts Discussed in Thread: TLC5927, TLC5958

I'm reading tidu370.pdf http://www.ti.com/general/docs/lit/getliterature.tsp?baseLiteratureNumber=tidu370&fileType=pdf and I have some doubt about how enable signal is generated by 74HC123.

The first mono is fire on every rising edge of A address line and it is on 1 state for 0,45 x 100x10^3 x 1x10^-6 = 45mseg. That pulse is the reset input for the second mono, wich is drived by B address signal.

I can't figure out what is the EN signal wave form.


thank and best regards!

  • 0
    TI__Intellectual 1820 points

    Hello Leonardo,

    In normal state, EN will be kept low always. It works as below:

    Each time signal A toggle high, the rising edge will trigger the 1st mono, there will be a high level pulse at signal 123_O, the pulse width is 45mS as you calculated. Since the time between one rising edge of A and the following rising edge of A is much smaller than 45mS (it's ~28uS if use 20MHz GCLK, and consider 1uS line change time), thus the first mono will re-triggered continuously, and signal 123_O will be always high.

    For the 2nd mono, since the frequency of signal B is half of that of A, the time between two rising edge of B is ~56uS, still much smaller than the width of mono pulse, and the 2nd mono will also be continously triggered by the rising edge of B, and EN will thus be kept low all the time in normal case.

    Regards

    Mike

  • 0 in reply to Mike(DC/DC) Wang
    Intellectual 420 points

    Ok, I understand, and if there is no signal going to the TLC address or if there is always the same address on the bus (for more than 46mseg) then the EN going HI and the mosfet are turned off. It's ment to be an automatic power save mode?


    One more question. why the drains mosfet have a 100ohm resistor and a zenner diode to GND? what is that zenner value?


    thank!

  • 0 in reply to Leonardo Garberoglio
    TI__Intellectual 1820 points

    Yes, your understading is correct, this is to protect LED from burning out in some abnormal condition.

    The 100ohm+Zener is used to solve ghost issue in multiplexing application. Each time after a row is finished displaying, the scan line (i.e, the power supply) for that row of LED is discharge through this 100ohm+Zener, thus, when OUTx go low to turn on the LED in next row, the corresponding LED is previoius row will not be turn on faultly

    let me know if any further question

    Regards

    Mike

  • 0 in reply to Mike(DC/DC) Wang
    Prodigy 50 points
    Hi mike,

    Now we are designing the full color display and single color display.

    Single color am using TLC5927

    full color TLC5958.

    Instead of FET why we don't use bjt (PNP) for power switching, is there any issue for single color (TLC5927).