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TPS929120-Q1: external PWM signal

Josef Warta
Expert 4310 points
Part Number: TPS929120-Q1

Hello Team, enclosed a question from my customer:

for diagnostic reasons customer wants to measure the forward voltage of the LEDs.
This is only possible during a HIGH-cycle.
According datasheet it is possible to feed in an external PWM signal from a microcontroller for example:

Means this that I can connect (for example) a 3kHz signals with duty cycle and this will be forwarded directly to the LEDs?

Many Thanks

Josef

  • 0
    TI__Genius 15776 points

    Hi Warta,

    1) you should make sure whether customer use the external pwm function; there is always a internal pwm generator.

    2) your understand is right, the ADC only work when PWM is on. And the ADC need a minimum period for operating, and that time is Ton-demand, you can find it in datasheet.

  • 0 in reply to Frank Qiu
    TI__Expert 4310 points

    Hello Frank, thanks for quick answer.

    But I think the idea is to feed in a PWM signal with duty cycle into the PWM pin and put this to the LED directly.
    So far I know the PWM pin is only to synchronize the internal PLL the the frequency which will feed in, true?

    Many Thanks

    Josef

  • 0 in reply to Josef Warta
    TI__Genius 15776 points

    That's right and i want to know why they choose external pwm input.

  • 0 in reply to Frank Qiu
    Prodigy 30 points

    Hello,

    as we have to measure the forward voltage of the LEDs externally with a microcontroller we need to know when there is a HIGH-level at the LED.
    So we need to synchronize the PWM of the LED driver / at the LED and the Analog Read-Out of the micro.
    For this the idea was to create a "main" PWM in the micro for the LEDs and give this as an input for the LED driver as a "common frequency".
    The Duty-Cycle for each LED should still be controlled by the LED driver as every LED needs to have a different intensity.

    Is this possible?
    What other options do we have to synchronize the PWM of the LEDs and the Microcontroller Analog input?

    Best regards,
    Marco Tauber

  • 0 in reply to Marco Tauber
    TI__Genius 15776 points

    1) You can use internal ADC to measure the output voltage;

    2) if you use external pwm pin you can't individually control the outputs' duty cycle;

  • 0 in reply to Frank Qiu
    Prodigy 30 points

    Regarding 1): it is not possible to use the ADC of the TPS929120, as this chip is not FUSI- / ASIL-capable. Therefore we must use another microcontroller

    Regarding 2): so what is the use of the PWM input Pin? Can you only control every LED together?

    Is there any other possibility to sync the PWM of the TPS929120 with another microcontroller?

    Best regards,
    Marco Tauber

  • 0 in reply to Marco Tauber
    TI__Genius 15776 points

    1)  TPS929120-Q1 is FS-capable device, it can support ASIL-B system; I don't know what's the meaning of "not FUSI- / ASIL-capable";

    2) Function as PWM0 input channel for OUT0-5 in internal address mode, function as PWM1 input channel for OUT6-11 in internal address mode. So you can control max 2 group from the PWM input pin. You can use that to sync the PWM signal,

  • 0 in reply to Frank Qiu
    Prodigy 30 points

    The driver is FS-capable, but it is not developed according ISO26262 and not suitable for ASIL C. This was not described correctly by me.

    The idea was to add an additional external LED-diagnosis to reach ASIL C, but as I see this is not as easy technically feasible as thought.

    Thanks & Best Regard,
    Marco Tauber

  • 0 in reply to Marco Tauber
    TI__Genius 15776 points

    Only 2 external PWM can be used as we talk above.