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TL494: TL494 maximum duty

wu chen ruei
Prodigy 60 points
Part Number: TL494

I use TL494 to design my circuit,I set the frequecny is 100kHz,CT is 4.7nF, RT is 2K variable resistance,

when I was test circuit,I find the duty isn't working in maximum,it was attenuation to 87%.

  • 0
    TI__Mastermind 31580 points

    Hi Wu Chen,

    What is the voltage on the DTC pin ? The voltage on the DTC sets the dead-time of the PWM signal, see section 9.3.3 on page 11 of the datasheet for the description of operation.

    To maximise the duty cycle the voltage on the DTC pin should be 0V.

    Regards

    Peter

  • 0 in reply to Peter Meaney
    Prodigy 60 points

    The DTC voltage is 0V , and FB voltage is 0V ,too.

    which is attenuation of  maximum duty?

  • 0 in reply to wu chen ruei
    TI__Mastermind 25570 points

    Hello Wu Chen

    The maximum duty cycle is 45% or 90% for both outputs so it looks like you are seeing the normal Dmax for this device. Can you post a schematic so that I can double check this please ?

    Regards

    Colin

  • 0 in reply to Colin Gillmor
    TI__Mastermind 31580 points

    Hi Wu Chen,

    I hope you have made some progress since Colin's resply. Since there has been no activity on this post for 5 days I will close the post.

    Please open a new post if you have more questions.

    Regards

    Peter

  • 0 in reply to Colin Gillmor
    Prodigy 60 points

    I know datasheet has that,but I look to duty 97% in other RTCT,

    so what is maximum duty 97%? for which CTRT is right?

    I really want to know why my circuit doesn't have maximum duty.

    I'm sorry, I'm busy to other thing.

  • 0 in reply to wu chen ruei
    Prodigy 20 points

    hi，wu

     I had the same problem,then i change CT LARGER.and  reduce the frequency. the duty is 91%.

    now i try to make the DTC VOLTAGE TO -0.3V. 

    BUT I am still finding the way to make the DTC VOLTAGE TO -0.3V

    DO you ALL have  some way to achieve this?

    Thanks a lot 

    JI YUE

  • 0 in reply to user6121785
    TI__Mastermind 25570 points

    Hello Wu,  Hello Ji

    I don't think that pulling DTC negative is a good idea - it may work in the lab but there is a danger that the negative voltage could forward bias some internal junctions in the IC and cause a malfunction. DTC should be tied to 0V to minimise the dead time.

    I don't see any way to increase the duty cycle beyond the device specification by making changes to RT or CT - there may be some small effect by reducing CT and increasing RT but it would be small.

    One thing you should experiment with is to directly control the voltage on the FEEDBACK pin to greater than 3.5V - be careful to keep the current into the pin to less than 3mA (the recommended operating maximum). If this works you may be able to add an external pull up resistor from FEEDBACK to VREF or  to VCC to increase the effective operating range of the IC error amplifiers.

    Regards

    Colin

  • 0 in reply to Colin Gillmor
    Prodigy 20 points

    Hello,Colin

    Thanks a  lot

    I will try the way you suggest.