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TPS61194: TPS61194

Shafeeq Muhammed
Expert 1200 points
Part Number: TPS61194

Hi,

As per the datasheet, the supported PWM freq is 100Hz to 20KHz.
Can we give PWM with a frequency more than 20KHz?

Because of the PWM switching, the ripple on the voltage rail is very high.
If we make the PWM as high, the ripple is less.
Also increasing the PWM frequency results in reduction in ripple voltage.
So whether we can use frequency higher than 20KHz?
It may help us to reduce the ripple.

Please suggest whether we can use higher frequencies.

Thanks & Regards,
Shafeeq

  • 0
    TI__Mastermind 22745 points

    Hi Sir,

        We don't suggest to use more than 20kHz dimming frequency, you can use an electrolytic capacitor at the output side for large ripple.

    Regards

    Sean

  • 0 in reply to Sean Zhou
    Expert 1200 points

    Hi Sean,

    What will happen if I give the frequency more than 20KHz?
    Just for curiosity, I am asking. Actually I have given 22Khz already.

    Regards,
    Shafeeq

  • 0 in reply to Shafeeq Muhammed
    TI__Mastermind 22745 points

    Hi Shafeeq,

        Yes, you can use 22kHz PWM dimming frequency, but the limitation is the minimum pulse width is about 5us, so this will limit the minium duty cycle. For 22kHz the minimum pwm cycle is about 11%.

    Regards

    Sean

  • 0 in reply to Sean Zhou
    Expert 1200 points

    Hi Sean,

    Thanks for your reply.
    So if we are using a minimum pulse width as 20%, then can we use PWM frequency up to 40KHz?

    Thanks & Regards,
    Shafeeq 

  • 0 in reply to Shafeeq Muhammed
    TI__Mastermind 22745 points

    Hi Shafeeq,

        From the technical side of view, yes, but you'd better to verify this with your application. What's your application? why need such as high dimming frequency?

    Regards

    Sean

  • 0 in reply to Sean Zhou
    Expert 1200 points

    Hi Sean,

    As I told earlier, the ripple is reducing when the frequency increases.
    As you suggested we can't use electrolytic capacitors in design.

    So I want to try with higher frequencies.

    Thanks & Regards,
    Shafeeq

  • 0 in reply to Shafeeq Muhammed
    TI__Mastermind 22745 points

    Hi Shafeeq,

        Sure, let me know when you get the test results. High switching frequency will do help to decrease the output voltage ripple.

    Regards

    Sean

  • 0 in reply to Sean Zhou
    Expert 1200 points

    Hi sean,

    I have tested with frequency up to 42Khz with 50% duty cycle.
    With all the frequency, the display backlight worked fine.
    And I have noticed that the ripple reduces when the frequency increases up to 32KHz.
    As I told I have tested up to 42KHz, but there is no change in the ripple voltage.
    The ripple voltage remains constant when the frequency is from 32KHz to 42KHz.
    So I have fixed my frequency as 32KHz.
    And I have tested with a minimum duty cycle of 14.28%.
    (In our application we are only using 7 brightness levels, so 100/7 = 14.28).
    I have tested with all 7 levels and it looks working fine.

    Thanks for all your support.
    I will let you know if any further support required.

    Thanks & Regards,
    Shafeeq

  • 0 in reply to Shafeeq Muhammed
    Expert 1200 points

    Hi Sean,

    I have attached a high-level block diagram of the regulator and LED driver.
    The LED driver and the backlight LED array is on an LCD.
    So we cannot change anything on the LED driver circuit design as the LCD is a standard module which we will purchase from other vendors.
    The boost regulator which I have shown in the diagram is there on our board.
    And we are observing the high ripple voltages on the 12V output rail of this regulator.
    So we have made some modification on this regulator circuit to reduce the ripple to a minimum.
    But the ripple didn't reduce to an expected level.
    And we have observed the ripple reduction with respect to PWM frequency (it is the only control we have on LED driver circuit).
    As I informed in the previous post I have tested with 32.5KHz and found the minimum ripple on that frequency.

    We will be using PWM with a 50% duty cycle in normal condition.
    It will reduce to 20% in some condition. Not less than that.

    I want to know that the operation with 32.5Khz will affect the reliability of the product. the minimum pulse width will be 6.14us.
    Whether It will affect the reliability of the product in the long term use or not?

    Thanks & Regards
    Shafeeq

  • 0 in reply to Shafeeq Muhammed
    TI__Mastermind 22745 points

    Hi Shafeeq,

         Don't worry about the dimming frequency and the only limitation the minimum on pulse is about 0.5us, so for your application, it is should be ok.

    Regards

    Sean