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TL494: error of oscillator frequency and duty

Yuto Sakai
Mastermind 6100 points
Part Number: TL494

Hi,

Could you tell me about the frequency of the oscillator(fOSC) and duty.
There seems to be an error between the calculated value and the measured value.

RT=9.1kΩ
CT=1000pF
fOSC (calculated value) = 110kHz
fOSC (measured value) = 122 kHz
*Customers calculate using Formula (3) on page 6 of the application report SLVS074H.

R1=18kΩ
R2=6.32kΩ
RT=9.1kΩ
CT=1000pF
Duty (calculated value) = 49.5%
Duty (measured value) = 44%
*Customers calculate using figure 29 on page 20 of application report SLVS074H.

Are these results reasonable?

Best regard,
Yuto Sakai

  • 0
    TI__Mastermind 34465 points

    Hello Yuto Sakai,

    The answer to your first question is that the formula for the osc frequency

    is only approximate and you can expect this variation between expected and measured results.

    Your second question is confusing because SLVS074H does not have a figure 28 page 20.

    There is a figure 9 page 13 and thas has R1,R2,RT and CT.

    This is a buck regulator example with 32V in and 5V out.

    The duty cycle is set by the loop response and D = 5/32 = 0.156 and this is calculated on page 17 of this design example.

    Regards

    John

  • 0 in reply to John Griffin1
    Mastermind 6100 points

    Hi John-san,

    Thank you for your reply.
    Sorry, I made a mistake in the name of the application note.
    ×:SLVS074H　〇:SLVA001E
    https://www.ti.com/lit/an/slva001e/slva001e.pdf?ts=1593685651709&ref_url=https%253A%252F%252Fwww.google.com%252F

    I have additional questions.

    ・About fOSC
    According to the part selected by the customer, the RT error is 0.5% and the CT error is 5%.
    When I take the error into account and calculate it with the formula for fOSC, fOSC is max116.3kHz and min109kHz.
    The measured result is not within this frequency range. Is this error reasonable?

    ・About DUTY
    I made a mistake in the name of the application and it was SLVA001E.
    Considering the figure 29 on page 20 of SLVA001E, is the error between the calculated Duty value and the measured value reasonable?

    Best regards,
    Yuto Sakai

  • 0 in reply to Yuto Sakai
    TI__Mastermind 34465 points

    Hello Yuto Sakai,

    The formula for the frequency is only approximate.
    There is no tolerance specified in the datasheet.
    The result you got is what is reasonable.

    The equation SLVA001E  is for dead time . Not duty cycle.

    And its is valid for R1+R2 = 5k

    Regards

    John

  • 0 in reply to John Griffin1
    Mastermind 6100 points

    Hi John-san,

    Thank you for your reply.
    How much variation is there in fOSC and dead time?
    It does not have to be the guaranteed value.

    Best regards,
    Yuto Sakai

  • 0 in reply to Yuto Sakai
    TI__Mastermind 34465 points

    Hello Yuto Sakai,

    I do not have those numbers available.

    Regards

    John

  • 0 in reply to John Griffin1
    Mastermind 6100 points

    Hi,

    There is already 10% variation between calculation and measurement.
    Is there any reason to think that this 10% is reasonable?
    Is it possible to have 20 to 30% variation due to individual differences?

    Our customers are concerned about the large variations.
    Since there is no problem with the reference value, could you comment?

    Best regards,
    Yuto Sakai

  • 0 in reply to Yuto Sakai
    TI__Mastermind 34465 points

    Hello

    As I explained the equation provided is an approximation 
    PWM regulators data sheets do not provide min/max limits for oscillator frequency.

    The only guaranteed data is what is specified as max/min in the parametric tables of the data sheet

    The TL494 is characterised at 10kHz test frequency
    The parametric tables provide data at a particular test frequency and the curves will give a typical frequency change with timing component variations.
    The guarantees you are looking for are not tested and cannot be provided

    If you look at the data sheet it specifies a typical variation is 100Hz/kHz. This is a typical variation.

    Regards

    John

  • 0 in reply to John Griffin1
    TI__Mastermind 34465 points

    Hello 

    I am closing this post since I believe all further communication is exhausted.
    Please open a new post if any new issue is discovered.

    Regards

    John