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TPS55340: Max Duty

K.Hamamoto
Guru 12155 points
Part Number: TPS55340

Hi

There is the maximum duty on the datasheet. However it is only for Rfreq=80kohm.

How much is the duty max when it is for Rfreq=22.9kohm(2MHz)?

If the duty max decrease with increasing the frequency, is the off time fixed for this device?

In my experience, the duty max decrease with increasing the frequency.

However the off time at each frequency is not same. (At faster frequency,the off time is shorter.) How does the off time define in that case?

Best Regards,

Koji Hamamoto

  • 0
    TI__Mastermind 40080 points

    The TPS55340 is of fixed frequency PWM control, its off time can be calculated by Toff=Ts*(1-D). The maximum duty is not limited by the off time, but by the internal delay of the logic circuit. The maximum Duty at 2MHZ is  85%

  • 0 in reply to Helen chen
    Guru 12155 points
    Hi Helen-san,

    Thank you for your support.

    >The maximum Duty at 2MHZ is 85%

    Is 85%@2Mhz the minimum value or typical value of the maximum duty?

    My concerns is that the off time (include internal logic delay) is changed or not changed by frequency. (the persentage of max duty shold be changed by frequency. because the cycle time is changed.)

    In the typical case, the max duty is 93% at 577kHz. (the off time is 69ns)
    As you mentioned above, if that is 85% at 2MHz, the the off time will be 75ns.
    On the other hand, if 85% is minimum value of the maximum duty, it is shown the off time will be changed by frequency. (min = 89% at Rfreq=80k, then min=85% at Rfeq=2.9k. The off time will be changed from 190ns(89%) to 75ns(85%).)

    Best Regards,
    Koji Hamamoto
  • 0 in reply to K.Hamamoto
    TI__Mastermind 40080 points
    the internal oscillator itself has 94% max duty, that is 30ns OFF time(this off time is different with the SW off time, it refers to the internal oscillator) at 2MHz and 50ns at 1.2MHz. there's also around 42ns logic delay time. so can get around 89% duty at 1.2MHz and 85.6% duty at 2MHz.
  • 0 in reply to Helen chen
    Guru 12155 points
    Hi Helen-san,

    Thank you. I understood.

    Best Regards,
    Koji Hamamoto
  • 0 in reply to K.Hamamoto
    TI__Mastermind 40080 points
    Great, good luck!