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TPS61170/PFM thresh

Tsukui Yusuke
Guru 21930 points
Other Parts Discussed in Thread: TPS61170, TPS61175, TPS61175-Q1

Hi,

Could you tell me about TPS61170 PFM thresh.
Customers want to know the switching timing of PFM and PWM.
Because,Customer is worried about the rpple in PFM.

Please tell me if there is a calculating formula.

Best Regards,
Yusuke/Japan Disty





  • 0
    TI__Mastermind 40080 points

    Yusuke-San,

    There's a minimum on-time requirement for TPS61170. At the point when the switch reaches minimum on-time is the threhold for PFM operation. This point is depend on your input and output voltage, the efficiency of the total circuit. Its hard to calculat it very accurately, just can calculate it in draft by Vin, Vo , efficiency.

    BR, Helen

  • 0 in reply to Helen chen
    Guru 21930 points

    Helen-san,

    Thank you for your support.
    Let me ask you one more thing.
    Under the conditions of light load,
    Does this device is in PFM mode?

    Customers are concerned about the pulse-skipping mode at light loads.
    Could you tell me your advice?

    Best Regards,
    Yusuke/Japan Disty

  • 0 in reply to Tsukui Yusuke
    TI__Mastermind 40080 points

    Yusuke-San, 

    In the light load , it will enter into the skip mode. But the lowest frequency is depend on the minimum outtput load. What does the customer really worried about? Output voltage ripple in skip mode?

    BR,Helen

  • 0 in reply to Helen chen
    Guru 21930 points

    Helen-san,

    Thank you for your response.
    Yes. Customer is reluctant to it (voltage ripple).

    Do the following ways of thinking fit this device?
    This device would enter PFM when the inductor valley current is closed to zero.

    Best Regards,
    Yusuke/Japan Disty

  • 0 in reply to Tsukui Yusuke
    Guru 21930 points

    Helen-san,

    Otherwise, TPS61175 datasheet is the following description.
    Can I use this expression for TPS61170?


    Best Regards,
    Yusuke/Japan Disty

  • 0 in reply to Tsukui Yusuke
    TI__Mastermind 40080 points

    Yusuke-San,

    This device would enter skip mode at the point of the switch's minimum on-time, please check the datasheet on page 5, the minimum on-time is 40ns.

    This is usually happended when input ouput voltage is very near to each other, or at the very light load. The ripple will a little bit higher in this condition. But this can be solved by adding more output capacitance at the output side.

    So the output ripple is not a problem.

    BR, Helen

  • 0 in reply to Tsukui Yusuke
    TI__Mastermind 40080 points

    Yusuke-San,

    From the TPS61175 datasheet on page 10, you can see the following decription:

    If the switching frequency is above 1.2 MHz, the pulse-skipping operation may not function. The TPS61175-Q1
    will always run in PWM mode with minimum ON pulse width. To keep the output voltage in regulation, a
    minimum load is required.

    So the equation you mentioned is a minimum load requirement for TPS61175 when its operation frequency is higher than 1.2MHz. This has nothing to do the TPS61170.

    If the customer is still worried about the ripple. We can help to check the ripple for them. What is the input voltage range? What is the output voltae and minimum load at the customer side?

    BR, Helen

  • 0 in reply to Helen chen
    Guru 21930 points

    Helen-san,

    Thank you for your advice.
    I understand your comment.
    (Expression of TPS61175 in this matter can not be used.)

    TPS61170 use conditions of the customer is the following：
    Vin：10-17V
    Vout：48V
    Iout：0-10mA
    (48V Phantom Power Application Circuit)



    Is there a way to somehow calculate the skip condition?

    Best Regards,
    Yusuke/Japan Disty

  • 0 in reply to Tsukui Yusuke
    TI__Mastermind 40080 points

    Yusuke-San,

    I will do the test on a modified  EVM board once the component were get ready. Hopefully, I can give the result to you next Monday or Thursday :)

    BR,Helen

  • 0 in reply to Tsukui Yusuke
    TI__Mastermind 40080 points

    Yusuke-San,

    Increase the output cap and increase the Rcomp will be helpful to the output ripple. But need to add to a very big value capacitor in order to make sure the loop stability is still OK under heavy load condition(PWM load condition). Suggest to let C3=C4=47uF/50V, Rcomp=100k.  And also the customer need to care about the noise from the capacitor because of the PFM operation.

    BR,Helen