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TPS61193-Q1: About DCM mode frequency

Tsukui Yusuke
Guru 21930 points
Part Number: TPS61193-Q1
Other Parts Discussed in Thread: TPS61193

Hi

Customers want to know the DCM mode frequency to calculate the inductor current.
Please tell me how to calculate DCM frequency.

(I also posted to the following E2E.)
e2e.ti.com/.../2470074

Best Regards,
Yusuke / Japan Disty

  • 0
    TI__Expert 4550 points
    Hi Yusuke-san,
    Thanks for reaching out, we will review your question and get back to you soon.
  • 0 in reply to Caryss Li
    Guru 21930 points
    Caryss-san,

    Thank you for you response and support.
    I look forward to hearing from you.

    Best Regards,
    Yusuke / Japan Disty
  • 0 in reply to Tsukui Yusuke
    TI__Mastermind 18550 points
    Hi Yusuke-san,

    Actual switching frequency in DCM mode will be same as switching frequency in normal CCM mode.
    DCM mode (also called PFM mode in this context ) occurs because either the load current is very light or the boost conversion ratio is small due to Vin closer to Vout. TPS61193 boost converter has a minimum on time for the switching cycle and if the commanded duty cycle is lower than minimum on time of the switch due to either very light load current or smaller conversion ratio, the device starts to skip pulses to keep the output voltage from rising out of regulation. So the so called DCM mode (PFM mode) is a bursty mode of operation where when the output of the converter starts to rise out of regulation, the converter stops switching and once the output falls below a certain threshold due to load or leakage, the converter generates a burst of switching pulses to maintain regulation. This behavior repeats until commanded duty cycle is greater than minimum on time of the switch at which point normal PWM operation resumes.
    As I mentioned earlier, switching frequency during this burst will be same as switching frequency in the normal PWM switching mode so the inductor current should be calculated based on the PWM switching frequency of the converter and not on the frequency of the DCM burst. How often the bust occurs (i.e. the burst's frequency of occurrence) in a given application depends on the load current and Vin/Vout of the boost and will vary based on these conditions.
    Hope this explanation is helpful.
    Regards,
    Liaqat
  • 0 in reply to Liaqat Khan
    Guru 21930 points

    Liaqat-san,

    Thank you for your explanation.
    Is it correct with understanding that there is no frequency calculation method considering pulse skipping?

    Change the approach.
    Could you tell me how to calculate the inductor current in DCM mode?

    Best Regards,
    Yusuke / Japan Disty

  • 0 in reply to Tsukui Yusuke
    TI__Mastermind 18550 points

    Hi Yusuke-san,

    Your understanding is correct that we do not have an accurate frequency calculation method for how often the pulse skipping bursts occur as it depends on applications conditions at the moment such as Vin, Vout, load current, output capacitance, leakage, boost compensation etc.

    Also, inductor current in DCM mode can not be well defined. Inductor current is zero in between bursts. During a burst, inductor current rises successively until output voltage reaches a high threshold at which point switching stops and inductor current becomes zero (see scope plot below). Even to get an average inductor current, we need to know the frequency and length of the DCM burst to get average current. So inductor current is only defined in CCM mode and method to calculate it is on page 21 of the datasheet.

    Regards,

    Liaqat

  • 0 in reply to Liaqat Khan
    Guru 21930 points

    Liaqat-san,

    Thank you for your kind support.
    I understood your explanation.

    Let me ask you one more thing.
    In the past postings, I learned that DCM is a normal operation.
    e2e.ti.com/.../2470074

    Under customer input voltage conditions,
    It is possible to start up from the DCM state.
    What is the concern and risk of starting up in DCM mode?

    Best Regards,
    Yusuke / Japan Disty

  • 0 in reply to Tsukui Yusuke
    TI__Mastermind 18550 points

    Yusuke-san,

    DCM is normal operation of the part and I do not have any concern if the part starts up in the DCM due to low conversion ratio (Vin, Vout) or light load current.

    Hope this explanation is helpful.

    Regards,

    Liaqat