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TPS61194-Q1: Pulse Skip Mode Condition

Hirotsugu Kobayashi
Mastermind 8485 points
Part Number: TPS61194-Q1
Other Parts Discussed in Thread: TPS61194EVM

Hi, team.

Our customer is estimating in TPS61194EVM.
The construction of the LED's is 3s8p, and output voltage of boost converter is about 24V.
When input voltage is set to more than 16 V, it enter to pulse skip mode.

Please tell the condition to transfer to a pulse skip mode.

Best, Regards,

Kobayashi

  • 0
    Mastermind 8485 points

    Hi, all.
    A waveform is attached.
    When VIN will be about 19 V, it'll be enter to pulse skip mode.

    Please tell me the condition which enters to pulse skip mode.
    Duty?  Inductor Current?

    TPS61194-Q1 Evaluation.pdf

  • 0 in reply to Hirotsugu Kobayashi
    TI__Intellectual 2180 points
    Hi Kobayashi-san,

    This is expected IC behavior that is associated to light load conditions or when Vin gets close to Vout level.

    The PFM functionality is mostly driven by an internal Light Load Comparator. A compensation current, dependent on sensed current from the internal switching FET and other boost parameters is compared to a reference LLC comparator current. If the compensation current is smaller than the reference current of LLC comparator then the internal loop prevents the switch to close ( => device enters to the PFM-mode). When the boost output voltage has decreased enough the compensation current is forced over the LLC-comparator triggering point (=> exit from PFM-mode). The compensation current depends not only on switching current (i.e. current sense through switching FET) but also on other boost parameters such the switching frequency, FB control loop, current ramp settings, blank time, etc. Exact conditions when it enters PFM are hard to determine, but this is expected behavior of device to prevent from output voltage to continue increasing above necessary level.

    Best Regards,

    Salome
  • 0 in reply to Salome Ramirez Carrion
    Mastermind 8485 points
    Hi Salome-san
    Thank you for your support.

    If inductance value is smaller, does the threshold which becomes a PFM mode rise?
    Our customer want to keep a PWM mode.

    Best Regards,
    Kobayashi
  • 0 in reply to Hirotsugu Kobayashi
    TI__Intellectual 2180 points
    Hello Kobayashi-san,

    We wouldn't recommend lowering the inductance any further. Actual recommended inductance for 400kHz is 33uH.

    What are the maximum Vin conditions for the application?

    If Vout = 28V and Vin can get as high as 26V as shown in the plots, the device most likely will be entering PFM regardless if we optimize external components to lower this PFM threshold due to the small Vout to Vin ratio condition.


    Best Regards,

    Salome
  • 0 in reply to Salome Ramirez Carrion
    Mastermind 8485 points
    Hi, Salome-san.
    Thank you for your support.
    Our Customer tyies to use it in 2MHz to avoid FM-band, so L is 4.5uH and Cout is 33uF.
    LED's configuration is 8s3p, Vf total is typically 24V, ILED is 80mA/string.
    In this condition, when input voltage up to more than 16V, it enters to PFM mode.

    Do you have the idea that the threshold of PFM mode is hihger?
  • 0 in reply to Hirotsugu Kobayashi
    TI__Intellectual 2180 points

    Hi Kobayashi-san,

    PFM threshold depends on several boost parameters so although higher frequency and smaller inductance is tested, you could still see PFM mode entering at similar conditions. Reducing inductor value and setting higher switching frequency does not necessarily mean PFM threshold will be decreased. 

    When a boost frequency is set by RFSET, the device automatically sets internal boost parameters accordingly such as minimum on/off times and current ramp settings which will influence when PFM mode is triggered. 

  • 0 in reply to Salome Ramirez Carrion
    Mastermind 8485 points

    Hi, Salome-san

    When RFSET is 27k, how many seconds is minimum on/off times set?

    Our costomer want to keep PWM mode up to 16Vin.

  • 0 in reply to Hirotsugu Kobayashi
    TI__Intellectual 2180 points

    Kobayashi-san,

    Please refer to Table 2. DC-DC Parameters for min switch off time. This table indicates the various boost parameters selected for every frequency range.