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TPS61033: IOUT to shift to PFM

Dice-K
Mastermind 6500 points
Part Number: TPS61033
Other Parts Discussed in Thread: TPS61023, TLV61070A, TPS61020

Hi support team,
I would like to calculate IOUT to shift to PFM.

Does it shift to PFM when the inductor valley current is 0A?

Calculating with the following conditions, when IOUT is 0.074A, the inductor valley current will be 0A.

Is 0.074A the IOUT to shift to PFM?

VIN = 5.3V
VOUT= 5.5V
IOUT= 0.074A
η = 0.85
L = 2.2μH
fSW = 2400kHz

Duty = 0.18
IL(DC) = 0.09A
ΔIL(P-P) = 0.18A
IL(Valley) = IL(DC) - ΔIL(P-P)/2 = 0A

Regards,
Dice-K

  • +1
    TI__Expert 8325 points

    Hi Dice,

    No, the low current limit mentioned in the d/s is not 0A, it is about 300mA.

    And we don't recommend to use 2.2uH inductor for TPS61033, 0.47uH is recommended. If the maximum load current is lower than 1A, 1uH is also acceptable. 

    Regards,

    Nathan

  • 0 in reply to Nathan Ding
    Mastermind 6500 points

    Hi Nathan,
    Thank you for your many replies.

    I have understood that 2.2μH is not recommended.
    If I calculate at 2.2μH, I can calculate that it will shift to PFM at IOUT = 0.31A.

    VIN = 5.3V
    VOUT= 5.5V
    IOUT= 0.31A
    η = 0.9
    L = 2.2μH
    fSW = 2400kHz

    Duty = 0.13
    IL(DC) = 0.36A
    ΔIL(P-P) = 0.13A
    IL(Valley) = IL(DC) - ΔIL(P-P)/2 = 0.3A

    Is "the low limit" of 300mA the same for recent devices TPS61023 and TLV61070A?
    Is the old TPS61020 different?

    Best regards,
    Dice-K

  • +1 in reply to Dice-K
    TI__Expert 8325 points

    Hi Dice,

    No, they are different, for TPS61023 and TLV61070A, it is about 200mA, for TPS61020, it is 0A.

    By the way, the low limit is not a very accurate value, because we don't make correlation, so the results is only for reference.

    And I recommend you can use this public tool instead of the calculation, this will be more convenient. 

    https://www.ti.com/tool/POWERSTAGE-DESIGNER?keyMatch=POWER%20STAGE%20DESIGNER%20TOOL

    Regards,

    Nathan

  • 0 in reply to Nathan Ding
    Mastermind 6500 points

    Hi Nathan,
    Thank you for your support.

    "The low limit" differs for each device, and if it is not written in the data sheet, it may be difficult to calculate the output current to the PFM on my own.
    I have understood that this value is approximate and cannot be defined as a spec.

    Best regards,
    Dice-K