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TLV61048: Boundary condition of PFM mode and PWM mode

Takuya Terada
Expert 5115 points
Part Number: TLV61048


Hi Team,

My customer would like to know the current when this IC become PFM mode.

Datasheet said "When the FB voltage hits the PFM reference voltage, the TLV61048 goes into power-save mode".

However, I cannot find any information about PFM reference voltage. 

Could you tell me the condition of this IC when this IC become PFM mode?

  • 0
    TI__Mastermind 21565 points

    Hi Takuya:

    Thanks for reaching out. 

    Yes, as you found, the condition is mentioned in the datasheet as: "When the inductor peak current hits the low limit (400 mA typical), the output voltage exceeds the set threshold voltage as the load current decreases further. When the FB voltage hits the PFM reference voltage, the TLV61048 goes into power-save mode."

    The Vout will around 1.0125*Vout_nom which showed in next page. And in the spec table, the Vref is 0.81V typ for PFM mode.

  • 0 in reply to Minqiu Xie
    TI__Expert 5115 points

    Hi Minqiu,

    Thank you for your support.

    My understanding is as followed.

    When the inductor current(output current) decrease,  out put voltage increase.

    Then Vout hit 1.0125*Vout_nom(output current would be 400mA typ?). As aresult, FB voltage also increase and hit 0.81V, and goes into PFM mode.

    Is it correct?

    If it is correct, I don't understand the difference of Vref( PWM mode) and Vref(PFM mode).

    Because Vref(PFM) is included in Vref(PWM).

    best regards,

    Takuya

  • 0 in reply to Takuya Terada
    TI__Mastermind 21565 points

    Hi Takuya:

    Let's assume Vout=Vout_nom, and Vref=0.8V with normal load. If the load current decrease,  the converter will decrease the peak current of the inductance. If the peak current of inductance limited around 400mA, it won't decrease anymore. It will make the power supplied much more than load needed. So it will make the Vout higher than target. Once the Vref detect 0.81V, The converter will enter into PFM. 

    And 400mA is the peak current of inductance, not output current. 

  • 0 in reply to Minqiu Xie
    TI__Expert 5115 points

    Hi Minqiu,

    Thank you for your support.

    so that mean, once the peak inductance current decrease 400mA, Vout become larger and FB voltage hit Vref pfm. then IC goes into PFM mode. is it correct?

    i have two more questions.

    >> If the peak current of inductance limited around 400mA,

    this inductance current is fixed, is it correct?

    and I think Vref(PFM) must be larger than Vref(PWM). 

    is it correct?

    best regards,

    Takuya

  • 0 in reply to Takuya Terada
    TI__Mastermind 21565 points

    Hi Takuya:

    "once the peak inductance current decrease 400mA, Vout become larger and FB voltage hit Vref pfm. then IC goes into PFM mode. is it correct?"

    [MQ]-> It's correct if the load is light. (Just in case that the load is just at the boundary.)

    "this inductance current is fixed, is it correct?" 

    [MQ]->The peak current of inductance will limited around 400mA typically, but the slope is determined by Vin/Vout/L. So the average current will change following these factors.

    "and I think Vref(PFM) must be larger than Vref(PWM). "

    [MQ]->The Vref(PFM) =1.0125*Vref(PWM). So you are right.

  • 0 in reply to Minqiu Xie
    TI__Expert 5115 points

    Hi Minqiu,

    Thank you for your support.

    I understand.

    thank you very much!

    best regards,

    Takuya