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TPS65911: What is the light load current to switch between PFM and PWM mode for VDD1 and VDD2?

Josef Rypar47
Prodigy 240 points
Part Number: TPS65911

Hello,

The PMIC's VDD1 +1.0V regulator that supplies CVDD1 rail of 66AKG12 processor produces a voltage ripple of about 50mV on our prototype. This is clearly the pulse skipping (PFM mode) power saving feature of the PMIC.

The PMIC datasheet says in chapter 7.2.2.3.2:

" At light load currents, the converter operates in PFM mode and the output voltage ripple is dependent on the output capacitor value and the PFM peak inductor current."

What is the level of "light load"? - this parameter is not defined in the datasheet.

In other words - what is the output current that causes transition from PFM to PWM mode?

We have followed the datasheet recommendation on the size of the output capacitor - 10uF.

The datasheet says:

"Higher output-capacitor values minimize the voltage ripple in PFM mode."

Does the capacitor value reduce the voltage ripple only or does it also reduce the output current threshold for PFM->PWM transition?

Thanks and Regards,

Josef Rypar

  • 0
    TI__Genius 10885 points

    Hi Josef,

    The exact PFM to PWM crossing threshold is not characterized, but will likely be related to the zero crossing current of the inductor. Looping in the device expert to comment.

    Increasing the output capacitor will absorb more of the ripple, but the crossing transition is likely more dependent on the inductor value, input voltage, and output voltage.

    Pulse skipping can also be manually disabled for all the regulators using the DCDCCTRL_REG register.

    Best Regards,

    Rick S.

  • 0 in reply to Richard Solarski
    TI__Mastermind 48861 points

    Hi Josef,

      Rick's answer is totally correct. Basically, it's decided by the device's internal CCM/DCM boundary detector which can be enable (PFM or pulse skipping for light load) or disable (force to PWM). Whenever it's enabled, it does its work automatically. 

  • 0 in reply to Phil Yi
    Prodigy 240 points

    Hi Phil and Rick,

    thank you for your answers. Now I understand the factors that drive the PFM-PWM transition.

    Having specific parameters: L=2.2uH, Vin=5V, Vout=1.0V, fsw=3MHz, can you please estimate (roughly) the desired value of the load current? Is there a formula to calculate that? Are there any other factors like temperature, production tolerances, etc., that affect this parameters?

    We basically need to know how far we are from the transition point in our current design.

    Thanks and regards,

    Josef

  • 0 in reply to Josef Rypar47
    TI__Mastermind 48861 points

    Hi Josef,

      Yes, there is a formula to calculate that as listed below: 

      The zero crossing current of the inductor = VOUT*(VIN-VOUT)/(Inductance*VIN*Switching frequency*1000000).

     Of course, the temperature, production tolerances, etc., can affect this parameters, but for rough estimate, those impacts can be ignored.