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TIDM-PSFB-DCDC: How does LMR38020 move between PWM and PFM as the load changes?

Woney Gu
Prodigy 265 points
Part Number: TIDM-PSFB-DCDC
Other Parts Discussed in Thread: LMR38020

Hello, i am reading the datasheet of LMR38020 and i find that LMR38020 has TWO FUNCTIONAL MODES: Auto Mode, where the device can move between PWM and PFM as the load changes, and FPWM Operation. So i am wondering how the device choose its functional modes and determine whether the load is light or not. If the chip code is LMR38020FSDDAR, does it mean that this device can only operate on FPWM mode? Also how can i know whether the device realizes the function of spread spectrum? 

I am looking forward to hear from you. Thank you!

  • 0
    TI__Expert 5290 points

    Hi Woney,

    You can refer to Datasheet Page3 Table 5 for the OPN variants of the device and to see if the variant integrates Spread Spectrum or not. LMR38020FSDDAR is FPWM and it will operate with FPWM under the all the load condition. If the device is PFM, at light load, the high-side MOSFET is turned on in a burst of one or more pulses to provide energy to the load. The duration of the burst depends on how long it takes the inductor current to reach IPEAK-MIN. More details are elaborated in Datasheet Page 18 Section 8.4.1 Auto mode.

    Regards,

    Hongjia

  • 0 in reply to Hongjia-Wu
    TI__Prodigy 265 points

    Thank you! So can I understand it that way?

    LMR38020FSDDAR is FPWM so it will operate with a fixed frequency (if spread spectrum is not taken into account) at any time (start up, steady state...) and under any load condition. And only when a IC is not FPWM then it can operate in auto mode (PFM at light load and PWM at heavy load). Also since spread spectrum is optional for LMR38020FSDDAR, the fsw can spread in a range of -8%~8% to improve the EMI performance.

    Is my understanding right?

    and how to enable the spread spectrum?

  • 0 in reply to Woney Gu
    TI__Expert 5290 points

    Hi Woney,

    Yes you are right. However Spread spectrum function is integrated with specific device variant. If you want this function, you need to choose the specific OPN with Spread Spectrum marked as yes.

    Regards,

    Hongjia

  • 0 in reply to Hongjia-Wu
    TI__Prodigy 265 points

    Thank you a lot!

    From this table i can see that  LMR38020FSDDAR's OPN with spread spectrum is marked as YES. So this device's spread spectrum function has already been enabled right? And it also means i cannot disable spread spectrum right?

  • +1 in reply to Woney Gu
    TI__Expert 5290 points

    Hi Woney,

    Yes your understanding is correct.

    Regards,

    Hongjia

  • 0 in reply to Hongjia-Wu
    TI__Prodigy 265 points

    Hello Hongjia, Will LMR38020FSDDAR operates in PFM mode during start up time, before tss_done (around 18ms)? 

  • +1 in reply to Woney Gu
    TI__Expert 5290 points

    Hi Woney.

    Yes, at startup it is operating with PFM.

    Regards,

    Hongjia