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LM65645-Q1: How much response time is until straring PWM mode from heavy load observed at light load operation?

Yoshiki Ochi
Genius 9365 points
Part Number: LM65645-Q1
Other Parts Discussed in Thread: LM65645

Tool/software:

Hi team, 

Could you share how much response time is until PWM mode starting from heavy load applied at light load PFM operation? Image is shown below. 

Regards,
Ochi

  • 0
    TI__Guru* 78070 points

    Hi Ochi-san,

    Thank you for posting.  There is no fixed timer for the IC to walk out of the PFM mode into the PWM mode. The transition is natural and smooth.  Note that IL_PEAK_MIN controls PFM, which is in very light load conditions.  Following a large step load, the inductor will increase beyond IL_PEAK_MIN threshold and the control moves out of PFM mode and into PWM mode.  It usually takes not more than two regular switching cycles.  For instance, if you set the operation in 400kHz, the transition completes within 5us.

    Hope this clarifies,

    Best Regards,

    Youhao

  • 0 in reply to Youhao Xi
    TI__Genius 9365 points

    Hi Youhao, 

    Thank you for feedback. We are care about voltage dip by heavy load step transient in PFM mode. Could we estimate how much bandwidth is in PFM mode by using external component calculation? We want to know "tp" in below figure. 

    https://www.ti.com/lit/ta/ssztal4/ssztal4.pdf 

    Regards,
    Ochi

  • 0 in reply to Yoshiki Ochi
    TI__Guru* 78070 points

    Hi Ochi-san,

    Depending on the starting load level in PWM, the bandwidth can be different. Instead, please ask the customer to refer to the LM65645 EVM user's guide.  Figure 3-6 shows the output response to the max step load, 0A to 4.5A in auto mode, namely in deep PFM (No load) to full load.  All other step load should result in smaller undershoot. 

    The customer can also simulate this in Pspice.  

    Best Regards,

    Youhao