• State Resolved
  • Locked Locked
  • Replies 2 replies
  • Answers 1 answer
  • Subscribers 62 subscribers
  • Views 191 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

LMR33620: PFM mode

Mark Guastaferro
Genius 11095 points
Part Number: LMR33620

I need to better understand what determines when the LMR33620C will drop into PFM mode. For example, is there a threshold that the peak inductor current must cross during the upper FET on-time to keep it in PWM mode? Some other peak current mode regulators do this. 

The reason I ask is if I use Webench to look at the Ops Values during different load settings, it seems to report PFM mode when the inductor current will drop below zero (diode emulation mode).  Whereas, some simulations I've run using the Pspice transient model I can see the switch node waveform is in PFM mode at load currents higher than what would cause negative inductor current and will stay in PWM mode longer if I reduce the inductor value. Since Webench doesn't support a circuit simulator for this device, I was thinking it might not capture the PFM mode accurately.

I don't have actual hardware to check this (yet), so validating the model would be really helpful for setting a target inductor value. 

  • 0
    TI__Mastermind 25590 points

    Mark,

    Here's a video I made regarding mode transitions for LMR33620. Here's what's going on.

    1. SW goes HIGH until IL ramps up to I_PEAK_MIN (0.65A for LMR33620 in datasheet).

    2. IL reaches I_PEAK_MIN and SW goes LOW. IL ramps down to 0A.

    3. IL reaches 0A and both FETs turn off. SW is high-z. Then the converter waits to switch again based on VOUT sensed on FB.

    The output current will be equal to the average value of this waveform. These pulses occur as often as necessary to keep up with the load demand (hence pulse-frequency modulation)

    You are in PFM as long as:

    A. The pulse frequency is less than the part's normal operating frequency (2.1MHz for the C version) - otherwise you're in fixed-frequency discontinuous conduction mode.

        AND

    B. There is some non-zero time when both FETs are off (IL is zero amps not counting ringing) which can also be seen as IOUT must be less than I_PEAK_MIN/2 (this is when the pulse waveform turns into a triangle waveform)

    Let me know if something is not clear.

    -Sam

  • 0 in reply to Samuel Jaffe
    TI__Genius 11095 points
    Hi Sam,
    This is exactly what I wanted to know. Thank you for the info and video. Sorry, I didn't notice the I_PEAK_MIN spec in the data sheet.