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Using the L5973D- I have followed the Schematic as the datasheet instructs, but I have tried two different inductors. I have found problems with other systems on the board with the one inductor (744 561 15), such as Microchip while running in debug mode. The Schottky Diode (D1) used is the STPS340-SMB instead of the STPS2L25.

enter image description here

The requirements of the inductor are 15uH and 3A. So the first inductor selected was the DO3316P-153 (as recommended by the L5973D datasheet). The frequency out of the L5973D at pin one (circled in red on the schematic) is given in the figure below:

enter image description here

When the inductor is replaced with the more affordable inductor (744 561 15), the frequency at pin one is given in the figure below:

enter image description here

With the 744 561 15, there is this added ripple before the square wave, can anyone explain what is happening here?

Is the older inductor (DO3316P-153) filtering or is the new inductor(744 561 15) adding in a delay?

EDIT:

After running the Board for more than an hour and the system keeps resetting itself. At this point, I am not sure what is going on, any suggestions?

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  • \$\begingroup\$ It is unclear what model the "new" inductor is, include a link. Also those plots do not show the frequency but the voltage (over time). \$\endgroup\$ Sep 4, 2018 at 10:22
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    \$\begingroup\$ Looks like you just entered DCM with your 744 561 15 inductor. Have you designed for CCM? \$\endgroup\$
    – winny
    Sep 4, 2018 at 12:29
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    \$\begingroup\$ @winny Seems so obvious now that you say it, it's in DCM all right. \$\endgroup\$
    – Barleyman
    Sep 4, 2018 at 14:41
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    \$\begingroup\$ As Inductance value is not changed, DCM will be there whether it is old inductor (DO3316P-153) or new inductor(744 561 15). as DCM behavior is not observed for OLd inductor and it is observed for new One, then at 250 Khz switching frequency the new inductor inductance is still less @ 250 Khz. at 250Khz switching freq with Vin 12, Vout 3.3V, with 15uH, current ripple p-p is ~ 683 mA , if default load is < ~ 300mA then DCM will be entered, \$\endgroup\$
    – user19579
    Sep 4, 2018 at 17:44
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    \$\begingroup\$ @Gareth T: Those are ruff calculations found in any general Buck design, V (Vinmax-Vout)= L * di(ripple)/dt(on time). Find min Lmin. Then with selected L calculate delta I , your current should be at least > delta I/2 to have CCM. \$\endgroup\$
    – user19579
    Sep 5, 2018 at 13:58

1 Answer 1

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It seems your load current is small enough that the buck converter enters discontinuous mode. This means in basic terms that the inductor charge is exhausted before the end of the cycle, hence you get a some ringing before the next cycle starts.

Usually it's recommended you'd want to stay near the transition between continuous and discontinuous mode in normal operation. Theoretically this leads into optimal component sizes and costs but practically it's very much a moving target.

Wikipedia gives reasonably good explanation of what's going on there: https://en.wikipedia.org/wiki/Buck_converter#Discontinuous_mode

To push the SMPS back to CCM, you'd want a larger inductance value. However changing the inductor size may also call for a smaller output capacitor. And then you need to re-adjust the feedback loop compensation as well.

If you're not comfortable with all that, I'd suggest

  • Leave it alone, DCM is not a problem as such, or
  • Use Texas Instruments webench tool to design you a SMPS circuit, it does practically everything for you. Kids these days.
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