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LM2735X Minimum Load

Nathaniel Wilson
Intellectual 1990 points
Other Parts Discussed in Thread: LM2735

Hi team,

I saw a post (here:http://e2e.ti.com/support/power_management/non-isolated_dcdc/f/196/t/298339.aspx ) that mentions that the LM2735 will stop switching until the output is back into regulation once it gets below a minimum load. It also mentions that this minimum load can be calculated using Vin, Vout and L. I did not see a way to calculate this in the data sheet. The closest I could find is on page 11 equation 7. However, for that you would need the peak induction current and inductor ripple current. Is this the correct way to calculate the minimum load?

Thanks!

  • 0
    TI__Genius 16025 points

    Hi Nathaniel,

    We've received your query and will get back to you shortly.

    Thanks,

    Anston

  • 0
    TI__Genius 16025 points

    Hi Nathaniel,

    Sorry for the delay in response, I've been pondering how to reply over the last few days. 

    At light load conditions, a single switching cycle will pass more energy into the output capacitor than is drawn by the load. From this point, the feedback loop enters a pulse skipping mode. Exact calculation is difficult because the single switching cycle is determined by the minimum duty cycle that the device can accommodate. There are two frequency versions of the LM2735 available. As they are of similar architecture, choosing the lower frequency device will improve the light load characteristic since while the ON-time is the same the resulting minimum duty cycle figure of the lower frequency part will be better. Additionally, if you increase the inductance value, the peak switch current per switching cycle will be lowered and also improve light load margin but to a lesser degree. Note that minimum duty cycle is not a guaranteed parameter as only a typical number is shown on the EC table.

    Exact calculation is difficult because it requires iteration. We suggest trying Webench for simulation results and progressively decreasing target load current until pulse skipping occurs.

    Thanks,

    Anston