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LMR51610: usage as Inverting Buck-Boost (or other SOT-23 regulator)

Alessio Caligiuri1
Prodigy 190 points
Part Number: LMR51610
Other Parts Discussed in Thread: LMR50410

Hello,

I would like to use the LMR51610 in Inverting Buck-Boost configuration, to build a DC/DC converter with the following specs:

  • Vin = 24V +/- 10%
  • Vout = -15V
  • Iout = 0.5A max

This would replace an old design with LM2594HVM-ADJ, in order to reduce size and BOM cost.

My questions are:

  • is this possible?
  • are there any drawback?

If not possible, I would like to have your suggestion for an alternate in SOT-23 package (I am currently using the LMR50410 converter with success, for a non inverting buck converter).

Thanks, best regards.

Alessio

  • 0
    TI__Genius 13290 points

    Hi Alessio,

    It should be possible. The main thing you want to make sure that the voltage the device is rated for is more than the maximum voltage variance. You need a 39V (24V – -15V) rated part for your application.

    It should technically be possible to implement an inverting buck boost with any buck as long as the voltage you expose the device to is not exceeding the abs max. For more information on how to implement an inverting buck boost, refer to this application note.

    https://www.ti.com/lit/an/snva856b/snva856b.pdf?ts=1689196566270&ref_url=https%253A%252F%252Fwww.google.com%252F

    Thanks,

    Andrew

  • +1
    Prodigy 190 points

    Hi Andrew,

    many thanks for your reply. That application note is really interesting and I have already had a look to that. The voltage, current and efficiency limitations are quite clear to me.

    I have many doubts and gaps for what concerns the closed-loop stability of the regulator operating as IBB. I mean, the app. note mentions a RHP zero appearing in the loop function; with this kind of new TI regulators that are internally compensated, it would be interesting to understand a bit better any possible chance to go to the instability region or to have a bad phase margin.

    Moreover, I see that, for example, LMR51610 is available in both PFM and FPWM version, where the former is modulating frequency and can go to DCM operation, and the latter is a constant frequency that operates in CCM even with light load, by inverting the inductor current in the same cycle. Do you have an idea on what is the impact, in terms of stability, of these two versions? In other words, what could be better for the IBB operation?

    Best regards,
    Alessio

  • 0 in reply to Alessio Caligiuri1
    TI__Genius 13290 points

    Hi Alessio,

    It mostly doesn't matter for FPWM vs PFM. The only thing reason you use FPWM over PFM is if there's some problematic frequency you want to avoid switching at  (ie 1MHz in an automotive application). I think that the light load efficiency benefits that you get in an PFM over a FPWM make it worth it to use that.

    By the way, I discussed this with my colleagues and there is a type of converter you don't want to use. COT devices behave in strange ways in the IBB configuration. I thought I should let you know, but you won't have to worry about that with the LMR51610.

    Here's another design resource for designing an IBB. Although he didn't explicitly test for stability, He used an internally compensated part and didn't run into any issues.

    https://www.ti.com/lit/an/snvaa65/snvaa65.pdf?ts=1689267817548&ref_url=https%253A%252F%252Fwww.ti.com%252Fproduct%252FLM63615-Q1

    Also if you don't want to commit to ordering a PCB for an IBB circuit you made to test it, then you can modify a TI EVM to test the parts IBB functionality without too much effort.

    Thanks,

    Andrew

  • 0 in reply to Andrew Kutzler
    Prodigy 190 points

    Hi Andrew,

    many thanks for your help. About the COT devices, I guess you are referring to "Constant-On-Time" control; isn't it something very similar to the PFM at light loads? I mean, if I understood correctly how PFM works, when there is a low output current, the high-side switch is kept with constant ON time (t_on,min) and the switching period is increased to maintain voltage regulation. This is COT control... am I right?

    Thanks also for the App. Note you linked, it is another good starting point for my design.

    My plan is to have at least some preliminary theoretical ideas, then to test this architecture with a modified EVM, as you suggested.

    Best regards,
    Alessio

  • 0 in reply to Alessio Caligiuri1
    TI__Genius 13290 points

    Hi Alessio,

    Not exactly. LMR51610 is a peak current mode device. It regulates the output by adjusting the peak current rather than having a constant on time (COT) and adjusting the duty cycle percentage. COT devices also require a special type of compensation. LMR51610 operates in a continuous conduction mode or discontinuous conduction mode while in PFM.

    Sounds good feel free to post a new thread if you run into any issues implementing your design.

    Thanks,
    Andrew

  • 0 in reply to Andrew Kutzler
    Prodigy 190 points

    Hi Andrew,

    thanks for the clarification. Could you please suggest me a document where these control strategies are explained? I know that TI has very good app. notes but I cannot find anything about COT.

    Best regards,
    Alessio