• State Resolved
  • Locked Locked
  • Replies 6 replies
  • Subscribers 63 subscribers
  • Views 753 views
  • Users 0 members are here
Support feedback
Options
Options
Related

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.

TPS61020 Induction calculations

David Linker
Prodigy 60 points
Other Parts Discussed in Thread: TPS61020, TPS60120, TPS62160, TPS61260, TPS61070

I am designing for a low-current application. Peak current will be ~ 30 mA, in very short bursts, and average current will be < 200 µA during operation. The power will be from a coin cell (BR probably but CR possible) at ~ 3V, while the circuit will run at 3.3V when operating.

In addition, I want a true shutdown that disconnects the circuit from the battery, with < 1µA draw when shut down, for a long shelf life, and finally want as much efficiency as I can design in, for the longest possible battery life when operating.

After trolling the datasheets, and trying some evaluation boards, it seems that the TPS61020 will give me the highest efficiency at this low current draw (73%). 

I have three questions:

- The first is whether I should be considering any other parts in the comparison. I have tested the EVM for TPS60120, which was 58% efficiency, and tried a two cell series configuration with the TPS62160, which had a 71% efficiency when accounting for two times as many cells needed.

- The second is that I did the calculations for the inductor from the datasheet, and came up with 82 µH!!! Does this mean that I should just use the design max of 22

µH and test how it works, or should I give up on using this chip?

- Finally, I have only used inductor less DC/DC converters before. Do I have to worry about EMI, and if so, is there any data on EMI from the evaluation board, for example?

Thanks in advance.

  • 0
    TI__Guru**** 151780 points

    Yes, you need a different IC for such low powers.  With load disconnect, I would recommend the TPS61070 or TPS61260.

    If you stay with the TPS61020 which I don't recommend, then you can use any value in the datasheet's range.  You will always operate in power save mode at such light load currents.

    Also, make sure you are measuring the efficiency with this app note's setup: http://www.ti.com/lit/an/slva236/slva236.pdf

  • 0 in reply to Chris Glaser
    Prodigy 60 points

    Thank you for your quick reply.

    I have ordered the EVMs for both of those parts.

    I looked at the datasheet for the TPS61070, and the calculation for the inductor is similar, although the frequency is higher, so that the calculation yeilds a recommended inductor of 41 µH.

    Can I get away with much less?

    The other datasheet was not available online, but needed to be requested, so I don't know if it is similar.

    Thanks.

  • 0 in reply to David Linker
    TI__Guru**** 151780 points

    The TPS6xxxx parts are easy to use.  Just pick an inductance within the recommended range in the datasheet and it will work, assuming you meet the other datasheet requirements for external components.  We recommend to use the default circuit, shown in every datasheet figure.

    Designing for a given ripple current is what some designers do when their load currents are higher.  It does not make sense to do this for your very low load currents.  You will always operate in power save mode, which gives the best efficiency.

  • 0 in reply to Chris Glaser
    Prodigy 60 points

    Thank you for this clear guidance. I have ordered the EVMs, and based on my results, I will use a circuit design based on the recommended design.

    Is there any data on EMI for the reference designs of EVMs?

    Thanks

  • 0 in reply to David Linker
    Prodigy 60 points

    I was wondering why you advise against using the TPS61020?

    I have tested the various EVMs and measured the current consumed versus delivered, which is the important metric for me.

    The ratio of current delivered versus current consumed is as follows for the TPS61020, and the two chips you recommended, TPS61070 and TPS1260

    TPS61020 74%

    TPS61070

    		 49%
    TPS61260 15%

    In other words, I would need 50% more battery capacity if I used the TPS61070, and 5x the battery capacity if I used the TPS61260.

    Thanks for any thoughts.

  • 0 in reply to David Linker
    TI__Genius 16870 points

    Hi David,

    First off, the current ratio is not the efficiency of the converter. The voltages in and out also come into play. 

    Although, assuming that all of your input and output voltages are the same between the compared converters one could make a fair comparison between the above mentioned converters with the current ratios.  

    After comparing the efficiencies of all of the EVMs, you are welcome to select the best one. For the TPS61070's case, the TPS61070 does have a lower Iqu and Ishdn current than the TPS61020. This should be considered if the the load is disconnected for long periods of time.