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Original question:

[FAQ] Is there a calculation tool for TPS923655 / TPS923654 / TPS923653 / TPS923652?

TPS923655: Excel calculation tool

Tomasz Jastrzebski
Prodigy 102 points
Part Number: TPS923655

Tool/software:

Hello TI,

Excel calculation tool mentioned in this post may have some issues.
1. 500 kHz frequency in row 18 is not on the list.
2. If I enter values taken right from the example in the official specification (chapter 9.2.1.1 Design Requirements), the tool comes with substantially different power inductor value.
I think either calculations presented in chapter 9.2.1.1 are simplified too much (e.g. switching diode forward voltage is not considered) or there is some mistake in the excel tool.

  • 0
    TI__Genius 15305 points

    Hi Tomasz,

    Sorry. I am currently out of office. I will reply to you tomorrow when I am back to office. Thanks for your understanding.

    Best Regards,
    Steven

  • +1
    TI__Genius 15305 points

    Hi Thomasz,

    Sorry for the late response. Please refer to my answers below.

    1. Table 8-1 from the datasheet does cannot list all the possible value. Actually you can set the switching frequency continuously between 100kHz to 2.2MHz. A simple rule is that when you time the switching frequency (left column) with the corresponding resistor value (right column), you will get an nearly constant value.
    2. The excel calculation tool just provide a basic reference for your circuit design. Some factors are not taken into consideration when designing this calculation tool, like the current rising / falling time. The design example in Section 9.2 Typical Application from the datasheet is carefully designed to meet the design parameters, with more factors taken into consideration, in comparison with the calculation tool. Therefore, the circuit components may be designed differently.
    3. The equations shown in Section 9.2 Typical Application from the datasheet are simplified for easy and quick calculation for some initial component values. You can then adjust the values based on your test results.

    Best Regards,

    Steven

  • 0 in reply to Steven Li
    Prodigy 102 points

    Hi Steven,
    Thank you for the clarification. Note that the example in the section 9.2.1.2 "Detailed Design Procedure" uses fsw=500kHz which, as you rightly pointed out, is not an available option. This mislead me.
    Kind regards,
    Tomasz


  • 0 in reply to Tomasz Jastrzebski
    TI__Genius 15305 points

    Hi Tomasz,

    Steven Li said:
    Table 8-1 from the datasheet does cannot list all the possible value. Actually you can set the switching frequency continuously between 100kHz to 2.2MHz. A simple rule is that when you time the switching frequency (left column) with the corresponding resistor value (right column), you will get an nearly constant value.

    Please be noticed that you can set the switching frequency continuously between 100kHz to 2.2MHz, which is not limited to the discrete values shown in Table 8-1 from the datasheet.

    500kHz is a valid switching frequency that you can set. For example, in order to set 500kHz switching frequency, you can use an R_FSET resistor of 400kHz * 59kohm / 500kohm = 47.2kohm. (I am using the 400kHz value here to calculate the nearly constant product).

    Please be noticed that this calculated resistor value (or the resistor value you look up from Table 8-1 from the datasheet) normally is just a rough value for the targeted frequency. If you need an accurate switching frequency, you may need to fine-tune this resistor value based on your test results. 

    Best Regards,

    Steven