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LM3492: Datasheet 9.2.2.7 RRT and RIREF

Siegmund Schlechter
Prodigy 20 points
Part Number: LM3492

Hi TI,

the resistors RRT and RIREF set the switching frequency fSW of the boost converter and the LED current ILED respectively.

Figure 16 shows values for ILED of 150mA only. Determining for other values is not possible.

A small variation of RRT show a huge deviation of fSW.

How can RRT be calculated?

Thanks a lot!

Siegmund

  • 0
    TI__Genius 16140 points

    Siegmund, 

    You may refer to figure 15 for frequency setting. thanks!

    Regards,

    Xiaoguang

  • +1
    Prodigy 20 points


    Dear Xiaoguang,

    thank you for your fast reply, however R_RT is not only dependent on f_sw,
    as can be seen when figure 15 is compared to figure 16 (see e.g. at 200 kHz / 800 kOhm).

    Also Formula 8 and 9 suggest a possible relation of R_RT to V_in, V_out or L1, I_in, V_in:
    [ f_sw = (1 - V_in/V_out) / t_on = (1 - V_in/V_out) / (L1 * (2 * I_in) / V_in) ]
    which would explain the different t_on values for several (V_in, V_out, R_rt) configurations
    listed in the table in chapter 7.5 Electrical Characteristics.
    Otherwise it would be unnecessary to list R_rt there.

    To my current understanding R_rt cannot be independent of V_out (whatever is internally attached),
    in the context of f_sw, since changes in V_out must trigger the switching at some point
    (increase in V_out => higher current flow into RT pin, due to constant R_rt).

    Since I require a different combination of V_in, V_out, I_led and L1 for my application
    than in your application note/datasheet and currently do not get the switching frequency I want,
    I require further information. Therefore a clarification in the datasheet regarding the
    dependencies/independencies of R_rt and also R_iref would be greatly appreciated.

    Thank you very much and have a nice day.

    Kind regards,

    Siegmund

  • 0 in reply to Siegmund Schlechter
    TI__Genius 16140 points

    Hi, Siegmund, 

     R_rt and R_iref  are independent. Riref sets LED current. 

    As you know, operation frequency can't be set very accurate and there is always signifcant tolerant.  My suggestion is that selecting a Rrt referring to Figure 15 with target frequency and fine tune it if necessary.

    LM3492 is old device that designed long time ago. I can't find out more information that can help you on this question.