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TPS92692: LED driver

Part Number: TPS92692
Other Parts Discussed in Thread: LM3424, INA149

Dear team,

I have some problem on TPS92692:

1, In data sheet, inductor, output capacitor and input capacitor selection are calculated based on output power, which are unlike others such as lm3424, and I can not understand the equation given in data sheet. (P27-P28 in datasheet)

2, In the over voltage protection circuit in buck-boost topology use a PNP, why?(P26 in datasheet)

3, What is the meaning of rD, "LED string dynamic resistance", how to calculate or acquire it?

thank you!

  • Hi Xiaoquing,

    Please see my answers:

    1. The TPS92692 is a different type controller (peak current fix frequency) than the LM3424 so the equations will be differnt than the LM3424. In section 9.2 (page 34-46) we have examples of how to design both a Boost and a Buck Boost regulators and you can follow those examples for your design requirement.

    2. For a Buck Boost regulator, the LED is connected to the ouput and input voltage. The PNP transistor provides the differential voltage across the LED (Output voltage - Input voltage) thus it can better protect the LED.

    3. LED dynamic resistance is the ratio of forward voltage of the LED relative to forward current of the LED. Attached here is a short explanation of it: webench.ti.com/.../Rd.pdf

    Thanks
  • Hi Tuan,

    thanks for your reply!

    on the first question, I want to know more about it, is there some application note for the analysis or derivation on these equation?

    similarly, could you please provide some more detail about the second question?

  • Hi Xiaoqing,

    The application notes are in the specification along with details example on how to design a boost or boost to battery controller. Please see section 9 of the specification.

    For the second questions: The transistor is used to do differential voltage from the output to the input so that you get the actual votlage on the LED to better protect the LED. What more detail do you need than that?

    Thansk
  • Hi Xiaoquing,

    We have not heard back from you so we believe you may have solved the issue.
    The thread will close now but will reopen if you reply.
    TI THINKS RESOLVED;
  • Hello Tuan,

    Thanks for your helps,

    I have some new question:

    1, when I test the loop response of TPS92692 on EVM, Have no PWM dimming and no ss the connection is shown as below, except INA149,A and B connected to each end of R7

    But the result may have something wrong:

    What should I do to test it correctly? 

  • 2, LM3424 can work in Buck-Boost topology (or Boost-to-Battery), whose topology is the same as Boost, the only difference is the cathode of load connecting to the input.

    In this topology, the inductor is directly connecting to the input, thus the ripple current on the input capacitor is equal to the ripple current on inductor, that is, the ripple current on the capacitor is continuous. This fact determines the calculation of input capacitor, the equation is given as below:

    In other word, the calculation of input capacitor in Boost-to-Battery topology should be the same as that in Boost topology. However, the equation for Boost-to-Battery input capacity calculation given in datasheet  is:

    This equation is used when the current is discontinuous, and it is right in the inverting Buck-Boost topology, but does not apply in this case. An experiment wave is shown below, which is a typical integral of continuous current, and has confirmed that the ripple current on input capacitor in Boost-to-Battery topology is continuous. 

    So is  there something wrong?

  • Hi Xiaoqing,

    Boost to battery is really a buck-boost and that's why the equations are different than a boost regulator and the waveforms will be different. For the Buck Boost, the input cap will experience current flowing back into it when the switch node is off. Since the output cap and load are tied back to the input votlage. The boost does not have current flowing back from the ouput since the output cap is and load is tied to ground.
  • Hi Tuan,
    What puzzles me is that in Boost-to-Battery circuit, the ripple current is connected or not?
    Besides, is INA149 necessary for loop response measurement? or there are some other wrongs when test?
    Thanks
  • Hi Xiaoqing,

    For Boost to Battery the output cap and load is tied back to the input cap and that's why the current to the input cap is different than that of the boost where the ouput cap and load is tied to ground.

    For the loop response as I answered before...You need the INA149 otherwise the voltage coming into the Network Analyzer will be too high and you will have issue.

    Thanks Tuan