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TPS61194: There are two problems in TPS61194's circuit.

Went Cao
Genius 5550 points
Part Number: TPS61194
Other Parts Discussed in Thread: , TPS61193-Q1

Hi, all

      There are two problems in TPS61194's circuit:

      1. When LED_BL don't connect with any LEDs, the voltage of LED_BL change to 13V from 27.6V, but, the EVM's circuit is aways 30V. Why?

      2.When LED_BL connect with LEDs,OUT1/2/3 is 80mA, but the SW pin's wave  of TPS61194 is  abnormal,like the picture.Is not continuous wave?

  • 0
    TI__Intellectual 2180 points
    Hello,

    Regarding your questions:

    1. When all LEDs open condition happens, the boost ramps up to max output level determined by FB resistor divider. Once Vout max is reached, since all LED channels are open, the device will disable all current sinks outputs (LED 0-3) and boost will completely stop switching. So you should expect for Vboost to discharge down to Vin level.

    2. Looks like boost is entering DCM mode due to relative light load and low conversion ratio condition, this is expected behavior and shouldn't be an issue. But if you would like to avoid operating in DCM mode, you could
    increase the load, by setting higher LED current or higher Vboost level (adding a series LED) or decreasing input voltage.

    Hope this helps.
  • 0 in reply to Salome Ramirez Carrion
    Genius 5550 points
    Hello
    1. But the EVM can always keep 30V,why ?
  • 0 in reply to Went Cao
    Genius 5550 points

    Hi Salome

          Sorry, because of my wrong operation, the EVM output 13V also,when don't have LEDs.

          But the second question, is there any data of test about that SW's wave by the load changing?

        

  • 0 in reply to Went Cao
    TI__Intellectual 2180 points
    Hello,

    We don't have data summary for SW by load, since there are many parameters that affect this such as switching frequency, load, input/output voltage.

    I realize that you mentioned 80mA/string. From schematic, RISET seems to be ~39k which would set the ILED current to ~60mA instead of 80mA. Please make sure that ~30kOhm is used for target 80mA current. This will increase the load and should help prevent device from going into DCM mode. Let me know if this fixes the switching behavior.

    Also, I wanted to point out that other device that would be better suited for your application is TPS61193-Q1, which has all same capabilities with TPS61194-Q1 but is a 3 channel device.