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Original question:

TPS61196: Is it possible to use TPS61196 under the condition that LED current per one IFB pin is less than 50mA?

TPS61196: Is it possible to use TPS61196 under the condition that LED current per one IFB pin is less than 50mA?

Shinsuke Tanaka
Intellectual 520 points
Part Number: TPS61196

Mr.Zhou,

I'm sorry for my delayed response and I appreciate your explanations.

>You need to set to 50mA per channel ,

>but you need to control the PWM duty not very large and not exceed your maximum current limit.

Above are your latest comments on this topic.

And regarding how we control the LCD panel whose LED current per one string is less than 50mA using TPS61196,

I now understand as shown below based on your comments;

(Here we assume that the LED current per one string is 35mA, just an example.)

   1)  LED current should be set to 50mA. Concretely speaking, the resistance connected to ISET pin should be set to around 100kohm.

          In other words, the LED current should NOT be set to 35mA, namely resistance connected to ISET pin should NOT be set to 140kohm.

   2) In addition to 1), the average current which flows LED string should be decreased using PWM so that

      the current value which one IFB pin sinks does NOT exceed LED current per one string, here 35mA.

      In other words, we have to configure the duty of PWM so that it never exceeds 70%(=35mA/50mA).

Are my above 1) and 2) understandings correct?

I'm sorry to bother you repeatedly, but please let me clarify my understandings.

Best regards,

Shinsuke Tanaka

  • 0
    TI__Mastermind 22745 points
    Hi Shinsuke,
    Yes, your understand is totally right. if you have any further question , please let me know.
    Regards
    Sean
  • 0 in reply to Sean Zhou
    Intellectual 520 points

    Dear Mr.Zhou,

    Could you please let me know the reasons why the current limit should NOT be set to less than 50mA

    from the viewpoints of internal structure of TPS61196 or behaviors of  that?

    For example, I now guess it is possible that 

    the internal circuits of TPS61196 are configured so that its IFB pin sinks 50mA

    even when the current limit is set to less than 50mA,

    namely the resistance connected to ISET pin is 140kohm ,200kohm etc.

    Of course, my above guess is just an example and may be incorrect,

    but I want to know the reasons why the current limit should NOT be set to less than 50mA

    from such viewpoints.

    Best regards,

    Shinsuke Tanaka

  • 0 in reply to Shinsuke Tanaka
    TI__Mastermind 22745 points
    Hi Shnsuke,
    TPS61196 can't support maximum current below 50mA. it can't start up.
    Regards
    Sean
  • 0 in reply to Shinsuke Tanaka
    TI__Mastermind 22745 points
    Hi Shinsuke,
    I'm sorry that I need to make some correction, because the datasheet of TPS61196 only has mentioned that TPS61196 can output upton 50mA within +/-1.5% accuracy. Actualy thise device can support below 50mA per channel. Last time I have tested with the EVM but I can't start up the LED, because I have made a mistake, I haven't provide enough headroom voltage for current sink. Recently I have tried again, t works fine, and I have also checked with our designer, it really can support below 50mA. And I have request them to help me identify is there any risk for below 50mA. I have tested 30mA and 45mA, it works fine. I'm sorrry for the mistake and misunderstand. I will update further information when I get from designers. Sorry again.
    Regards
    Sean
  • 0 in reply to Sean Zhou
    Intellectual 520 points
    Dear Mr.Zhou,

    I appreciate your updates on this topic.

    I understand that TPS61196 can be used even under the condition that
    the sink current on IFB pin is set to be less than 50mA.
    According to my colleague who also has discussed this topic with other TI engineers ,
    the current matching accuracy becomes a bit bigger than 1.5%, which is shown on the datasheet,
    when TPS61196 is used at less than 50mA.
    I now guess that the reason why TPS61196 datasheet says the current should be bigger than 50mA
    is that current matching accuracy 1.5% can NOT be assured at less than 50mA.

    If there are any misunderstandings on my above comments, please let me know.

    Best regards,
    Shinsuke Tanaka
  • 0 in reply to Shinsuke Tanaka
    TI__Mastermind 22745 points
    Hi Shinsuke,
    Yes, Your understand is totally right. For TPS61196, if the full scale current is below 50mA, it can't reach the target matching accuracy. That's the main reason. And another reason is that when we define this device, it's mainly target high voltage and high current backlight application. Most customer will use this upon 50mA. So our spec guarantee the +/-1.5% mathcing accuracy above 50mA. if you can help to click "resolved" button, I will appreciate. We can discuss this question in new "post".
    Regards
    Sean