• State TI Thinks Resolved
  • Locked Locked
  • Replies 3 replies
  • Answers 2 answers
  • Subscribers 62 subscribers
  • Views 226 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

TLC5940: Power on details

Franz Schellhase
Expert 3605 points
Part Number: TLC5940
Other Parts Discussed in Thread: TLC5944,

Hi team,

Can you answer the following customer question please?

Unfortunately, the data sheet of the TLC5940 does not provide very detailed information about the Power-ON behavior. For the TLC5944 I found the following passage in the Grayscale (GS) Shift Register and Data Latch section, which seems to be valid for the TLC5940 as well, since the power-on behavior of both devices is identical.

Could you please confirm that this statement also applies to the TLC5940 in terms of content?


Thank you,

Franz

  • 0
    TI__Genius 12915 points

    Hi Franz,

    Yes it also could be applied to TLC5940. 

    The reason why TLC5944 require BLANK be a high level when powering up is that the DC and GS registers have no default value. Therefore to prevent random channel turn-on, BLANK should be pulled high firstly.

  • 0 in reply to Hardy Wu
    TI__Expert 3605 points

    Hi Hardy,

    Thank you for the quick reply! Can you also answer this follow-up question?

    The only mode that can be used for our application is "VUP to OPEN" (as it was suggested on E2E before). However, I noticed that in OFF state (i.e. BLANK is High), the TLC5944 impresses a largely CONSTANT LED forward voltage of about 1.63V on all connected LEDs, regardless of the actual LED type used (R, G, B). This is independent of the LED supply voltage and also largely independent of the supply voltage of the TLC5944 device itself (apart from a few hundredths of a volt). However, I could not understand this behavior in the datasheet (or simply overlooked it). However, we use the LED controller in our application only as a pure low-side driver for the LEDs, which are all operated with a "permanently applied, i.e. not switched" LED supply voltage of about 5V.

    I suspect the LED forward voltage in OFF-state to be related to LED open detection. Can you also confirm and elaborate on this behavior of the TLC5944 for me?

    Thank you,

    Franz

  • 0 in reply to Franz Schellhase
    TI__Genius 12915 points

    Hi Franz,

    I think it is not affected by LOD function, since it only samples voltage on output pin but not supplies.

    This phenomenon can be explained as, when VUP open, the internal pre-charge FET will still turn on and off, but it will not have any affect on LED performance because it is only connecting an open circuit to the OUT pins. During BLANK high time, the pre-charge FET is on and all cathode of LEDs connect to the same net VCUP. Since VCUP is float, the leakage current flow from VLED to VCUP raise VCUP up and makes all LEDs have the same forward voltage.