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TLC5947: Constant current drive

Part Number: TLC5947

Hello, 

We have a project in which QTB QBTO46-UVA LEDs are used. We arrange the LEDs as a 6x4 array and use TI LED driver chip TLC5947 to implement constant current drive.

I need your help. I have 2 questions:

1. We use TLC5947 4 outputs. each output drive 6 LEDs, the nominal LED VF is 3.5V, So 6 LEDs is 21V, VLED = 21V. If we apply VLED 21V, it seems to be constant voltage drive, no constant current drive. And I don't see any feedback (in side the chip?) to stabilize the current. Would you please educate me ? 

2. Our customer needs to have LED fault notification when LED burnout.  6 LEDs are serial together. When one LED is burnt out, The LED is in OPEN state or SHORT state?  Is there still current flowing the 6 LEDs. I need this information to design our fault detection circuit. Should I detect the voltage cross the 6 LEDs (the total voltage is less then normal voltage if the burnout LED is SHORT) or the current flowing the 6 LEDs (no current if the burnout LED is OPEN)?

By the way, the customer doesn't want to use a microcontroller, so no SPI interface and no grayscale .

Thanks.

Regards;

Chang

Chang

  • Another question: because we don't use SPI interface, so I will leave SIN/SOUT/SCLK pins float. But I am not sure how to deal with XLAT pin, tired to GND?

  • Hi Chang,

    1) You need to left some margin for saturation voltage on output pins, which could be referred at Figure 6 in datasheet. Each output is current sink, which can drive stable current when the voltage on output pin is enough.

    2) You could detect the voltage on output pin for fault state. For example, if VLED is 23V and Vf is 3.5V, normally Vo should be 2V. When LED short happen, Vo would be driven up and higher than a threshold. When LED open happen, Vo would be float and lower than a threshold.

    3) Not got your point. How can you control the IC if you do not use the interface?