TLC5940: TLC5940 LED driver

Chang Cao

Part Number: TLC5940

I need help from experts.

We use TLC5940 fro LED driver and have 2 questions:
1. The customer gave us a TLC5940 reference design: The TLC5940 OUTx connects LED through a serial resistor. And we follow their design. As I know the constant-current drive doesn't need a serial resistor for LED, the chip internal circuit will adjust the current. Will this serial resistor interfere the constant current drive? At least it will cause TLC5940 thermal problem, because now the Vout=LED forward voltage + resistor voltage drop. it will increase heat created on TLC5940.
2. The customer writes the software. When we build the prototype, can we test the board without their software? ie, the default grayscale brightness is set at full brightness, so the LED can light up? If we can test it, would you please instruct us how to configure (pull up or pull down) BLANK/DCPRG/VPRG/XLAT signals. Attached is our preliminary schematic (HWI schematic preliminary.pdf) and customer sample schematic (xxxx.png).

Thanks.

Chang

over 3 years ago

+1 Frank Qiu over 3 years ago

TI__Genius 15776 points

Hi Monet could you help in this case.

0 Monet Xu over 3 years ago

TI__Expert 7755 points

Hi Chang,

1. If you don't want to series the resistor in the LEDx pin, please choose the proper VLED which can provide the forward voltage and headroom voltage for current sink. if the VLED is too small, current sink can't provide the programmed current which will lead to wrong status. If the VLED is to high, there will be too much voltage on the OUTx pin which will increase the power and temperature of device.

2. The attached file is missed, could you please provide them again?

And these device is too old so the reference information and files is limit and there is a device is more proper and easy used, would you consider replacing the chip?

If you provide more information about your application, I can choose the more proper device for you.

Best Regard

Monet Xu

0 Chang Cao over 3 years ago in reply to Monet Xu

Intellectual 520 points

Hi, Monet:

0 Chang Cao over 3 years ago in reply to Monet Xu

Intellectual 520 points

Hi, Monet:

0 Chang Cao over 3 years ago in reply to Chang Cao

Intellectual 520 points

Thank you for your quick response.

0 Chang Cao over 3 years ago in reply to Chang Cao

Intellectual 520 points

It is our customer to choose this chip. Another question: without the grayscale software, once powered up, will the LED light up? ie, is the default grayscale the max brightness if the EEPROM is not programmed? Because we want to test the board before we send it to customer.

0 Chang Cao over 3 years ago in reply to Frank Qiu

Intellectual 520 points

I attach the .dsn schematic. please let me know if you can open it. Thanks.

very strange, when I press <RETURN> key, it is posted. so you get several posts.

0 Frank Qiu over 3 years ago in reply to Chang Cao

TI__Genius 15776 points

Hi Monet could you help follow this case.

0 Monet Xu over 3 years ago in reply to Chang Cao

TI__Expert 7755 points

Hi Chang,

The files still can't be saw.

Since I don't have any kit about TLC5940 so please have the following try:

according to the table 3 in the datasheet of TLC5940, the default data of DC register is 3Fh. After power on and enable the device, Set DCPRG to H and set VPRG to V(VPRG), observe the LED and waveform on the LEDx PIns.

Best Regard

Monet Xu

0 Chang Cao over 3 years ago

Intellectual 520 points

Monet:

Thanks.

Chng

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TLC5940: TLC5940 LED driver