TLC5951 only on when hand near package

Leon Loeser

Other Parts Discussed in Thread: TLC5951

I am trying to power an led with the TLC5951. One RGB Led is connected with the three cathodes to the first three outputs of the TLC and the anode at 5V. The TLC seems to be working correctly, but only when my fingers are near the package. when i remove my fingers, the led begins to flash in sometimes the wrong colors. The exposed pad (TSSOP) isn't soldered on.

Are there any pins that need certain pullups?

PWM and serial in both work at roughly 20MHz

over 10 years ago

0 Leon Loeser over 10 years ago

Prodigy 90 points

I am controlling the chip with only the GS data lines, not the DC line. WHen tying DCSIN and DCSCK together, the chip laso turns on the led when i am not near it. Is there some suggested state for DCSIN and DCSCK when unused?

0 Leo Bosch over 10 years ago in reply to Leon Loeser

Guru 23000 points

Never leave unused input pins open (floating), even when they are internally pulled up/down with a (high value) resistor. Connect the unused pins to Vcc or GND or in the direction of the internal pull-up/down resistor.

In your case they don’t have a pull-up/down resistor so I would connect them to GND.

0 Leon Loeser over 10 years ago in reply to Leo Bosch

Prodigy 90 points

I pulled DCSIN and DCSCK to VCC/GND (not with resistor, but wire) and it didn't stop the flickering :(

0 Leo Bosch over 10 years ago in reply to Leon Loeser

Guru 23000 points

Did you connect all open input pins (not only DCSIN and DCSCK) to GND?

0 Leon Loeser over 10 years ago in reply to Leo Bosch

Prodigy 90 points

the other pins are all being used

0 Leo Bosch over 10 years ago in reply to Leon Loeser

Guru 23000 points

Then the last I can imagine is improper GND connection and the unconnected Thermal Pad. Only the Data Sheet doesn’t specify the Thermal Pad connection, Vcc or GND but I guest to GND.

Does the same occur when you are not updating data to the TLC, no communication?

How have you connect the TLC, from MCU on same PCB or by cable?

0 Leon Loeser over 10 years ago in reply to Leo Bosch

Prodigy 90 points

by cable

0 Leo Bosch over 10 years ago in reply to Leon Loeser

Guru 23000 points

Then, with 20MHz, your cable connection must be very well done. Try with everything (additionally) connected with a separate solid ground wire reduce or solve the problem.

0 Leon Loeser over 10 years ago in reply to Leo Bosch

Prodigy 90 points

the problem doesnt seem to be signal integrity because the generated colors and "rythms" (timing of light) is right. I feel like there is *somewhere* a floating pin, but mine are all on a known state...

0 Leon Loeser over 10 years ago in reply to Leon Loeser

Prodigy 90 points

ok, it was a signal error, but not integrity. I was polling the wrong SPI flag (TXE, checks the buffer emptiness). I now poll the right flag (BSY, spi busy) and it works correctly. I however needed to pull DCSCK high in order to prevent that one from randomly latching around^^

Thanks for your help!

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TLC5951 only on when hand near package