Any way to avoid TLC5947 flicker?

Rick Mann

Other Parts Discussed in Thread: TLC5947, TLC5951, TLC5946, TLC5941, TLC5943

I'm successfully driving a TLC5947 from an AT91SAM7X using its SPI peripheral. The problem is that the TLC5947 always blanks all the outputs when XLAT goes high. That is, you can't latch a new value in to the 5947 without it turning off all the outputs.

The problem is then that there is a perceptible flicker each time the 5947 is updated. Sometimes you can't detect it, because XLAT goes high at just the right time (very near the start of the PWM count). But most of the time, it's very noticeable, and distracting.

This can be seen most readily when writing the same values to the 5947 over and over. Try writing 0x7ff to one channel at 30 Hz. You'll see what I mean.

This makes the driver nearly unusable. Is there any workaround? Is there another part that doesn't have this problem? The best thing the part could do is wait internally to latch the values until the PWM count rolls over. Obviously, that can't be done now.

Has anyone else dealt with this problem?

over 12 years ago

0 Brigitte over 12 years ago

TI__Guru 58215 points

Hello Rick,

The IC is intended for applications which show static pictures for a longer period of time. So whenever the picture is changing, BLANK can be set high to turn the outputs off and reset the GS counter and during BLANK=high, XLAT can be used to latch the new picture in.

If you want to view videos which means that you need to have control over the GS counter to know when to latch in the data, other parts like TLC5941, TLC5943, TLC5946 or TLC5951.

Best regards,
Brigitte

0 sky54664 over 12 years ago in reply to Brigitte

Prodigy 70 points

Hello,

I have the same problem. My application is for ligthing LEDs with a ramp. When I start programming the TLC5947, I set BLANK high but when I load the data on SIN or SCLK or XLAT I can see an important flicker on the LEDs (so that BLANK goes down).

Best regards,

Sky,

0 sky54664 over 12 years ago in reply to sky54664

Prodigy 70 points

Hi again,

I do not understand why BLANK goes low when SCLK or XLAT or SIN are changing.

Thanks

0 Brigitte over 12 years ago in reply to sky54664

TI__Guru 58215 points

Hello sky54664,

Is it possible that there is crosstalk on your board between SCLK/XLAT/SIN and BLANK? Crosstalk is the only reason that I can think of for a change in BLANK if you change one of the other pins. Please observe all 4 pins at the same time with an oscilloscope and check if one or the other shows the same frequency as the wrong signal on BLANK.

Another idea would be to use a flying wire for BLANK and check if the problem is gone. If this is the case, then there is crosstalk on the board and you need to redesign the board.

Best regards,
Brigitte