LM2794: LED's turning off with 8 second delay from LED_EN going low

Derek Roosken77

Part Number: LM2794

Hi,

We are having an issue with the LM2794 LED driver. On a few boards (not all) that we have built, we see the LEDs do not turn off for up to 8 seconds after LED_EN is pulled low. We also see this behavior get worse the higher we set the brightness control. We have swapped U3 and the LEDs and this did not fix the issue. We are in the process of swapping the capacitors to see if that has an affect. Does anyone have any good troubleshooting ideas or have seen this behavior before?

over 8 years ago

0 Angelos Tsiros over 8 years ago

TI__Intellectual 2140 points

Hi,

I have not encountered this issue before. Can you check the LED_EN signal to ensure there is enough drive and that it is clean (not bouncing during the transition)? I cannot think of anything else at this time.

Regards,
Angelos

0 Derek Roosken77 over 8 years ago in reply to Angelos Tsiros

Prodigy 20 points

Hi Angelos, I took some measurements on the Vin rail and the LED_EN rail and the signals look pretty clean (<120mV).

One thing we saw was the switching frequency on the capacitors was ~555kHz?

0 Angelos Tsiros over 8 years ago in reply to Derek Roosken77

TI__Intellectual 2140 points

Hi Derek,

The frequency being 555kHz should not be an issue, that's well within the specification margin. I cannot think of a reason this behavior is observed, Can you obtain some scope shots of: V(SD), V(ISET), V(Dx) and V(Cfly)? Please try to probe these signals as close to the LM2794 as possible. Could there be a soft short between another voltage rail and the SD pin?

Regards,

Angelos

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LM2794: LED's turning off with 8 second delay from LED_EN going low