TLC5940: Maximum Current

Hi Nathan,

Nathan Said :

I am currently working with the TLC5940, and I see on the datasheet that it is specified to only run up to 60 mA if Vcc < 3.6V. Can someone please explain the reasoning behind this specification?

[Jared]: It depends on the internal circuit limit.

Actually, I am not sure what's your question here, can you please describe it clearly?

BR, Jared

Hi Jared,

I am referring to the above spec under section 6.3 of the datasheet. I am wondering why this distinction exists as it does not exist for other LED drivers such as the TLC59116 (which has just one value for the maximum recommended current regardless of Vcc).

My main quesiton is that I am trying to understand the underlying reason for this spec, and what the expected behavior would be if it is not followed (whether it is just shortened lifespan or something else). I am having issues with thermal performance and my LED's shutting off after a couple of minutes. I also do not see on the datasheet anywhere that specifies the TLC5940 will actually turn off in the event of over-temperature detection. Is this expected?

Thank you,

-Nathan

Hi Nathan,

As I answered before, the reason is because the internal circuit design is different. TLC59116 doesn't have this spec, but if you have looked at much more datasheet, you can find the same limit condition for some LED driver, such TLC5940, LP5860, etc.

For your question, the behavior is as the figure shows, if VCC<3.6V, the maximum output current will only be 60mA. But if VCC>3,6V, the maximum output current can be 120mA. For the thermal shutdown behavior, I think it's possible.

BR, Jared

Hi Jared,

This is in the recommended operating conditions of the datasheet, nowhere does datasheet make it sound like internal circuitry will actually limit the current based on Vcc. The current is set by Rext, and this part of the datasheet makes it sound like if Vcc is less than 3.6V, you should not use an Rext that will result in a current that it greater than 60 mA. In other words, if Vcc < 3.6V, Rext should be greater than 650 ohms. 

I confirmed that this is indeed the case, my power supply shows ~90 mA per channel and my Rext is set to give that. My Vcc is 3.3V 

This is why I am wondering why this recommendation exists. What is the reasoning that spec was put on the datasheet? Is the driver far less efficient with a lower Vcc or is there some other reason?

Thanks,

-Nathan

Hi Nathan,

I want to clarify it again, it's the internal design circuit limit. I think we can't show for public since it's related to internal how to design the circuit. Maybe you can tell me what's concern from you and then I can help to answer it.

For your question so far, I am not sure what do you mean about "less efficient"?

BR, Jared

Jared, 

Thanks for your clarification. If it is an internal current limit, then I am not sure how I could be measuring 90 mA from each channel with a Vcc of 3.3V. My experimental setup is clean - I have a separate benchtop supply providing only the power to the LEDs so I am very doubtful that this measurement could be in error.

As I mentioned above, my concern is with the thermal performance of the device. By less efficient, that would mean a greater amount of heat generated by the driver at any given LED current.

Thank you,

-Nathan 

Hi Nathan,

Do you mean you measure 90mA when power 3.3V? I will check with our designer later.

And for the concern about thermal, please forgive my stupid, I don't really understand you mean. Would you mind help to describe clearly? Thanks very much!

BR, Jared

Hi Jared,

Yes I measure 90 mA when Vcc is 3.3V, I would be interested to hear what the designer has to say.

For the thermal, the device is operating significantly hotter than expected and so I am trying to isolate possible causes. I am wondering if the device generates more heat for a Vcc of 3.3V than it does for 5V.

Thanks,

-Nathan

Hi Nathan,

For the behavior you measure, I think it doesn't make sense. But please skip this result to let us focus on thermal first, can you please tell me your VCC and current from VCC and the voltage dropped on output and the output current, then I can calculate the thermal according to these data.

BR, Jared