TLC59731: Maximum Vf for given VCC?

Rafael Send

Part Number: TLC59731
Other Parts Discussed in Thread: TLC5973, , LM3699, REG71050, TPS60231

Hi,

I'm looking for some help setting the current for each channel of this device. If I want to supply my device with 3.3V and my blue LEDs have a Vf of 2.8V at 20mA, am I right in saying that I would need a 0.01Ohm resistor?

That seems a little odd. Can anyone advise as to what the minimum difference between Vf and VCC is?

For example, in the 5973 device, it says VCC = Vf + 1V. Is it the same for the 59731, just per channel?

Thanks,

Rafael

over 5 years ago

0 Clemens Ladisch over 5 years ago

Guru 243180 points

A 0.01 Ω resistor would result in a current of 20 A.

The TLC5973 has internal current regulators that need 1 V to work. The TLC59731 has no such regulators and requires current-limiting resistors. There is not really a minimum voltage difference, but when it becomes too small, any small deviation in VCC, Vf, VOUT, or R results in a large deviation in the current. What are the allowed tolerances on your 20 mA target?

Consider using a 5 V boost converter (at this current, usually a charge pump, e.g., REG71050), or a LED driver with integrated boost converter (e.g., LM3699/TPS60231 if you do not need to control the LEDs individually).

0 Kenneth Du over 5 years ago

TI__Genius 14295 points

Hi, Rafael,

As you can see from Figure 4, the minimum voltage for Vout at 20mA is around 0.3V.

If you use 3.3V for Vled, there's will be 3.3V-2.8V=0.5V drop on the driver, which is enough for 20mA constant current.

And you don't need to add a resistor since this chip output constant current and 0.5V drop is OK for the device.

Please help to click "Resolve" if the problem is solved.

Thanks.

Regards,

Kenneth

0 Clemens Ladisch over 5 years ago in reply to Kenneth Du

Guru 243180 points

Kenneth Du said:

And you don't need to add a resistor since this chip output constant current

That would be correct for the TLC5973, but not for the TLC59731.

0 Kenneth Du over 5 years ago in reply to Clemens Ladisch

TI__Genius 14295 points

Hi, Clemens,

Thanks for pointing this out.

You're correct.

And in such case, according equation 2, there should be a resistor of (3.3V-2.8V-0.3V)/0.02A=10ohm.

Kenneth

0 Rafael Send over 5 years ago in reply to Kenneth Du

Prodigy 80 points

In that case, the Equation 2 in the datasheet is mislabelled.

It says the current units are in mA, which is how I arrived at the 0.01Ohm from my first post.

So it should be A, not mA in the formula?

0 Clemens Ladisch over 5 years ago in reply to Rafael Send

Guru 243180 points

Equations 1 and 2 indeed use a wrong unit.

0 Rafael Send over 5 years ago in reply to Clemens Ladisch

Prodigy 80 points

So would I be correct in stating that I can NOT use the 5973 with my LED and 3.3V, but I CAN use the 59731?

0 Clemens Ladisch over 5 years ago in reply to Rafael Send

Guru 243180 points

Yes, there is not enough voltage left for the 5973's regulator, and the 59731 can work with a resistor that is designed to drop 0.2 V. But that is not a good regulator: it can easily happen that the tolerances of VCC, Vf, or Vout make a difference of 0.1 V, which would result in a current change of 50 %.

Do you actually need to control the three LEDs separately?

0 Rafael Send over 5 years ago in reply to Clemens Ladisch

Prodigy 80 points

Yes, I need to drive an RGB LED off of 1 I/O pin using a single primary lithum cell...

0 Clemens Ladisch over 5 years ago in reply to Rafael Send

Guru 243180 points

Then you need to use the 59731. (Or the 5973 with a charge pump.)

0 Rafael Send over 5 years ago in reply to Clemens Ladisch

Prodigy 80 points

Yup, 59731 is currently in my design. I'll play around with the minimum useable VCC for my LED; ideally I don't want to boost too high for efficiency reasons. If I can stick with 3.3V that would be better.
Thanks!

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TLC59731: Maximum Vf for given VCC?