LM3643: Can LM3643 drive RED LED?

Kai Nomiyama

Expert 5655 points

Part Number: LM3643

Hi team,

I have a question about LM3643.

Can LM3643 drive red led?

Vf of red led is 2.4V and LM3643 has pass mode so I think LM3643 can drive red led but my concern is thermal issue.

e.g.

Vin=5V

ILED=1.5A

Vf=2.4V(red led)

Voltage drop of current source = (VIN – ILED x RPMOS) – Vf = 4.871 – 2.4 =2.471V

Power loss = (ILED x PRMOS) x ILED + 2.471V x ILED = 3.9W

3.9W x 67.8C/W = 264.42C

This is beyond Tj(125C).

Does above my calculation is correct?

If yes, LM3643 would not be able to use as red led driver.

If you have alternative LED driver that can meet below customer requirement, please let me know.

Vin=3.3~5V

ILED=1.5A

strings: 2 string and independent control

LED: 1 LED per 1 string. not decided using White led or red led.

Thanks,

Regards,

Kai

over 8 years ago

0 Angelos Tsiros over 8 years ago

TI__Intellectual 2140 points

Hi,

Your calculations seem to be correct. We will get back to you with a possible solution.

Regards,
Angelos Tsiros

0 Greg Lubarsky over 8 years ago

TI__Intellectual 1940 points

Hello,
Your math looks correct if we assume a 1.5A constant current. The LM3643 can only supply the 1.5A load for 400ms by design. This was done in part to manage the thermal time-constants on chip. If your application needs a constant 1.5A current source, a different device will be needed. If you can clarify the use case for the application, we can further help identify a solution. Thanks.

Greg

0 Kai Nomiyama over 8 years ago in reply to Greg Lubarsky

TI__Expert 5655 points

Hi Greg and Angelos,

Thank you for your comment.
I will get back to you when I get detail use-case of RED LED.

Thanks,

Rregards,
Kai

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LM3643: Can LM3643 drive RED LED?