LM3407: Maximum String Length of LEDs

John Griffith

Part Number: LM3407

Hello,

I have a customer that is looking at the LM3407 and it is not clear what the maximum string length for the LEDs is. The picture on the first page shows "1 to 7 LEDs" but then it says the output voltage range can be 90% the input voltage.

More specifically, my customer is looking at having one string of 9 green LEDs with a total forward drop of 18.9V (2.1V / LED) and another string of 9 red LEDs with a total forward drop of 17.55V (1.95V / LED).

The picture seems to indicate that the device cannot handle 9 LEDs but the specifications seem to show that as long as the input voltage is greater than 21V for the green LEDs and 19.5 for the red LEDs it should work. Can you confirm if two LM3407 devices will work for the above mentioned strings of 9 red and 9 green LEDs?

Thanks!

John

over 4 years ago

0 Tuan Tran74 over 4 years ago

TI__Genius 17620 points

Hi John,

It should work for the LED loads and LED voltages you are specifying above. Tt

Thanks Tuan

0 John Griffith over 4 years ago in reply to Tuan Tran74

TI__Mastermind 21314 points

Hi Tuan,

So is total forward voltage drop the only thing that matters? If that is the case, can the team consider updating the datasheet graphic? It is misleading to show 7 LEDs max in one spot on the datasheet and then a voltage limitation in another location.

Thanks,

John

0 John Griffith over 4 years ago in reply to John Griffith

TI__Mastermind 21314 points

Hi Tuan,

Any update on this? Is it safe to tell the customer that the limitation is only with the total voltage drop of the string and it has nothing to do with the number of LEDs?

Is this something that can be updated in the datasheet as it is currently misleading.

John

0 Tuan Tran74 over 4 years ago in reply to John Griffith

TI__Genius 17620 points

Hi John,

As I mentioned before the operation should be fine. As far as changing the data sheet we will talk to the team about it.

Thanks Tuan

0 John Griffith over 4 years ago in reply to Tuan Tran74

TI__Mastermind 21314 points

Hi Tuan,

To be honest your answer hasn't been terribly clear. Both times you have said "should work". You haven't stated any concerns or made it clear if the only limitation with this part the forward voltage drop on the LED string. That is why I have asked for a more direct or detailed answer.

John

0 Tuan Tran74 over 4 years ago in reply to John Griffith

TI__Genius 17620 points

The operation is fine...If you need anymore details then please email me directly...Tuan

Power management

Power management forum

LM3407: Maximum String Length of LEDs