LM3509: SCH review

Amelie Zheng

Genius 10865 points

Part Number: LM3509

Hi Team,

My customer now use LM3509 to drive three LED in their project. May I ask whether the SCH is ok?

LED IF= 30mA, in this application, they limit the current to 20mA by RESET.

over 3 years ago

0 Yuting Shao over 3 years ago

TI__Genius 13001 points

Hi Amelie,

If the customer wants to power the internal logic circuit (VIO, pin4) with 3.3V instead of power supply (IN, pin8), R169 should be removed.

BR,

Robin

0 Amelie Zheng over 3 years ago in reply to Yuting Shao

TI__Genius 10865 points

Hi Robin,

Customer has removed the R174 so only supplied by 5V.

Besides this, is there other thing need to change?

0 Yuting Shao over 3 years ago in reply to Amelie Zheng

TI__Genius 13001 points

Hi Amelie,

I think there is nothing that need to be changed.

BR,

Robin

0 Amelie Zheng over 3 years ago in reply to Yuting Shao

TI__Genius 10865 points

Hi Robin,

The reason why customer ask for SCH review is that their project have been RTM 20K and found 5-7 have problem.(D26 or D27 was broken but D23 is working normally). So customer is worried about whether it is correct in his SCH.

I have a question that VF of these three LED are the min 2.6V and max 3.6V. Since their Vin is only 5V, D26 and D27 voltage is total 5.2-7.2V and D23 is only 2.6-3.6V, which is lower than 5V.

Since the Vin is 5V, whether it will have risk to destroy the LED?

thanks！

LED灯 H2012AW32B0C24.pdf

0 Yuting Shao over 3 years ago in reply to Amelie Zheng

TI__Genius 13001 points

Hi Amelie,

Considering that two LED string's power consumption is larger, you can check the PCB layout situation and near heat source to see if too high temperature results in LED damage.

BR,

Robin

0 Amelie Zheng over 3 years ago in reply to Yuting Shao

TI__Genius 10865 points

Hi Robin，

Thanks for your reply.

I am also curious. I haven't found the control option about the output of PIN OVP/MAIN/SUB FB. We don't need to adjust the output of these PIN?

Do they adjust automatically based on detecting the LEDs? If not, how could I change the MAIN/SUB? How could I calculate the voltage of MAIN/SUB/OVP?

Besides the above question, I am also confused about the power consumption you mention. Since only three LED is connected to LED driver, the power consumption is sure to be lower then five LED connected, right?

0 Yuting Shao over 3 years ago in reply to Amelie Zheng

TI__Genius 13001 points

Hi Amelie,

You could find the MAIN/SUB FB configuration from datasheet as below. As for OVP, the threshold is constant.

MAIN/SUB FB are input pin, so you cannot adjust the output. Once you have configured the output current(LED mode) or feedback voltage(OLED mode), the device will automatically adjust these pins' voltage to target value which set internally.

As for power consumption, besides LED quantity, the PCB layout and other heat sources will significantly influence the hear dissipation and LED lifecycle.

0 Amelie Zheng over 3 years ago in reply to Yuting Shao

TI__Genius 10865 points

Hi Robin

Thanks for your reply!

Sorry. But I can't see the photos you share...

0 Yuting Shao over 3 years ago in reply to Amelie Zheng

TI__Genius 13001 points

Hi Amelie,

Sorry for that!

You can refer to Table 1,2,3 and Figure 42 of LM3509 datasheet.

BR,

Robin

Power management

Power management forum

LM3509: SCH review