LM3409: Application issues of chips

Hi Tonyx,

Could you please share the complete schematic? 

BR,

Hailee.

Using TI's official LM3409EVAL/NOPB - LM3409 demonstration board，You can see relevant information on the TI website.

The following are updates on the issues encountered during the debugging process, mainly including:
1. Test1 and Test2 respectively light up two different types of LEDs, and the phenomena they appear are also different. Please help to see if there are any improvement plans.
2. Test3 recorded the lighting experiment of the third LED model, but its working state remained abnormal. Could you please help analyze and solve this problem.
Additionally, due to our current usage scenario, the power is relatively low. Is there a design improvement plan for application scenarios with current below 100mA to improve the linearity of Vadj debugging?

Debugging records
Test 1
Test conditions
Vin 10.3V
Vadj 0.2V
PWM frequency 30Hz/5kHz 10% duty cycle, Vp-p=5V

LED information
Forward voltage drop (single lamp)=2.7~3.2V
Forward current=100~700mA
Number of series connections: 2

phenomenon
oscilloscope
Ch 1 yellow PWM control signal
Ch 2 blue LED positive voltage
Ch 4 Green LED Luminescence Response (Using PD Test)

① 30Hz switching frequency image

② 5kHz switching frequency image

Problems that occurred:
1. The ripple problem is quite serious. Are there any measures and suggestions to reduce the ripple?
2. After the frequency increases, the LED shuts off and causes tailing, resulting in severe LED afterglow. Is there any way to improve it?

Test 2
Test conditions
Vin 10.5V
Vadj 0.2V
PWM frequency 30Hz/5kHz 10% duty cycle, Vp-p=5V

LED information
Positive voltage drop (single lamp)=2.6~3.0V
Forward current=5-100mA
Number of series connections: 2

phenomenon
oscilloscope
Ch 1 yellow PWM control signal
Ch 2 blue LED positive voltage
Ch 4 Green LED Luminescence Response (Using PD Test)

① 30Hz switching frequency image

② 5kHz switching frequency image

Problems that occurred:
At 30Hz, there is a 300us delay in the LED voltage compared to the control signal. During the delay period, the LED positive electrode has a smaller level, similar to the phenomenon of pre opening. After increasing the frequency to 5kH, this phenomenon disappears
At 5kHz, the falling edge of the LED response oscillates and produces afterglow
3. There are no significant ripples, which is significantly different from the phenomenon in test1

Test 3
Test conditions
Vin 10.3-15V
Vadj 0.2V
PWM frequency 30Hz/5kHz 10% duty cycle, Vp-p=5V

LED information
Forward voltage drop (single lamp)=2.6~3.4V
Forward current=5-100mA
Number of series connections: 2

phenomenon
oscilloscope
Ch 1 yellow PWM control signal
Ch 4 green LED positive voltage
Ch 3 Powder LED Luminescence Response (Using PD Test)

① 30Hz switching frequency image

② 5kHz switching frequency image

Problems that occurred:
LED working status incorrect
Vin is adjusted within 10V~15V, Vadj is adjusted within 0.2-0.7V, and there is no difference in the phenomenon

Please help to see how to solve all the above problems. Thank you!

Any update?

Hi Tonyx, 

Since this device is a constant current device, it would be more helpful if you can provide the output current waveforms for analysis.

Please also provide a test record, including the Iout, Vcoff, and Vsw point under different test conditions.

AN-1954 LM3409 Demonstration Board (Rev. D) is supposed to be the board you use. There is no output capacitor in this design, if you want to reduce the output voltage ripple, you can add the output cap - C5 and test again.

Hailee.

The debugging of LM3409 is still ongoing, and now each type of LED can light up, but there is a delay of 300us when turned on. May I ask how this is caused?

Hi Tonyx,

What is the meaning of each line? the “Turn on” means PWM on or Vin on? 

If you use the EN pin to apply a PWM signal, when the EN pin is pulled low, VCC begins to discharge; When VCC discharges below its UVLO threshold, the part goes into low power shutdown mode. When you pull EN high again, there will be a delay and then a normal startup sequence. Please refer to the 8.3.6 PWM Dimming Using the EN Pin and 8.4.1 Low-Power Shutdown in the datasheet.

BR,

Hailee.

Channel 2 is the measured light of the LED
Channel 4 is the LED polarity voltage
Channel 1 is PWM

Do you have any other suggestions for handling the problem?

Hi Tonyx,

If this delay time is not acceptable in your application. you can use shunt dimming feature to avoid this delay, rather than using EN pin for PWM dimming.

BR,

Hailee.