LM3424: Need help for LED driver

Hi Mitesh,

Thank you so much for reaching out. An engineer will respond to this next week.

Best Regards,

Caroline

Hello Mitesh,

Did they go thru the calculations in the data sheet for their design?  Please forward the schematic with values that they used with the calculations.  Did they use the appropriate saturation rated inductor please give part number for that?  

Thanks Tuan

Hello Tuan,

We are following the calculations as per datasheet as well as EVM.

I have attached the schematic with modified values using for design trials.

let me know if you need any more details.

like Mitesh mention above, for Input > 6.5V it is working fine. plz refer below waveform.

for Input less than 6.5V, its become unstable or missing its regulation. as shown below.

Hello Saga,

Please give me a few days to go thru your circuit to get you an answer.

Thanks Tuan

Hello Sagar,

I've asked for the part number along with the saturation current of the inductor that you use in the previous post.  Can you please provide that for us.

Thanks Tuan

Hello Tuan,

The Inductor Part no is "7447709220" and specifications are below.

let me know if you need any more details.

Also like to mention few observations,

while Increasing voltage from 5.3V to 6.8V it is unstable but once it turn on properly it is maintain the regulation till 6.V input if we slowly decrease the voltage.

Inductor waveform at 6.4V when increasing the input voltage from 5.3V 

Inductor waveform at 6.7V when increasing the input voltage from 5.3V, its stable at this point.

Now Inductor waveform at 6.0V when decreasing the input voltage from 7V, As per this, it is try to maintain the shape till 6.0V but below this its again unstable or reducing the output current.

all these trial i have taken with R6=300mR. Rest everything is same.

one more thing, i have replace the IC also to check the variation but no change in behavior.

also try to change the switching frequency but no change in behavior.

kindly guide me.

I insist you if you can check this at your side practically on EVM that will help us more to understand the issue.

Hello Tuan,

In my above comment, there is typo error of R6 value, I used R6 = 30mR.

Also as per above waveform, the CRO is setting is for high side duty cycle hence it is showing 16%.

but actually it is off time, So duty cycle is 84% in our case. so there is no ton-min time issue here.

I adjusted the slope values as per RT only. 

but slope changing has not help me out here.

Hello Sagar,

I did a simulation and based on the 0.03 Ohm and the values you have above the gate drive and loop was stable.  However, I noticed that with a 0,03 Ohm, VIS feeding back is really small.  The peak is about 90mV and turn on is about 75mV so that is about 15mV of delta sensing on the current sense.  This can be a noise issue here with such low signal to noise.  As a test can you please change RLIM to 0.06 Ohm to see if it solves the issue.  You will also need to adjust Rslope accordingly to this also.  Note that this is just a test here to figure out the real problem.  

Thanks Tuan

Hello Tuan,

Two Observations need to discussed.

Observation 1:

Modification on schematic: R6 change to 50mR. R9 = 250mR & Slope (R14)=10K.

It is working fine for below volt range.

Input voltage  - 5.2V to 8.0V

Output  - 30V/500mA.

But when I started Dimming through nDIM, then there is noise from Inductor.

I try to change the Dimming Frequency (from 100Hz to 10KHz) but no luck, above 10KHz its being unstable so not tested.

Kindly Guide.

Observation 2:

with Same Values when I change the Load to 50V/250mA & do the 50% Diming for adjusting the current value to 250mA then

Current is not constant for full Input voltage range.

Means, When I start increase the voltage from 5.3V, output current increases by 5mA for every 100mV increase in input voltage.

Hello Tuan, 

Thanks for your valuable inputs.

For Observation1: will it be any impact of audible noise on Emission part ?

For Observation 2: I will look into design again.

One more issue need to understand, when i do the dimming with Load of 50V/250mA, I have seen the noise in the output current, this will be persistent till the input voltage range of 6.5V, above this noise has not observed.

why this is happening?

Ch1- Switch

Ch2- Input Voltage

Ch3- Output Current

Hello Tuan,

will it be possible for you to share the Simulation file of LM 3424, I have the TI Tina simulation software. it will be helpful for me to try the different combinations.