LM3481: Issue in Output Waveform with Load

Hi Sahil,

1. Could you please let me know the input voltage you run the test so that I can help check the loop for you?

2. Also, it would be helpful if you could provide the COMP waveform with the output voltage you had here.

3. Why the load you design for is 1.5A? The voltage across the LED is about 3V, which makes the current about 5mA. I just want to make sure I understand the operating condition for the circuit.

Thanks,

Yinsong

Hi Yinsong,

Thank you for your response.

Please find my response as below:

1. We are running the test on 12V input voltage.

2. Please find the COMP pin waveform as below:

3. To clarify your doubts, LED is connected only to indicate that power is good; it isn't the actual load. The actual load is LCD. LCD has typical current of 600 mA and inrush current of 1.5A.

Hi Sahil,

Thanks for the info.

If this is the case, it would be helpful if you could provide the information on the real load.

And you said it is working fine with constant load with the LED working? I want to make sure the LED is not causing the trouble.

Regards,

Yinsong

Hi Yinsong,

LED isn't causing trouble as we have checked by removing the series resistor (R174 in schematic) of LED.

After removing that resistor, we are facing the same issue.

Hi Sahil,

From the output voltage and COMP waveform you sent, it looks like the load transient response. Hence,

1) still, could you give more information on the load? The LCD.

2) You can quickly verify this by giving a load transient using an external electronic load (disconnect the LCD). Set it to worst case like 0-1.5A and high slew rate and compare the response with the one you had.

Regards,

Yinsong

Hi Yinsong,

The LCD specs are as below:

Also, if it is load transient issue, what is the soultion for this? How can we reduce this dip?

Also, one of our observation is whenever LCD is connected, inductor heats up very rapidly.

Hi Sahil,

I'm not familiar with the LCD characteristics. But you can easily test it by probing the load current of LCD. It might be the inrush current.

If it is the load transient issue, you can reduce it by

1. Improve the compensator
2. Increase the output cap

For the inductor, what is the temperature it rises to? You can do a rough calculation based on the DCR and core loss of the inductors being used. And you can check if the temperature rise match with the calculation.

Thanks,

Yinsong

Hi Yinsong,

Can you please provide the calculation sheet for LM3481 to tune compensator network?

Hi Sahil,

Sorry but unfortunately, we do not have calculation sheet released for LM3481.

Thanks,
Yinsong

Hi Yinsong,

Can you suggest any solution for inductor heating issue?

We are using SRP6540-220M inductor in our design.