LM3697EVM: LM3697 sink current is not reaching max set value

The input voltage is kept at 4V and input current limit is 2A from the power supply. When I turned on the leds, I see huge 1V ripple at the Vin.

Hardy, could you help in this case.

Hi Abhilash,

Could you help to capture the waveform of VIN/SW/VOUT/HVLEDx?

TI_board.pdf

Hi Hardy, attached waveforms in the pdf doc.

Thanks,

Abhilash

Hi Abhilash,

What's the switching frequency now? If it's not 1MHz, you can set the switching frequency to be 1MHz and try again.

Robin

I'm using default 500KHz frequency for now. I tested with 1MHz, still the behavior is same.

Hi Abhilash,

Would you mind showing your schematic for analysis? Thanks!

Robin

Hi Robin,

I'm using LM3697 TI evaluation board. 

Hi Abhilash,

I tested on EVM board with your GUI setting and VIN=3A but the 30mA output current is work. Looking at your waveform that your VOUT seems clamped at 16V. I think we need to determine whether the insufficient current caused low forward voltage, or reverse that limited VOUT causes IOUT cannot be higher.

For suspect 1, you could change 6 LEDs to 5 or 4 in one string. If the device still only output 4mA, there may something wrong happened inside the IC.

For suspect 2, you could loose your power supply current limitation to see if the system can work normally.

Hi Robin,

Thanks for backup.

Hi Hardy,

Thanks for the reply.

I tested with 5 strings, with 1 LED configuration. Still the LED current is not reaching to max value or even with higher supply current it is the same. I'll replace with new IC and test i will take some time to get new part and test.

But if something is wrong with the chip, will it clamp the output voltage to 16V and limit the output current? I'm not even seeing any status faults.

-Abhilash

Correction, 5LEDs and 1 string. 

Hi Abhilash,

It is still weird that usually the IC on EVM is fine since the board would be tested after manufacture. 

Here is a quick test tag support some typical setting test, some would set at high current. Could you also use these test on your side to see if they are normal? 

I tested few quick tests also, it is not reaching maximum currents.

Hi Abhilash,

When you changed full-scale current, how the output current influenced? The current did not change, or linear with setting but maximum is 4mA, or clamped below 4mA but under control when setting register below 4mA?

HI Hardy,

LED current is changing for below 4mA. I mean to say I can see brightness change at lower settings.