LM3553: Faster ramp-up/ramp-down times with Rset < 14 kOhm?

Hi Lorenzo,

Sorry to late reply. I can't give you a clear answer now, can you give me a few days to test on the EVM board and give you an answer?

Thanks and best regards,

Jared

Thank you, Jared. Appreciated.

Hi Lorenzo,

I discussed about this case with our design and according to the previous projects, there should be no problem with halving Rset. Since the EVM board has not yet arrived at my office, I have not verified it. I check the logistics and it shows that it is already on the way. I should be able to give you the result the day after tomorrow at the latest.

Best Regards,

Jared

That's good, thank you.

The datasheet does not characterise the dynamic behaviour of the LM3553 apart from the specification of tFD-MIN (Minimum Flash Duration Step) = 12.8us.

There must be limits to the transient response and, in an application like camera flash, it's critical to know how fast the LED current can change.

Hi Lorenzo,

Thanks for your explanation, I will pay attention to the ramp up speed of the current during the test and will reply to you ASAP.

Best Regards,

Jared

Hi Lorenzo,

I verified it on the EVM. As you think, LM3553 be able to ramp up the output current to 1.2A in 64 steps corresponding to 1.6ms when I use Rset = 7kR and only set FC6-FC0 < 64.

But I found there is a problem on the waveform. The detail is that the current will ramp up to 1.2A at first and fall down to 400mA. I am not sure about the reason since we can't find the database of this device.

BR

Jared

Hi Lorenzo,

I will discuss about this with our design tomorrow and see if we can find some useful information on the datasheet.

BR

Jared

Appreciated.
I hope you'll be able to find out the reasons for the odd behaviour.

Hi Lorenzo,

I find the reason why the current will fall down to 400mA, it's because the current limit of VIN is only 1A. So, when the input current is ramp up to 1A, the input voltage will be dropped down and the boost start, resulting in a sudden drop in output current. I change another power supply with a higher current capacity and no longer had this issue.

As for the noise of the current, I think it's caused by my flying leads on the board. Because I find the same noise with both Rset 7KR and 14KR, so it's not the different Rset caused. I also find using different flying leads, the noise will be different. So I suspect it's caused by the flying leads.

Hope it can help you. Thanks!

BR

Jared

OK, I understand.

For clarity: is the LM3553 in your circuit driving two white LED in series?

Hi Lorenzo,

I add a flying lead on the EVM, so only one white LED on my board for aiming to measure the output current. The schematic diagram is as follows：

BR Jared

Thank you for the picture. My application is more demanding. Hopefully I'll be able to run my own tests with the evaluation kit soon.
Could you get any comment from TI chip designers?