LM5118: LM5118 blew with load

Hello John,

If you use an LED string as load, you cannot set the converter for regulating voltage. The LED string defines the voltage and you need to regulate the current. Please have a look at this app note: https://www.ti.com/lit/pdf/slva419

The power rating of the used LEDs in this app note are a lot lower than the ones you are using, but I hope that the information helps to design the converter the way that it does not blow up.

Best regards,
Brigitte

Thanks for that input.  We are drive some high power LED's, 16 in series.

What is not shown here is that there is a Tee connection on the FB pin that is driven with an OPAMP.

It will reduce the voltage and thus the current of the LED's.  48VDC is the top output current (LED's can go much higher), but we want to take it down to 30VDC.   We cannot use PWM dimming as this is used in very high speed photography.

The ripple of a adjustable Voltage DC/DC cannot be seen by the camera's.

I have done testing with a EVAL board at a lower output voltage and it works great.

It looks like I miss read part of my design and need to add more cap's on the output.

Hello John,

If you have the capability to change the output voltage, I would try to start with the lowest voltage as this normally should translate into the lowest current through the LEDs and should cause the lowest input surge current during startup when the output caps are empty.

More caps might be helpful, but more caps mean more inrush current at startup. You might increase the softstart time to reduce the peak current during startup.

After the converter started and is in regulation, you should not change the regulation voltage too fast to not get into some protection features.

Best regards,
Brigitte

I liked the app you sent in the first reply, but being limited on input VDC is an issue.

The housing we get comes with 2 AC/DC supplies at 48V 5A.  It can be adjusted to output 52.6VDC.

We have 64 led's we are trying to control. 4 strings of 16 LED's (2A our full on, they can take 3A).  This is used in the high end applications like Broad Cast Sport, High Speed Testing,  Remove TV location and movie production.  A lot of light.

I wanted to break it up to 8 strings of 8 but then we would have to get new supplies and just through the others away or resell them.  My prof of concept was done with an Eval board that did 8 LED's with 2 strings.

The one thing I am thinking is is to go with strings of 14 or 15 to give the LM5118 margin.

Also I have a micro on this that will feed into the FB to reduce the output voltage.  I can set this up to start very low and slowly ramp up. 

I think this will be my next try:

1.  Setup very lower voltage and ramp up.

2.  Do the same with 1 or 2 less LED's

3.  Run a webeench on LM5118 for 24.5VDC output at 4.2A and reconfigure my LED Plate.

Hello John,

Looking forward to see the result with starting up into very low voltage and ramping up slowly.

Best regards,
Brigitte

This has been put on hold with my client. But I would like to ask this.

You have a task to driver 64 Cree White LED at 2A max and it has to be dimmable.

You have 2 Mean Well AC/DC 48VDC @ 5A as your source voltage.

The light will be used in high speed camera application with shutter speed over 200K to 1M /s.

Breaking the strings up is fine.  Can be individual strings are some in parallel.

Multiple DC/DC modules are fine.  Dimming input is PWM @20KHZ but can be converted to analog.

Has to operate in temp of 125F and down to -30F, and using a heat sink is ok as well.  IP67+ environment.

And parts have to be available.

What would you use?

Hello John,

Difficult question as there are such a lot more things to keep in mind, e.g. what do you need to do when one LED is damaged and creating either a short or an open?

What are the size, cost, component constraints?

I am sorry, but my knowledge on IP67+ environment does not really exist, but the web tells me that the surrounding needs to be save against dust and water, so efficiency inside seems to be important to make sure that the closed box does not get too hot inside. Correct me if I am wrong.

If cost is least important, my first check would be on the available LED drivers, e.g. here: https://www.ti.com/power-management/led-drivers/illumination-led-drivers/products.html

If cost is most important, I would check for buck converters (least amount of external components and highest efficiency).

But other constraints like maximum amount of LEDs that are allowed to not enlight when the system turns on or other reasons would possibly bring me to the point to say, a buck-boost controller like LM5176 is the best choice.

What exactly do you mean with "And parts have to be available."? If you mean that there is no time between ordering and delivering, I do not have an answer that would be based on knowledge, but the list above or in any other page that lists products, you would get this information immediately, see above link.

Best regards,
Brigitte

IP67 is for outdoor products that will see rain, snow and harsh weather.  IP68 is where we get into full water tight enclosures.

Available:  Sock of the part.  the LM5176 is out of stock everywhere and TI site does not have a date posted.

Not a good way to start a design.  We have to know what stock is right now when starting a design and buy what is need for 1 to 2 years now as how know when this issue of supply will be back to normal.

Back to my work.  I am going to take off 1 LED on the string which will put my output at 44.  That should be go for testing this.  I will also modify the design to 24V and 4A and run 2 LED strings in parallel.

We the other work load I have this may take a week or two as more parts are needed.

Hello John,

I am sorry, but I do not have insight in when parts are back in stock and especially at the moment when they are in stock they are out very fast again. On the different subpages with the list of devices, e.g. below for all DC/DC regulators, you can see the actual TI stock. For further information, please contact customer support center here: https://ticsc.service-now.com/csm 

Best regards,
Brigitte