I'm going to make a speculative guess. It appears something has changed and the only thing that I can think of is Linear Technology has made a revisional change to the die either die shrinking to improve yield or some other die change. It's possible they shrunk the switch element transistor and retained the same specifications. Where before the switch element had a lot of safety margin it may now be hard to achieve full specifications. This has happened to me before with the Badboy converter IC. There were the good early years where the idle current was high, but, the internal switch was beefy and could handle 2A without issues. They made a die change to reduce "idle" current to improve lower power based designs which increased efficiency numbers. The internal switch as no longer beefy and exhibited this same similar symptom with thermal overload on loads that it used to handle without issue.
I got the Red Shark raw fab boards today and assembled one and mounted it to a test heatsink. I was not able to get past 3.2A on the input side without using a strong fan blowing on the fins of the heatsink (on high). When I ran the fan on high near the heatsink the heatsink temperature was not more than a few degrees higher than ambient.
There is one specifications that is not given in the datasheet for this particular footprint style and that is the die to power pad thermal transfer ratio. I'm going to guess it's in the 10C/W range as the new Red Shark has 16 vias in the power pad and they are silver filled. This was a very expensive proto board, but, there is nothing better than silver filled vias for a standard PCB process unless you want to go ape bonkers on cost. Then there are other alternatives that I'm sure would exponentially raise the raw board cost.
As it stands now the local fab shop quoted me $1200.00 and $1800.00 for 20 boards to have the vias on the power pad silver plated. I did not use the local fab proto shop even though they are one of the best in the industry and can do anything and then some.
I was able to get the cost down. I won't mention who, where or how, but, I'm going to have to trust they are silver filled. I can't see them and I'm not going to perform some metalurgy test to see if they are silver filled or not.
I have tried solder fill, silver paste fill (70% silver) and now 100% silver filled vias. There is not much more that can be done with the current design to improve the situation.
The thermal transfer from the bottom-side of the board to copper C to heatsink is around 2C/W or so if the copper C is mounted correctly.
Even turning down the output to 800mA I was still able to put it in a situation where it thermally shutdown.
This one situation is:
2 li-ion batteries and 6 LEDs in series. At 800mA the input reached 3A around 6.6V. This is still in the range of the li-ion cell being able to deliver heavy current demand. This will shut down thermally under this condtion.
For a 6 LED in series configuration you need to use at least 3 li-ion batteries and keep the dead cell voltage above 7V or so so the Blue Shark is not taxed to thermal overload (that is with 800mA LED drive current).
Either that or use two Blue Sharks each set for 500mA. This is a safe condition for a 2 li-ion cell driving 6 LEDs each converter driving 500mA each to the string.
I am going to take the blue shark off hold status. I'm sorry if this change affects your build and please contact us so we can work on any resolution that you currently have or will have due to this change.
Remember the silent BE prefix on all converter boards still applies. All converter boards are on the "Bleeding Edge" of technology and thus some risk is given.
The absolute safe max input current is now around 2.5A. This is significantly different than before, but, as of now I don't believe it is something we have control over.
I am going to start work on new design that will not have these issues. I will post any updates as they become available.
I got the Red Shark raw fab boards today and assembled one and mounted it to a test heatsink. I was not able to get past 3.2A on the input side without using a strong fan blowing on the fins of the heatsink (on high). When I ran the fan on high near the heatsink the heatsink temperature was not more than a few degrees higher than ambient.
There is one specifications that is not given in the datasheet for this particular footprint style and that is the die to power pad thermal transfer ratio. I'm going to guess it's in the 10C/W range as the new Red Shark has 16 vias in the power pad and they are silver filled. This was a very expensive proto board, but, there is nothing better than silver filled vias for a standard PCB process unless you want to go ape bonkers on cost. Then there are other alternatives that I'm sure would exponentially raise the raw board cost.
As it stands now the local fab shop quoted me $1200.00 and $1800.00 for 20 boards to have the vias on the power pad silver plated. I did not use the local fab proto shop even though they are one of the best in the industry and can do anything and then some.
I was able to get the cost down. I won't mention who, where or how, but, I'm going to have to trust they are silver filled. I can't see them and I'm not going to perform some metalurgy test to see if they are silver filled or not.
I have tried solder fill, silver paste fill (70% silver) and now 100% silver filled vias. There is not much more that can be done with the current design to improve the situation.
The thermal transfer from the bottom-side of the board to copper C to heatsink is around 2C/W or so if the copper C is mounted correctly.
Even turning down the output to 800mA I was still able to put it in a situation where it thermally shutdown.
This one situation is:
2 li-ion batteries and 6 LEDs in series. At 800mA the input reached 3A around 6.6V. This is still in the range of the li-ion cell being able to deliver heavy current demand. This will shut down thermally under this condtion.
For a 6 LED in series configuration you need to use at least 3 li-ion batteries and keep the dead cell voltage above 7V or so so the Blue Shark is not taxed to thermal overload (that is with 800mA LED drive current).
Either that or use two Blue Sharks each set for 500mA. This is a safe condition for a 2 li-ion cell driving 6 LEDs each converter driving 500mA each to the string.
I am going to take the blue shark off hold status. I'm sorry if this change affects your build and please contact us so we can work on any resolution that you currently have or will have due to this change.
Remember the silent BE prefix on all converter boards still applies. All converter boards are on the "Bleeding Edge" of technology and thus some risk is given.
The absolute safe max input current is now around 2.5A. This is significantly different than before, but, as of now I don't believe it is something we have control over.
I am going to start work on new design that will not have these issues. I will post any updates as they become available.
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