Re: TEC and current driver chips for laser?

miso@xxxxxxxxx wrote:

Joerg wrote:

miso@xxxxxxxxx wrote:

Joerg wrote:


Hello Chris,

[ ... ]


Those fancy new "thermally effective" packages often do require some
heat to be conducted into a copper pour plane. The chips I have seen
this way were within temp specs but way too hot for my taste.

Well, if there is no alternative to the ADN2830 there is always the
method of using two in parallel, setting each to half the current. They
are just kind of expensive for a controlled current source. North of $10
a pop. If this all goes into a larger production run it'll have to
become discrete but we are far away from that point.


I think the LFCSP packages with the exposed paddle should be soldered to a
square of metal which is connected to the ground plane by lots of vias. It
is important that the paddle is really soldered properly. When soldering
them by hand, I put some flux on the paddle of the IC and then tack down
some pins of the device before soldering the ground connection from the
back side of the PCB. I feed solder down one via and make sure it comes up
the other vias, that way I can be pretty sure it has wetted the IC package.
Other people do it in different ways and the assembly contractors seem to
have no trouble with them, I think they use solder paste. The thermal
resistance in the package would be pretty low because the paddle is just a
square of solid copper with the die attached to one side and the PCB
soldered to the other side. (or at least that applies to the ones that I
have seen, though those were not really high-power devices.)


No doubt this will be fine in production but for prototyping this is a
bear. Also, I like stuff to run at lower temps than those LFCSP. I am
just about ready to go the usual route by doing it all in
discrete/jelly-bean again.


Do you know how to measure die temperature? [I can explain the
procedure if you don't.] I suspect things are better than you think
regarding heat. Parts that are hot to the touch are still quite
reasonable designs. That is, the designer takes temperature into
account. The real problem would be some problem caused by the heat of
this chip as it effects the rest of the system.


Well, I've got a good 20 years under the belt now and parts with
temperatures too hot to the touch had higher failure rates than the
others. Also, there are applications where there are strict limits on
how hot something can get anywhere in the system (medical etc.). Even
where there aren't I prefer things to remain in a range where touching
doesn't lead to a blister ;-)

--
Regards, Joerg

http://www.analogconsultants.com


I've designed linear power chips They get hot. No big deal IF the
design is good. I can't believe AD doing a schlock job. You don't need
heat for a part to fail. There are plenty of failure mechanisms that
don't involve the chip getting warm to the touch.


True, but more heat does increase the chances of failure. So why should I accept higher temperatures when I can come up with a solution that runs much cooler?


The next time you are on an airplane, watch the wing wiggle a bit. You
are looking at something that is engineered properly. If the wing was
stiff enough not to wiggle, it would be much heavier. It is the same
case with your chip. If it ran cool, it would have to be bigger, have a
slug in the package, etc.


Watch it on take-off (or at rotation, as pilots say) when the wings get loaded. They bow upwards a whole lot. But that's meant to be. However, the same goes here: The more flex you design in the faster fatigue can set in. For airplanes the time frames are usually in excess of 30 years. Come to think of it, same for electronics. Some aircraft are more prone to structural failures than others. The worst one out here (California) was a CDF plane where both wing struts gave up after decades of service. Unfortunately the crew did not survive that. The other (larger) accident of a jetliner was more likely due to maintenance issues. With that one the threads for the stabilizer screw gear head had stripped out.


I would bet AD has burn in data to show their design is good. It is
that kind of a company.


AD is an excellent company IMHO. I trust them and they have never disappointed that trust. Can't say that for some other companies. But there are some parts with every manufacturer where I feel a bit uneasy and decide to go another way.

--
Regards, Joerg

http://www.analogconsultants.com
.

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