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Topic: Why is there such a large difference in the current capacity of these vregs? (Read 2 times) previous topic - next topic

scswift

These are both 3.3v regs, they have the same package, they appear to have nearly identical specs, yet the first one is rated for 250mA and $1.50, and the second is rated for 150mA, and $0.50:
http://www.ti.com/lit/ds/symlink/lp2992.pdf
http://www.ti.com/lit/ds/symlink/lp2985-18.pdf

The first one seems to have a slightly lower dropout.  350mV @ 125 degrees, versus 400mV @ 125 degrees for the second. 

But the second one has a 150 degrees max junction temperature, while the first has only a 125 degrees max junction temperature, and I know this is used in the power dissipation calculations.

They both can handle up to 16v, and both list their PEAK output current as 350mA.

So what's the difference?  Is it the dropout voltage?  If so, where does that factor into any power dissipation calculations?  I looked up how to calculate the power dissipation, and found this equation:

PD = IOUT  * (VIN - VOUT) + VIN*IQ
where VIN = input voltage; VOUT = output voltage; IOUT = output current, A; and IQ = quiescent current, A

I assume quiescent current is also known as ground current as that seems to match up with the minumum quiescent current listed in the absolute maximum ratings and there's no other quiescent current listed.  In which case for both vregs, it's 850uA typical at 150mA output.  Which means as far as the power dissipation equation goes, the two regulators are identical.

So why is the current rating so different for these parts?

johnwasser

Maybe they just justify the price difference by degrading the specs on the lower-cost option.
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James C4S

It's a result of TI acquiring National.  Two different companies, two different products, two different manufacturing processes, two different designs, for the essentially the same end product.
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scswift

So you're saying they're actually identical? 

I took another look at the spec sheet and I noticed they had some graphs labeled "short circuit current".  If I'm not mistaken those show a worst case scenario of the reg trying to put out as much current as it can at 6v and 16v.  I think the graph also shows the thermal regulation kicking in shortly after the start of the test.

If that's the case, then for the first reg @ 6v, it peaks at 450mA, and drops to 300mA over the course of 2 seconds, then takes a nose dive, leveling out again just under 250mA.
For the second reg, it peaks at 350mA, drops to 250mA after half a second, then over the next 1.5 seconds slowly drops to 225mA.

That would seem to indicate the first part is slightly better.  It can provide an average of 325mA for 2 seconds before thermal regulation kicks in, whereas the second part really can't provide more than an average of 250mA over that same period.

But it also seems to indicate the parts are largely the same, if all you're interested in is their long term current handling ability, and that the second part can handle quite a bit more than 150mA if you've got the input voltage low enough.  (Seems at 16V they both crap out around 100mA.)

[edit]

On the other hand, I just noticed the graphs for the two regulators aren't measuring exactly the same stuff.  The first is measuring 2.8v out, and the second 3.3v out.  I'm not sure how the difference would affect the graphs.  It could either make the two regs more equal or more different.  But still, with the second reg putting out 3.3v it's serving up quite a bit more current than I expected.

johnwasser


So you're saying they're actually identical? 


No, he said they are different because they were designed and made by different companies before the companies merged.
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