From: John Larkin
Subject: Re: Reading temperature characteristics
Date: Thu, 26 Sep 2002 08:47:52 -0700
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On Thu, 26 Sep 2002 11:36:49 GMT,
firstname.lastname@example.org (The little lost angel) wrote:
>I've been looking through the specs sheet for fixed 5v regulators.
>For some strange reasons, the spec sheets don't list the same stuff
>On one, I get charts for output/temp, on the other, I get a
>So it's a bit confusing to a noob like me.
>When a chart says junction temperature, case temperature, what's the
>If I'm going to put the regulator in a small enclosure with moving air
>(at about 60C~80C? two heatsinks too hot to touch for more than a
>split sec). Is the following thinking correct?
>LM1117 junction temperature range 0-125C
>Junction to ambient 79C/W
>Max quiscent current 10mA
>The formula given is P(dissipation) is Vin*I(ground) + Vdrop*I(load)
>I have no idea how to determine the current flowing to ground but
>assume that's the quiscent current, plus the capacitors (which I've no
>idea dissipate how much current or whether I need them if voltage
>stability is not critcal).
>My usage would take a max of 4 LED with resistors, nominally set for
>This would indicate 13V*10mA + 8V * 18mA * 4(4xLED+Resistors?)=
>0.13+0.58 = 0.71W
>Temp Rise allowed = TmpMax - AmbientExpected = 45C (125-80)
>Junction/Ambient TmpResistor = 45/0.71 = 63C/W (lower than the allowed
>Therefore The LM1117 would not be acceptable for my application
>without a heatsink?
>Thanks for any corrections!
most of the idle current of the 1117 flows out of the Vout pin, so
goes into the load. So the power dissipation is essentially
P = (13-5) * 0.018 * 4 = 0.58 watts.
You can see this on the National datasheet's simplified schematic.
So if the part is not heatsinked, the junction temp rise above ambient
is 0.58 * 79 (assuming a TO-220) = 45 K, so even if ambient is 55
you're not over 100C junction temp, which is OK. So no heatsink is OK,
and even a tiny sink will cool things nicely.
Oh, an LM1117 will oscillate like gangbusters if it doesn't have a
pretty big (10 uF or so) output cap.