That 65 C/W is for a SOCKET without no pcb heatsinking copper. If soldered, with appropriate copper layout, it goes down to 45°C/W, or less, Junction to Ambient.
As for your heatsinks, they are ~20°C/W, but you forgot to add the Junction to Case rating of 2°C/W. So 25 Ambient + 20 Heatsink + 2 Junction to Case = 47°C/W * 1.2W = 56.4°C Junction Temperature.
Key points, Look at notes 8 - 11 on page 7 of the pdf, and consider the board layout (you have tons of empty board spacing no need to have everything so close together).
Page 19 also has good information:
HEAT SINK/THERMAL CONSIDERATIONS
In many cases, no heat sink is required to keep the LM2575 junction temperature within the allowed operating range. For each application, to determine whether or not a heat sink will be required, the following must be identified:
1. Maximum ambient temperature (in the application).
2. Maximum regulator power dissipation (in application).
3. Maximum allowed junction temperature (150°C for the LM1575 or 125°C for the LM2575). For a safe, conservative design, a temperature approximately 15°C cooler than the maximum temperature should be selected.
4. LM2575 package thermal resistances θJA and θJC.
But then you realize, The LM2575 is characterized for operation over the virtual junction temperature range of -40°C to 125°C. At 1.2W (I'm rounding up a bit) and worst case 65°C/W Junction to Ambient, that's still only 78°C Junction Temperature. Almost 50°C below it's maximum operating temperature. Worst case, socket, no proper pcb copper sizing, no heatsink, and you're still good to go. ** Rearrange the traces, and throw on your heatsink, and you're golden. You might need to move the L1/L2 inductors or the heatsink won't attach right. Ideally, you would have the Ground Pin 3 connected directly to the large ground plane.**
Just bare in mind, I hope you have selected the right layout for the 2575 you are getting, as it has multiple versions.
Finally, TI has the Switchers made Simple software here: http://www.ti.com/ww/en/simple_switcher_dc_dc_converters/index.html?DCMP=simple_switcher&HQS=switcher that can help (Though the LM2575 is not included). Also, this article http://store.curiousinventor.com/blog/pcb-as-a-heat-sink-calculating-trace-width-for-given-current can help give you some ideas.
Refer to wikipedia
The SI units of thermal resistance are kelvins per watt or the equivalent degrees Celsius per watt (the two are the same since as intervals Δ1 K = Δ1 °C).
K/W is the same as C/W and that's because they represent a temperature difference per watt rather than an absolute temperature.
The result for your calculation using a 40K/W heatsink is:
\$21 + (2.5 \times (5 + 40)) = 112.5°C \$
There seems to be some misconception regarding the meaning of the K/W rating and the cooling ability of a given heatsink.
When you compare two heatsinks, the lower the K/W rating the better the heatsink, a lower K/W rating means it can dissipate more power with less temperature increase.
As an example:
A 40K/W heatsink increases the temperature 40 degree Celsius (above the ambient temperature) for every Watt. A more efficient heatsink (regarding cooling ability) is a model that has lower K/W rating like for example 20K/W because the temperature will increase only by 20 degree Celsius for each dissipated watt.
Best Answer
That should be okay, in a TO-220 case with no heatsink, assuming moderate environmental temperature.
You don't need to add junction to case to junction to ambient, all you need is junction to ambient, which should heat it to less than 48°C (assuming 25°C ambient). Remember, it's the temperature rise you are calculating.
Too hot to touch is around 60°C, at least for my calibrated finger. If you're getting it too hot to touch there are several possibilities:
a) Your 200mA measurement is not accurate (perhaps the meter drops the voltage)
b) Your 7.2V measurement is not accurate
c) You have really sensitive fingers
d) It's really hot where you are testing it (30-40°C)
To answer your question about the maximum temperature and thermal shutdown- the data sheet specifies an abs. max. junction temperature of 150°C, but to be conservative I like to see no more than 100°C. That would imply a maximum ambient temperature of 75°C, which is reasonable.
The actual thermal shutdown is kind of a last ditch thing, and does not come on until you've well exceeded the absolute maximum junction temperature, typically around Tj = 170°C. It's very unhealthy for the chip to experience this, but it does prevent immediate destruction. That would correspond to an ambient temperature of 145°C which seems mighty hot, even for southern Europe.