interpreting power dissipation table of an LDO

I'm looking for an LDO in an SC70 package and when looking at the power dissipation rating table, it lists the following information.... RΘja=315C/Watt, derating factor above Ta=25C is 3.18mW/C, Ta<25C power rating=320mW, Ta=70C power rating=175mW, and Ta=85C power rating=130mW. What exactly does all this mean?

bigfarmerdave said:

I'm looking for an LDO in an SC70 package and when looking at the power dissipation rating table, it lists the following information.... RΘja=315C/Watt, derating factor above Ta=25C is 3.18mW/C, Ta<25C power rating=320mW, Ta=70C power rating=175mW, and Ta=85C power rating=130mW. What exactly does all this mean?

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hi,
If you plotted the Temp Ambient versus mWatts on graph. X axis temp and Yaxis mWatt it will make more sense to you.

It basically means at Ta < 25C the max heat the regulator can tolerate/dissipate is 320mW.
and at Ta=70C its less ie: 175mW
and at Ta=85C its even less ie: 130mW.
These values are without any heatsink.

Assume you have a LVDO regulator that has a drop out of 0.5V
At Ta<25C thats 0.32/0.5 = 640mA
At Ta=70C thats 0.175/0.5=350mA
At Ta=85C thats 0.26/0.5 = 260mA

So you can see as the Ta rises the max current that can flow thru the reg should be reduced.... Heat sinks help to keep the Ta down.

If you recall Watts = V * I, where in this case V is the voltage across the regulator ie: Vin to Vout and the I is the current thru the regulator.

OK.?