LM393: Maximum power dissipation

Hello Devon,

Maximum power dissipation is determined by the package thermal resistance and your maximum ambient temperature.

You need to determine the total load and quiescent currents, convert to watts, then multiply by the package thermal resistance (thetajA).

This will give you the *rise* in temperature over ambient. Remember that "ambient" also needs to include your maximum system temperature (say 70°C). Keep in mind that the board temperature may be higher than the system ambient air temp if there are power dissipating devices on the board.

For example, if the total calculated dissipation is 100mW, and you are in the TSSOP-8 package, then the total temp rise will be 0.1W * 149C/W = 14.9°C over ambient.

The calculated total maximum temperature cannot be higher than the Maximum Junction Temperature (150°C). In practice, you want to keep the junction temp to less than that, preferably to the maximum specified temp range spec (125°C).

We can work the formula backwards to find the maximum permissible power dissipation.

Lets use the TSSOP-8 package as the worst case:

Assuming:

TSSOP package at 149°C/W

Maximum System temp: 70°C

Maximum Junction Temp: 125°C

So:

125°C - 70°C = 55°C maximum allowable rise

55 / 149 = 0.37W maximum power (seems reasonable)

Remember that the total power must include the power for *each* output, as well as the quiecent supply current of the comparator.

Figuring out the power of each output gets a little complicated. You must multiply the output current times the amount of voltage dropped across the output.

So, looking at figure 3, if the load is 10mA, the voltage dropped is 200mV. 10mA * 200mV = 2mW (or 4mW total for both channels).

The supply power is the total quiescent current times the supply voltage (use worst case supply current).

Assuming a 5V supply, total quiescent supply current is 1mA (be careful of per-channel specs - you have two channels). 5V * 1mA = 5mW

So your total power is 2mW + 2mW + 5mW = 9mW - well below the 370mW maximum. 9mW * 149C/W = 1.3°C rise! Not worth worrying about..

Again - you need to run these with your own numbers - particularly if using high supply voltages and heavy (>20mA) loads.