AD8616/AD8618

Rev. A | Page 15 of 16

These thermal resistance curves were determined using the

AD8616 thermal resistance data for each package and a

maximum junction temperature of 150°C. The following

formula can be used to calculate the internal junction

temperature of the AD8616/AD8618 for any application:

where:

To calculate the power dissipated by the AD8616/AD8618, use

the following equation:

where:

The quantity within the parentheses is the maximum voltage

developed across either output transistor.

POWER CALCULATIONS FOR VARYING OR

UNKNOWN LOADS

Often, calculating power dissipated by an integrated circuit to

determine if the device is being operated in a safe range is not

as simple as it might seem. In many cases, power cannot be

directly measured. This may be the result of irregular output

waveforms or varying loads; indirect methods of measuring

power are required.

There are two methods to calculate power dissipated by an

integrated circuit. The first can be done by measuring the

package temperature and the board temperature. The other is to

directly measure the circuit’s supply current.

Calculating Power by Measuring Ambient and Case

Temperature

Given the two equations for calculating junction temperature:

where:

data sheet.

The two equations can be solved for P (power):

Once power has been determined, it is necessary to go back and

calculate the junction temperature to assure that it has not been

exceeded.

The temperature measurements should be directly on the

package and on a spot on the board that is near the package but

not touching it. Measuring the package could be difficult. A very

small bimetallic junction glued to the package could be used; an

infrared sensing device could be used if the spot size is small

enough.

Calculating Power by Measuring Supply Current

Power can be calculated directly if the supply voltage and

current are known. However, supply current may have a dc

component with a pulse into a capacitive load. This could make

rms current very difficult to calculate. This can be overcome by

lifting the supply pin and inserting an rms current meter into

the circuit. For this to work, the user must be sure that all of the

current is being delivered by the supply pin being measured.

This is usually a good method in a single-supply system;

however, if the system uses dual supplies, both supplies may

need to be monitored.