AD8615/AD8616/AD8618

Rev. C | Page 14 of 20

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 tem-

perature of the AD8615/AD8616/AD8618 for any application:

where:

= junction temperature

= power dissipation

= package thermal resistance, junction-to-case

= ambient temperature of the circuit

To calculate the power dissipated by the AD8615/

AD8616/AD8618, use

where:

= output load current

= supply voltage

= output voltage

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 is to measure the package

temperature and the board temperature. The second is

to directly measure the circuits supply current.

Calculating Power by Measuring Ambient and Case

Temperature

The two equations for calculating junction temperature are

where:

= junction temperature

= ambient temperature

= the junction-to-ambient thermal resistance

data sheet.

The two equations for calculating P (power) are

P

Once power has been determined, it is necessary to recalculate

the junction temperature to ensure that it has not been

exceeded.

The temperature should be measured directly on and near the

package, but not touching it. Measuring the package can be

difficult. A very small bimetallic junction glued to the package

can be used, or an infrared sensing device can 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, the supply current can have a dc

component with a pulse directed into a capacitive load, which

could make the rms current very difficult to calculate. This

difficulty can be overcome by lifting the supply pin and

inserting an rms current meter into the circuit. For this method

to work, make sure the current is 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.