AD8615/AD8616/AD8618

Rev. E | Page 14 of 20

The following formula can be used to calculate the internal

junction temperature 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 the following:

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 may 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 circuit’s supply current.

Calculating Power by Measuring Ambient Temperature

and Case Temperature

The two equations for calculating the junction temperature are

where:

= junction temperature

= ambient temperature

= the junction-to-ambient thermal resistance

where:

is case temperature.

The two equations for calculating P (power) are

P

Once the power is determined, it is necessary to recalculate the

junction temperature to ensure that the temperature was not

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

If the supply voltage and current are known, power can be

calculated directly. However, the supply current can have a dc

component with a pulse directed into a capacitive load, which

can 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.