Datasheet

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:
T
J
= P
DISS
× θ
JA
+ T
A
where:
T
J
= junction temperature
P
DISS
= power dissipation
θ
JA
= package thermal resistance, junction-to-case
T
A
= ambient temperature of the circuit
To calculate the power dissipated by the AD8615/AD8616/
AD8618, use the following:
P
DISS
= I
LOAD
× (V
S
– V
OUT
)
where:
I
LOAD
= output load current
V
S
= supply voltage
V
OUT
= 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
T
J
= T
A
+ P θ
JA
where:
T
J
= junction temperature
T
A
= ambient temperature
θ
JA
= the junction-to-ambient thermal resistance
T
J
= T
C
+ P θ
JC
where:
T
C
is case temperature.
θ
JA
and θ
JC
are given in the data sheet.
The two equations for calculating P (power) are
T
A
+ P θ
JA
= T
C
+ P θ
JC
P = (T
A
− T
C
)/(θ
JC
− θ
JA
)
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.