AD8615/AD8616/AD8618
The following formula can be used to calculate the internal
junction temperature of the AD8615/AD8616/AD8618 for any
application:
Calculating Power by Measuring Ambient Temperature
and Case Temperature
The two equations for calculating the junction temperature are
TJ = PDISS × θJA + TA
TJ = TA + P θJA
where:
TJ = junction temperature
where:
TJ = junction temperature
TA = ambient temperature
θJA = the junction-to-ambient thermal resistance
PDISS = power dissipation
θJA = package thermal resistance, junction-to-case
TA = ambient temperature of the circuit
TJ = TC + P θJC
To calculate the power dissipated by the AD8615/AD8616/
AD8618, use the following:
where:
TC is case temperature.
θJA and θJC are given in the data sheet.
PDISS = ILOAD × (VS – VOUT
)
where:
The two equations for calculating P (power) are
TA + P θJA = TC + P θJC
ILOAD = output load current
VS = supply voltage
VOUT = output voltage
P = (TA − TC)/(θJC − θJA)
Once the power is determined, it is necessary to recalculate the
junction temperature to ensure that the temperature was not
exceeded.
The quantity within the parentheses is the maximum voltage
developed across either output transistor.
POWER CALCULATIONS FOR VARYING OR
UNKNOWN LOADS
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.
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.
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.
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.
Rev. E | Page 14 of 20
ADI [ ADI ]
