AD8531/AD8532/AD8534

Rev. F | Page 12 of 20

The thermal resistance curves were determined using the

AD8531/AD8532/AD8534 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 AD8531/AD8532/AD8534 for any application:

where:

To calculate the power dissipated by the AD8531/AD8532/

AD8534, the following equation can be used:

where:

The quantity within the parentheses is the maximum voltage

developed across either output transistor. As an additional

design aid in calculating available load current from the

AD8531/AD8532/AD8534, Figure 5 illustrates the output

voltage of the AD8531/AD8532/AD8534 as a function of

load resistance.

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, which 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, and the

other is to directly measure the supply current of the circuit.

CALCULATING POWER BY MEASURING AMBIENT

AND CASE TEMPERATURE

Given the two equations for calculating junction temperature

where:

where:

The two equations can be solved for P (power)

Once power is determined, it is necessary to go back and calculate

the junction temperature to ensure 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 can be used, or

measurement can be done using an infrared sensing device if

the spot size is small enough.

CALCULATING POWER BY MEASURING SUPPLY

CURRENT

Power can be calculated directly, knowing the supply voltage

and current. However, supply current may have a dc component

with a pulse into a capacitive load, which can make rms current

very difficult to calculate. It can be overcome by lifting the supply

pin and inserting an rms current meter into the circuit. For this

to work, be sure 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.

INPUT OVERVOLTAGE PROTECTION

As with any semiconductor device, whenever the condition

exists for the input to exceed either supply voltage, the input

overvoltage characteristic of the device must be considered.

When an overvoltage occurs, the amplifier can be damaged,

depending on the magnitude of the applied voltage and the

magnitude of the fault current. Although not shown here, when

the input voltage exceeds either supply by more than 0.6 V, pn

junctions internal to the AD8531/AD8532/AD8534 energize,

allowing current to flow from the input to the supplies. As

illustrated in the simplified equivalent input circuit (see Figure 36),

the AD8531/AD8532/AD8534 do not have any internal current

limiting resistors; therefore, fault currents can quickly rise to

damaging levels.

This input current is not inherently damaging to the device, as

long as it is limited to 5 mA or less. For the AD8531/AD8532/

AD8534, once the input voltage exceeds the supply by more than

0.6 V, the input current quickly exceeds 5 mA. If this condition

continues to exist, an external series resistor should be added.

The size of the resistor is calculated by dividing the maximum

overvoltage by 5 mA. For example, if the input voltage could

reach 10 V, the external resistor should be (10 V/5 mA) = 2 kΩ.

This resistance should be placed in series with either or both

inputs if they are exposed to an overvoltage condition.