AD8531/AD8532/AD8534

REV. D

–9–

Power Dissipation

Although the AD8531/AD8532/AD8534 is capable of providing

load currents to 250 mA, the usable output load current drive

capability will be limited to the maximum power dissipation

allowed by the device package used. In any application, the

absolute maximum junction temperature for the AD8531/

AD8532/AD8534 is 150

∞C, and should never be exceeded for

the device could suffer premature failure. Accurately measuring

power dissipation of an integrated circuit is not always a

straightforward exercise, so Figure 34 has been provided as a

design aid for either setting a safe output current drive level or

in selecting a heatsink for the package options available on the

AD8531/AD8532/AD8534.

TEMPERATURE – C

1.5

1

0

0100

25

50

75

0.5

85

FREE AIR

NO HEATSINK

SOIC

TSSOP

SOT-23

SC70

Figure 34. Maximum Power Dissipation vs. Ambient

Temperature

These 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

junction-to-case; and

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 1 illustrates the AD8531/

AD8532/AD8534 output voltage 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 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 can be done by measuring the pack-

age temperature and the board temperature. The other is to

directly measure the circuit’s supply current.

Calculating Power by Measuring Ambient and Case

Temperature

Given the two equations for calculating junction temperature:

ture.

data sheet.

The two equations can be solved for P (power):

Once power has been determined it is necessary to go back and

calculate the junction temperature to assure that it has not been

exceeded.

The temperature measurements should be directly on the pack-

age and on a spot on the board that is near the package but

definitely not touching it. Measuring the package could be diffi-

cult. A very small bimetallic junction glued to the package could

be used or it could 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. This could make rms current

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

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

this to work you must be sure all of the current is being deliv-

ered by the supply pin you are measuring. 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 device’s

input overvoltage characteristic must be considered. When an

overvoltage occurs, the amplifier could 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 (Figure 32), the AD8531/

AD8532/AD8534 does not have any internal current limiting

resistors, so fault currents can quickly rise to damaging levels.