AD8531A Analog Devices, AD8531A Datasheet - Page 9

• Manufacturer Part Number: AD8531A
• Description: Low Cost, 250 ma Output Single-supply Amplifiers
• Manufacturer: Analog Devices

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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 al-
lowed by the device package used. In any application, the abso-
lute 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 straightfor-
ward exercise, so Figure 34 has been provided as a design aid
for either setting a safe output current drive level or in selecting
a heat sink for the three package options available on the
AD8531/AD8532/AD8534.
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 fol-
lowing formula can be used to calculate the internal junction tem-
perature of the AD8531/AD8532/AD8534 for any application:
where T
To calculate the power dissipated by the AD8531/AD8532/
AD8534, the following equation can be used:
where I
The quantity within the parentheses is the maximum voltage
developed across either output transistor. As an additional de-
sign 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 mea-
Figure 34. Maximum Power Dissipation vs. Ambient
Temperature
P
junction-to-case; and
T
V
V
1.5
0.5
LOAD
JA
DISS
1
0
J
A
S
OUT
0
= junction temperature;
= Ambient temperature of the circuit.
= is supply voltage; and
TSSOP
SOIC
= package thermal resistance,
PDIP
JA
JA
JA
= power dissipation;
= is output voltage.
= is output load current;
= 240 C/W
= 103 C/W
= 158 C/W
P
DISS
25
T
J
= I
= P
TEMPERATURE – C
LOAD
DISS
50
(V
JA
S
–V
+ T
OUT
A
)
T
FREE AIR
NO HEAT SINK
75
J
MAX = 150 C
85
100
–9–
sured directly. This may be the result of irregular output wave-
forms or varying loads. So indirect methods of measuring
power are required.
Here are two methods to calculate power dissipated by an inte-
grated circuit. The first can be done by measuring the package
temperature and the board temperature. The other method 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:
where T
ture.
where T
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
package and on a spot on the board that is near the package
but definitely not touching it. Measuring the package could be
difficult. 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 compo-
nent with a pulse into a capacitive load. This could make rms
current very difficult to calculate.
This can be overcome by lifting the supply pin and inserting an
rms current meter into the circuit. For this to work you must
be sure all of the current is being delivered by the supply pin
that you are measuring. This is usually a good method in a
single supply system. However, if the system uses dual sup-
plies, both supplies may need to be monitored.
Input Overvoltage Protection
As with any semiconductor device, whenever the condition ex-
ists 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.
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
JA
J
C
is the junction to ambient thermal resistance.
is junction temperature, and T
is case temperature and
P = (T
T
AD8531/AD8532/AD8534
A
+ P
T
T
A
J
J
= T
= T
– T
JA
C
A
C
= T
)/ (
+ P
+ P
C
JA
+ P
JC
JA
JC
and
–
A
is ambient tempera-
JC
JA
JC
)
are given in the