
6
OPA501
CURRENT LIMITS
The OPA501 has independent positive and negative current
limit circuits. Current limits are set by the value of R
+SC
 and
R
–
SC
. The approximate value of these resistors is:
0.65
I
LIMIT
I
 is the desired maximum current at room temperature in
Amperes and R
 is in ohms. The current limit value de-
creases with increasing temperature—see typical perfor-
mance curves. The current limit resistors conduct the full
amplifier output current. Power dissipation of the current
limit resistors at maximum current is:
The current limit resistors can be chosen from a variety of
types. Most wire-wound types are satisfactory, although
some physically large resistors may have excessive induc-
tance which can cause instability.
SAFE OPERATING AREA
Stress on the output transistor is determined by the output
current and the voltage across the conducting output transis-
tor. The power dissipated by the output transistor is equal to
the product of the output current and the voltage across the
conducting transistor, V
CE
. The Safe Operating Area (SOA),
APPLICATIONS INFORMATION
Grounding techniques can greatly affect the performance of
a power op amp. Figure 1 shows grounds connected so that
load current does not flow through signal ground connec-
tions. Power supply and load connections should be physi-
cally separated from the amplifier input and signal connec-
tions.
Power supply connections to the amplifier should be by-
passed with 10
μ
F tantalum capacitors connected close to the
device pins. The capacitors should be connected to load
ground as shown.
Figure 2, shows the permissible range of voltage and cur-
rent. SOA is reduced at high operating temperature—see
Figure 3.
The safe output current decreases as V
CE
 increases. Output
short-circuits are very demanding. A short-circuit to ground
forces the full power supply voltage (positive or negative
side) across the conducting transistor. With V
S
 = 
±
30V, the
current limit must be set for 2A to be safe for short-circuit
to ground. For further information on SOA and evaluating
signal and load conditions, consult Applications Bulletin
AB-039.
HEAT SINKING
Most applications require a heat sink to assure that the
maximum junction temperature of 200
°
C is not exceeded.
The size of the heat sink required depends on the power
dissipated by the amplifier and ambient temperature condi-
tions. Application Bulletin AB-039 explains how to find
maximum power dissipation for DC, AC, reactive loads, and
other conditions. Applications Bulletin AB-038 shows how
to determine heat sink requirements.
The case of the OPA501 is isolated from all circuitry and can
be fastened directly to a heat sink. This eliminates cumber-
some insulating hardware that degrades thermal perfor-
mance. See Applications Bulletin AB-037 for information
on mounting techniques and procedures.
FIGURE 3. Transistor Safe Operating Area at +125
°
C Case
Temperature.
FIGURE 1. Basic Circuit Connections.
FIGURE 2. Transistor Safe Operating Area at +25
°
C Case
Temperature.
P
MAX
=
 (I
LIMIT
)
2
R
SC
1
2
4
6
10
20
40
60 80
10
8
6
4
2
1.0
 0.6
0.4
0.2
0.1
O
Voltage Across Output Transistor (V)
100
Maximum
Specified
Current
8
T
CASE
 = +25°C
T
JUNCTION
 = +200°C
  JC
θ
1ms
0.5ms
Maximum
Specified
Voltage
RM, AM
SM, BM
5ms
DC
Power
Dissipation
Limit
Second
Breakdown
Limit
1
2
4
6
10
20
40
60 80
10
8
6
4
2
1.0
0.8
 0.6
0.4
0.2
0.1
O
Voltage Across Output Transistor (V)
100
Maximum
Specified
Current
8
T
CASE
 = +125°C
T
JUNCTION
 = +200°C
  JC
θ
0.5ms
1ms
Maximum
Specified
Voltage
RM, AM
SM, BM
5ms
DC
+
(1)
+
(1)
OPA501
R
SC
–
R
SC
+
R
2
R
1
V
IN
V–
V+
G = 1 +
R
2
R
1
NOTE: (1) 10μF Tantalum.
Load
R
SC
=
 0.0437