Datasheet

OPA549
SBOS093E
10
www.ti.com
Thermal Shutdown Status
The OPA549 has thermal shutdown circuitry that protects the
amplifier from damage. The thermal protection circuitry dis-
ables the output when the junction temperature reaches
approximately 160°C and allows the device to cool. When the
junction temperature cools to approximately 140°C, the output
circuitry is automatically re-enabled. Depending on load and
signal conditions, the thermal protection circuit may cycle on
and off. The E/S pin can be monitored to determine if the
device is in shutdown. During normal operation, the voltage on
the E/S pin is typically 3.5V above Ref. Once shutdown has
occurred, this voltage drops to approximately 200mV above
Ref. Figure 4 shows an example implementing this function.
FIGURE 4. Thermal Shutdown Status.
FIGURE 5. Output Disable and Thermal Shutdown Status.
FIGURE 6. Safe Operating Area.
External logic circuitry or an LED can be used to indicate if
the output has been thermally shutdown, see Figure 10.
Output Disable and Thermal Shutdown Status
As mentioned earlier, the OPA549’s output can be disabled
and the disable status can be monitored simultaneously.
Figure 5 provides an example of interfacing to the E/S pin.
SAFE OPERATING AREA
Stress on the output transistors is determined both by the
output current and by the output voltage across the conduct-
ing output transistor, V
S
– V
O
. The power dissipated by the
output transistor is equal to the product of the output current
and the voltage across the conducting transistor, V
S
– V
O
.
The Safe Operating Area (SOA curve, Figure 6) shows the
permissible range of voltage and current.
The safe output current decreases as V
S
– V
O
increases.
Output short circuits are a very demanding case for SOA. A
short circuit to ground forces the full power-supply voltage
(V+ or V–) across the conducting transistor. Increasing the
case temperature reduces the safe output current that can be
tolerated without activating the thermal shutdown circuit of
the OPA549. For further insight on SOA, consult Application
Report SBOA022 at the Texas Instruments web site
(www.ti.com).
POWER DISSIPATION
Power dissipation depends on power supply, signal, and load
conditions. For dc signals, power dissipation is equal to the
product of output current times the voltage across the con-
ducting output transistor. Power dissipation can be mini-
mized by using the lowest possible power-supply voltage
necessary to assure the required output voltage swing.
For resistive loads, the maximum power dissipation occurs at
a dc output voltage of one-half the power-supply voltage.
Dissipation with ac signals is lower. Application Bulletin
SBOA022 explains how to calculate or measure power
dissipation with unusual signals and loads.
THERMAL PROTECTION
Power dissipated in the OPA549 will cause the junction
temperature to rise. Internal thermal shutdown circuitry shuts
down the output when the die temperature reaches approxi-
mately 160°C and resets when the die has cooled to 140°C.
Depending on load and signal conditions, the thermal protec-
tion circuit may cycle on and off. This limits the dissipation of
the amplifier but may have an undesirable effect on the load.
Any tendency to activate the thermal protection circuit indi-
cates excessive power dissipation or an inadequate heat
sink. For reliable operation, junction temperature should be
limited to 125°C maximum. To estimate the margin of safety
in a complete design (including heat sink) increase the
ambient temperature until the thermal protection is triggered.
12 510
V
S
 – V
O
 (V)
20 50 100
10
20
1
Output Current (A)
0.1
Pulse Operation Only
(Limit rms current to ≤ 8A)
Output current can
be limited to less
than 8A—see text.
T
C
= 125°C
T
C
= 85°C
T
C
= 25°C
PD = 90W
PD = 47W
PD = 18W
OPA549
E/S
HCT
Logic
Ground
Ref
E/S pin can interface
with standard HCT logic
inputs. Logic ground is
referred to Ref.
OPA549
E/S
Open Drain
(Output Disable)
HCT
(Thermal Status
Shutdown)
Logic
Ground
Ref
Open-drain logic output can disable
the amplifier's output with a logic low.
HCT logic input monitors thermal
shutdown status during normal
operation.