Datasheet
"#$
SBOS206E − DECEMBER 2001 − REVISED FEBRUARY 2007
www.ti.com
9
OUTPUT PROTECTION
Reactive and EMF-generation loads can return load
current to the amplifier, causing the output voltage to
exceed the power-supply voltage. This damaging
condition can be avoided with clamp diodes from the
output terminal to the power supplies, as shown in
Figure 4. Schottky rectifier diodes with a 3A or greater
continuous rating are recommended.
OPA561
Flag
I
LIM
E/S
47
µ
F
Output Protection Diode
Output Protection Diode
V
O
R
O
100k
Ω
+V
−
V
5
4
8
7
6
3
2
0.1
µ
F
0.1
µ
F
47
µ
F
Figure 4. Output Protection Diode
THERMAL PROTECTION
The OPA561 has thermal sensing circuitry that helps
protect the amplifier from exceeding temperature limits.
Power dissipated in the OPA561 will cause the junction
temperature to rise. Internal thermal shutdown circuitry
shuts down the output when the die temperature reaches
approximately 160°C, resetting when the die has cooled to
140°C. Depending on load and signal conditions, the
thermal protection 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 indicates excessive power
dissipation or an inadequate heatsink. For reliable,
long-term, continuous operation, junction temperature
should be limited to +125°C, maximum. To estimate the
margin of safety in a complete design (including heatsink),
increase the ambient temperature until the thermal
protection is triggered. Use worst-case loading and signal
conditions. For good, long-term reliability, thermal
protection should trigger more than 35°C above the
maximum expected ambient condition of your application.
This produces a junction temperature of +125°C at the
maximum expected ambient condition.
The internal protection circuitry of the OPA561 was
designed to protect against overload conditions; it was not
intended to replace proper heatsinking. Continuously
running the OPA561 into thermal shutdown can degrade
reliability. The E/S pin can be monitored to determine if
shutdown has occurred. During normal operation the
voltage on the E/S pin is typically above (V−) + 2V. During
shutdown, the voltage drops to less than (V−) + 0.8V.
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 conducting output transistor. Dissipation with ac
signals is lower. Application Bulletin AB−039 (SBOA022)
explains how to calculate or measure power dissipation
with unusual signals and loads, and can be dowloaded
from www.ti.com.
HEATSINK AREA
The relationship between thermal resistance and power
dissipation can be expressed as:
where:
T
J
= Junction Temperature (°C)
T
A
= Ambient Temperature (°C)
q
JA
= Junction-to-Ambient Thermal Resistance (°C/W)
P
D
= Power Dissipation (W)
To appropriately determine required heatsink area,
required power dissipation should be calculated and the
relationship between power dissipation and thermal
resistance should be considered to minimize shutdown
conditions and allow for proper long-term operation
(junction temperature of +125°C). Once the heatsink area
has been selected, worst-case load conditions should be
tested to ensure proper thermal protection.
For applications with limited board size, refer to Figure 5
for the approximate thermal resistance relative to heatsink
area. Increasing heatsink area beyond 2in
2
provides little
improvement in thermal resistance. To achieve the
32°C/W stated in the Electrical Characteristics, a copper
plane size of 9in
2
was used. The HTSSOP-20 PowerPAD
package is well suited for continuous power levels from
2W to 4W, depending on ambient temperature and
heatsink area. Higher power levels may be achieved in
applications with a low on/off duty cycle, such as remote
meter reading.