POWER SUPPLIES
The power being dissipated (PD) in the output transistor of
the amplifier can be calculated:
The OPA551 and OPA552 may be operated from power
supplies of ±4V to ±30V, or a total of 60V with excellent
performance. Most behavior remains unchanged throughout
the full operating voltage range. Parameters that vary sig-
nificantly with operating voltage are shown in the Typical
Performance Curves.
PD(output stage) = IO • (VS – VO) = 25mA • (30 – 15) = 375mW
PD(total) = PD(internal) + PD(output stage) = 432mW + 375mW = 807mW
The resulting junction temperature can be calculated:
TJ = TA + PD θJA
For applications that do not require symmetrical output
voltage swing, power supply voltages do not need to be
equal. The OPA551 and OPA552 can operate with as little
as 8V between the supplies or with up to 60V between the
supplies. For example, the positive supply could be set to
50V with the negative supply at –10V or vice-versa.
TJ = 40°C + 807mW • 100°C/W = 120.7°C
Where,
TJ = junction temperature (°C)
TA = ambient temperature (°C)
θJA = junction-to-air thermal resistance (°C/W)
The SO-8 package outline shows three negative supply (V–)
pins. These pins are internally connected for improved thermal
performance. Pin 4 is to be used as the primary current
carrier for the negative supply. It is recommended that
pins 1 and 5 not be directly connected to V– but, instead
be connected to a thermal mass. DO NOT lay out the PC
board to use pins 1 and 5 as feedthroughs to the negative
supply. Doing so can result in a reduction of performance.
For the DDPAK package, the θJA is 65°C/W with no heat
sinking, resulting in a junction temperature of 92.5°C.
To estimate the margin of safety in a complete design
(including heat sink), increase the ambient temperature until
the thermal protection is activated. Use worst-case load and
signal conditions. For good reliability, the thermal protec-
tion should trigger more than +35°C above the maximum
expected ambient condition of your application. This en-
sures a maximum junction temperature of +125°C at the
maximum expected ambient condition.
The tab of the DDPAK-7 package is electrically connected
to the negative supply (V–), however, this connection should
not be used to carry current. For best thermal performance,
the tab should be soldered directly to the circuit board
copper area (see heat sink text).
If the OPA551 or OPA552 is to be used in an application
requiring more than 0.5W continuous power dissipation, it
is recommended that the DDPAK package option be used.
The DDPAK has superior thermal dissipation characteris-
tics and is more easily adapted to a heat sink.
POWER DISSIPATION
Internal power dissipation of these op amps can be quite
large. Many of the specifications for the OPA551 and
OPA552 are for a specified junction temperature. If the
device is not subjected to internal self-heating, the junction
temperature will be the same as the ambient. However, in
practical applications, the device will self-heat and the junc-
tion temperature will be significantly higher than ambient.
After junction temperature has been established, perfor-
mance parameters that vary with junction temperature can be
determined from the performance curves. The following
calculation can be performed to establish junction tempera-
ture as a function of ambient temperature and the conditions
of the application.
Operation from a single power supply (or unbalanced power
supplies) can produce even larger power dissipation since a
larger voltage can be impressed across the conducting output
transistor. Consult Application Bulletin AB-039 for further
information on how to calculate or measure power dissipation.
Power dissipation can be minimized by using the lowest
possible supply voltage. For example, with a 200mA load,
the output will swing to within 3.5V of the power supply
rails. Power supplies set to no more than 3.5V above the
maximum output voltage swing required by the application
will minimize the power dissipation.
SAFE OPERATING AREA
Consider the OPA551 in a circuit configuration where the
load is 600Ω and the output voltage is 15V. The supplies are
at ±30V and the ambient temperature (TA) is 40°C. The θJA
for the 8-pin DIP package is 100°C/W.
The Safe Operating Area (SOA curves, Figures 3, 4, and 5)
shows the permissible range of voltage and current. The
curves shown represent devices soldered to a circuit board
with no heat sink. The safe output current decreases as the
voltage across the output transistor (VS – VO) increases. For
further insight on SOA, consult Application Bulletin AB-039.
First, the internal heating of the op amp is as follows:
PD(internal) = IQ • VS = 7.2mA • 60V = 432mW
The output current (IO) can be calculated:
IO = VOUT/RL = 15V/600Ω = 25mA
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 and
produces a typical output current of 380mA. With ±30V
OPA551, OPA552
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