PA74/76 • PA74A/76A
GENERAL
P r o d u c t I n n o v a t i o nF r o m
Please read Application Note 1 "General Operating Consider-
ations" which covers stability, supplies, heatsinking, mounting,
SOA interpretation, and specification interpretation. Visit www.
Cirrus.com for design tools that help automate tasks such as
calculations for stability, internal power dissipation, heatsink
selection; Apex Precision Power's complete Application Notes
library; Technical Seminar Workbook; and Evaluation Kits.
PARALLEL CONFIGURATION CONSIDERATIONS
LOSSES
The PA74 and PA76 utilize a parallel configuration to achieve
the desired current output requirements.The parallel configura-
tion inherently creates internal losses due to circulating currents.
The circulating currents generate power losses through the
current sharing resistors when delivering current to the load.
STABILITY CONSIDERATIONS
All monolithic power op amps use output stage topologies
that present special stability problems. This is primarily due to
non-complementary (both devices are NPN) output stages with
a mismatch in gain and phase response for different polarities
of output current. It is difficult for the opamp manufacturer to
optimize compensation for all operating conditions.
SUPPLY CURRENT
The parallel configuration used in the PA74 and PA76 also
generates supply currents while high voltage sign waves are
seen at the output. Listed below are the supply currents expected
while running at a particular frequency and when VO ≈ 15Vpp,
note that the outputs are not loaded.
Frequency
100Hz
1KHz
5KHz
10KHz
15KHz
Supply Current
18mA
20mA
32mA
50mA
75mA
SOA
OUTPUT CURRENT FROM +V
S
or -V
S
, (A)
10
TWO AMPLIFIERS LOADED
DC, T
C
= 25°C
ONE AMPLIFIER
LOADED
1.0
DC, T
C
= 85°C
SATURATION OPERATION
The parallel configuration used in the PA74 and PA76 is
sensitive to operation in the saturation region. The PA74 and
PA76 may exhibit large peak currents; this is mainly due to
thermal protection limitations.
70
60
50
40
30
20
10
0
0
25
50
75
100
CASE TEMPERATURE, T
C
(°C)
125
ONE AMPLIFIER LOADED
POWER DERATING
TWO AMPLIFIERS LOADED
0.1
1.0
10
100
SUPPLY TO OUTPUT DIFFERENTIAL VOLTAGE, V
S
- V
O
(V)
SAFE OPERATING AREA (SOA)
The SOA curves combine the effect of all limits for this power
op amp. For a given application, the direction and magnitude
of the output current should be calculated or measured and
checked against the SOA curves. This is simple for resistive
loads but more complex for reactive and EMF generating loads.
THERMAL CONSIDERATIONS
Thermal grease or a Apex Precision Power TW03 thermal
washer, RCS = .1 to .2°C/W, is the only recommended interface
for the PA74/76. Internal power dissipation increases directly
with frequency therefore it is critical to sufficiently heat sink
the PA74 and PA76. Even unloaded the PA74 and PA76 can
dissipate up to 3 watts while running at higher frequencies.
4
INTERNAL POWER DISSIPATION, P (W)
PA74-76U
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