OPERATING
CONSIDERATIONS
PA08 • PA08A
GENERAL
Please read the “General Operating Considerations”, which
covers stability, supplies, heatsinking, mounting, current limit,
SOA interpretation, and specification interpretation. Additional
information can be found in the application notes. For informa-
tion on the package outline, heatsinks, and mounting hardware,
see the “Package Outlines” and “Accessories” sections of the
handbook.
SAFE OPERATING AREA (SOA)
The output stage of most power amplifiers has two distinct
limitations:
1. The current handling capability of the transistor geometry
and the wire bonds.
2. The second breakdown effect which occurs whenever the
simultaneous collector current and collector-emitter voltage
exceeds specified limits.
The SOA curves combine the effect of these limits. 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. However, the
following guidelines may save extensive analytical efforts.
1. Under transient conditions, the following capacitive and
inductive loads are safe with the current limits set to the
maximum:
±VSC(MAX) L(MAX)
150V .4µF 280mH
125V .9µF 380mH
100V 2µF 500mH
75V 10µF 1200mH
50V 100µF 13H
2. The amplifier can handle any EMF generating or reactive
load and short circuits to the supply rails or simple shorts to
common if the current limits are set as follows:
SHORT TO ±VSC, SHORT TO
±VSC, L, OR EMF LOAD COMMON
150V 20mA 67mA
125V 27mA 90mA
100V 42mA 130mA
75V 67mA 200mA
50V 130mA 200mA
These simplified limits may be exceeded with further analysis
using the operating conditions for a specific application.
200
150
100
80
50
40
30
2080 100 120 150 170 200 250 300
SUPPLY TO OUTPUT DIFFERENTIAL VOLTAGE (V)
OUTPUT CURRENT FROM
+V OR –V (mA)
S
steady state
t = 5ms
t = 1ms
t = 0.5ms
t = 200µs
S
3. The output stage is protected against transient flyback.
However, for protection against sustained, high energy
flyback, external fast-recovery diodes should be used.
INDUCTIVE LOADS
Two external diodes as shown in Figure 1, are required to
protect these amplifiers from flyback (kickback) pulses
exceeding the supply voltages of the amplifier when driving
inductive loads. For component selection, these external
diodes must be very quick, such as ultra fast recovery diodes
with no more than 200 nanoseconds of reverse recovery time.
The diode will turn on to divert the flyback energy into the supply
rails thus protecting the output transistors from destruction due
to reverse bias.
A note of caution about the supply. The energy of the flyback
pulse must be absorbed by the power supply. As a result, a
transient will be superimposed on the supply voltage, the
magnitude of the transient being a function of its transient
impedance and current sinking capability. If the supply voltage
plus transient exceeds the maximum supply rating or if the AC
impedance of the supply is unknown, it is best to clamp the
output and the supply with a zener diode to absorb the transient.
INPUT PROTECTION
The input is protected against common mode voltages up to
the supply rails and differential voltages up to ±50V. Increased
protection against differential input voltages can be obtained by
adding 2 resistors, 2 capacitors and 4 diode connected FETs
as shown in Figure 2.
CURRENT LIMITING
Proper operation requires the use of two current limit resis-
tors, connected as shown in the external connection diagram.
The minimum value for RCL is 3.24Ω. However, for optimum
reliability it should be set as high as possible. Refer to the
“General Operating Considerations” section of the handbook
for current limit adjust details.
PROTECTION, INDUCTIVE LOAD PROTECTION, OVERVOLTAGE
IN4936 OR
UES1106
+V
S
–V
S
100pf/200V
100pf/200V
150KΩ
150KΩ
+IN
–IN
Fig. 1 Fig. 2
+
_
Q1
Q2
Q3
Q4
This data sheet has been carefully checked and is believed to be reliable, however, no responsibility is assumed for possible inaccuracies or omissions. All specifications are subject to change without notice.
PA08U REV. I FEBRUARY 1998 © 1998 Apex Microtechnology Corp.