PA84A View Datasheet(PDF) - Cirrus Logic
Part Name
Description
MFG CO.
'PA84A' PDF : 5 Pages View PDF
PA84 • PA84A • PA84S
Product Innova tionFrom
GENERAL
Please read Application Note 1 "General Operating Consider-
ations" which covers stability, supplies, heat sinking, mounting,
current limit, SOA interpretation, and specification interpretation.
Visit www.Cirrus.com for design tools that help automate tasks
such as calculations for stability, internal power dissipation,
current limit and heat sink selection. The "Application Notes"
and "Technical Seminar" sections contain a wealth of informa-
tion on specific types of applications. Package outlines, heat
sinks, mounting hardware and other accessories are located
in the "Packages and Accessories" section. Evaluation Kits
are available for most Apex Precision Power product models,
consult the "Evaluation Kit" section for details. For the most
current version of all Apex Precision Power product data sheets,
visit www.Cirrus.com.
SAFE OPERATING AREA (SOA)
The bipolar output stage of this high voltage operational
amplifier has two output limitations:
1. The internal current limit which limits maximum available
output current.
2. The second breakdown effect, which occurs whenever the
simultaneous collector current and collector-emitter voltage
exceeds specified limits.
SOA
50
40
STEADY STATE
t = 5ms
t = 1ms
35
30
25
SAFE OPERATING AREA CURVES
20
150
170
200
250
300
SUPPLY TO OUTPUT DIFFERENTIAL VOLTAGE (V)
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. The following capacitive and inductive loads are safe:
±VS
150V
C(MAX)
1.2µF
L(MAX)
.7H
125V
6.0µF
25H
100V
12µF
90H
75V
ALL
ALL
2. Short circuits to ground are safe with dual supplies up to
±150V or single supplies up to 150V.
3. Short circuits to the supply rails are safe with total supply
voltages up to 150V (i.e. ±75V).
OUTPUT PROTECTION
Two external diodes as shown in Figure 1, are required
to protect these amplifiers against 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. Be
sure the diode voltage rating is greater than the total of both
supplies. 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.
FIGURE 1. PROTECTIVE,
+VS
INDUCTIVE LOAD
–VS
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.
STABILITY
Due to its large bandwidth the PA84 is more likely to oscillate
than lower bandwidth Power Operational Amplifiers such as
the PA83 or PA08. To prevent oscillations, a reasonable phase
margin must be maintained by:
1. Selection of the proper phase compensation capacitor and
resistor. Use the values given in the table under external
connections and interpolate if necessary.The phase margin
can be increased by using a large capacitor and a smaller
resistor than the slew rate optimized values listed in the
table. The compensation capacitor may be connected to
common (in lieu of +VS) if the positive supply is properly
bypassed to common. Because the voltage at pin 8 is only a
few volts below the positive supply, this ground connection
requires the use of a high voltage capacitor.
2. Keeping the external sumpoint stray capacitance to ground
at a minimum and the sumpoint load resistance (input and
feedback resistors in parallel) below 500Ω. Larger sumpoint
load resistance can be used with increased phase compen-
sation (see 1 above).
3. Connecting the amplifier case to a local AC common thus
preventing it from acting as an antenna.
4
PA84U
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