OPERATING
CONSIDERATIONS
PA85 • PA85A
SAFE OPERATING CURVES
The safe operating area curves define the maximum ad-
ditional internal power dissipation the amplifier can tolerate
when it produces the necessary output to drive an external
load. This is not the same as the absolute maximum internal
power dissipation listed elsewhere in the specification since
the quiescent power dissipation is significant compared to
the total.
GENERAL
Please read Application Note 1 "General Operating Con-
siderations" which covers stability, supplies, heat sinking,
mounting, current limit, SOA interpretation, and specification
interpretation. Visit www.apexmicrotech.com for design tools
that help automate tasks such as calculations for stability, in-
ternal power dissipation, current limit and heat sink selection.
The "Application Notes" and "Technical Seminar" sections
contain a wealth of information 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 product
models, consult the "Evaluation Kit" section for details. For
the most current version of all Apex product data sheets, visit
www.apexmicrotech.com.
INPUT PROTECTION
Although the PA85 can withstand differential voltages up to
±25V, additional external protection is recommended. Since
the PA85 is a high speed amplifier, low leakage, low capaci-
tance JFETs connected as diodes are recommended (e.g.
2N4416, Q1-Q4 in Figure 2). The differential input voltage will
be clamped to ±1.4V. This is sufficient overdrive to produce
maximum power bandwidth.
CURRENT LIMIT
For proper operation, the current limit resistor (R
CL
) must be
connected as shown in the external connection diagram. The
minimum value is 1.4 ohm, however for optimum reliability the
resistor value should be set as high as possible. The value
is calculated as follows; with the maximum practical value of
30 ohms.
.7
R
CL
=
I
LIM
- .016
POWER SUPPLY PROTECTION
Unidirectional zener diode transient suppressors are recom-
mended as protection on the supply pins. The zeners clamp
transients to voltages within the power supply rating and also
clamp power supply reversals to ground. Whether the zeners
are used or not, the system power supply should be evaluated
for transient performance including power-on overshoot and
power-off polarity reversals as well as line regulation.
Conditions which can cause open circuits or polarity reversals
on either power supply rail should be avoided or protected
against. Reversals or opens on the negative supply rail is
known to induce input stage failure. Unidirectional transzorbs
prevent this, and it is desirable that they be both electrically
and physically as close to the amplifier as possible.
SAFE OPERATING AREA (SOA)
The MOSFET output stage of this power operational ampli-
fier has two distinct limitations:
1. The current handling capability of the MOSFET geometry
and the wire bonds.
2. The junction temperature of the output MOSFETs.
NOTE: The output stage is protected against transient flyback.
However, for protection against sustained, high energy flyback,
external fast-recovery diodes should be used.
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STABILITY
The PA85 is externally compensated and performance can
be tailored to the application. Use the graphs of small signal
response and power response as a guide. The compensation
capacitor C
C
must be rated at 500V working voltage. An NPO
capacitor is recommended. The compensation network C
C
R
C
must be mounted closely to the amplifier pins 7 and 8 to avoid
spurious oscillation.
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This data sheet has been carefully checked and is believed to be reliable,
SHANNON ROAD •
is assumed
ARIZONA 85741
omissions. All specifications are subject
1 (800) 546-2739
APEX MICROTECHNOLOGY CORPORATION • 5980 NORTH
however, no responsibility
TUCSON,
for possible inaccuracies
•
or
USA • APPLICATIONS HOTLINE:
to change without notice.
4
PA85U REV M OCTOBER 2003
© 2003 Apex Microtechnology Corp.
APEX [ CIRRUS LOGIC ]
