PA86 Datasheet, PDF (4/4 Pages) Cirrus Logic – POWER OPERATIONAL AMPLIFIER

PA86
OPERATING
CONSIDERATIONS
GENERAL
POWER SUPPLY BYPASSING
Please read Application Note 1 “General Operating Consid-
erations” which covers stability, power 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,
internal power dissipation, current limit, heat sink selection,
Apex’s complete Application Notes library, Technical Seminar
Workbook and Evaluation Kits.
Bypass capacitors to power supply terminals +Vs and –Vs
must be connected physically close to the pins to prevent lo-
cal parasitic oscillation in the output stage of the PA86. Use
electrolytic capacitors at least 1μF. Bypass the electrolytic
capacitors with high quality ceramic capacitors (X7R) 0.1μF
or greater.
SUPPLY CURRENT
SPECIAL CONSIDERATIONS
The PA86 features a class A/B driver stage to drive the output
MOSFETs and an innovative input stage to achieve very high
It is very important to realize that in order to achieve
high frequency performance the heat sink tab has to be
tied to a stable, low impedance reference, i.e. power sup-
ply or GND. An AC connection through a 0.1µF capacitor
is also sufficient. Internal to the PA86, the heatsink tab
Y _{is electrically isolated to more than 350V . This may help}
allay some electrical isolation concerns in tying the heat
sink to Vs or GND.
R CURRENT LIMIT
For proper operation, the current limit resistor, Rlim, must
A ^{be connected as shown in the external connections diagram.}
For maximum reliability and protection, the resistor should
be set as high as possible. The value of the resistor is calcu-
lated as follows, with Ilim in A; the maximum practical value
IN is 1500Ω.
Rlim = 0.7 / Ilim
SAFE OPERATING AREA
The MOSFET output stage of the PA86 is not limited by
IM ^{second breakdown considerations as in bipolar output stages.}
Only thermal considerations and current handling capabilities
limit the SOA. The output stage is protected against transient
flyback by the parasitic body diodes of the output stage MOS-
L _{FET structure. However, for protection against sustained high}
energy flyback external fast-recovery diodes must be used.
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slew rates. The supply current drawn by the PA86, even with
no load, varies with the slew rate of the output signal.
STABILITY
The PA86 is externally compensated and performance
can be tailored to the application. Use the graphs of small
signal response and power response as a guide. Due to the
innovative design of the PA86, two compensation networks
are required. The values of these components should be the
same to provide symmetric slew rate characteristics. The
compensation capacitor Cc must be rated at 500V working
voltage. NPO capacitors are recommended. The compensa-
tion networks CcRc must be mounted closely to the amplifier
pins x & y and z & w to avoid spurious oscillation.
The PA86 may require an external 33 pF capacitor (mini-
mum breakdown of 200 V) between CC- (pin 3) and –Vs (pin
5) to prevent oscillations in the falling edge of the output. This
capacitor is provided with the evaluation kit. Please refer to
EK60 datasheet for details.
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PA86U REV 3 FEBRUARY 2006  © 2006 Apex Microtechnology Corp.