OPERATING
CONSIDERATIONS
PA78
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 PA78. Use
electrolytic capacitors at least 1μF. Bypass the electrolytic
capacitors with high quality ceramic capacitors (X7R) 0.1μF
or greater.
SUPPLY CURRENT
ThePA78featuresaclassA/Bdriverstagetodrivetheoutput
MOSFETs and an innovative input stage to achieve very high
slew rates. The supply current drawn by the PA78, even with
no load, varies with the slew rate of the output signal.
SPECIAL CONSIDERATIONS
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 PA78, the heatsink tab
is electrically isolated to more than 350V . This may help
allay some electrical isolation concerns in tying the heat
sink to Vs or GND.
STABILITY
The PA78 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 PA78, 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.
CURRENT LIMIT
For proper operation, the current limit resistor, Rlim, must
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
is 1500Ω.
The PA78 may require an external 33 pF capacitor (mini-
mum breakdown of 350V) between Cc- and –Vs to prevent
oscillations in the falling edge of the output. This capacitor
is provided with the evaluation kit. Please refer to EK60U
datasheet for details.
Rlim = 0.7 / Ilim
SAFE OPERATING AREA
The MOSFET output stage of the PA78 is not limited by
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-
FET structure. However, for protection against sustained high
energy flyback external fast-recovery diodes must be used.
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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.
4APEX MICROTECHNOLOGY CORPORATION • 5980 NORTH SHANNON ROAD • TUCSON, ARIZONA 85741 • USA • APPLICATIONS HOTLINE: 1 (800) 546-2739
PA78U REV 4 FEBRUARY 2006 © 2006 Apex Microtechnology Corp.
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