APEX MICROTECHNOLOGY CORPORATION • 5980 NORTH SHANNON ROAD • TUCSON, ARIZONA 85741 • USA • APPLICATIONS HOTLINE: 1 (800) 546-2739
4
OPERATING
CONSIDERATIONS
PA84 • PA84A • PA84S
GENERAL
Please read Application Note 1 "General Operating Con-
siderations" which covers stability, supplies, heat sinking,
mounting, current limit, SOA interpretation, and specication
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 specic 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.
SAFE OPERATING AREA (SOA)
The bipolar output stage of this high voltage operational
amplier 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 specied 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. The following capacitive and inductive loads are safe:
±VS C(MAX) L(MAX)
150V 1.2µF .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).
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OUTPUT PROTECTION
Two external diodes as shown in Figure 1, are required
to protect these ampliers against yback (kickback) pulses
exceeding the supply voltages of the amplier 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 yback 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 yback
pulse must be absorbed by the power supply. As a result, a tran-
sient 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 oscil-
late than lower bandwidth Power Operational Ampliers 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 amplier case to a local AC common thus
preventing it from acting as an antenna.
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.
PA84U REV O OCTOBER 2006 © 2006 Apex Microtechnology Corp.