PA81J Datasheet, PDF (4/4 Pages) Cirrus Logic – HIGH VOLTAGE POWER OPERATIONAL AMPLIFIERS

OPERATING
CONSIDERATIONS
PA81J • PA82J
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.
SAFE OPERATING AREA (SOA)
For the PA80J and PA81J, the combination of voltage capa-
bility and internal current limits mandate that the devices are
safe for all combinations of supply voltage and load. On the
PA82J, any load combination is safe up to a total supply of 250
volts. When total supply voltage equals 300 volts, the device
will be safe if the output current is limited to 10 milliamps or
less. This means that the PA82J used on supplies up to 125
volts will sustain a short to common or either supply without
danger. When using supplies above ±125 volts, a short to
one of the supplies will be potentially destructive. When us-
ing single supply above 250 volts, a short to common will be
potentially destructive.
Safe supply voltages do not imply disregard for heatsinking.
The thermal calculations and the use of a heatsink are required
in many applications to maintain the case temperature within
the specified operating range of 0 to 70°C. Exceeding this case
temperature range can result in an inoperative circuit due to
excessive input errors or activation of the thermal shutdown.
INDUCTIVE LOADS
Two external diodes as shown in Figure 2, are required
to protect these amplifiers against flyback (kickback) pulses
exceeding the supply voltage of the amplifier when driving in-
ductive 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.
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.
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SINGLE SUPPLY OPERATION
These amplifiers are suitable for operation from a single sup-
ply voltage. The operating requirements do however, impose
the limitation that the input voltages do not approach closer
than 10 volts to either supply rail. This is due to the operating
voltage requirements of the current sources, the half-dynamic
loads and the cascode stage. Refer to the simplified schemat-
ics.Thus, single supply operation requires the input signals
to be biased at least 10 volts from either supply rail. Figure 3
illustrates one bias technique to achieve this.
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Figure 4 illustrates a very common deviation from true single
supply operation. The availibility of two supplies still allows
ground (common) referenced signals, but also maximizes the
high voltage capability of the unipolar output. This technique
can utilize an existing low voltage system power supply and
does not place large current demands on that supply. The 12
volt supply in this case must supply only the quiescent current
of the PA81J, which is 8.5mA maximum. If the load is reactive
or EMF producing, the low voltage supply must also be able to
absorb the reverse currents generated by the load.
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PA81_82JU REV H MARCH 2001 © 2001 Apex Microtechnology Corp.