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4

OPERATING

CONSIDERATIONS

PA81J • PA82J

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.

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.

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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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.

PA81_82JU REV H MARCH 2001 © 2001 Apex Microtechnology Corp.