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4

OPERATING

CONSIDERATIONS

PA09 • PA09A

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,

internal power dissipation, current limit; heat sink selection;

Apex’s complete Application Notes library; Technical Seminar

Workbook; and Evaluation Kits.

SUPPLY VOLTAGE

The specified voltage (±V

having equal voltages. A nonsymmetrical (ie. +70/–10V) or a

single supply (ie. 80V) may be used as long as the total volt-

age between the +V

of the voltages of the specified dual supply.

SAFE OPERATING AREA (SOA)

The MOSFET output stage of this power operational ampli-

fier has two distinct limitations:

1. The current handling capability of the MOSFET geometry

and the wire bonds.

2. The junction temperature of the output MOSFETs.

SAFE OPERATING AREA CURVES

The SOA curves combine the effect of these limits and allow

for internal thermal delays. For a given application, the direc-

tion 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. The following guidelines may save

extensive analytical efforts:

1. Capacitive and inductive loads up to the following maximums

are safe:

±V

S

CAPACITIVE LOAD

INDUCTIVE LOAD

40V

.1µF

11mH

30V

500µF

24mH

20V

2500µF

75mH

15V

∞

100mH

2. Short circuits to ground are safe with dual supplies up to

±20V.

3. The output stage is protected against transient flyback.

However, for protection against sustained, high energy

flyback, external fast-recovery diodes should be used.

BYPASSING OF SUPPLIES

Each supply rail must be bypassed to common with a

tantalum capacitor of at least 47µF in parallel with a .47µF

ceramic capacitor directly connected from the power supply

pins to the ground plane.

OUTPUT LEADS

Keep the output leads as short as possible. In the video

frequency range, even a few inches of wire have significant

inductance, raising the interconnection impedance and limit-

ing the output current slew rate. Furthermore, the skin effect

increases the resistance of heavy wires at high frequencies.

Multistrand Litz Wire is recommended to carry large video

currents with low losses.

GROUNDING

Single point grounding of the input resistors and the input

signal to a common ground plane will prevent undesired current

feedback, which can cause large errors and/or instabilities.

"Single point" is a key phrase here; a ground plane should

be used as shielding rather than a current path. Leaving the

case of the PA09 floating will cause oscillations in some ap-

plications.

COMPENSATION

The PA09 is extremely flexible in terms of choice of com-

pensation capacitor for any given gain. The most common

ranges are shown in the COMPENSATION typical performance

graph. Swinging closer to the supply rails, heavier loads, faster

input signal rise and fall times and higher supply voltages all

tend to demand larger values of compensation capacitor. This

capacitor must be rated at least as high as the total voltage

applied to the amplifier. In making specific value choices, use

the square wave stability test presented in APPLICATION

NOTE 19, Figures 40 and 41.

In addition to small signal testing, if the application includes

step functions in the input signal, use this circuit to measure

large signal response. By increasing square wave amplitude to

the maximum of the application, this test may show significant

distortion of the output waveform following the square wave

transitions. In this case the faster input stages of the PA09

are out-running the output stage and overload recovery time

creates the distortion. This speed relationship is also why

slew rate does not increase for compensation values below

about 27pF.

SUPPLY CURRENT

When swinging large signals, the output stage of the PA09

demands extra supply current. The following graphs illustrate

this current for several conditions for both sine and square

wave signals. Current is exclusive of any load current and will

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